Trends in the alkaline-earth amide series: The millimetre-wave spectrum ofMgNH2 and MgND2 ((X)over-tilde(2)A(1))

The pure rotational spectra of MgNH2 ((X) over tilde (2)A(1)) and its deute rium isotopomer have been recorded using millimetre/sub-millimetre wave dir ect 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 f or 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 r(0) 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, ther e appears to be subtle differences between MgNH2 and its calcium and stront ium 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 th is molecule relative to that in calcium or strontium amide. Such difference s may result from more covalent character in the bonding of MgNH2 in compar ison to the heavier alkaline-earth amide species.