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 -aa(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 -aa(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.
