Formic acid ͑HCOOH, FA͒ and acetic acid ͑CH 3 COOH, AA͒ are studied in a nitrogen matrix. The infrared ͑IR͒ spectra of cis and trans conformers of these carboxylic acids ͑and also of the HCOOD isotopologue of FA͒ 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 C v O bands.

Six trans-trans and five trans-cis dimeric structures of formic acid (HCOOH) are revealed by ab initio calculations. Four trans-trans and two trans-cis dimers are identified in the IR absorption spectra in argon matrices. The trans-cis dimers are obtained by narrow-band IR excitation of the vibrational transitions of the trans-trans dimers. Two trans-trans (tt3 and tt6) and one trans-cis (tc4) dimer are characterized experimentally for the first time. The tunneling decay rates of two trans-cis dimers (tc1 and tc4) are evaluated at different temperatures. A greater lifetime of the trans-cis dimers at elevated temperatures compared to the cis-monomer suggests that the high-energy conformers can be stabilized upon hydrogen bonding.

The higher energy conformer (cis) of HCOOD is prepared by vibrational excitation of the trans form. The cis conformer decays back to the conformational ground state (trans) via tunneling of deuterium. The tunneling process in HCOOD in rare gas matrices is extremely slow (in scale of weeks). We present new measurements of the tunneling rate constants, which characterize the efficiency of the cis-to-trans conversion process in Ne, Ar, Kr, and Xe matrices. The tunneling rates of HCOOD follow the trend k(Xe) approximately = k(Kr)>k(Ar) approximately = k(Ne), which is anomalous with respect to the reaction barrier of the solvated molecule. We propose a semiempirical energetic scheme of solid state solvation, which is consistent with all experimental observation. The temperature dependence of the tunneling constants rates of HCOOD is very weak compared to HCOOH in all matrices. The fundamental vibrational frequencies of the cis and trans conformers of HCOOD in various matrices are reported.

The highest frequency hydrogen bond fundamental of formic acid dimer, ν24 (Bu), is experimentally located at 264 cm−1. FTIR spectra of this in-plane bending mode of (HCOOH)2 and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three Bu combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D0 = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.