A b i n i t i o study of the vibrational dependence of hyperfine coupling constants in the methyl, silyl, and formaldehyde anion radicals

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Abstract
The influence of vibrational effects on the isotropic hyperfine coupling constants is studied by ab initio methods for the CH3, SiH3, and H2CO− radicals. The calculations are carried out using double‐zeta plus polarization basis sets of contracted Gaussian orbitals. A spin‐restricted SCF plus perturbative configuration interaction method including all spin adapted configurations with three uncoupled electrons is employed. The perturbation treatment is extended to second order which yields a definitive improvement of the results over previous first order calculations. A quantum mechanical treatment of the vibrational problem shows that the influence of the nuclear motion depends on the shape of the potential. The vibrational effects are found necessary to account for the experimental findings and the overall results are in very good agreement with experiment. It is found that the usual classical approach fails to reproduce the correct behavior of the splittings, especially for low temperature experiments.

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