Comparing electron-sping-tensor results of first-row radicals with those of higher rows

The electron-spin Δg values of radicals from the first and second row with 9–15 valence electrons (VE) were calculated using second-order perturbation theory and a Breit–Pauli Hamiltonian, in conjunction with multireference configuration interaction (MRCI) wave functions. The radicals studied include CO+, SiO+, BO, AlO, BeF, MgF, and CaF, with 9 valence electrons; M+O and M+S (M = Li, Na), with 13 VEs; and F, Cl, and FCl−, with 15 VEs. Our method works well for first- and second-row systems. Comparing with experimental results for 9 and 15 VE radicals trapped in rare-gas matrices, Δg⊥ components (usually large) are described within 10%, whereas most calculated Δg‖ values (usually small) are much smaller than the experimental ones. The Δg⊥ values of the mixed first–second row systems SiO+ and AlO, due to a mutual cancellation of the contributions from the lowest two 2Π states, are relatively small (≈ −2500 ppm). Also, reliable g shifts for SiO+ and AlO can only be calculated using natural orbitals, as the molecular orbital treatment gives incorrect ground-state spin-density distributions. Difficulties previously found by a density functional theory/gauge including atomic orbitals (DFT/GIAO) study on the g shifts of AlO are thus most probably caused by the use of inacurrate spin densities rather than the possible failure of the perturbation expansion truncated at second order. For the π radicals MO2 and MS2, Δg‖ is very large due to the coupling of two close-lying states having similar spin-density distributions.