Terahertz spectroscopy of ²⁵MgH (X²Σ ⁺) and ⁶⁷ZnH (X²Σ⁺): Bonding in simple metal hydrides

Pure rotational spectra of ²⁵MgH (X²Σ ⁺) and ⁶⁷ZnH (X²Σ⁺) were recorded using direct absorption techniques. These free radicals were synthesized by the reaction of metal vapor, generated in a Broida-type oven, with H₂ in a dc discharge. The N = 0 → 1 and N = 1 → 2 transitions were recorded for both species in the frequency range 342-789 GHz. Spin-rotation and metal and proton hyperfine interactions were resolved in the spectra. From these data, rotational, fine structure, and hyperfine constants were determined, including the Fermi contact, dipolar, and electric quadrupole parameters of the ²⁵Mg and ⁶⁷Zn nuclei. Comparison of the metal and proton hyperfine terms suggests that the unpaired electron resides in a σ molecular orbital that has significant s contributions from both the metal and the hydrogen atoms. The dipolar terms for both metals are relatively large, in contrast to those of the proton, and indicate spσ and possibly sdσ (zinc only) orbital hybridization. The quadrupole constants arise principally from the p/d orbital character of the unpaired electron, although there is a non-negligible polarization contribution. These results suggest significant covalent character in both MgH and ZnH, in contrast to their fluoride analogs.