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.

The infrared spectrum and conformational flexibility of benzil, (C 6 H 5 CO) 2 , are studied by matrix-isolation FTIR spectroscopy, supported by DFT calculations. It is shown that the low-frequency (ca. 25 cm -1 ), largeamplitude torsion around the C-C central bond strongly affects the structural and spectroscopic properties exhibited by the compound. The equilibrium conformational distribution of molecules with different OdC-CdO dihedral angles, existing at room temperature in the gas phase, and trapped in a low-temperature (T ) 9 K) inert matrix can be changed either by in situ irradiation with UV light (λ > 235 nm) or by annealing the matrix to higher temperatures (T ≈ 34 K). In the first case, the increase of the average OdC-CdO angle results from conformational relaxation in the excited electronic states (S 1 and T 1 ), whose lowest-energy conformations correspond, for both S 1 and T 1 states, to a nearly planar configuration with the OdC-CdO dihedral angle equal to 180°. In the second case, the decrease of the average value of the OdC-CdO dihedral angle is a consequence of the change in the S o C-C torsional potential, resulting from interactions with the matrix media, which favors the stability of the more polar structures with smaller OdC-CdO dihedral angles.