Near-UV absorption cross sections and trans/cis equilibrium of nitrous acid

Abstract

The A 1A″ ← X 1A′ absorption spectrum of gaseous nitrous acid has been measured in the 300-400-nm range. Absolute cross sections were determined by a combination of gas-phase and wet chemical analysis. The cross sections of prominent bands are 25% larger than the recommended values of Stockwell and Calvert. The influence of spectral resolution on absolute and differential absorption cross sections was also investigated. The integrated band area of the nπ* transition yields an oscillator strength f = (8.90 ± 0.36) × 10-4, less than the reported liquid phase value of 2 × 10-3. The equilibrium constant K = ptrans/pcis, based on the assumption that the oscillator strength of the nπ* transition is the same for both rotamers, was found to be 3.25 ± 0.30 at 277 K. This yields an energy difference ΔE between trans- and cis-HONO of -2700 J mol-1 in the electronic ground state, and -6000 J mol-1 in the excited state. © 1991 American Chemical Society.