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Torsional Dependence of Molecular Electrical Properties and Long-range Interactions: CH3OH- Rare Gas Systems

Stephen L. Davis
E. Dennison
Journal / Anthology

Journal of Chemical Physics
Year: 1994
Volume: 101
Issue: 2
Page range: 1293-1301

Self-consistent-field (SCF) values are reported for all Cartesian tensor components of the dipole, quadrupole, and octupole moments (mu, Theta, Omega) and polarizabilities alpha and A of the methanol molecule in its staggered, eclipsed, and three intermediate conformations. The methanol geometry was held rigid except for a single internal rotation angle gamma, describing the relative orientation of the methyl rotor and the hydroxyl framework. Three different basis sets were used, including a 110 contracted Gaussian set based on the electrical properties (ELP) basis of Dykstra et al. [Adv. Chem. Phys. 75, 37 (1989), and references therein]. It was found that the tensor components Omega_xxx, Omega_xyy, A_xxx, A_xyy, and A_yxy vary as cos 3gamma, while the components Omega_yyy, Omega_yxx, A _yyy , A _yxx , and A_xyx vary as sin 3gamma. All other components of Omega and A, as well as all components of mu, Theta, show little variation with gamma. This dependence was explained using a simple model that treats each property as a sum of a constant, hydroxyl framework contribution and a conformation-dependent, methyl rotor contribution. Torsional averages of these properties were computed from torsional wave functions obtained by diagonalization of the internal axis method (IAM) Hamiltonian. It was found that the large amplitude internal rotation in methanol gives rise to large vibrational effects on the A_xxx, A_xyy, and A_yxy polarizabltiey components. The conformational dependence of the electrical properties was used to describe the conformational dependence of long-range interactions involving a near-symmetric, nonrigid molecule such as methanol. The leading gamma-dependent interaction term was shown to vary as R^-8 and R^-7 for the induction and dispersion interactions (respectively) between a methanol molecule and a structureless atom. Cartesian tensor expressions are given for the long-range dispersion interaction within second-order perturbation theory, and the leading torsionally dependent interaction is shown to vary as sin^3 cos 3(phi - gamma), where theta, phi are the spherical coordinates of the atom.

*Science > Chemistry