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

ESR spectra of MgF, CaF, SrF, and BaF molecules in their ground ${}^2\Sigma$ 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 $\mathrm{\Delta g\perp }$ for the MgF molecule indicates that the lowest‐lying ${}^2\Pi$ state is regular, in support of Walker and Richards' reinterpretation of the λ‐type doubling.