Hyperfine coupling constants, electron-spin g-factors and vertical spectra of the X-2 Sigma(+) radicals BeH, MgH, CaH and BZ(+), AlZ(+), GaZ(+) (Z = H, Li, Na, K). A theoretical study

Hyper. ne coupling constants (hfcc), electron-spin g-factors, excitation energies ( DE) and oscillator strengths (f-values) are calculated for the X(2)Sigma(+) (1sigma(2)2sigma) radicals YH (Y = Be, Mg, Ca) and BZ(+), AlZ(+), GaZ(+) (Z = H, Li, Na, K). Single reference ab initio and density functional theory methods are used for the hfcc's, and multireference configuration interaction wavefunctions for DeltaE's, f-values and g-shifts. In XZ(+) (X = B to Ga; Z = Li to K), the 2sigma unpaired electron is 60-90% p(sigma)(X), resulting in \A(perpendicular to)\ being one order of magnitude larger than \Ak(parallel to)\. The Deltag(perpendicular to)'s are relatively large for just 3 valence electrons, from -3300 to -6100 ppm for BZ(+), -14 100 to -29 600 ppm for AlZ(+), and -93 700 to -198 300 ppm for GaZ(+). Such large Deltag(perpendicular to)(XZ(+))'s relate to the strong coupling between the close-lying X(2)Sigma(+) and I(2)Pi(2sigma --> 1pi) states, which results from the splitting of the P-2(s(2)p) state of X due to its interaction with Z(+). ESR spectra are not available for any of the XZ(+)'s. Calculation on the hydrides YH/XH+ reproduce well the experimental hfcc's and g factors, except for AlH+, which appears to be strongly perturbed in Ar matrices. Several transitions X(2)Sigma(+) --> n(2)Sigma(+) and X(2)Sigma(+) --> n(2)Pi in XZ(+) have large oscillator strengths, mainly caused by intramolecular charge-transfer processes X <----> Z and atomic-like s(g) <----> p(u) excitations.