Hyperfine Interaction and Chemical Bonding in MgF, CaF, SrF, and BaF molecules

ESR spectra of MgF, CaF, SrF, and BaF molecules in their ground Σ2 states were measured while trapped in solid neon and argon matrices at 4°K. g tensors and hyperfine structure (hfs) constants for interaction with 19F, and in the case of SrF and BaF, with 87Sr and 137Ba, were determined. In all cases less than 4% of the spin density is on the F atom, supporting the evidence for their extreme ionicity. The variation in the anisotropic character of the 19F hfs probably indicates a slight increase in covalency in the lighter molecules. The hf interaction with the metal nucleus is quite anisotropic, demonstrating that the unpaired s electron on the M+ ion actually has appreciable p and d character, i.e., that the spherical s electron orbit in the free M+ ion is highly distorted by the F− ion in the molecule. This was supported by an approximate variational calculation considering the F− ion as a point charge. A negative Δg⊥ for the MgF molecule indicates that the lowest-lying Π2 state is regular, in support of Walker and Richards' reinterpretation of the λ-type doubling.