ALBERT

Notes: Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–C¨H. The transition structure in the interconversion HC≡C–C¨H(arrow-right-and-left)HC(overdot)=C=C(overdot)H(arrow-right-and-left)HC¨–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6-31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.

Notes: Variational calculations of vibrational levels of three Ag modes are reported for cyclobutadiene, its vicinally 13C-dilabeled derivative, and the tetralabeled derivatives 13C4H4 and C4D4. These calculations use the ab initio two-configuration GVB/4-31G potential surface for the antisymmetric CC stretch (automerization), symmetric CC stretch, and symmetric CCH bend. The three vibrational modes are strongly coupled so that one-dimensional and adiabatic multidimensional approaches are not justifiable in this case. The splitting of the ground vibrational state of di-13C -cyclobutadiene is calculated to be 4 cm−1, which implies the rate constant for automerization is k=2.4×1011 s−1 at temperatures near absolute zero. The Raman spectrum of cyclobutadiene is predicted to have two strong bands in the region of 3100–3200 cm−1. The other transitions are predicted to be of considerably lower intensity, though three of them should be strong enough to be observed. Two of the three are predicted to be accompanied by satellite lines which are due to the automerization splitting of the Ag levels. Uncertainties involved in the theoretical approach are discussed.

Notes: Results of ab initio two-configuration CI-SD/[3s2p1d/2s], MBPT(4), CCSD+T(CCSD), and CCSDT-1 calculations are reported for the rectangular D2h equilibrium and square D4h transition structures of cyclobutadiene. The latter is a classic example of a multireference correlated method. The optimum CC and CH bond lengths found for the D4h transition structure are 1.448 and 1.093 A(ring), respectively. The activation barrier for the automerization is 9.0 kcal/mol at the two-reference GVB-CISD level while the single reference CCSD gives 19.9, 14.4 for CCSD+T(CCSD) and finally a dramatic change to 9.5 at the highest CCSDT-1 level. The importance of triples in overcoming the multireference character at the transition state is apparent. On the other hand, GVB-CISD is simpler than CCSDT-1 which attests to the importance of a qualitatively correct multireference starting point for this example. A less sophisticated computational method, GVB/4-31G, which also gives a reasonable barrier of 10.2 kcal/mol was used for the construction of the two-dimensional potential surface of automerization. The following lowest vibrational energies were obtained for this surface (v1 and v2, the symmetric and antisymmetric CC stretches in D4h symmetry, are given in parentheses): 0 and 4.2 cm−1 (00+; 00−); 1526.1 and 1607.6 cm−1 (01+; 01−), 1480.9, and 1485.5 cm−1 (10+; 10−), and 791.6 cm−1 for the zero-point energy (00+). The computed splitting of the vibrational ground state implies the rate of automerization is k=2.5×1011 s−1 for temperatures close to absolute zero.

Notes: The Ar–NO+ ionic complex is studied using ab initio calculations. The complex geometry and harmonic vibrational frequencies are calculated using second order Møller–Plesset perturbation theory (MP2) calculations, employing a variety of basis sets. The calculated intermolecular bond length supports the experimental value, whereas the calculated Ar–N–O bond angle suggests a possible reinterpretation of the experimental result. The vibrational frequencies are then recalculated using an anharmonic approach and the fundamentals are found to be in much better agreement with the experimental values [obtained from zero-kinetic-energy (ZEKE) spectroscopy] than are the harmonic values. However, the calculations suggest that the potential energy surface of this complex cation is very anharmonic, and that the experimental assignment of the vibrational features in the ZEKE spectrum may have to be revised. The interaction energy of the complex is calculated, both with and without the full counterpoise (CP) correction; the CP-corrected values are in much closer agreement with experiment than are the uncorrected values. The final value of the stabilization energy, taking into account the MP4 correction is ca. 950 cm−1, in excellent agreement with the (re-evaluated) experimental value of 920±20 cm−1.

Notes: Prospects for higher order perturbation theory in evaluating nonadiabatic corrections to the adiabatic energy levels are investigated by performing calculations on a series of two-dimensional anharmonically coupled oscillators. The convergence properties of the perturbation series are demonstrated for different harmonic frequencies and magnitudes of perturbation. It is found that the perturbation series corresponding to the adiabatic levels which are away from any intersections can accurately be summed, by means of the Padé summation technique, even in the case of "giant'' couplings. The proposed "nondegenerate'' perturbation theory procedure may also provide accurate results, under certain conditions, even in the case of intersecting energy levels. It fails to converge only in the regions around the crossing points. Elsewhere, it converges safely (especially when using high-accuracy arithmetics) and provides the basis for relatively accurate interpolation over the crossing regions. © 1995 American Institute of Physics.

