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: Δgxx=3571, Δgyy=−10296, Δgzz=−537; H2O+: Δgxx=−249, Δgyy=15733, Δgzz=4105; CO+: Δg⊥=−2383, Δg∥=−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 Δg-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⊥-shift only differs from the equilibrium value by 83 ppm.