Ab initio quadratic configuration interaction calculation of the isotropic hyperfine coupling constants in the ethyl radical

Abstract

The isotropic hyperfine coupling constraints in the electronic ground state of the ethyl radical have been determined by means of ab initio molecular orbital theory. Extensive inclusion of electron correlation in a single-determinant unrestricted Hartree-Fock (UHF) description is coupled with finite (Fermi contact) field perturbation theory to derive the required spin density distribution. Results obtained with a modest polarized double-{zeta} basis set are already in quantitative accord with experiment. Augmentation of this basis set improves the level of agreement still further. An analysis is made of the correlation contributions to the magnetic coupling at each nucleus. At the radical center, {sup 13}C{sub {alpha}}, a large ({approximately}245 MHz) positive spin polarization estimated by the UHF method to be developed in the valence shell is seen to be sharply reduced (to {approximately} 155 MHz) by the inclusion of electron correlation. Taken together with the computed core polarization ({approximately}-75 MHz), the direct spin density ({approximately}20 MHz) from the highest occupied {alpha} molecular orbital, and a vibrational correction ({approximately}15 MHz) due to zero-point out-of-plant wagging of the methylenic hydrogens, the predicted carbon-13 splitting is within 1% of observation. At the adjoining methylenic hydrogens almost the entire coupling constant derives from spin polarization which ismore » overestimated at the UHF level and which again is moderated by the inclusion of electron correlation. A small (<2 MHz) vibrational correction brings the computed value into essentially exact agreement with the observed splitting.« less