Hyperfine coupling in alkaline earth monofluorides. Limitations of the ionic model

Isotropic hyperfine coupling constants for the electronic ground state X 2 Σ of BeF, MgF, CaF and CaOH are calculated at SCF, MP2 and QCI ab initio levels of theory using moderate size basis sets. At calculated equilibrium geometries the following A iso ( 19 F) are obtained: 204.6, 187.1 and 113.1 MHz for BeF, MgF and CaF, respectively. The effects of vibration and centrifugal distortion on the spin densities are also calculated. The results are discussed in terms of electron delocalization. Charge transfer from F − and OH − to M + and electron overlap give rise to covalent mixing which causes hyperfine coupling and demonstrates the intrinsic limitations of the ionic model.