High resolution spectroscopy of MgOH (X ²Σ⁺) in its V₂ mode: Further evidence for quasilinearity

Pure rotational spectra of the MgOH and MgOD radicals have been recorded in the v₂ bending vibration of their X ²Σ⁺ ground electronic states using millimeter-wave direct absorption spectroscopy. Multiple rotational transitions arising from the v¹₂= 1¹, 2², 2⁰, 3¹, 3³, 4², and 4⁴ substates have been measured in the frequency range 240-520 GHz for these species. Both the spin-rotation and l-type doubling interactions have been resolved in the spectra. The complete data sets for MgOH and MgOD have been analyzed using a linear model for the Hamiltonian which takes into account higher order (l=±4) l-type interactions. The global analyses were adequate, but anomalous behavior was apparent in both molecules. In particular, the B_v vs v₂ relation was found to be highly nonlinear, large variations in the l-type doubling constant q were observed with vibrational level, and r₀, r_s, and r̃_e structures determined differed substantially. Such findings suggest that MgOH is highly quasilinear, comparable to HNCO. The competition between ionic and covalent bonding is therefore becoming apparent in the lighter alkaline earth hydroxide species.