Detection of Nitrogen-Protonated Nitrous Oxide HNNO+by Rotational Spectroscopy

The rotational spectrum of nitrogen-protonated nitrous oxide (\ce{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 $\chi_{aa}$(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 $\chi_{aa}$(N) values for the two isomers of protonated nitrous oxide are qualitatively consistent with the valence bond description of \ce{H\bond{-}N\bond{=}N+\bond{=}O} for the electronic structure of the nitrogen-protonated form and \ce{N\bond{#}N+\bond{-}O\bond{-}H} for the oxygen-protonated form. \ce{HNNO+} is found to be 2-4 times less abundant than \ce{NNOH+} under a range of experimental conditions, as might be expected because this metastable isomer is known to be only $\sim$6~kcal mol$^{-1}$ less stable than ground state \ce{NNOH+} from kinetic measurements by Ferguson and co-workers.