Showing papers by "Wesley D. Allen published in 2005"

Model identity SN2 reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2): Marcus theory analyzed.

Abstract: The structures of seven gas phase identity SN2 reactions of the form CH3X + X- have been characterized with seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple ζ quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree−Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are = −0.8, = 1.6, = 28.7, = 14.3, = 13.8, = 28.6, and = 25.7 kcal mol-1. General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a techniqu...

The highly anharmonic BH5 potential energy surface characterized in the ab initio limit.

Abstract: The strong sensitivity to level of theory of the salient features of the ground state potential energy surface of BH(5) has been overcome by rigorously converged ab initio computations employing correlation-consistent basis sets cc-p(C)VXZ (X=2-6), explicitly correlated R12 corrections, and coupled-cluster theory complete through quadruple excitations (CCSDTQ). Extrapolations via our focal point method yield a C(s)-symmetry global minimum of BH(3)-H(2) type featuring interfragment B-H distances of (1.401, 1.414) A, an H(2) bond length elongated to 0.803 A, and a BH(3)+H(2) dissociation energy D(e)(D(0))=6.6 (1.2) kcal mol(-1). The classical barriers for H(2) internal rotation and hydrogen scrambling are 0.07 and 5.81 kcal mol(-1), respectively, above the BH(5) minimum. Our thermochemical computations yield Delta(f)H(0) ( degrees )[BH(5)(g)]=-111.3+/-0.2 kcal mol(-1)+Delta(f)H(0) ( degrees )[B(g)], which is limited in accuracy only by persistent uncertainties in the enthalpy of formation of gaseous boron. As a first step in investigating the extremely anharmonic 12-dimensional vibrational dynamics of BH(5), a complete quartic force field has been computed at the all-electron cc-pCVQZ CCSD(T) level of theory. Previous matrix isolation infrared assignments of the nu(2) and nu(9) stretching modes of BH(5) compare favorably with our computed vibrational fundamentals, but the experimental assignment of the nu(6) bending mode of the BH(3) moiety is not supported by computed isotopic shifts.

The ab initio limit quartic force field of BH3.

Abstract: The complete quartic force field of BH(3) has been converged to the ab initio limit by extrapolation of core-valence correlation-consistent basis set series (cc-pCVXZ, X = T, Q, 5) of all-electron CCSD(T) (coupled-cluster singles and doubles with perturbative triples) energy points. Additional computations including full coupled-cluster treatments through quadruple excitations (CCSDTQ), scalar relativistic effects, and diagonal Born-Oppenheimer corrections (DBOC) were concurrently executed. Within second-order vibrational perturbation theory (VPT2) our quartic force field yields the fundamental frequencies nu(1) = 2502.3 cm(-1), nu(2) = 1147.2 cm(-1), nu(3) = 2602.1 cm(-1), and nu(4) = 1196.5 cm(-1), in excellent agreement with observed gas-phase fundamentals, displaying a mean absolute error of only 0.3 cm(-1). Our converged prediction for the equilibrium bond length of BH(3) is r(e) = 1.1867 A.

On the nature of the Møller-Plesset critical point.

Abstract: It has been suggested [F. H. Stillinger, J. Chem. Phys. 112, 9711 (2000)] that the convergence or divergence of Moller-Plesset perturbation theory is determined by a critical point at a negative value of the perturbation parameter z at which an electron cluster dissociates from the nuclei. This conjecture is examined using configuration-interaction computations as a function of z and using a quadratic approximant analysis of the high-order perturbation series. Results are presented for the He, Ne, and Ar atoms and the hydrogen fluoride molecule. The original theoretical analysis used the true Hamiltonian without the approximation of a finite basis set. In practice, the singularity structure depends strongly on the choice of basis set. Standard basis sets cannot model dissociation to an electron cluster, but if the basis includes diffuse functions then it can model another critical point corresponding to complete dissociation of all the valence electrons. This point is farther from the origin of the z plane than is the critical point for the electron cluster, but it is still close enough to cause divergence of the perturbation series. For the hydrogen fluoride molecule a critical point is present even without diffuse functions. The basis functions centered on the H atom are far enough from the F atom to model the escape of electrons away from the fluorine end of the molecule. For the Ar atom a critical point for a one-electron ionization, which was not previously predicted, seems to be present at a positive value of the perturbation parameter. Implications of the existence of critical points for quantum-chemical applications are discussed.

H−C−SiH3: Direct Generation and Spectroscopic Identification of Ethylidene's Cousin

Abstract: Ground-state triplet silaethylidene, generated directly by the reaction of 3P carbon atoms with silane under matrix isolation conditions in solid Ar (10−12 K), has been thoroughly characterized by ...