ALBERT

Notes: We report results of quantum wave packet calculations of the O(1D)+HCl(v=0,j)→ClO+H, OH+Cl, reactions for zero and nonzero total angular momentum, J, (using the centrifugal sudden approximation), and using a new fit to extensive ab initio calculations of a global potential [K. A. Peterson, S. Skokov, and J. M. Bowman, J. Chem. Phys. 111, 2445 (1999)]. Initial state-selected and cumulative reaction probabilities to form each set of products for J=0 are calculated by a direct summation of the initial state-selected reaction probabilities. We propose and test a simple energy-shifting approximation that relates the initial state-selected reaction probability for arbitrary j to the one for j=0. Extensions of standard J- and K-shifting methods are suggested and applied to both reaction channels. In doing this extension the adiabatic rotation approximation is used to determine the rotational barriers in the entrance and exit channels. The energy dependence of the reaction cross sections to form the two products is calculated for O(1D)+HCl(v=0,j=0) using J- and K-shifting and compared at two translational energies to results of quasiclassical trajectory calculations. The thermal rate constants for the two reaction channels are calculated from 200 to 400 K and compared to experiment. © 2000 American Institute of Physics.

Notes: An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for the ground and low-lying excited states of the HI molecule by employing a relativistic effective core potential for the iodine atom. The computed spectroscopic constants for the X 1Σ+ ground and b 3ΠΩ Rydberg states are in good agreement with available experimental data, as are the vertical excitation energies for the repulsive a 3Π1, a 3Π0+, and A 1Π1 states of the A band. The a 3Π0+ state is found to possess a shallow minimum of 600 cm−1 depth outside the Franck–Condon region, at (approximate)5.1 a0. The electric-dipole moments have also been calculated for transitions from the ground to the A band states. Contrary to what is usually assumed, the a 3Π1, A 1Π1←X0+ transition moments are found to depend strongly on internuclear distance. Employing the computed potential energy and transition moment data, partial and total absorption spectra for the A band are calculated and the I* quantum yields, ΦI*(ν), are determined as a function of excitation energy. The maximal ΦI*(ν) values are calculated to be 0.55–0.59 and lie at 39 000–40 000 cm−1, which agrees well with experimental results. The influence of the t 3Σ1+ state and of the nonadiabatic effects on the ΦI*(ν) values is found to be negligible in the essential part of the A band. Finally, it is shown that significantly higher I* quantum yield values (up to 0.8–0.9) may be achieved when vibrationally hot HI molecules are excited in the appropriate spectral range. © 2000 American Institute of Physics.

Notes: A new idea of controlling molecular photodissociation branching by a stationary laser field is proposed by utilizing the unusual intriguing quantum-mechanical phenomenon of complete reflection. By introducing the Floquet (or dressed) state formalism, we can artificially create potential curve crossings, which can be used to control molecular processes. Our control scheme presented here is summarized as follows. First, we prepare an appropriate vibrationally excited state in the ground electronic state, and at the same time by applying a stationary laser field of the frequency ω we create two nonadiabatic tunneling (NT) type curve crossings between the ground electronic bound state shifted up by one photon energy (h-dash-bar)ω and the excited electronic state with two dissociative channels. In the NT-type of curve crossing where the two diabatic potential curves cross with opposite signs of slopes, it is known that the complete reflection phenomenon occurs at certain discrete energies. By adjusting the laser frequency to satisfy the complete reflection condition at the NT type curve crossing in one channel, the complete dissociation into the other channel can be realized. By taking one- and two-dimensional models which mimic the HOD molecule and using a wave packet propagation method, it is numerically demonstrated that a molecule can be dissociated into any desired channel selectively. Selective dissociation can be realized even into such a channel that cannot be achieved in the ordinary photodissociation because of a potential barrier in the excited electronic state. © 2000 American Institute of Physics.

Notes: State-to-state rotational energy transfer of ground state NH(X 3Σ,v=0,J,N) in collisions with Ne is examined. NH is exclusively generated in the metastable NH(a 1Δ) state via photodissociation of hydrazoic acid at a wavelength of 266 nm. The strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm is used to generate single state NH(X 3Σ−,v=0,J,N) applying the stimulated emission pumping technique. The ground state radicals are detected after a certain delay time with laser induced fluorescence (LIF) using the intense NH(A 3Π←X 3Σ−) transition around 336 nm with respect to all quantum states. The collision induced energy flux between the different rotation and spin levels is studied in detail and a comprehensive set of state-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J,N) with Ne up to N=7 which include the effect of multiple collisions is given. The state-to-state rate constants are obtained by the use of an iterative integrated profiles method. We find a propensity for (ΔN=0, Δi=±1) and (ΔN=±1, Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. In most cases the energy transfer which changes the spin component and conserves the nuclear rotation quantum number N (ΔN=0, Δi=±1), is the most effective energy transfer in collisions with Ne. The energy dependence of the transition efficiency concerning only the nuclear rotation quantum number N obeys an energy-gap law (EGL). © 2000 American Institute of Physics.

Notes: The results of ab initio calculations of static electric properties of a series of cyclic conjugated ketones and thioketones are presented. Dipole moments, dipole polarizabilities, as well as the first and second hyperpolarizabilities, were evaluated at different levels of theory: SCF, CASSCF, MP2, CCSD, CCSD(T), and CASPT2 methods were used. Both MP2 and CASPT2 methods have been shown to perform well in the calculation of such properties. The dependence of the electric properties on the extent of the π-electron conjugation and on the replacement of the keto by the thioketo group is analyzed. Cyclic conjugated thioketones are shown to exhibit significant nonlinear electric properties which make them prospective building blocks for nonlinear optical materials. © 2000 American Institute of Physics.

Notes: Doppler-free UV-visible optical–optical double resonance polarization spectra of the 2 1Σu+←X 1Σg+ and C 1Πu←X 1Σg+ transitions in 7Li2 molecules were measured. In the case of the double minimum 2 1Σu+ state transitions to all vibrational levels (except the lowest two) localized in the inner potential well, to several levels in the outer well and some levels beyond the potential barrier were observed. Potential energy curves of the C 1Πu(v=0–10) state and the inner well of the 2 1Σu+ state are determined by the RKR method. A V(R) curve describing the whole potential of the 2 1Σu+ state (including both wells) is constructed using the Inverted Perturbation Approach (IPA). Tunneling splittings and energy shifts of the 2 1Σu+ levels were observed, and their magnitudes are well represented in a semiclassical model using the IPA potential. Perturbations between the C 1Πu(v,J) and 2 1Σu+(v′,J) levels were detected in the region above the potential barrier of the 2 1Σu+ state, where the rotational constant of the 2 1Σu+ state is much smaller than that of the C 1Πu state. © 2000 American Institute of Physics.

Notes: The study of the interaction energy and its many-body decomposition in a broad distance interval for the Ben, Mgn, and Can (n=2,3) clusters at the SCF and MP4 levels are performed. A comparative analysis of the obtained results allows one to conclude that the only stabilization factor in the dimers is the dispersion forces. So, the alkaline–earth dimers can be attributed to the van der Waals molecules. The trimers are stabilized by the two-body localized dispersion forces and three-body delocalized exchange forces. The binding in the alkaline–earth trimers has a mixed physical (van der Waals) and chemical (nonadditive exchange) nature. An NBO population analysis reveals a relatively large p-population in all clusters. A surprisingly large p-population at the MP4 level is also obtained for the isolated atoms. © 2000 American Institute of Physics.

Notes: We present a quasiclassical trajectory study of the title reaction using a potential energy surface that is derived from ab initio calculations, and which has previously been shown to yield accurate dynamical results for the H2+CN reaction. Results presented include integral and differential cross sections, and product vibrational and rotational distributions for ground and vibrational excited HCN. Vector correlations are also discussed. Detailed comparisons with all available experiments are presented, and most of the theoretical results are in excellent agreement with experiment. © 2000 American Institute of Physics.

Notes: We extract the exact on-resonance position and local dynamical information of the 1H network from a dramatic variation of the peak amplitude of 13C signals as a function of the carrier frequency of the decoupling field for 1H. Its Lorentzian dependence is explained within a simple theory. © 2000 American Institute of Physics.

Notes: We implemented a coarse-graining procedure to construct mesoscopic models of complex molecules. The final aim is to obtain better results on properties depending on slow modes of the molecules. Therefore the number of particles considered in molecular dynamics simulations is reduced while conserving as many properties of the original substance as possible. We address the problem of finding nonbonded interaction parameters which reproduce structural properties from experiment or atomistic simulations. The approach consists of optimizing automatically nonbonded parameters using the simplex algorithm to fit structural properties like the radial distribution function as target functions. Moreover, any mix of structural and thermodynamic properties can be included in the target function. Different spherically symmetric inter-particle potentials are discussed. Besides demonstrating the method for Lennard-Jones liquids, it is applied to several more complex molecular liquids such as diphenyl carbonate, tetrahydrofurane, and monomers of poly(isoprene). © 2000 American Institute of Physics.

Notes: We use molecular dynamics to study the nucleation of AgBr in water. After first testing our Born–Mayer–Huggins potentials for Ag+ and Br− by looking at bulk AgBr and at AgBr clusters in vacuo, we consider small numbers of Ag+ and Br− ions immersed in water. The system shows the expected qualitative features of nucleation form solution, including a critical cluster size that decreases with increasing concentration. However, we find that for cluster sizes at least as large as Ag18Br18, the most stable cluster is disordered. This is in stark contrast to clusters in vacuo where clusters as small as Ag4Br4 from ordered fragments of the lattice. These results lend some support to the conjecture that nucleation of crystals from solution is a two-stage process with the first stage consisting of the formation of disordered clusters of solute and the second stage involving the nucleation of a crystal from this solute "melt." © 2000 American Institute of Physics.

Notes: The ultrafast dynamics following one-photon UV photodetachment of I− ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump–probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ∼15 Å to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the "ejected" electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due to an electron in a contact pair. The contact pair is surprisingly long-lived and the nonadiabatic recombination is rather slow considering the proximity of the partners. Experiments in mixed solvents confirm our assignment of the two distinct ejection mechanisms. The detachment mechanism is therefore fundamentally different in the resonant (one photon) charge-transfer-to-solvent (CTTS) process from the multiphoton detachment of aqueous iodide ions, which bears more similarity to the direct solvent ionization. © 2000 American Institute of Physics.

Notes: A model is presented for the electrostatic component of solvatochromic shifts in vertical electronic excitation energies. The model, which makes use of the mean-field approximation, combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. The method is implemented at the multiconfigurational self-consistent field level. We present illustrative applications to the (n→π*)1 transitions of formaldehyde, acetaldehyde, and acetone in liquid water. The solvent shifts obtained compare well with other ab initio QM/MM calculations and when the electron correlation components are included with the experimental solvent shift, but differ from the results obtained with semiempirical QM/MM and continuum models. © 2000 American Institute of Physics.

