Formic and acetic acids in a nitrogen matrix: Enhanced stability

Abstract

Formic acid HCOOH, FA and acetic acid CH3COOH, AA are studied in a nitrogen matrix. The infrared IR spectra of cis and trans conformers of these carboxylic acids and also of the HCOOD isotopologue of FA are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the OH and COH modes and the relative intensities of the OH and CvO bands