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

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 J_Ka,Kc = 1₀₁ → 0₀₀ transition was measured for both molecules, as well as the J_Ka,Kc = 2₀₂ → 1₀₁ transition for NaNH_2, 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, χ_aa 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.