Notes: We investigate the effect of roughness degree of a protein surface on the solvent diffusion near the surface by carrying out the molecular dynamics simulations. As expected, the solvent molecules experience the larger frictional drag on a more corrugated protein surface. However, the roughness degree of a protein surface can be measured differently by different solvent molecules. It is shown that on the same protein surface the smaller solvent molecules experience the larger frictional drag than the larger ones. The observed trends in the mobility of solvent molecules on the protein surface correlates nicely with the correlation dimension of the protein surface, which we proposed recently as a new measure of the molecular surface roughness.© 2000 American Institute of Physics.

Notes: The Jahn–Teller effect in X˜ 2E CH3O is theoretically investigated in considerable detail. Making use of ab initio calculations carried out at the MRC-level we present the global shape of the adiabatic potential surfaces. Both sheets of the Jahn–Teller split X˜ 2E electronic state are used to determine the linear, quadratic and bilinear coupling constants of the e and a1 modes, as well as selected third-order constants. Dynamical calculations for the vibronic structure have been performed with these parameters. The dynamical Jahn–Teller problem is solved by taking into account all the three e modes or, alternatively, two e modes and two a1 modes. The resulting vibronic spectra are compared with those of previous work. Furthermore, ab initio calculated transition moments for A˜ 2A1↔X˜ 2E are used to give an explanation for the forbidden Δj=〈fraction SHAPE="CASE"〉32 transitions which were found by different experimental groups. © 2000 American Institute of Physics.

Notes: The influence of elongational flow on the association behavior of binary mixtures of functionalized polymers capable of forming single reversible orientationally dependent bonds, such as hydrogen bonds, is studied analytically. Applying a mean-field approach with an external potential representing the effect of the elongational flow, the orientation distribution functions for the dumbbell model and the freely jointed model of a polymer chain were obtained. Two opposite factors determine the association of "linear" diblock copolymerlike chains: the unfavorable extra stretching under flow of associated polymer chains and the favorable orientation of the chains (segments) along the flow direction. The former dominates and the fraction of associated "linear" chains decreases with increasing flow rate. For mixtures of polymers which are capable of forming associated T-chains, the association also decreases, however, more slowly, and this time due to unfavorable orientational effects. If the formation of associated linear and T-polymers as well as complex linear/T-polymers is possible, a strong preference for the formation of associated T-chains is found. At high flow rates any type of association becomes unfavorable. © 2000 American Institute of Physics.

Notes: We consider the average injection rate from a metal electrode into disordered organic media, and its field dependence for a fixed density of carriers in the metal. At sufficiently high field the injection occurs through particular pathways, lowering the energetic barrier between the metal Fermi energy and the molecular levels of the organic layer. Assuming a Gaussian distribution of random energies, we find the dependence of current j on field E j∝exp(βE) in the relevant domain of electric fields. This behavior agrees both with numerical simulations of three-dimensional systems and with experimental data. © 2000 American Institute of Physics.

Notes: We investigate the relationship between the memory functions that arise in stochastic theories of fluctuations at equilibrium and those appropriate for an underlying microscopic (deterministic) description. We consider the class of stochastic theories that are Markovian with transition rates that satisfy the detailed balance condition. This class includes, for example, Smoluchowski dynamics, kinetic lattice gas models, and kinetic Ising models. When a time autocorrelation function is calculated using stochastic and deterministic descriptions, and the projection operator method of Mori is used, first and second order memory functions arise in both descriptions. We find a close and simple relationship between the first order memory functions of the two descriptions but not for the second order memory functions. Instead, the second order memory function of the microscopic description is simply related to the so-called irreducible memory function of the stochastic description. The latter was introduced for Smoluchowski dynamics by Cichocki and Hess and generalized by Kawasaki. This explains the empirical findings that for stochastic dynamics the irreducible memory function, rather than the second order memory function, has a more fundamental physical interpretation and is more useful for constructing mode coupling theories. © 2000 American Institute of Physics.

Notes: An efficient procedure based on the reproducing kernel Hilbert space interpolation method is presented for constructing intermolecular potential energy surfaces (PES) using not only calculated ab initio data but also a priori information on long-range interactions. Explicitly, use of the reciprocal power reproducing kernel on the semiinfinite interval [0,∞) yields a set of exact linear relations between dispersion (multipolar) coefficients and PES data points at finite internuclear separations. Consequently, given a combined set of ab initio data and the values of dispersion (multipolar) coefficients, the potential interpolation problem subject to long-range interaction constraints can be solved to render globally smooth, asymptotically accurate ab initio potential energy surfaces. Very good results have been obtained for the one-dimensional He–He potential curve and the two-dimensional Ne–CO PES. The construction of the Ne–CO PES was facilitated by invoking a new reproducing kernel for the angular coordinate based on the optimally stable and shape-preserving Bernstein basis functions. © 2000 American Institute of Physics.

Notes: The torsional dependence of the CH3 stretching and bending modes of methanol has been explored in terms of a local mode internal coordinate picture [X. Wang and D. S. Perry, J. Chem. Phys. 109, 10795 (1998)]. First, the torsional variations of the small-amplitude vibrational frequencies along the mass weighted intrinsic reaction coordinate from the top to the bottom of the torsional potential barrier were calculated by means of ab initio frequency projection utilizing GAUSSIAN 98. The resulting curves for the three C–H stretch ab initio frequencies as functions of the torsional angle cannot be reproduced by the original 3×3 local mode model incorporating stretch–torsion and stretch–stretch couplings at lowest-order only, but are well-fitted if the model is extended to include higher-order coupling terms. For the CH-bending modes, with internal coordinates chosen to give a high degree of localization, bend–torsion and bend–bend coupling parameters were determined from the ab initio projected frequencies, and were then used to predict torsional tunneling splittings. Just as observed for the C–H stretch modes, the two higher-frequency asymmetric CH-bend modes are predicted to have inverted tunneling splittings with reduced amplitudes, while the splitting pattern for the lower frequency symmetric-bend mode is predicted to be normal. © 2000 American Institute of Physics.

Notes: We present a practical scheme for the evaluation of nonstandard two-electron molecular integrals that appear in ab initio theories employing explicitly correlated wave functions with linear r12 terms ("linear r12" methods). In contrast with previous efforts, the target integrals are evaluated recursively via intermediates formulated solely in terms of Cartesian Gaussian functions. All working equations fit conveniently the framework of highly efficient Head-Gordon and Pople method of evaluation of electron repulsion integrals. Thus, only straightforward modifications of existing codes that employ HGP or HGP–PRISM scheme are necessary to implement our approach. High potential of the pathway is realized in a robust practical implementation. © 2000 American Institute of Physics.

Notes: In this communication, we present the first measurement of the electric dipole polarizability of alkali–halide clusters with one excess electron. The polarizability is strongly size dependent. Very large values are observed for certain sizes. Ionization potential measurement is also presented for comparison. © 2000 American Institute of Physics.

Notes: The classical limit is shown to provide a description exactly equivalent to the quantum mechanical one in the approximation where each electron is assigned to an orbital. Strictly speaking it is therefore not a limit but an alternative way of solving the problem. There are some merits of this reformulation, most notably in that it brings the phase of the orbitals to the forefront, on equal footing as the occupancies. This allows one to discuss, e.g., electron localization, in a clearer manner. But computationally the classical description is not superior. There will be a definite advantage for more realistic electronic Hamiltonians, i.e., for implementing configuration interaction, and/or when the nuclear motion is coupled to the electronic dynamics. In this paper we limit attention to a derivation and discussion of the simple orbital approximation. © 2000 American Institute of Physics.

Notes: We show that, for condensation in an almost ideal vapor, the nucleation theorem is essentially a consequence of the law of mass action. The usual form of the theorem, with the effects of the translational degrees of freedom of the cluster included, is then derived using statistical mechanics and molecular theory, but only under the assumptions that the cluster excludes a volume to the surrounding vapor and that the vapor is ideal. The form of the result obtained via molecular theory is such that it appears unlikely (but not impossible) that the theorem remains valid for cases when the vapor is nonideal. This suggests that further work is necessary before the theorem can be regarded as established. We also consider the effects of the presence of a carrier gas. © 2000 American Institute of Physics.

Notes: Within the framework of the quantum phase-space representation established by Torres-Vega and Frederick, the rigorous solutions of the Schrödinger equation of the diatomic molecule oscillator with an empirical potential function are solved and discussed, and the Heisenberg uncertainty principle is interpreted in this physical system. © 2000 American Institute of Physics.

Notes: We present new calculations on the Renner–Teller induced decay of the vibrational states of HCO(A˜ 2A″) using accurate ab initio potential energy surfaces. The dynamics calculations are performed by employing filter diagonalization and an absorbing optical potential in the exit channel. The objective of this investigation is twofold: the completion of earlier time-dependent wave packet calculations by determining resonance widths for all vibrational states for projection quantum number K=1—up to 2.75 eV above the H+CO(re) dissociation threshold—and the determination of the widths for the long-lived K=0 states. In the latter case, a clear-cut J2(J+1)2 dependence, where J is the total angular momentum, is observed indicating that the rate determining step is K-resonance interaction between K=0 and 2 states. The experimentally observed J-independent contribution (0.22–0.5 cm−1), which dominates the linewidth for small values of J, is not accounted for by our calculations. Arguments are put forward, that it is caused by spin–orbit interaction, which is not included in our treatment. © 2000 American Institute of Physics.

Notes: Microwave-infrared double-resonance spectroscopy has been used to probe the solvation environment and its influence on the rotational relaxation of a cyanoacetylene molecule embedded in a superfluid 4He nanodroplet. The results support a model in which (within any given rotational state) the guest molecules are distributed over a set of spectroscopically inequivalent states which are most likely "particle-in-a-box" states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave double-resonance data suggests that transitions between these states occur at a rate which is comparable to the rotational relaxation rate, but not fast enough as to produce motionally narrowed, homogeneous absorption lines. The relative intensities of the rotational lines in the microwave-infrared double-resonance spectra are observed to depend strongly on the average droplet size. In the large droplet limit we can explain the observed pattern by invoking a "strong collision" regime, i.e., one in which the branching ratios of the rotational relaxation do not depend on the initial rotational state. For small droplets we speculate that, because of finite size effects, the density of (surface) states may become discontinuous, producing deviations from the "thermal" behavior of the larger systems. © 2000 American Institute of Physics.

