High-resolution Fourier transform study of the ν3 fundamental band of trans-formic acid

Using high-resolution Fourier transform spectra of trans-HCOOH recorded at 5.6 μm, we carried out an extensive analysis of the strong ν3 fundamental band (carbonyl stretching mode) at 1776.83 cm−1, starting from results of a previous analysis [Weber WH, Maker PD, Johns JWC, Weinberger E. J Mol Spectrosc 1987; 121: 243-60]. As pointed out in the literature, the ν3 band is significantly perturbed by resonances due to numerous dark bands. We were able to assign series belonging to the ν5+ν7, ν5+ν9, ν6+ν7 and ν6+ν9 dark bands, located at 1843.48, 1792.63, 1737.96 and 1726.40 cm−1, respectively. The model used to calculate energy levels accounts partly for the observed resonances, and enabled us to reproduce most of the observed line positions, within their experimental uncertainties. We also determined absolute line intensities with an accuracy estimated to 15%. Finally, we generated, for the first time, a list of line parameters for the 5.6 μm region of trans-formic acid.