Notes: The phenol-water cation radical has been investigated by ab initio theory using the spin-restricted open-shell Hartree–Fock and spin-restricted open-shell second-order Møller–Plesset theories with 3-21G*(O) and 6-31G* basis sets. The full geometrical optimization was performed for several hydrogen-bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol-localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol-water cation radical are compared with those of the corresponding neutral complex.

Notes: Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabilization energies are used to parameterize the nonempirical model (NEMO) empirical intermolecular potential of the benzene dimer in the ground electronic state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy minimum, corresponding to the T-shaped structure, is found. A parallel displaced structure is the first-order transition structure separating the molecular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (∼170 cm−1), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spectra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is related to the T-shaped structure with two feasible internal rotations and nonequivalent monomers). The dimer undergoes a nearly free internal rotation along the axis connecting the benzene centers of mass in the T-shaped equilibrium geometry and a hindered internal rotation (the barrier being ∼46 cm−1) along the axis that is perpendicular to the "nearly free" internal rotation axis. The tunneling splittings observed in the rotational spectrum are likely due to this hindered rotation. An analysis assuming the latter rotation as an independent motion and using purely vibrational tunneling splittings (obtained by extrapolating to zero values of the rotational quantum numbers) indicates that the genuine value of the hindered rotation barrier is nearly twice higher than its ab initio value. Similarly, the difference ΔR=0.25 Å between the ab initio (equilibrium) and experimental (ground state) values for the distance of the mass centers of the benzene monomers is strong evidence that our theoretical potential is much shallower than the genuine one. The Raman bands observed at the 3–10 cm−1 region seem to involve states associated with the nearly free rotation and the "energy minimum path" bending motion. Unambiguous assigning of the weaker Raman features is infeasible, partly due to limitations in the accuracy of the theoretical potential, and partly due to the lack of knowledge of the polarizability tensor of the dimer and temperature at which the spectra were taken. © 1999 American Institute of Physics.

Notes: The amino group nonplanarity in nucleic acid bases, aniline, aminopyridines, and aminotriazine was investigated by ab initio methods with and without inclusion of correlation energy utilizing medium and extended basis sets. For all the systems studied, the amino group was found to be nonplanar and the coupled cluster method [CCSD(T)] "nonplanarities'' and inversion barriers slightly higher than their second-order many-body perturbation-theory (MP2) counterparts. To assess the reliability of the calculations, inversion splittings for aniline and aniline-ND2 were evaluated by solving a two-dimensional vibrational Schrödinger equation for the large-amplitude inversion and torsion motions, while respecting the role of small-amplitude C–N stretching and H–N–H bending motions. Because a large number of points is required for the description of the aniline potential energy surface, the Hartree–Fock (HF) method with 6-31G* basis set was utilized. The vibrational calculations were performed within the framework of the semirigid bender Hamiltonian of Landsberg and Bunker. Excellent agreement between experimental and theoretical inversion-torsion frequencies for fundamental, overtone, and combination modes was found, which gives strong evidence for the adequacy of the theoretical model used in general, and potential energy surface in particular. Similarity between the HF, MP2, and CCSD(T) aniline inversion barriers and amino group nonplanarities gives us confidence that the MP2 and CCSD(T) inversion barriers and amino group nonplanarities of the DNA bases, aminopyridine, and aminotriazine, are close to the actual values which are still experimentally unknown. © 1996 American Institute of Physics.

Notes: The orthogonally spin-adapted linear-response coupled-cluster (LRCC) theory with singly and doubly excited clusters (CCSD) was employed to calculate quadrupole moment functions of the HF and N2 molecules in their ground electronic states. We also calculated several potential energy curves for both systems using various CC and non-CC methods, ranging from the limited and full configuration interaction (CI) and first-order CI (FOCI) to finite-order many-body perturbation theory. FOCI and related complete active space self-consistent field (CASSCF) methods were used in both energy and quadrupole moment calculations. Most of the calculations were performed using the medium-size basis set of TZ+2P quality devised by Sadlej [A. Sadlej, Coll. Czech. Chem. Commun. 53, 1995 (1988)] for high-level ab initio calculations of electrostatic molecular properties. In addition, a number of model CC calculations using small basis sets were performed, for which the exact full CI results, both for the energy and multipole moments, are available. It was demonstrated that the CCSD approach provides a realistic description of quadrupole moment functions, for all relevant geometries in the case of HF and for internuclear separations up to 1.5 times the equilibrium bond length for N2. The results of this study will be used for the analysis of the rovibrational dependence of quadrupole moments and for the calculation of quadrupole transition moments for both HF and N2. © 1996 American Institute of Physics.