Theoretical study of the electron–spin magnetic moment (g-tensor) of FCl−

The electronic g-shift of FCl−(X 2Σ+) is calculated using second-order perturbation theory, a Breit–Pauli Hamiltonian and MRCI wavefunctions. Second-order contributions to Δg⊥ are determined by the coupling with the two lowest 2Π states. The Δg⊥-values generally increase linearly with R(F–Cl), but show a plateau near Re. At equilibrium, the calculated values (in ppm) are Δg‖=−370, Δg⊥=23 965, and 〈Δg〉=15 855. Experimental values for FCl− trapped in crystals are −450±500, 27 400±2000, and 18 100 ppm, respectively. Allowing for crystal effects, the agreement is good. An optical absorption in the crystal at 4.13 eV is assigned to the transition 2 2Σ+ ← X 2Σ+ (ΔE=4.99 eV and f=0.39).