6198
J . Phys. Chem. 1991, 95,6198-6201
Ab Initio Quadratic Configuration Interaction Calculation of the Isotropic Hyperfine Coupling Constants in the Ethyl Radical+ Ian Carmicbael Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 7 , 1991) The isotropic hyperfine coupling constants 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 singladetenninant unrestricted HartreeFock (UHF) description is coupled with finite (Fermi contact) field perturbation theory to derive the required spin density dstribution. Results obtained with a modest polarized double-{ 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, I3Ca,a large (-245 MHz) positive spin polarization estimated by the UHF method to be developed in the valence shell is seen to be sharply reduced (to -155 MHz) by the inclusion of electron correlation. Taken together with the computed core polarization (--75 MHz), the direct spin density (-20 MHz) from the highest occupied a molecular orbital, and a vibrational correction ( 15 MHz) due to zerqoint out-of-plane wagging of the methylenic hydrogens, the predicted carbon-I3 splitting is within 1% of observation. At the adjoining methylenic hydrogens almost the entire coupling constant derives from spin polarization which is overestimated at the UHF level and which again is moderated by the inclusion of electron correlation. A small (