Notes: The results of theoretical calculations for the ground state and low-lying excited states of SiCu, SiAg, and SiAu, and their ions SiCu+, SiAg+, SiAu+ and SiCu−, SiAg−, SiAu− are presented. Calculations were carried out with high-level correlated methods including relativistic corrections at the level of the Douglas–Kroll approximation. The ground state data are compared with the recent experimental findings and they differ in the assignment of the ground-state symmetry. All neutral silicides are predicted to have the electronic ground state of 2Π symmetry, in agreement with earlier theoretical data. The neutral species and both negative and positive ions of silicides are found to be quite stable in the ground electronic state and in several low-lying excited states. The relativistic effects bring significant contribution to the stabilization of the gold silicide and its ions in all electronic states investigated in this paper. © 2000 American Institute of Physics.

Notes: The ultrafast optical Kerr-response of water and heavy water has been measured at 1 bar in the temperature range between 273 and 373 K. The nuclear Kerr response of the liquid exhibits a pronounced double exponential decay on longer time scales after dephasing of impulsively perturbed acoustic modes is completed. The time constant, τ2, characterizing the slowly decaying exponential component of the Kerr-response function is in quantitative agreement with rotational diffusion time constants of the water molecules obtained form nuclear magnetic resonance (NMR) spin-lattice relaxation rates. A detailed comparison with THz time domain spectroscopy demonstrates that the reorientational dynamics responsible for the long time tail of the Kerr response are due to single molecule as opposed to collective effects. Furthermore, a good agreement between the single molecule rotational diffusion and the Stokes–Einstein–Debye equation is found in the temperature range of thermodynamic stability of the liquid. The time constant, τ1, characterizing the fast exponential component of the Kerr-response of water is found to be in qualitative agreement with central Lorentzian linewidths obtained from frequency-domain, depolarized Raman scattering experiments. The temperature dependence of τ2 does not follow an Arrhenius-type behavior, which was previously taken as evidence for thermally activated crossing of a librational barrier with concomitant hydrogen-bond breakage. Instead, the temperature dependence of the fast relaxation time constant can be represented adequately by the Speedy–Angell relation which has been shown to accurately describe a number of transport parameters and thermodynamic properties of water. © 2000 American Institute of Physics.

Notes: Ab initio calculations have been performed in order to investigate a recently proposed polarizable model [M. C. C. Ribeiro, Phys. Rev. B 61, 3297 (2000)] for molecular dynamics (MD) simulation of the molten salt Ca0.4K0.6(NO3)1.4. On the basis of the electronegativity equalization method, polarization effects in the MD simulations have been introduced by a fluctuating charge (FC) model for the nitrate ion. Partial charges in the nitrate ion are obtained by ab initio calculations at several levels of theory, and compared with previously proposed models for MD simulations of nitrate melts. Charge fluctuation is achieved in the ab initio calculations by using positive probe charges placed around a nitrate ion. The parameters of the FC model are corroborated by comparison of the ab initio partial charges with the ones obtained directly by the electronegativity equalization method. Simulated annealing of a cluster including two double-charged cations and two nitrate ions shows that very different structures are obtained depending on whether the FC model or its nonpolarizable counterpart is considered. Ab initio calculations show that the structure of this cluster is strongly dependent on polarization effects in the nitrate ions. © 2000 American Institute of Physics.

Notes: Neutron diffraction measurements were carried out on ∼1.4 molal solutions of NiCl2 in methanol under ambient conditions. First- and second-order difference methods with isotopic substitution on nickel, chlorine and hydroxyl hydrogen (Ho) were applied to derive the Cl−-dependent partial structure factors and radial distribution functions, RDFs. The nearest-neighbor Cl–(H/D)o distance and the solvation number of Cl− were determined to be 2.18(1) Å and ≅2, respectively, as compared to 2.21(3) Å and 3.6 in a 5.8 M solution of LiCl in methanol. The joint analysis of the pair RDFs, gNiCl(r) and gClCl(r), reveals that the electrolyte in its 1.4 molal methanolic solution mainly exists in the form of solvated contact ion pairs, [NiCl]s+, with very small content of solvated [NiCl2]s〈sup ARRANGE="STAGGER"〉o ion-triples. © 2000 American Institute of Physics.

Notes: Ab initio calculations, of the ground and low-lying electronic states of O4 along the minimum energy paths (MEP) for the reactions O2(X 3Σg−)+O2(X 3Σg−)→O4(X 1A1) and O2(X 3Σg−)+O2(X 3Σg−)→O3(X 1A1)+O(3P) have been performed. Our CASSCF(16,12)+CASMP2 calculations using the 5s4p3d2 f ANO basis set provide a solid basis to establish the stability of the O4 chemically bound molecule. Surface crossings between singlet and triplet states have been found and further characterized by evaluating their spin–orbit coupling matrix elements. Our calculations show that for the ozone formation reaction there should be spin–orbit coupling transitions allowing vibrational to electronic energy transfer around the saddle point region. The formation of vibrationally excited O2(a 1Δg,v) is predicted to contribute to the dark channel reported in experiments. © 2000 American Institute of Physics.

Notes: In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH4 and CH3D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a 1A1 and not in the ground 3B1 state. © 2000 American Institute of Physics.

Notes: The first experimental study of the spectral dependence of the nucleation of cesium vapor caused by light absorption is carried out in a diffusion cloud chamber. The spectral dependence of the nucleation rate is compared with the absorption and ionization spectra of cesium vapor. The results evidence that the observed structure in the nucleation spectrum at photon energies above the ionization threshold is correlated to the light-induced ionization of cesium dimers Cs2+. At lower photon energies light-induced nucleation can be either due to generation of ions by direct ionization of cesium clusters, to various two-step ionization processes, or to optical excitation of cesium atoms. The spectral dependence of the nucleation rate provides a new and powerful tool to study ionization spectra of metal vapors. In addition to results obtained by conventional methods a more consolidated insight into energy levels of atoms and clusters under equilibrium conditions is possible. © 2000 American Institute of Physics.

Notes: The one-particle models of crystal-field theory provide a qualitative interpretation for the observed ground state splitting of four Kramers doublets of the 8S7/2 of Bk4+ doped into CeF4. A set of nine nonzero (Bqk) parameters corresponding a C2v point symmetry provide a very good correlation between the experimental data and simulated energy level schemes within a rms deviation of 13.7 cm−1. The calculated and experimental energy values have the same order-of-magnitude for the ground state and excited components. The total ground state splitting of the S-state ions of f-elements such as Bk4+ in CeF4 is larger when compared with Cm3+:LaCl3 and Gd3+:La(C2H5SO4)3⋅9H2O ions. The so-called crystal-field strength parameter, Nv, increases as a function of the increasing maximum splitting of the ground state due to a decrease in the participation of the pure 8S7/2 in the final composition of the ground state eigenvector. © 2000 American Institute of Physics.

Notes: We introduce a potential model for MgCO3 and CdCO3 which reproduces experimental structural parameters and solution energies. We employed atomistic simulation techniques to model the absorption and segregation of magnesium and cadmium ions to the low-index surfaces of calcite. Both magnesium and cadmium ions absorb at the surfaces from solution. The absorption energies are surface dependent, due to distinct relaxations of the different surfaces, and ion dependent with the smaller sized magnesium ion showing larger relaxations. The absorption energies are larger for magnesium than for cadmium, mainly due to the closer coordination of the water molecules to the magnesium substituted surfaces. The cadmium ions segregate to the bulk crystal more easily than the magnesium ions which prefer to remain at the surface. However, segregation energies for both cations in second and further layers of some surfaces are positive, indicating that once a calcium carbonate layer has overgrown the substituted surface layer, segregation to the bulk is energetically possible. © 2000 American Institute of Physics.

Notes: We report Monte Carlo simulations of dipolar hard-sphere fluids with a nonuniform distribution of the dipole strengths or different hard-sphere diameters. Dipolar polydispersity significantly reduces the spontaneous orientational order observed in the ferroelectric phase of monodisperse dipolar hard-sphere fluids at high densities and low temperatures. Equimolar binary mixtures of dipolar hard spheres of different diameters σa and σb also exhibit ferroelectric order. However, the order parameters are very dependent on the ratio σa/σb. Chains of dipolar hard spheres, typical of strongly interacting dipolar hard spheres at low densities and low temperatures, were observed in binary fluids of dipolar and neutral hard spheres of different sizes. © 2000 American Institute of Physics.

Notes: Molecular field theory predicts the induction of a smectic A phase by the application of a field, either magnetic or electric, to a nematic phase. This intriguing behavior results from an enhancement of the orientational order which is coupled to the translational order and so shifts the smectic A-nematic transition. To test this prediction we have investigated a system of Gay–Berne mesogenic molecules subject to an applied field of second rank using isothermal–isobaric Monte Carlo simulations. The results of our calculations are compared with the Kventsel–Luckhurst–Zewdie molecular field theory of smectogens, modified to include the effect of an external field. We have also used the simulations to explore the possibility of inducing more ordered smectic phases with stronger fields. © 2000 American Institute of Physics.

Notes: Using scaling theory, we investigate the equilibrium behavior of a melt of functionalized chains, nonfunctionalized polymers, and two infinite, planar surfaces, which model clay sheets. The functionalized chains contain two end groups (one at each chain end) that are highly attracted to the surfaces. We calculate the free energy of the system as a function of the separation between the surfaces for various values of the following parameters: the end-group adsorption energy, the length and volume fraction of the end-functionalized chains, and the interaction energy between the different chains. For polymers confined between the surfaces, we also determine the fraction of loops, tails, bridges, and free chains within this gap. Finally, we isolate conditions for the formation of "exfoliated," "intercalated," and immiscible polymer/clay mixtures. © 2000 American Institute of Physics.

Notes: The derivation of solute diffusion coefficient from the quasiequilibrium probability density function of the underlying stochastic quantity-characteristic size of subcritical solute clusters, was carried out within the classical formalism of metastable state relaxation (nucleation). It has been demonstrated that (a) the solute diffusion coefficient is necessarily concentration dependent; (b) the concentration dependent solute diffusivity is the function of only two parameters, solute diffusivity at saturation and total surface energy of a solute molecule. This energy, in principle, can be also a function of solute concentration for supersaturated solutions such as protein supersaturated solutions. In addition to that it has been demonstrated for supersaturated solutions of the proteins such as lysozyme and Bovine Pancreatic Trypsin Inhibitor (BPTI) solutions that (a) the total surface energy of lysozyme and BPTI molecules in the metastable state is concentration dependent. (b) The solute diffusivity as a function of solute concentration, observed in supersaturated protein solutions such as lysozyme and BPTI, can be accurately reproduced (with error less than 1.2%) on the base of classical theory of the metastable state relaxation, given that the surface energy of the protein molecules is concentration dependent. © 2000 American Institute of Physics.

Notes: We have used atomic force microscopy to investigate the patterns obtained by isothermal crystallization of quasi-2-D monolayers of poly(ethylene oxide) adsorbed onto bare silicon wafers. These monolayers were prepared by a pseudo-dewetting process. Fingerlike branched structures were observed with a characteristic width which increased rapidly with crystallization temperature. The patterns are explained by a simple model considering the interplay of transport on the surface and the probability of attachment to the crystal. The influence of molecular weight was also studied. The average stem length increases with increasing temperature for the longer polymer. The shorter one crystallized in the (almost) fully extended form for all temperatures investigated. Temperature jumps during crystallization resulted in patterns with two different length scales. © 2000 American Institute of Physics.

