INFRARED-ABSORPTION OF CIS-CIS PEROXYNITROUS ACID (HOONO) IN SOLID ARGON

Nitric acid (HONO2) isolated in solid argon at 12 K was irradiated with 193 nm emission from an ArF excimer laser. Recombination of the photofragments led to formation of peroxynitrous acid (HOONO) in various conformers. In addition to previously reported lines due to trans-perp HOONO, lines at 3285.4, 1600.3, 1395.0, 927.2, 794.3, and 629.1 cm(-1) were observed; they are assigned to cis-cis HOONO having a five-membered hydrogen-bonded ring. The observed vibrational frequencies and the corresponding isotopic shifts of both conformers are in agreement with recent theoretical calculations. Cis-cis HOONO was photolyzed much more rapidly than trans-perp HOONO upon irradiation at 308 nm with a XeCl laser. Lines due to a HO2...NO complex were also observed; they disappeared upon irradiation with the globar source of the infrared spectrometer. The mechanism of formation of various conformers of HOONO is discussed.