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要旨:
Infrared intensities of the fundamental bands of the d0 and d6 species of zinc, cadmium and mercury dimethyls are calculated ab initio using effective core potentials. These are compared with new experimental data for Zn(CH3)2, Zn(CD3)2, Cd(CH3)2 and Hg(CH3)2. Assisted by ab initio-determined signs for ∂p/∂Q, the experimental data are used to determine values of ∂p/∂S, electro-optical parameters, atomic polar tensors and effective atomic charges, all of which are compared with those in ethane. CH bending bond moments μCH vary widely both in magnitude and sign according to the symmetry coordinate from which they are determined, and electro-optical parameterization procedures employed earlier in hydrocarbons appear to be inappropriate here. CH stretching intensities are similar to those in ethane, but the direction of the CH dipole derivative lies increasingly off the bond direction as the size of the metal atom increases. The metal-carbon bond moment decreases from Zn to Hg in line with the diminishing electronegativity difference. However, its derivative increases slightly from Zn to Hg. Despite major differences within the atomic polar tensors for a representative hydrogen atom between the metal dimethyls and ethane, the effective charges are very similar. The concept of charge deformability is examined and its value in the present compounds questioned.
