ALBERT

Notes: The possible connection between the equilibrium structure of a solution and the chemical reaction dynamics that occur in that solution has been discussed by Adelman and co-workers. In this work, we present a computational demonstration of this connection using molecular dynamics simulations and the generalized Langevin equation (GLE). A favorable example of a reaction loosely based on thermally activated Cl+Cl2→Cl2+Cl in argon solvent is used for this demonstration by (1) computing equilibrium solution structural information in terms of the Ar–Ar and Ar–Cl radial distribution functions, both from integral equations and from molecular dynamics; (2) deriving a memory function for Cl in argon solvent from the radial distribution functions and the Ar–Cl potential; and (3) using this memory function in a simple GLE to compute the dynamics of the reaction. Energy flow results both for climbing and descending the barrier are in gratifying agreement with the dynamics of the same reaction as computed by full deterministic molecular dynamics.

Notes: Much of the behavior of simple gas phase reaction dynamics can be understood in terms of simple pictures based on the shapes of the underlying potential energy surfaces and the masses of the reagent atoms. Our aim here is to investigate to what extent such gas phase models can be used to understand properties of solution reactions. In particular, we examine for a solution reaction the validity of the Evans–Polanyi rule that an early potential barrier favors translational excitation of the reactants and vibrational excitation of the products, with the converse holding true for a late barrier. The test is performed by using molecular dynamics simulations for an asymmetric linear transition state A+BC→AB+C atom exchange reaction in argon solvent. We calculate for both gas and solution reactions the partitioning among translational, rotational, and vibrational energy during the reaction process. We find that for a short time period (−65 to 65 fs where t=0 is at the barrier top), in which the forces from the intrinsic gas phase potential dominate, the Evans–Polanyi rule can be carried over into the solution reaction. The gas phase vibrational energy distributions persist in solution over a much longer period. In particular, this calculation illustrates for an early barrier linear transition state potential in solution that a solvent induced fluctuation in the reagent translational energy is considerably more effective than a fluctuation in vibrational energy in prompting reaction. The resulting reaction products are formed highly vibrationally excited. For the reverse late barrier reaction, a solvent induced fluctuation in vibrational energy is needed for reaction and the resulting products are initially highly translationally excited. We expect that on the proper time scale, many other gas phase reaction dynamics rules will also carry over to solution reactions, particularly in cases in which the reagent–solvent interaction is weak.

Notes: The effects of geometrical confinement on the molecular dynamics of liquid toluene in porous sol–gel silica glasses have been studied by the NMR method. The NMR deuteron spin–lattice relaxation times for toluene-d8, ortho-toluene-d1, and para-toluene-d1 were measured as a function of temperature for bulk and confined liquids. The rotational diffusion tensors for overall, internal, and anisotropic rotational diffusion have been determined. The effect of geometrical confinement results in a considerable decrease of the rotational diffusion constants and leads to a lowering of the symmetry of the anisotropic rotational diffusion tensor.

Notes: The microwave spectra of three isotopic Ge species of germyl azide, GeH3N3, have been assigned. The splittings due to internal rotation of the germyl group have been analyzed yielding a threefold barrier of 86.6(1) cm−1. An electric dipole moment of 2.58(2) D has been determined from the Stark effect. The following refined structural parameters have been obtained with a fixed and slightly bent azide group: r(GeN)=1.866(12) A(ring), r(GeH)=1.497(8) A(ring), α(GeNN)=115.9(14)°, α(HGeH)=112.2(10)° and a tilt of the symmetric germyl group of 4.0° away from the azide group.

Notes: Absolute state-selected total cross sections σv', v'=0 and 1, for the reaction N+2(X˜,v'=0,1) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2) [reaction (1)] over the center-of-mass collisional energy (Ec.m.) range of 1.2–140 eV have been measured using the photoionization mass spectrometric and radio frequency ion guide methods. These measurements, together with the relative values for σv', v'=0–2, and spin-orbit-state distributions of product Ar+ ions determined using the crossed ion-neutral beam photoionization apparatus, allow the determination of the absolute values for σ2 and partial state-to-state cross sections σv'→J, v'=0–2, for reaction (1). Absolute values for σv', v'=0–2, at Ec.m.=8 and 20 eV are in good agreement with those determined previously by the threshold photoelectron secondary ion coincidence method. Absolute values for σv'→J, v'=0–2, at Ec.m.=8 and 20 eV are also found to be in satisfactory accord with the predictions of the semiclassical multistate calculation which uses the ab initio potential energy surfaces of the [N2+Ar]+ system. Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction Ar+(2P3/2)+N2(X,v=0)→Ar(1S0)+N+2 (X˜,v') from the consideration of microscopic reversibility.

Notes: The vibrational state distributions of N+2(X˜,v') ions resulting from the reactions, Ar+(2P3/2)+N2(X˜,v=0)→Ar(1S0) +N+2(X˜,v') [reaction (1)] and Ar+(2P1/2)+N2(X˜,v=0)→Ar(1S0) +N+2(X˜,v') [reaction (2)], over the center-of-mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly ((approximately-greater-than)85%) in the v'=1 state and that the production of N+2(X˜,v'=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that (approximately-greater-than)80% of N+2 product ions are in the v'=2 state. However, the calculation is found to either over-estimate the populations for N+2(v'〈2) or underestimate the populations for N+2(v'〉2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF octopole gas cell arrangement can be strongly susceptible to gas cell pressure effects whereas the experimental values for σ3/2 are reliable. The values for σ1/2 deduced by multiplying the values for σ3/2 and the ratios σ1/2/σ3/2 determined in the crossed ion–neutral beam experiment are in agreement with the theoretical cross sections. Both σ3/2 and σ1/2 are found to increase as Ec.m. is increased from 41.2 eV. This observation is interpreted as due to the formation of N+2 in the A˜ 2Πu state at high Ec.m. . Combining the measured vibrational state distributions of product N+2(X˜,v') ions and the absolute state-selected total cross sections, absolute state-to-state total cross sections for reactions (1) and (2) at selected Ec.m. are determined.