Notes: The flexibility of a model polyimide, pyromellitic-dianhydride 1,4-oxydianiline (PMDA-ODA) is investigated using fully ab initio methods. Hartree–Fock, second-order Møller–Plesset theory (MP2) and density-functional theory (DFT) methods are employed along with both numerical and Gaussian basis sets. A hybrid scheme which combines energies and first derivatives is also used and appraised for this system. Energies of monomer fragments of the polyimide as a function of torsional angle are calculated using geometry optimizations. Extensive comparisons are made with maleanil, a smaller fragment of PMDA-ODA. © 2000 American Institute of Physics.

Notes: The coil α-helix (and reverse) transition in peptides is modeled as a sequential diffusive kinetic process in which the fundamental event is the diffusion back and forth over a square barrier to propagate or dissolve a single hydrogen bond. The model is solved exactly numerically in one-dimension (the reaction coordinate), for helix and coil probabilities as a function of (1) time, (2) the number of hydrogen bonds, and (3) temperature. In addition, a modified first-passage time is calculated as the time scale of the coil to helix transition. The results of the diffusion model calculations are compared with recent experiments and we show how the model may give insight into protein folding kinetics. The mechanistic diffusion model complements the Master equation model applied previously to the coil–helix folding problem and provides insight into the choice of a useful reaction coordinate for the process. © 2000 American Institute of Physics.

Notes: A novel simulation model for crystallization of polymer mono-layers is presented and compared with experiments on short PEO chains. We assume that crystallized chains can exist in states of different degrees of order. The resulting morphologies are nonequilibrium intermediate structures which can further relax on increasingly long time scales. The parameters of the model are the maximum stretching length of the chains M, the energy of the crystal–crystal interaction in terms of the Metropolis rate p0 and the barrier for increasing the degree of chain order pS. Since flatly adsorbed chains in a mono-layer must be oriented upright in order to crystallize, diffusion controlled growth patterns emerge which change their morphology depending on the model parameters. Different morphologies can be used to compare the model phase diagram with experiments. For large values of M, the interplay of the ordering barrier pS and the unbinding barrier p0 controls the growth morphologies as well as internal structures such as the average stem length L. We find the variation of L with temperature in agreement with empirical relations proposed from experimental observations. When the free chains of the reservoir are exhausted, the edges of the crystal start to relax into energetically favored states which stabilize the growth patterns at long time scales. Such patterns are frequently observed in experiments. For high barrier factors the chains are poorly ordered and relax into a pronounced hole-rim morphology as observed experimentally at low temperatures. At higher temperatures secondary relaxation processes result in a destruction of the growth morphology into dropletlike structures having a higher degree of internal chain order. © 2000 American Institute of Physics.

Notes: One-color, two-color resonant two-photon ionization (R2PI), and mass analyzed threshold ionization (MATI) spectroscopic methods have been used to study the electronic transition and the threshold ionization of the 35Cl and 37Cl isotopomers of p-chloroaniline. The band origins of the S1←S0 electronic transitions of both species are found to be 32 573±1 cm−1. The ionization energies (IEs) of both isotopomers of p-chloroaniline are determined to be 62 409±7 cm−1 by the two-color R2PI spectroscopy and 62 410±4 cm−1 by the MATI spectroscopy. Analyses on the spectral features show that most of the active modes are related to the in-plane ring vibrations in the S1 state and cationic ground state. Isotope effect on the ring deformation vibrations 1, 6a, and 12 gives rise to a frequency shift of 1–3 cm−1 in the S1 state and 3–9 cm−1 in the ion state. The experimental results are well supported by the computed ones on the basis of ab initio and density functional theory calculations. © 2000 American Institute of Physics.

Notes: The valence state dependence of the Ar–HF interaction potential is extended to υHF=4. Three new ArHF (υHF=4) states, (4000), (4100), and (4110), are observed between 14 780 and 14 880 cm−1 using intracavity laser induced fluorescence. The term values and rotational constants of these states are the following: (4000) ν0=14 783.603 23(30) cm−1, B=0.103 606 8(68) cm−1; (4100) ν0=14 867.419 06(70) cm−1, B=0.102 612(27) cm−1; and (4110) ν0=14 875.048 30(39) cm−1, B=0.103 217(19) cm−1, respectively. The spectral red shifts of ArHF (υ000) dramatically increase from 9.654 cm−1 at υ=1 to 48.024 cm−1 at υ=4. The rotational constant of ArHF(4000) increases essentially linearly with HF valence excitation, becoming 1.3% (40 MHz) greater than that observed at υ=0. At υ=4, the outer classical turning point of HF is extended by 0.4 Å from re, and there is no evidence for Ar–H repulsion. The spectral red shift for linear hydrogen bonded Ar–HF(υ000) indicates a strong enhancement of binding energy upon HF valence bond excitation, while the rotational constant reveals an almost surprising decrease in heavy atom separation. Both the T-shaped ArHF(υ110) and antilinear Ar–FH(υ100), however, show very little dependence of binding energy upon υHF valence excitation. These observations are in good accord with the ab initio intermolecular potential surface. © 2000 American Institute of Physics.

Notes: We have investigated seven different reaction channels for each of the two symmetric charge-state reactants CO+N2 and N2++CO. For all the 14 reactions we have measured the integral cross sections as a function of relative energy and determined the corresponding reaction enthalpies. We make use of these values to find the heats of formation of NCO, NCO+, CNN, and CNN+. Similar branching ratios are observed for both charge-state reactions. This gives some hints that all reactions proceed through the same intermediate N2CO+. © 2000 American Institute of Physics.

Notes: State-selected photodissociation dynamics of CS2+ molecular ion has been investigated by an optical–optical double resonance technique. The CS2+ molecular ions were prepared by [2+1] REMPI and the predissociative C˜ 2Σg+ state was populated by the B˜ 2Σu+←X˜ 2Πg,3/2 followed by C˜ 2Σg+←B˜ 2Σu+ transition. The product branching ratios, CS+/S+, for selected vibrational levels in the C˜ state and average kinetic energy releases in the CS+ and S+ production channels were measured from the time-of-flight mass spectra. It has been found that excitation of the bending vibration enhances the CS+ production channel more than the S+ channel. In addition, an isotropic fragment distribution for different polarizations of the dissociating laser light has been observed, from which the dissociation time was estimated to be longer than 20 ps. Dynamical aspect of the reaction revealed by the present investigation is discussed together with the previous spectroscopic results. © 2000 American Institute of Physics.

Notes: Vibrationally mode-selected phenol cations (C6H5OH+ and C6D5OH+) were reacted with ND3 in a guided-ion-beam instrument. Integral cross sections and recoil velocity distributions are reported as a function of collision energy and vibrational state. Three reactions are observed. A small signal is found for the [PhOH:ND3]+ adduct at low total energies, indicating the formation of a very long-lived complex. The major reaction is H/D exchange, generating PhOD++ND2H. Exchange is ∼40% efficient at low energies, strongly inhibited by collision energy, and strongly enhanced by excitation of PhOH+ vibrations. Recoil velocity distributions suggest that H/D exchange proceeds through a statistical complex at all energies. A precursor complex is invoked to explain the energy and vibrational state dependence. The endoergic proton transfer reaction is a minor channel at all energies, with dynamics intermediate between the direct and complex limits. Quantum chemistry and RRKM calculations are reported, providing an additional mechanistic insight. © 2000 American Institute of Physics.

Notes: High-resolution photoelectron spectroscopy is used to study the branching ratios for vibrational autoionization of Rydberg states of aniline (C6H5NH2) converging to the ground electronic state of the ion. By using two-color double-resonance excitation, it is possible to prepare autoionizing resonances in which two different vibrational modes are excited. Determination of the vibrational state distribution in the product ion provides information on the relative rates of autoionization for the two modes. It is found that some normal modes appear to be especially effective at promoting vibrational autoionization, while others appear to be completely ineffective. © 2000 American Institute of Physics.

Notes: The first rigorous, variational results for the nonadiabatic (i.e., non-Born–Oppenheimer) ground states of the six isotopomers of the hydrogen molecule are reported. Ground-state energies in Hartrees are: H2[−1.164 025 023 2] [this result was reported by us earlier in Phys. Rev. Lett. 83, 2541 (1999)], D2[−1.167 168 78], T2[−1.168 535 65], HD[−1.165 471 906], HT[−1.166 002 033], and DT[−1.167 819 642]. Expectation values for the kinetic and potential energies, the internuclear distance and the square of the internuclear distance, the virial coefficient, and the square of the energy gradient norm for the optimized wave functions are also reported. The calculations were performed with a direct nonadiabatic variational approach using a new diatomic correlated Gaussian basis set exponentially dependent on interparticle distances and including pre-exponential powers of the internuclear distance. © 2000 American Institute of Physics.

Notes: The potential energy surface (PES) for the 2,2′-bithiophene molecule was investigated using Hartree–Fock, correlated MP2, MP4(SDQ), CCSD, and density functional theory levels. Distinct basis sets ranging from double-zeta to triple-zeta quality, with polarization functions added on all atoms, were employed as well as the Dunning correlated consistent polarized valence double-zeta (cc-pVDZ) basis set. Single point configuration interaction CISD calculations were also performed using the cc-pVDZ basis set. Harmonic frequency calculations were performed for the unambiguous characterization of the stationary points located on the PES and also to calculate thermal Gibbs free energy corrections. Regarding the structural predictions we found that the B3LYP/6-311G** and MP2/cc-pVDZ fully optimized geometries exhibit the best agreement with the gas phase electron diffraction data. The calculated B3LYP/6-311G**, MP2/cc-pVDZ and experimental torsional angle for the syn-gauche structure are, respectively, 37.4° (B3LYP), 39.9° (MP2), and 36°±5° (expt.) with the corresponding values for the anti-gauche form being, respectively, 150.3° (B3LYP), 146.0° (MP2), and 148°±3° (expt.). The relative energy between the two minima and torsional barriers are sensitive both to the size of the basis set and the level of the quantum mechanical method used. Therefore, larger basis sets are needed to assess the ability of the DFT approach for describing torsional barriers. The MP4(SDQ) and CCSD relative energy results, reported in this work, can be considered as the most reliable torsional potential data available for the 2,2′-bithiophene molecule. Our results indicate that the experimentally estimated relative energy value for the two equilibrium structures present on the PES for the bithiophene molecule, and consequently the relative abundance of the anti-gauche species, is somewhat underestimated. By comparison with MP4(SDQ) and CCSD results we have shown that single point DFT/6-311G** calculations using HF/6-31G* geometries is the most computationally efficient procedure to study bithiophene like systems, with energy barriers agreeing within 2 kJ/mol. © 2000 American Institute of Physics.

