Ab initiodetermination of the torsional spectra of acetic acid

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Abstract
The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-p VTZ ab initio calculations. The molecule shows two planar trans and cis conformers whose energy difference is 1882.7 cm−1. Both minimum energy geometries are separated by a barrier of 4432.1 cm−1. The most stable trans-conformer shows a quite low methyl torsion barrier of 169.8 cm−1. The roto-torsional energy levels have been calculated up to J = 10. The two torsional fundamental frequencies of the trans-conformer, the methyl and the OH torsion are 82.857 (A2) and 77.050cm−1 (E) and 568.532 (A2) and 568.418cm−1 (E). The V3 barrier causes a splitting of 0.315cm−1 in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be DJ = 90.4kHz, DJK = −301.5kHz and DK = 165.4kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.