Notes: The microwave spectrum of 3,3,3-trifluoro-2-methylpropene, H2C=C(CH3)CF3 has been recorded from 18.5 to 39.0 GHz. Only a-type transitions were observed and R-branch assignments have been made for the ground vibrational state as well as for three vibrational excited states of the CF3 torsion and one excited state of the CH3 torsion. The rotational constants for the ground vibrational state were found to have the following values: A=3549.25±0.35, B=2465.82±0.05, and C=1978.25±0.02 MHz. The dipole moment components were determined from the Stark effect to be ||μa||=2.44±0.01, ||μb||=0.59±0.09, and ||μt||=2.51±0.03 D. From an analysis of the internal rotational splittings, the threefold barrier for the methyl group was found to be 603±19 cm−1 (1.72 kcal/mol). This value is consistent with the value of 610±4 cm−1 (1.74 kcal/mol) obtained from the far infrared spectrum where the fundamental was assigned at 160.9 cm−1, but only if kinetic coupling between the perfluoromethyl and methyl torsion is taken into account. The CF3 torsional mode was observed at 55.9 cm−1 from which a threefold periodic barrier of 743±7 cm−1 (2.12 kcal/mol) was calculated. No appreciable potential coupling was found between the two C3v rotors. The infrared (3500 to 30 cm−1) and Raman spectra (3500 to 10 cm−1) have been recorded for both the gas and solid states. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. All of the normal modes have been assigned based on band contours, depolarization values, and group frequencies. These results are compared to the corresponding quantities in some similar molecules.

Notes: The microwave spectra have been recorded from 18.0 to 26.5 GHz for 12CH3GeH212C14N, 13CH3GeH212C14N, 12CH3GeH212C15N, 12CH3GeH213C14N, 12CH3GeD212C14N, and 12CD3GeH212C14N of the four naturally occurring isotopes of germane: 70Ge, 72Ge, 74Ge, and 76Ge. Only a-type transitions were observed and R-branch assignments have been made for all the isotopic species in the ground vibrational state from which the rotational constants were determined. From these data the complete structural parameters were determined to be r0(CME–Ge)=1.933±0.002 A(ring), r(Ge–CCN)=1.927±0.004 A(ring), r(C≡N)=1.155±0.004 A(ring), r(Ge–H)=1.521±0.001 A(ring), r(C–Hs)=1.092±0.005 A(ring), r(C–Ha)=1.092±0.005 A(ring), (arc left)CGeC=107.36°±0.60°, (arc left)HGeH=111.41°±0.04°, (arc left)HGeCMe=112.92°±0.08°, (arc left)HsCGe=109.5°±2.0°, and 〈HaCGe=109.3°±0.8°, where all of the parameters are rs values except for the carbon–hydrogen distances and angles. The dipole moment components were determined from the Stark effect to be ||μa||=4.20±0.14, ||μb||=0.39±0.03, and ||μt||=4.22±0.14 D. From the microwave splitting method, the threefold barrier to internal rotation was determined to be 1148±23 cal/mol (402±8 cm−1). The infrared (3200 to 50 cm−1) and Raman spectra (3200 to 10 cm−1) of gaseous and solid CH3GeH2CN, CH3GeD2CN, and CD3GeH2CN have been obtained. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values have been obtained. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and isotopic shifts. These results are compared to similar quantities in some related molecules.

Notes: A model is developed to describe the kinetics of the three scattering channels—unreactive scattering and dissociative chemisorption via single atom abstraction and two atom adsorption—that are present in the interaction of F2 with Si(100). The model provides a good description of the non-Langmuirian coverage dependence of the probabilities of single atom abstraction and two atom adsorption, yielding insight into the dynamics of the gas–surface interaction. The statistical model is based on the premise that the two dissociative chemisorption channels share a common initial step, F atom abstraction. The subsequent interaction, if any, of the complementary F atom with the surface determines if the overall result is single atom abstraction or two atom adsorption. The results are consistent with the orientation of the incident F2 molecular axis with respect to the surface affecting the probability of single atom abstraction relative to two atom adsorption. A perpendicular approach favors single atom abstraction because the complementary F atom cannot interact with the surface, whereas a parallel approach allows the F atom to interact with the surface and adsorb. The fate of the complementary F atom is dependent on the occupancy of the site with which it interacts. The model distinguishes between four types of dangling bond sites on the Si(100)(2×1) surface, based on the occupancy of the site itself and that of the complementary Si atom in the Si surface dimer. The results show that the unoccupied dangling bond sites on half-filled dimers are about twice as reactive as those on empty dimers, which is consistent with an enhanced reactivity due to a loss of a stabilizing π interaction between the two unoccupied dangling bonds on a dimer. © 2000 American Institute of Physics.

Notes: In the interaction of low energy F2 with Si(100) at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction (∼3%) of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an "attractive" interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas–surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels (single atom abstraction, two atom adsorption, and unreactive scattering) for an incident F2 are determined as a function of F2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel [P2=0.83±0.03(95%, N=9)] in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P1=0.13±0.03) at low coverage but increases to a maximum (P1=0.35±0.08) at about 0.5 monolayer (ML) coverage before decaying to zero. The reaction ceases at 0.94±0.11(95%, N=9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder. © 1999 American Institute of Physics.