Notes: We calculate the ground state and a class of zero orbital angular momentum (L=0) vibrationally excited state energies for NeN and ArN clusters using an adiabatic hyperspherical representation to solve the nuclear Schrödinger equation. The Schrödinger equation in the hyperangular coordinates is solved for a sequence of fixed hyperradii by diffusion Monte Carlo techniques, which determines the lowest effective potential curve. We monitor structural properties such as the pair and angle distribution as a function of the hyperspherical radius. These structural studies allow us to identify configurational changes as the N atom cluster fragments into an (N−1)-atom cluster plus an atom. We also determine separately the ground state of the full 3N-dimensional nuclear Schrödinger equation for the ground state, and compare the resulting structural properties with those calculated in the adiabatic hyperspherical approximation. © 2000 American Institute of Physics.

Notes: The self-epitaxial nucleation and growth origin of the formation and transformation of crystalline fractal pattern is identified for the first time. Two universal parameters, so-called relative elemental branching period 〈l˜〉 and the anisotropy or structural match parameter m, are introduced to describe the pattern transformation. These two parameters can be predicted according to our model. Fractal patterns arise when 〈l˜〉≤1. Fractal patterns have a strong anisotropy (m→1) at relatively low surface supersaturations, and a very weak anisotropy (m→−1) at high surface supersaturations. The theoretic results are in excellent agreement with experiments. © 2000 American Institute of Physics.

Notes: Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial fields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electrostatic plus Pauli) repulsion, and donation and back-donation orbital components. The increasing tendency towards multifold CO coordination seen at more negative fields is due primarily to enhanced back-donation. The decreasing propensity for atop vs multifold CO binding seen in moving from the lower-left to the upper-right Periodic corner of the Pt-group elements is due to the combined effects of weaker donation, stronger back-donation, and weaker steric repulsion. The uniformly hollow-site binding seen for NO arises from markedly stronger back-donation and weaker donation than for CO. The metal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field ("Stark-tuning") slopes. Decomposition of the DFT bond frequencies shows that the redshifts observed upon chemisorption are due to donation as well as back-donation interactions; the metal-dependent trends, however, are due to a combination of several factors. While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding site and metal again reflect the interplay of several interaction components. © 2000 American Institute of Physics.

Notes: Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular, PN(z), and parallel components, PT(z), of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ in the expression for PT(z) (z denotes the distance from the wall), but yield the same formula for the normal component PN(z). When evaluated by molecular dynamics (or Monte Carlo)-simulations PN(z) is constant, as required by mechanical stability. Contrary to that, the IK1-method leads to strong oscillations of PN(z). However, all methods give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, whereas the so-called surface of tension, zs, depends on the applied method. The difference is small for the IK- and H-methods, while the IK1-method leads to values that are in conflict with the interpretation of zs as the effective position of the interface. © 2000 American Institute of Physics.

Notes: A low-resolution structural model based on the packing geometry of α-carbons is utilized to establish a connection between the flexible and rigid parts of a folded protein. The former commonly recognizes a complementing molecule for making a complex, while the latter manipulates the necessary conformational change for binding. We attempt analytically to distinguish this control architecture that intrinsically exists in globular proteins. First with two-dimensional simple models, then for a native protein, bovine pancreatic trypsin inhibitor, we explicitly demonstrate that inserting fluctuations in tertiary contacts supported by the stable core, one can regulate the displacement of residues on loop regions. The positional fluctuations of the flexible regions are annihilated by the rest of the protein in conformity with the Le Chatelier–Braun principle. The results indicate that the distortion of the principal nonbonded contacts between highly packed residues is accompanied by that of the slavery fluctuations that are widely distributed over the native structure. These positional arrangements do not appear in a reciprocal relation between a perturbation and the associated response; the effect of a movement of residue i on residue j is not equal to that of the same movement of residue j on residue i. © 2000 American Institute of Physics.

Notes: The rotator-II to rotator-I phase transition in alkanes is studied within Landau theory. The effect of pressure on the transition is also examined. The transition is found to be first order, even at elevated pressure, with the transition temperature increasing with pressure. The various thermodynamic quantities are calculated near the transition. The Landau coefficients are calculated for various chain lengths from experimental data. The values of the coefficients are decreasing with increasing chain length. © 2000 American Institute of Physics.

Notes: Density-functional theory with interaction site model is applied to study binary mixtures consisting of Lennard-Jones atoms with hard-spherical cores (monomers) and bonded Lennard-Jones atoms of two hard-sphere sites (dimers). Strong amphiphilic interaction is shown to result in lamellae in the bulk condensed phase. A phase diagram showing disorder-lamellar phase boundaries and tricritical points is presented for a system with reduced interaction between the molecular species. Density distributions in the lamellar phase indicate that the mole fraction in the lamellae is determined by the concentration of the monomers. © 2000 American Institute of Physics.

Notes: Fluorescence decay of gaseous oxalylfluoride (COF)2 excited to the A˜ 1Au(7151) level was measured as a function of gas pressure in absence and in presence of magnetic field, B=0.3 T. On excitation to this level, the dynamics in both zero and nonzero fields may be described in the intermediate-molecule limit, with the fluorescence exhibiting biexponential and triexponential decay, respectively. The fast component decay rate constant kf=(5.31±0.22)⋅107 s−1 is independent on the (COF)2 gas pressure or magnetic field strength, while that of the first slow component depends on both. The second slow component lifetime is independent of the field strength, while dependent on the gas pressure. The Stern–Volmer dependence, measured with a field in the 0.3–12 mTorr pressure range, is nonlinear. The results obtained were explained by the indirect (electronic- and nuclear-spin-decoupling) mechanism, proposed earlier. © 2000 American Institute of Physics.

Notes: The excitation spectra of the 15V 31 344.9 band of the CS2 V 1B2←X 1Σg+ transition and the changes in these spectra with the application of a magnetic field of up to 12 kG have been measured with sub-Doppler resolution. The radiative lifetimes of rotationally resolved single lines and single Zeeman components were measured under collision-free conditions. All of the fluorescence decays were observed to be of a single exponential. Large Zeeman splittings were observed for many lines. The only symmetry allowed spin–orbit interaction is that of the 3A2(B2) component with the 1B2 state. The 3A2(B2) component has no magnetic moment, but a magnetic moment is induced when it is mixed with the 3A2(A1,B1) components. The mixing of the 3A2(B2) and 3A2(A1,B1) components is facilitated by spin–rotation interaction and the Zeeman interaction. From analysis of the observed Zeeman splittings of the perturbed levels, the 3A2(B2) component was determined to lie 14 cm−1 below the nearly degenerate 3A2(A1) and 3A2(B1) components in the energy region where the 15V band is observed. Irregular energy shifts and splittings of rotational lines were observed, and these were attributed to (a) Coriolis interaction between the V1B2(v′(a1);K=0JM) and V 1B2(v(b2);K=1JM) levels and (b) resonant spin–orbit interaction between the rotational levels V 1B2(v′(a1);KJM) and R 3A2(v(a1);KJM). These interactions become appreciable when two levels lie close in energy. Large Zeeman splittings were observed in case (b). Many vibrational lines with irregular intensity and spacing were observed in each band. These were attributed to (c) Fermi resonance between the vibrational levels in the V 1B2 state and (d) resonant spin–orbit interaction between vibrational levels in the V 1B2 and R 3A2 states. In case (d), large Zeeman splittings were observed for a series of rotational lines in a vibrational band. The background lines were identified from observed Zeeman splittings as the transitions to levels of the R 3A2 state, which are induced by resonant spin–orbit interaction with the levels of the V 1B2 state. The intensity of the excitation spectrum of the V 1B2←X1Σg+ transition was observed to decrease as the magnetic field increases. This was attributed to a mixing of the 3A2 state with the V 1B2 state and the resulting triplet–triplet emission, which was not detected in this experiment. It was possible to evaluate the lifetime of the radiative triplet–triplet emission via deperturbation analysis of the perturbed lines. © 2000 American Institute of Physics.

Notes: Ab initio molecular orbital calculations have been performed to investigate the structures and energies of the adduct isomeric radicals arising from the addition reaction of chlorine atom to isoprene. Geometry optimizations of the four isomers of the Cl–isoprene adduct were carried out with density function theory (DFT)-B3LYP and the energies were computed using the single-point calculation of several methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomer was found to be that with Cl addition to the terminal C1-position (isomer I). At the CCSD(T)/6-311G** level of theory, the energies of the other three isomeric radicals relative to I were 1.4, 14.2, and 14.7 kcal mol−1 for Cl additions to C4- (isomer IV), C3- (isomer III), and C2-positions (isomer II), respectively. The activation energy for Cl migration from the more energetic isomers to the lower energy ones (i.e., from isomers II to I or III to IV) was found to be relatively low (2.8–4.7 kcal mol−1), indicating that isomerization of the Cl-isoprene adduct is likely to dominate the chemistry. The present theoretical results are compared to available experimental data based on product analysis for the Cl-isoprene reaction system to infer the atmospheric degradation mechanism of isoprene initiated by chlorine atoms. © 2000 American Institute of Physics.

Notes: A high-resolution rotational coherence spectroscopy (RCS) investigation of para-cyclohexylaniline (pCHA) was performed with a solid-state picosecond laser setup, which allowed for the determination of rotational constants with unprecedented precision for a RCS experiment. The technique of time-resolved fluorescence depletion was used for the RCS measurements. The unique structural features of pCHA enabled the determination of both ground and excited state rotational constants. Three different sets of recurrences were observed in the spectrum and assigned to K″-, K′-, and J″-type transients. From a detailed analysis by a grid search procedure based on the numerical simulation of RCS spectra and a nonlinear least-squares fitting routine the following rotational constants for the ground state were obtained: A″=2406.5±0.6 MHz, (B+C)″=714.9±0.4 MHz. For the electronic excited state two different sets of constants were found to fit the experimental data within the reported uncertainties: set (I) A′=2343.6±1.3 MHz, (B+C)′=714.4±1.7 MHz and set (II) A′=2346.3±1.3 MHz, (B+C)′=719.3±2.1 MHz. From additional information set (II) was found preferable for the description of the excited state. Furthermore, the fluorescence lifetime and the alignment of the transition dipole moment in the molecular frame were obtained from the fit procedure. For a structural characterization of pCHA we performed ab initio calculations of the electronic ground and excited state using HF/6-31G(d) and CIS/6-31G(d) levels of theory, respectively. These results were compared with the experiments and used to investigate the dependence of the rotational constants on characteristic intramolecular coordinates. © 2000 American Institute of Physics.

