Ab initio determination of the torsional spectra of acetic acid

The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-pVTZ 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 (A(2)) and 77.050 cm(-1) (E) and 568.532 (A(2)) and 568.418 cm(-1) (E). The V-3 barrier causes a splitting of 0.315 cm(-1) in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be D-J = 90.4 kHz, D-JK = 301.5 kHz and D-K = 165.4 kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.