Trends in the alkaline-earth amide series: The millimetre-wave spectrum of MgNH₂ and MgND₂ (X̃²A₁)

The pure rotational spectra of MgNH₂ (X̃²A₁) and its deuterium isotopomer have been recorded using millimetre/sub-millimetre wave direct absorption techniques. Both species were synthesized by the reaction of magnesium vapor and either NH₃ or ND₃ in a dc discharge. Twelve rotational transitions were measured for MgNH₂, and eight transitions were observed for MgND₂, both in the range 120-530 GHz. Asymmetry components with K_a = 0-5,7, and sometimes 6, were measured for nearly every transition. Rotational, centrifugal distortion, and spin-rotation parameters have been accurately determined for both molecules, as well as an r₀ structure for magnesium amide. The data indicate that MgNH₂ is a planar molecule with C_2v symmetry, similar to CaNH₂ and SrNH₂, and does not undergo inversion. However, there appears to be subtle differences between MgNH₂ and its calcium and strontium analogs. Magnesium amide, for example, has a significantly larger H-N-H bond angle than CaNH₂ and SrNH₂. In addition, the spin-rotation constants indicate slightly more in-plane p character for the unpaired electron in this molecule relative to that in calcium or strontium amide. Such differences may result from more covalent character in the bonding of MgNH₂ in comparison to the heavier alkaline-earth amide species.