Notes: Exchange holes in molecules can be delocalized over several centers, thus throwing into question their approximation by local density functionals. This work introduces a simple model which detects delocalization in molecules through a local variable related to kinetic energy density. A local exchange functional is derived that reproduces exact-exchange atomization energies of molecules with relatively low error. This has important implications for the simplification of "hybrid" density-functional theories which contain an exactly computed exchange term. © 2000 American Institute of Physics.

Notes: The photodissociation/ionization dynamics of O2 around 193 nm have been studied using a narrowband tunable ArF excimer laser and the velocity map imaging technique. Angular and kinetic energy distributions of the product O+ ions and O(3P2) atoms are recorded and analyzed. The production of O(3P2) atoms is resonance enhanced on the one-photon level by the B 3Σu−(v=4) state, which is part of the B 3Σu−←X 3Σg− Schumann–Runge bands. Angular distribution measurements for individual rotational levels of the B state yield values for the anisotropy parameter, βSR, which are in good agreement with the values predicted by independent measurements of predissociation lifetimes from spectral linewidths. An average value of β=0.48 is found for the underlying Herzberg continuum at 193 nm. O2+ production is enhanced on the two-photon level via members of the nsσg(n−1)dπg1Σg+ Rydberg series terminating at higher vibrational levels of the ion. The high Rydberg states autoionize into the O2+ X 2Πg ground state or absorb one more photon and then autoionize into the A 2Πu and b 4Σu− states of O2+, which subsequently fluoresce. Production of O+ from one- and two-photon dissociation of the O2+ formed after two-photon absorption is also observed and characterized. © 2000 American Institute of Physics.

Notes: Using a linear-response approach, the recently introduced spin-restricted coupled-cluster (SR-CC) theory is extended to the treatment of excited states of high-spin open-shell molecules. Explicit equations are given within the usual singles and doubles approximation and our implementation (within an existing spin–orbital code) is described. It is shown that in SR-CC theory, due to spin constraints, the spin-expectation value for the excited states calculated as corresponding energy derivatives always corresponds to the exact value. In addition, the SR-CC singles and doubles (SR-CCSD) approach is extended to include also the so-called pseudotriple excitations (best described as double excitations with an additional spin–flip in one open-shell orbital) which are important for the description of so-called low-spin excited states. Exploratory calculations for a few diatomic systems (BeH, OH, NO, CN, and CO+) show that problems due to spin contamination in the unrestricted Hartree–Fock (UHF) CCSD treatment of excited states are rectified by using a restricted open-shell Hartree–Fock (ROHF) reference, as it is done in the SR-CC approach. While SR-CCSD performs well for high-spin excited states, the closely related partially spin-adapted (PSA) CC approach is shown to be inferior and errors in the computed excitation energies are generally larger than the typical accuracy of about 0.2 eV in CCSD excited state treatments. So-called low-spin states (e.g., the 2 2B1 state of NH2) are shown to require inclusion of pseudotriple excitations for even a qualitatively correct description. If they are included, ROHF-CC, SR-CC, and PSA-CC give essentially identical results. © 2000 American Institute of Physics.

Notes: Clusters of hydroxide ion, HO−(H2O)n=1–4, have been studied by high level ab initio calculations in order to better understand the first coordination shell of OH− ions. Geometry optimizations were performed at Hartree–Fock, density functional theory and second order Møller–Plesset perturbation theory levels using the 6-31+G(d,p) basis set. Single point energy calculations were carried out on the optimized geometries using the more extended 6-311+G(2df,2p) basis set and a higher level of electron correlation, namely fourth-order Møller–Plesset perturbation theory. For the n=1–3 clusters, only structures with the hydroxide ion hydrogen bonded to all waters molecules were considered. For the n=4 cluster, three minima were found; the most stable species has all four waters directly bound to the hydroxide ion, while the other two clusters have only three waters in the first coordination shell. In addition, the transition state connecting the cluster containing four waters in the first coordination shell to the species having three waters in the coordination shell was characterized. The barrier for this rearrangement is very low (1.82 kcal/mol), and we predict this process to occur on the picosecond time scale. The thermodynamic properties (enthalpy, entropy and Gibbs free energy) for the formation of the clusters have been calculated for all the species (including the fully deuterated clusters). Comparison of our calculations with experimental data reveals good agreement in the free energy. Nevertheless, our ab initio results suggest that for the n〉1 clusters, both −ΔH0 and −ΔS0 are larger than those reported from experiment and new experiments may be necessary to obtain accurate experimental values. © 2000 American Institute of Physics.

Notes: The full quartic force field of the ground electronic state of the silyl anion (SiH3−) has been determined at the CCSD(T)-R12 level employing a [Si/H]=[16s11p6d5f/7s5p4d] basis set. The vibrational energy levels, using the quartic force field as a representation of the potential energy hypersurface around equilibrium, have been determined by vibrational perturbation theory carried out to second, fourth, and sixth order. The undetected vibrational fundamental for the umbrella mode, ν2, is predicted to be 844 cm−1. High-quality ab initio quantum chemical methods, including higher-order coupled cluster (CC) and many-body perturbation (MP) theory with basis sets ranging from [Si/H] [5s4p2d/3s2p] to [8s7p6d5f4g3h/7s6p5d4f3g] have been employed to obtain the best possible value for the inversion barrier of the silyl anion. The rarely quantified effects of one- and two-particle relativistic terms, core correlation, and the diagonal Born–Oppenheimer correction (DBOC) have been included in the determination of the barrier for this model system. The final electronic (vibrationless) extrapolated barrier height of this study is 8351±100 cm−1. © 2000 American Institute of Physics.

Notes: For the first time, millimeter wave absorption spectra of the van der Waals complexes 3He–CO and 4He–CO were detected in a supersonic jet. Altogether four rotational transitions of 3He–CO and six rotational transitions of 4He–CO were recorded between 110 GHz and 127 GHz with an intracavity spectrometer based on the millimeter wave generator, called OROTRON. The obtained results were included in a global fit together with previously known data enabling a more precise determination of the energy levels of the 3He–CO and 4He–CO complexes. In extremely cold, dark, and dense interstellar clouds the He–CO complex may have astrophysical relevance. © 2000 American Institute of Physics.

Notes: We have measured pressure broadening and line shift parameters for the J=1←0 and J=2←1 transitions of carbon monoxide (CO) in collision with hydrogen (H2) at various temperatures between 8 and 600 K. Because of the abundance of both species in the interstellar medium, the CO–H2 collision system is of astrophysical importance. Measurements below 35 K were made using the collisional cooling technique, while measurements at higher temperatures were made in a conventional equilibrium cell. These measurements were compared with theoretical results of quantum scattering calculations using the currently best available potential surface for the CO–H2 collision system. We find a good agreement between theory and experiment at higher temperatures (〉30 K) while for lower temperatures considerable deviations occur. © 2000 American Institute of Physics.

Notes: Photoacoustic spectra of the second (3νCH), third (4νCH), and fourth (5νCH) overtones of the methyl C–H stretches in CH3CF2Cl and CH3CFCl2 were measured. The spectra are characterized by a multiple peak structure of partially resolved triplets and quartets with an anomalous linewidth decrease in the 4νCH region. The results are interpreted in terms of a simplified local mode model for C–H stretching vibrations, including also the stretch-deformation Fermi resonances. The model accounts for most spectral features and allows determination of the time scale for vibrational redistribution. © 2000 American Institute of Physics.

Notes: The method of kinetically controlled selective ionization (KCSI) for investigating collisional energy transfer in highly vibrationally excited molecules is presented in detail. In this first paper of a series the focus is on the key concepts and the technical realization of KCSI experiments to provide a common basis for following reports on our available results of KCSI studies on the vibrational relaxation of a variety of larger molecules. The KCSI technique directly monitors the energetic position and shape of the population distributions g(E,t) during the relaxation process by means of an energy selective two photon ionization process via an electronic intermediate state. Such measurements allow—for the first time—to extract complete and accurate experimental sets of transition probability distributions P(E′,E) even at quasicontinuous densities of states. Basic energy transfer quantities are already obtained from a straightforward analysis of the arrival time and width of the KCSI curves. A master equation formalism is outlined which is the basis of a data inversion providing a complete evaluation of the experimental information content. Various examples of characteristic KCSI data on collisional deactivation of highly vibrationally excited molecular populations are used to discuss important aspects of the quality and the general character of P(E′,E) parameters deduced from such measurements. The conditions for a successful modeling of these data are very tightly bound, and the resulting energy transfer parameters 〈ΔE(E)n〉 are therefore of high precision. In Paper II [J. Chem. Phys. 112, 4090 (2000), following article] we give a full account of the toluene KCSI experiments. We will deal with our completed studies on azulene, azulene-d8, pyrazine and pyridine in follow-up publications of this series. © 2000 American Institute of Physics.

Notes: Complete and detailed experimental transition probability density functions P(E′,E) have been determined for the first time for collisions between a large, highly vibrationally excited molecule, toluene, and several bath gases. This was achieved by applying the method of kinetically controlled selective ionization (KCSI) (Paper I [J. Chem. Phys. 112, 4076 (2000), preceding article]). An optimum P(E′,E) representation is recommended (monoexponential with a parametric exponent in the argument) which uses only three parameters and features a smooth behavior of all parameters for the entire set of bath gases. In helium, argon, and CO2 the P(E′,E) show relatively increased amplitudes in the wings—large energy gaps |E′−E|—which can also be represented by a biexponential form. The fractional contribution of the second exponent in these biexponentials, which is directly related to the fraction of the so-called "supercollisions," is found to be very small (〈0.1%). For larger colliders the second term disappears completely and the wings of P(E′,E) have an even smaller amplitude than that provided by a monoexponential form. At such low levels, the second exponent is therefore of practically no relevance for the overall energy relaxation rate. All optimized P(E′,E) representations show a marked linear energetic dependence of the (weak) collision parameter α1(E), which also results in an (approximately) linear dependence of 〈ΔE〉 and of the square root of 〈ΔE2〉. The energy transfer parameters presented in this study form a new benchmark class in certainty and accuracy, e.g., with only 2%–7% uncertainty for our 〈ΔE〉 data below 25 000 cm−1. They should also form a reliable testground for future trajectory calculations and theories describing collisional energy transfer of polyatomic molecules. © 2000 American Institute of Physics.

Notes: Jet-cooled diatomic PtSi, produced in a laser ablation supersonic expansion source, has been spectroscopically investigated between 17 400 and 24 000 cm−1 by resonant two-photon ionization spectroscopy. Two vibrational progressions are observed and identified as the [15.7]Ω′=1←X 1Σ+ and [18.5]Ω′=1←X 1Σ+ band systems. Three bands in the former system and six bands in the latter system were rotationally resolved and analyzed, leading to bond lengths of re′=2.1905(13) Å and re′=2.2354(3) Å for the [15.7]Ω′=1 and [18.5]Ω′=1 states, respectively. The Ω″=0 ground state of PtSi is assigned as a 1Σ+ state, in agreement with previous work and with the assigned ground states of the isovalent NiC, PdC, PtC, and NiSi molecules. The ground state bond length of PtSi is given by r0″=2.0629(2) Å. A Rydberg–Klein–Rees analysis of the ground and excited state potential energy curves is presented, along with a discussion of the chemical bonding and a comparison to the isoelectronic molecule, AlAu. Evidence is presented for a double bond in PtSi, as opposed to a single bond in AlAu. © 2000 American Institute of Physics.

