HYPERFINE SPLITTING IN HOCO FROM AB-INITIO CALCULATION

Results are reported from an ab initio molecular orbital study of the isotropic hyperfine coupling, a, in trans- and cis-HOCO. Equilibrium radical structures are obtained from large-basis-set second-order Moller-Plesset perturbation theory (UMP2) calculations. Vibrational frequencies are predicted for the analogous deuterated species from scale factors determined by comparison with the results of similar calculations on, and the known experimental vibrational spectra of, various isotopomers of formic acid. The isotropic splittings due to the possible magnetic nuclei, H-1, C-13, and O-17, are then computed at these optimized geometries. An extensive investigation of the basis-set dependence of the computed magnetic coupling constants is carried out at the unrestricted Hartree-Fock (UHF) and UMP2 levels of theory. Further electron correlation effects on the calculated splittings are determined from the results of coupled-cluster (UCCD) and quadratic configuration interaction (UQCISD) treatments with moderately large basis sets. For the more readily observed couplings, a(1H) and a(13C), value of -4.7 and 533 MHz are obtained for the trans-isomer. Experimentally a(1H) = +/-6.8 MHz is reported. [Sears et al. J. Chem. Phys. 1993, 98, 6624]. Couplings in cis-HOCO are predicted to be 77 MHz at hydrogen and 608 MHz at carbon.