Spectroscopic and ab initio investigation of the nu(OH) overtone excitation in trans-formic acid

Fourier transform spectroscopy and intracavity laser absorption spectroscop y are used to record the absorption spectrum of formic acid at high spectra l resolution, in the ranges of the 2 nu(OH) (6968.258 cm(-1)) and 4 nu(OH) (13 284.075 cm(-1)) vibrational bands of the trans-rotamer, respectively. N umerous perturbations combined with a large line density limit the extent t o which the vibration-rotation analysis is performed. Some 689 lines are as signed in the first overtone band and related vibration-rotation constants are determined. Only the band origin and upper state principal A-rotational constant are determined for the n=4 overtone band, because of much higher spectral density. Interpolation, helped by literature data, provides all mi ssing principal rotational constants in the n(OH) series, for n = 1 to 4. A ll major vibration-rotation parameters appear to evolve very smoothly along the series. This trend is fully supported by ab initio calculations perfor med at the MP2/cc-pVTZ level of theory and based on an effective one-dimens ional model describing the OH local mode stretching vibration. Results of t hese calculations are detailed in terms of the mean nuclear structure in th e excitation series, up to n = 4. The related calculated dipole moment surf ace allows the experimental a:b subband intensity ratio in the series to be interpreted in terms of a decrease in the HOC angle, from Theta approximat e to 90 degrees (n = 1) to 44 degrees(n = 4). The potential occurrence of a proton exchange mechanism between the two oxygen nuclei, close to a tautom erism mechanism, is discussed. (C) 2000 American Institute of Physics. [S00 21-9606(00)00824-2].