Notes: Detailed ab initio calculations on the lowest vibrational state of the electronic C˜ 2T2 manifold of CF4+ show a small Jahn-Teller effect in all asymmetric modes. This results in only small distortions from tetrahedral symmetry, but due to strong coupling between electronic and nuclear motions (dynamical Jahn-Teller effect), the spin–orbit splitting is reduced by an order of magnitude. This answers the outstanding question on the large differences between the results of electronic structure calculations and measurements on spin-orbit splittings in XY4 molecules or molecular ions [R. N. Dixon and R. P. Tuckett, Chem. Phys. Lett. 140, 553 (1987); J. F. M. Aarts and J. H. Callomon, Mol. Phys. 81, 1383 (1994)]. Complete agreement with experiment is however not reached because the dynamical Jahn-Teller effect is very sensitive to small errors in the potential energy surface. © 2000 American Institute of Physics.

Notes: The lowest energy equilibrium structure for Ne19, Ar19, Ne9Ar10, and Ne12Ar7 clusters adsorbed on model surfaces was determined using simulated annealing techniques. In all cases, two-dimensional structures were obtained and in the case of the alloy clusters, a core structure was observed. Phase transitions were identified by calculating anomalies in the constant volume heat capacity as a function of temperature. In all cases, the usual melting and evaporation transitions were identified. In the case of alloy systems, various low temperature transitions were observed. These transitions are associated with order–disorder transitions which are similar to what is observed in alloy solids in bulk. The effect that surface corrugation has on these transitions was also analyzed. © 2000 American Institute of Physics.

Notes: Canonical ensemble molecular dynamics simulations were conducted for methane diffusion in AlPO4-5 in order to assess the role of the lattice motion on adsorbate diffusivity in straight pore zeolites. Both a static lattice model and a full dynamic lattice model were used at a loading of 1.5 methane/unit cell at 295 K. Although recent simulation work has asserted that there should be a difference, we show that there is little difference in the observed methane diffusivity (1.26×10−7 m2/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33×10−7 m2/s and 0.328). Furthermore, we introduce a methodology for handling lattice motion in molecular simulations by utilizing the normal vibrational modes in a harmonic crystal approximation. © 2000 American Institute of Physics.

Notes: Electron transfer reaction rate constants at semiconductor/liquid interfaces are calculated using the Fermi Golden Rule and a tight-binding model for the semiconductors. The slab method and a z-transform method are employed in obtaining the electronic structures of semiconductors with surfaces and are compared. The maximum electron transfer rate constants at Si/viologen2+/+ and InP/Me2Fc+/0 interfaces are computed using the tight-binding type calculations for the solid and the extended-Hückel for the coupling to the redox agent at the interface. These results for the bulk states are compared with the experimentally measured values of Lewis and co-workers, and are in reasonable agreement, without adjusting parameters. In the case of InP/liquid interface, the unusual current vs applied potential behavior is additionally interpreted, in part, by the presence of surface states. © 2000 American Institute of Physics.

Notes: We present an application of Wertheim's thermodynamic perturbation theory (TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones monomers. We use both the reference hypernetted chain (RHNC) and mean spherical approximation (MSA) integral equation theories to describe the properties of the reference fluid. The equation of state, the density dependence of the excess chemical potential, and the critical points of the liquid–vapor transition are compared with simulation results and good agreement is found. The RHNC version is somewhat more accurate, while the MSA version has the advantage of being almost analytic. We analyze the scaling behavior of the critical point of chain fluids according to TPT1 and find it to reproduce the mean field exponents: The critical monomer density is predicted to vanish as n−1/2 upon increasing the chain length n while the critical temperature is predicted to reach an asymptotic finite temperature that is attained as n−1/2. The predicted asymptotic finite critical temperature obtained from the RHNC and MSA versions of TPT1 is found to be in good agreement with the aitch-theta point of our polymer model as obtained from the temperature dependence of the single chain conformations. © 2000 American Institute of Physics.

Notes: Equilibrium polymerization is studied here as a prototype for clustering transitions that commonly occur in systems of interacting particles at equilibrium. These transitions are often difficult to locate because of transition "rounding" associated with a limited extent of cluster growth, competing association or dissociation processes that initiate or inhibit clustering, and other constraints on the particle clustering dynamics. Instead of singularities in thermodynamic and transport properties, more subtle property changes signal the onset of particle clustering, explaining why clustering transitions are often overlooked or misinterpreted. We utilize a Flory–Huggins model for the equilibrium ("living") polymerization of linear polymer chains to identify experimental signatures (features in the osmotic pressure, osmotic compressibility, and specific heat) that can be used to locate and quantify the transition "rounding" in general clustering transitions. The computation of a flattening in the concentration dependence of the osmotic pressure in the one-phase region motivates our consideration of the temperature dependence of the second virial coefficient and the variation of the theta temperature Tθ with "sticking energy" Δh as possible important indicators of particle clustering. The ratio of the critical temperature Tc for phase separation to Tθ, along with other "critical constant" ratios, such as the osmotic compressibility factor Zc, are also calculated and discussed in connection with establishing criteria for identifying particle clustering transitions and for quantifying the relative "strength" of these transitions. © 2000 American Institute of Physics.

Notes: We examine the second order split-operator propagator of Fleck et al. as it functions to solve the time-dependent Schrödinger wave equation. In particular we examine the usefulness of dividing the propagator into its two lower order component propagators each of which propagates the wavepacket on one half of the time-step of the full propagator. © 2000 American Institute of Physics.

Notes: The existence of integral (Hi) in the surplus function variational quantum Monte Carlo approach is discussed and a method is proposed for avoiding the infinity in the calculation of Hi. A rigid model of nucleus was adopted to avoid the infinity appearance of r−i. Discussion of a new solution for Schrödinger's equation of hydrogen atom, which may make the calculation of the Hi value more accurate is also presented. © 2000 American Institute of Physics.

Notes: Bound and quasibound states of HeH2+ and HeHD+ in three dimensions, for zero total angular momentum, have been computed using a time-dependent quantum mechanical approach. Time evolution of a carefully chosen wave packet in the interaction region is followed and the time correlation function evaluated and its Fourier transform obtained. The resulting eigenvalue spectrum and the corresponding eigenfunctions are examined to characterize the nature of the dynamical resonances for the system. It becomes clear that at low energies the quasibound states can be assigned readily in terms of local modes. While some of the higher energy state eigenfunctions resemble the hyperspherical modes, a large number of them cannot be assigned easily, suggesting irregular dynamics, in keeping with a large number of unstable periodic orbits known for the system. © 2000 American Institute of Physics.

Notes: We determined the self part of the intermediate scattering function in liquid polyethyleneoxide (PEO) and PEO–alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics (MD) computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm−1 to approximately 20 nm−1. We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt (1 metal ion per 15 monomers) leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data. © 2000 American Institute of Physics.

Notes: The effect of the interatomic interaction on the static dipole polarizability has been reexamined in detail for the He dimer at the equilibrium internuclear distance of RHe... He=5.6 a0 by using high-precision explicitly correlated R12 approaches. Our most accurate CCSD(T)-R12 results for the increments of the polarizability are 61.79×10−3 a.u. for the anisotropy and −1.04×10−3 a.u. for the trace, not significantly different from recent literature CCSD data and confirming their high accuracy. © 2000 American Institute of Physics.

Notes: In an extension of recent work [Y. Kim, K. Patton, J. Fleniken, and H. Meyer, Chem. Phys. Lett. 318, 522 (2000)], overtone pumping followed by resonantly enhanced multiphoton ionization (REMPI) detection is used to record spectra corresponding to excitation of the lowest five bend–stretch vibrational levels of the NO(X 2Π,v=2)–Ar complex. High-quality ab initio potential energy surfaces, coupled-electron-pair (CEPA) and coupled cluster single double triple [CCSD(T)], are used to predict the positions of these same five states, but in the NO(X 2Π,v=0)–Ar complex. The vibrational wave functions and basis set expansion coefficients, determined within the adiabatic bender model, are then used to simulate the observed spectrum for excitation of the NO(X 2Π, v=2)–Ar complex. The overall position and rotational substructure matches the experiment extremely well, particularly when the simulation is based on the presumably more accurate CCSD(T) potential energy surfaces. © 2000 American Institute of Physics.

Notes: When a Ne:SO2 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned with the aid of isotopic substitution studies to SO, SO2+, SO2−, (SO2)2−, and, tentatively, SO−. The fundamental and first overtone absorptions of SO lie 0.9 and 1.8 cm−1, respectively, below the gas-phase band centers. Ab initio calculations at the Hartree–Fock level show an instability in the v3 vibration of SO2+ which is avoided by higher-level calculations. The ν3 and ν1 fundamentals of SO2− isolated in solid neon are identified at 1086.2 and 990.8 cm−1, respectively. In agreement with an earlier proposal, the 1042 cm−1 absorption originally assigned to ν3 of SO2− trapped in solid argon is reassigned to MSO2, with M an alkali metal. Near the photodetachment threshold for SO2− isolated in a neon matrix, electron capture by SO2 nearest-neighbor pairs results in growth of infrared absorptions of (SO2)2−, which has been shown by gas-phase studies to have a significantly higher photodetachment threshold than does SO2−. The isotopic substitution studies require that the two sulfur atoms in (SO2)2− be nonequivalent, favoring the linking of the two SO2 units by a S (centered ellipsis) O bond. © 2000 American Institute of Physics.

Notes: Electron impact total ionization cross-sections of small silicon hydrides, SiHn(n=1–4), and fluorides, SiFn(n=1–3), have been calculated by the application of a recently developed theoretical model. The binary-encounter-Bethe (BEB) model has a simple structure and requires information from calculations on the parent ground-state molecule only (binding energies, orbital kinetic energies, and occupation numbers). Previous applications of the BEB theory to the silicon hydrides and fluorides have employed a combination of experimental and Koopman's theorem binding energies. In the current work binding energies have been calculated using the explicitly correlated multiconfigurational spin tensor electron propagator (MCSTEP) method which gives highly accurate ionization potentials for closed- and open-shell systems. Calculations have been performed using cc-pVDZ and cc-pVTZ basis sets with multiconfigurational self-consistent field (MCSCF) reference wave functions. Comparisons are made between our MCSCF/MCSTEP and previous Hartree–Fock (HF)/Koopman's theorem results and available experimental data. The use of improved theoretical data does not have a significant effect on the resultant cross-sections; however, our new technique is a viable method for calculating electron impact ionization cross-sections for systems where Koopman's theorem is known to be unreliable or no experimental data is available. © 2000 American Institute of Physics.

