Spectroscopic and ab initio investigation of the νOH overtone excitation in trans-formic acid

Fourier transform spectroscopy and intracavity laser absorption spectroscopy are used to record the absorption spectrum of formic acid at high spectral 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. Numerous perturbations combined with a large line density limit the extent to which the vibration-rotation analysis is performed. Some 689 lines are assigned 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 missing principal rotational constants in the n(OH) series, for n = 1 to 4. All major vibration-rotation parameters appear to evolve very smoothly along the series. This trend is fully supported by ab initio calculations performed at the MP2/cc-pVTZ level of theory and based on an effective one-dimensional model describing the OH local mode stretching vibration. Results of these calculations are detailed in terms of the mean nuclear structure in the excitation series, up to n = 4. The related calculated dipole moment surface 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 approximate 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 tautomerism mechanism, is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)00824-2].