Quadrupole coupling in alkali metal amides MNH2 (X~1A1): An experimental and computational study

[Display omitted] •First experimental determination of the quadrupole coupling in metal amide species.•First FTMmmW measurements (22–58 GHz) of LiNH2 and NaNH2.•Further evidence for strong ionic bonding in alkali amide molecules.•First computed structure for KNH2 (X~1A1) with a modern basis set for future work. Rotational spectra of LiNH2 and NaNH2 have been recorded using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the range 22 – 59 GHz. The species were created from the reaction of metal vapor and ammonia, diluted in argon, using a Discharge-Assisted Laser Ablation Source (DALAS). The JKa,Kc = 101 → 000 transition was measured for both molecules, as well as the JKa,Kc = 202 → 101 transition for NaNH2, all of which exhibited quadrupole coupling splittings. The two data sets were each analyzed with an S-reduced asymmetric top Hamiltonian, establishing the lithium and sodium electric quadrupole coupling parameter, χaa for the first time, and refining previous rotational constants. Quadrupole and nuclear-spin rotation interactions were also computationally investigated at the MP2/6-311G++(3df,2pd) level for LiNH2, NaNH2 and KNH2. These calculations suggest that the major contributor to the quadrupole interactions is the alkali metal nucleus, not that of nitrogen, as confirmed experimentally. Comparison of quadrupole coupling constants suggest that LiNH2 and NaNH2 are principally ionic molecules with a charge distribution similar to LiF and NaF.