The Ultraviolet Absorption Spectrum of Nitrous Acid in the Vapor State

The diffuse absorption bands of HON 14 O, HON 16 O, DON 16 O, and DON 15 O in the near ultraviolet have been examined. Transitions from both rotational isomers of nitrous acid contribute to the vibrational structure, with the excited state σ(N=O) stretching vibrations active in the most intense progressions. The cis–trans energy separation increases from 500 cal/mole in the ground state to 1200 cal/mole in the upper state. This is attributed to weakening of the internal hydrogen bond in the cis state upon an electronic promotion of primarily n O → π* type. The accompanying change in geometry is responsible for the appearance of the cis ground-state δ(ONO) bending mode in the spectrum.