Trends in hyperfine coupling constants and electron‐spin g factors for X \documentclass{article}\pagestyle{empty}\begin{document}$^{2}\Sigma^{+}_{(g,u)}$\end{document} diatomics with 1,3, and 5 valence electrons

A comparison of electron spin resonance (ESR) parameters calculated for X 2 Σ ( g , u ) + diatomic radicals is presented, including Li 2 + , LiK + with one valence electron (VE); Li 2 − , LiK − , LiBe, LiCa, LiB + , LiGa + , Be 2 + , and BeCa + with 3 VEs; and B 2 + , BGa + with 5 VEs. The A iso and A dip constants are studied using 6‐311+G(2df) basis sets and several methods (UHF, CISD, MP2, B3LYP, PW91PW91). For the s ‐rich radicals with 1 and 3 VEs, it is found that A iso ≫ A dip ≈0, whereas for those of p σ ‐type (such as LiB + /LiGa + with 3 VEs, and all present 5 VE radicals), the values of A iso and A dip are generally similar. The electron‐spin g factors are calculated using perturbation expansions up to second order, a Hamiltonian based on Breit–Pauli theory, and correlated (MRCI) wave functions. All radicals have negative values of Δ g ∥ and Δ g ⊥ . The sum‐over‐states expansions for Δ g ⊥ are dominated by the coupling with just one or two 2 Π excited states. The Δ g ⊥ 's of the first‐row radicals Li 2 + , Li 2 − , Be 2 + , LiBe, LiB + , and B 2 + are about −60, −5, −120, −215, −3290, and −1300 ppm, respectively, while for the mixed first/third‐row counterparts LiK + , LiK − , BeCa + , LiCa, LiGa + , and BGa + they are about −1200, −2480, −1000, −7840, −93700, and −800 ppm, respectively. These results show the strong influence heavier atoms with larger spin‐orbit constants have on g ⊥ shifts. The smaller Δ g ⊥ of BGa + (when compared with B 2 + ) is caused by the 3σ→1π and 3σ→2π excited states contributing positively and negatively, respectively, to this shift. Such positive contribution is at variance with the rule stating that an excitation of type σ→π (SOMO→virtual MO) should contribute negatively. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002