Detection of nitrogen-protonated nitrous oxide (HNNO+) by rotational spectroscopy

The rotational spectrum of nitrogen-protonated nitrous oxide (HNNO(+)), an isomer whose existence was first inferred from kinetic studies more than 30 years ago, has now been detected by Fourier transform microwave spectroscopy, guided by new high-level coupled-cluster calculations of its molecular structure. From high-resolution measurements of the hyperfine splitting in its fundamental rotational transition, the rotational constant (B + C)/2 and the quadrupole tensor element χaa(N) for both nitrogen atoms have been precisely determined. The derived constants agree well with quantum-chemical calculations here and others in the literature. The χaa(N) values for the two isomers of protonated nitrous oxide are qualitatively consistent with the valence bond description of H-N═N(+)═O for the electronic structure of the nitrogen-protonated form and N≡N(+)-O-H for the oxygen-protonated form. HNNO(+) is found to be 2-4 times less abundant than NNOH(+) under a range of experimental conditions, as might be expected because this metastable isomer is known to be only ∼6 kcal mol(-1) less stable than ground-state NNOH(+) from kinetic measurements by Ferguson and co-workers.