Multireference configuration interaction calculations of electronic g-tensors for NO2, H2O+, and CO+

Electronic g-tensors parametrize the Zeeman splitting observed in the EPR spectra of radicals. In this work, we report g-tensor calculations for NO2, H2O+, and CO+ at the multireference CI level. Deviations of the tensor elements (g-shifts) from the free-electron value are computed via a perturbation expansion, complete to second order in relevant Breit-Pauli terms. The g-shifts we obtain for these molecules are as follows: NO2: Delta g(xx)=3571, Delta g(yy)=-10296, Delta g(zz)=-537; H2O+: Delta g(xx)=-249, Delta g(yy)=15733, Delta g(zz)=4105; CO+: Delta g(perpendicular to)=-2383, Delta g(parallel to)=-181 [all values in parts per million (ppm)]. These results are in reasonable agreement with gas phase experimental data. Larger g-shifts are typically within 20% of experiment, whereas smaller g-shifts generally differ by no more than several hundred ppm. Basis set effects and gauge dependence are examined in the case of CO+. For this molecule, a good valence description is vital for achieving accurate Delta g-values and small gauge-dependence. Polarization functions are of some use in these calculations, but diffuse functions have little effect on the gauge dependence of a cationic radical such as CO+. Vibrational effects are also examined for CO+. The vibrationally averaged g(perpendicular to)-shift only differs from the equilibrium value by 83 ppm. (C) 1997 American Institute of Physics.