Photoelectron Spectroscopy of cis- Nitrous Acid Anion ( cis- HONO – )

We report photoelectron spectra of cis-HONO– formed from an association reaction of OH– and NO in a pulsed, plasma-entrainment ion source. The experimental data are assigned to the cis-HONO– isomer, which is predicted to be the global minimum on the anion potential energy surface. We do not find evidence for a significant contribution from trans-HONO–. Electron photodetachment of cis-HONO– with 1613, 1064, 532, 355, and 301 nm photons accesses the ground X̃1A′ (S0) and excited ã3A″ (T1) states of neutral HONO. The photoelectron spectrum resulting from detachment forming cis-HONO (S0) exhibits a long vibrational progression, dominated by overtones and combination bands involving the central O–N stretching and ONO bending vibrations. This indicates that there is a significant change in the central O–N bond length between cis-HONO– and cis-HONO (S0). The electron affinity (EA) of cis-HONO is determined to be 0.356(8) eV. We also report the dissociation energy (D0) of cis-HONO–, forming OH– + NO, as 0.594(9) eV, which is a factor of 4 decrease in the central O–N bond strength compared to neutral cis-HONO. The T1 state of cis-HONO is shown to be ∼2.3 eV higher in energy than cis-HONO (S0). Electron photodetachment to form cis-HONO (T1) accesses a transition state along the HO–NO bond dissociation coordinate. The resulting photoelectron spectrum exhibits broad peaks spaced by the terminal N═O stretching frequency. Electronic structure calculations and photoelectron spectrum simulations reported here show very good agreement with the experimental data.