A b i n i t i o (STO-3G) dipole moment derivative properties of the CHn series

Abstract

The atomic and bond dipole moment derivative properties associated with the total infrared absorption intensity of the methylidyne, methylene, and methyl free radicals, and of methane, have been calculated by means of the ab initio STO-3G algorithm. These properties include the coordinate-dependent atomic polar tensors and the following invariant quantities: the dipole derivatives for a bond stretch, mean atomic dipole moment derivatives, atomic anisotropies, and the effective atomic charges. Three sets of calculations were conducted for each species by combining the experimental or theoretically determined equilibrium molecular geometry with the standard or optimum orbital exponent scaling factors. The relationship between each of these dipole derivative properties and, in turn, the equilibrium geometry, the carbon and hydrogen scaling factors, and the number of bonded hydrogen atoms is analyzed. According to any calculation, it is found that the calculated dipole derivative value for a C–H stretch in methane is negative, which agrees with the experimentally deduced results, but is positive for all the radicals, except the doublet state of CH. Furthermore, it is shown that the optimum scaling factors must be used in the calculations in order to consistently obtain good agreement between theory and experiment. Based on both the theoretical equilibrium geometry and the optimum scaling factors of a species, the following trends are exhibited by this homologous series of hydrocarbons: (1) The dipole derivative magnitude for a C–H bond stretch is a monotonic increasing function of the product of the carbon and hydrogen scaling factors. (2) The effective charge of carbon increases as the optimum size of the carbon valence shell increases, and decreases as additional hydrogens are bonded to the carbon. (3) The hydrogen effective charge generally decreases as the number of hydrogens increases. (4) The mean dipole derivative of hydrogen is a monotonic decreasing function of the number of C–H bonds, and is negative only for methane. (5) Calculations on the doublet and quartet electronic states of CH yield different values for the dipole moment, the effective charge, and the anisotropy, the dipole derivatives for a bond stretch have opposite directions.