Theoretical study of hyperfine coupling constants and electron spin g factors for X2Σ+ diatomics from Groups 1 and 2

The ESR parameters of the cations Be-2(+), Mg-2(+), Ca-2(+), BeMg+ BeCa+, MgCa+ and the mixed radicals ZBe, ZMg, ZCa (Z = Li, Na, K), all having a X (2)Sigma(u)(+)(1sigma(g)(2)1sigma(u))/X(2)Sigma(+) (1sigma(2)2sigma) ground state, have been studied theoretically. The A(iso) and A(dip) constants have been calculated with UHF, CISD, MP2, B3LYP, PW91PW91 wavefunctions, and 6-311 + G(2df) basis sets. The electron spin g factors (magnetic moment mu(s)) have been evaluated from correlated (MRDCI) wavefunctions, using a Hamiltonian based on Breit-Pauli theory with perturbation expansions up to second order, and 6-311+ G(2d) basis sets. As expected for s-rich radicals, the hyperfine spectra are governed by the A(iso) terms. Both Deltag(parallel to) and Deltag(perpendicular to) values are negative, but Deltag(parallel to) lies close to zero. For Deltag(perpendicular to) the coupling with 1 (2)Pi((u)) dominates the sum-over-states expansions. Although the singly occupied MOs (SOMO) are mostly of s character, the \Deltag(perpendicular to)\ are relatively large, up to 5200 ppm for cationic, and up to 7850 ppm for neutral radicals. These large values are caused by low excitation energies and high magnetic transition moments, the latter due to the fact that the sigma*(s-s) SOMO has the same nodal properties as a p(sigma) orbital. Of the radicals considered here, an ESR spectrum is available only for Mg-2(+). Our theoretical A(iso) of -287 MHz reproduces well the matrix result (-291 MHz). Calculated values of -10 ppm for Deltag(parallel to) and of -1280 ppm for Deltag(perpendicular to) give an average <Deltag> = -860 ppm that lies within the experimental range of -600(+/-300) ppm in Ne, and of -1300(+/-500) ppm in Ar matrices.