Notes: A new global optimization method, multicanonical jump walk annealing (MJWA), is proposed and applied to the geometric optimization of Lennard-Jones and Morse clusters and the hydrophobic (B), hydrophilic (L), and neutral (N) (BLN) protein model. The method efficiently finds the global minima of these systems. In four comparative studies, MJWA greatly outperforms the conventional simulated annealing in locating the global minima. Theoretical comparison with other global optimization methods is discussed. Through this paper, we demonstrate a criterion for devising stochastic global optimization schemes. Namely, a stochastic global optimization method must favor the global minimum thermodynamically and at the same time be able to cross the high energy barriers. © 2000 American Institute of Physics.

Notes: The potential energy surfaces of the three lowest electronic triplet states of CO2 which lead to O(3P)+CO(1Σ+), 3A′, 1 3A″, and 2 3A″, have been computed at the completeactive-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level with a modest 631+G(d) basis. Potential energy surfaces are fit with a global functional form. The 3A′ state has a well 0.9 eV deep and the 1 3A″ state has a 0.2 eV well with respect to the O(3P)+CO(1Σ+) dissociation threshold. The 3A′ and 1 3A″ states are both bent at their minima and have a barrier at 0.2 eV and 0.3 eV above threshold, respectively. The 2 3A″ state is mostly repulsive, and has a saddle at C2v geometries. We have run classical trajectory calculations for O(3P)+CO(1Σ+) collisions using these surfaces. Results agree well with available vibrational relaxation and oxygen atom exchange measurements except at low temperature. Comparisons are also made with measured vibrational excitation cross sections and infrared emission spectra of the nascent CO products at 3.4 eV collision energy. These results show a high degree of vibrational and rotational excitation with a nearly statistical population which is evident in a distinct spectral "bandhead" signature. Analysis of the trajectories show that almost all collisions which lead to oxygen atom exchange and/or vibrational energy transfer occur when the O(3P) approaches the CO at OCO angles between 80° and 140°, passes over the barrier and through the wells of the 3A′ and 1 3A″ states, and interacts with the repulsive wall of the carbon end of the CO nearly perpendicular to the CO bond. © 2000 American Institute of Physics.

Notes: An approach based on finite perturbation theory is proposed for deriving models of distributed polarizabilities from quantum mechanically determined induction energies. It relies on the construction of a grid of points, over which the induction energy resulting from the interaction of a charge with the molecule of interest is evaluated at the desired level of approximation. Distributed polarizabilities of any order are then fitted by solving the normal equations of the least-squares problem, the solution of which provides an optimal description of induction effects. The method is probed by examining several models of distributed polarizabilities of increasing complexity in the case of water, methanol, acetonitrile, and benzene. At a reasonable order of multipole expansion, atomic polarizabilities derived using this approach are found to reproduce the corresponding molecular polarizabilities with an appropriate accuracy. The careful choice of the parameters to be fitted appears to be a key factor for obtaining physically realistic models. © 2000 American Institute of Physics.

Notes: We present an efficient algorithm for generating semiglobal potential energy surfaces of reactive systems. The method takes as input molecular mechanics force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locations are determined by an iterative process. These Hessian expansions might come, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A 2×2 electronic diabatic Hamiltonian matrix is constructed from these data such that, by construction, the lowest eigenvalue of this matrix provides a semiglobal approximation to the lowest electronically adiabatic potential energy surface. The theory is illustrated and tested by applications to rate constant calculations for three gas-phase test reactions, namely, the isomerization of 1,3-cis-pentadiene, OH+CH4→H2O+CH3, and CH2Cl+CH3F→CH3Cl+CH2F. © 2000 American Institute of Physics.

Notes: Three-dimensional quantum reactive scattering calculations have been carried out for the (D+H2)+ nonadiabatic ion–molecule collision. The calculations have been done using the time-independent close-coupling formalism with hyperspherical coordinates. The (3×3) diatomics-in-molecule potential energy surfaces have been employed. The result of the accurate quantum scattering calculations have been compared to the results of the quasiclassical trajectory surface hopping method. Two versions of the method have been used; one uses Tully's fewest switches algorithm and the other is the trajectory surface hopping method of Tully and Preston, in which electronically nonadiabatic hopping is only allowed at the predefined crossing seams. We have found that the agreement between the quantum result and the result of Tully's method is generally good, but the Tully and Preston method significantly underestimates the nonadiabatic transition probability. © 2000 American Institute of Physics.

Notes: The potential curves of low-lying singlet states of Be2 are computed in the hierarchy of coupled-cluster (CC) models: CCS, CC2, CCSD, and CC3 from the ground-state energy and linear response excitation energies. The results are compared with full configuration interaction (FCI) values. For single-excitation dominated transitions the convergence of the excitation energies toward FCI is smooth with a reduction in the error of about a factor of 3 at each level of the CC hierarchy. This reduction is only seen from CCSD to CC3 for the double-excitation dominated transitions. CC3 gives accurate potential curves and spectroscopic constants of the excited states dominated by single excitations. For the double-excitation dominated states the CC3 equilibrium distances and vibrational frequencies agree reasonably well with FCI, while the bonding energies and the 0-0 excitation energies have larger errors. We analyze the influence of the frozen-core approximation, basis-set effects and the counterpoise correction on the ground-state energy, the excitation energies, and molecular constants. Considering all these effects, we provide an improved estimate of the calculated properties. © 2000 American Institute of Physics.

Notes: A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J=17/2) and O 3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate. © 2000 American Institute of Physics.

Notes: The angular momentum polarization of the products of the reaction Cl(2P3/2)+CD4(v=0,j=0)→DCl(v′=0,j′=1)+CD3 is calculated via the quasiclassical trajectory method (QCT) based on extended London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES) at a collision energy of 0.28 eV (6.46 kcal/mol). In the stationary target frame (STF), the rotational alignment factor A0(2)stf has been calculated. In the meantime, we also present four polarization dependent "generalized" differential cross sections (PDDCS) (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), and (2π/σ)(dσ21−/dωt) in the center of mass frame. Furthermore, the distribution of dihedral angle P(φr), the distribution of angle between k′ and J′, P(θtr), and the angular distribution of product rotational vectors in the form of polar plots in θr and φr are calculated as well. The calculated results are in good agreement with the experimental data. © 2000 American Institute of Physics.

Notes: Rydberg electron transfer spectroscopy (RET) has been used to determine the dipole-bound electron affinity of the indole molecule, and the value of 3 meV was obtained. RET has also been employed to study [indole–(water)N]− cluster anions and the results have been interpreted with the help of ab initio calculations. It has been shown that for N=1 and 2 only dipole-bound anions are formed and that the electron attachment induces large amplitude motions in these systems. [Indole–(water)N]− anions with N=3 and 4 have not been observed. This finding for N=3 is consistent with a low theoretically predicted dipole moment of the neutral indole–(water)3 complex, which is insufficient for the formation of a stable dipole-bound anion. Above N=5, RET experiments showed formation of valence [indole–(water)N]− anions. From the observed size threshold for the formation of these anions, the negative value of the valence electron affinity of indole equal to −1.03±0.05 meV was deduced. © 2000 American Institute of Physics.

Notes: The 2-pyridone dimer, (2PY)2, which is linked by two antiparallel N–H⋅⋅⋅O hydrogen bonds, is a model for hydrogen bonded nucleotide base pairs, e.g., the uracil dimer. Mass- and isomer-selected S2←S0 vibronic spectra of supersonically cooled (2PY)2 were measured by laser two-color resonant two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone⋅2-hydroxypyridine mixed dimer, present at (approximate)5% relative concentration. S2→S0 fluorescence emission spectra show dominantly the hydrogen bond shearing vibration ν4″ at 98.5 cm−1 and the stretching vibration ν6″ at 163.5 cm−1. The hydrogen bond stretching vibration force constant was determined to be 75.4 N/m, or 37.7 N/m per hydrogen bond, a very high value. The ν2″ (au) torsional and the ν3″ (bu) slanting vibrations were also identified. Ground state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, interaction, and dissociation energies were calculated using Hartree–Fock and density functional (B3LYP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement with all experimental observations. © 2000 American Institute of Physics.

Notes: Ab initio calculated coordinate and internal valence coordinate coefficients for each of the four spin–spin coupling surfaces of the acetylene molecule—1J(C, H), 1J(C, C), 2J(C, H), and 3J(H, H) are presented. Calculations were carried out at the SOPPA(CCSD) level using a large basis set. Couplings were calculated at 35 geometries (including equilibrium) giving 35 distinct sites on the 1J(C, C) and 3J(H, H) surfaces and 53 distinct sites on the 1J(C, H) and 2J(C, H) surfaces. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. All couplings are sensitive to geometry with the principal features being (a) an even steeper increase of J(C1, H1) with CC bond stretching than with CH bond stretching—an example of "unexpected differential sensitivity" (or UDS), (b) very opposite variations of 2J(C1, H2) with variations of the CC and C2H2 bond lengths, (c) very opposite variations of 1J(C, C) with a CC stretch and a CH stretch and (d) very opposite variations of 1J(C1, H1) with variations of the H1C1C2 and C1C2H2 angles with the latter variation being three times more effective (another example of UDS). The surfaces are averaged over a very accurate force field to give values of all couplings in the ten isotopomers containing all possible combinations of 12C, 13C, 1H, and 2H nuclei at 0 K and at a number of selected temperatures in the range accessible to experiment. The dominant nuclear motion effect comes from bending at the carbon atoms with stretching being of greater importance only for 1J(C, H). Agreement with recent experimental data both for the absolute values of the couplings and for isotope effects on them is generally very good although there are some disappointments for 1J(C, H). Values of the reduced coupling constants and their derivatives for the carbon–carbon and the one-bond carbon–proton coupling in acetylene are compared with recent results for some other molecules. © 2000 American Institute of Physics.

Notes: The A 2Δu←X 2Πg, B 2Σu−←X 2Πg, and C 2Σu+←X 2Πg electronic transitions of C3− were observed in a neon matrix and in the gas phase, although the energy of the excited electronic states involved in these transitions is 1–1.5 eV above the photodetachment threshold. The excited Feshbach states are sufficiently long-lived that some of the bands in the gas-phase photodetachment spectrum exhibit rotational structure. Assignment of the transitions is made on the basis of rotational analysis or profile simulations and theoretical calculations. The b 4Πu←X 2Πg transition is also weakly observed. The presence of such discrete bands, though in the continuum, provides a means of detection for anions in the interstellar medium. © 2000 American Institute of Physics.