Identification of New Dimers of Formic Acid: The Use of a Continuous-Wave Optical Parametric Oscillator in Matrix Isolation Experiments

We report on the first photochemical use of a continuous-wave optical parametric oscillator (OPO) in a matrix isolation experiment. The narrow-band high-power OPO light promotes efficient conformational changes of matrix-isolated formic acid by selective vibrational excitation, which is combined with thermal annealing of matrices. This approach allows us to find five new dimers of formic acid in an argon matrix (presumably two trans-trans and three trans-cis structures), as shown by infrared spectroscopy. The assignment of the newly observed absorptions is discussed on the basis of the recent ab initio calculations. As a result of this work, all of the trans trans and trans-cis dimers of formic acid theoretically predicted to date have now been observed in matrix isolation experiments. The structure of dimers can be influenced by the solid matrix surrounding, which may explain differences from the computational spectra obtained for species in vacuum.