Comparing electron‐spin g ‐tensor results of first‐row radicals with those of higher rows

The electron‐spin Δ g 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   − 2and M + S   − 2(M = Li, Na), with 13 VEs; and F   − 2 , Cl   − 2 , 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, Δ g ⊥ components (usually large) are described within 10%, whereas most calculated Δ g ‖ values (usually small) are much smaller than the experimental ones. The Δ g ⊥ values of the mixed first–second row systems SiO + and AlO, due to a mutual cancellation of the contributions from the lowest two 2 Π 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 MO 2 and MS 2 , Δ g ‖ is very large due to the coupling of two close‐lying states having similar spin‐density distributions. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 324–335, 2000