A comparison of IR fourier spectroscopy and analysis of the vibrational structure of UV spectra as methods for determining torsional potential parameters

Two methods, UV and IR Fourier spectroscopy, used to determine torsional frequencies, torsional vibration levels, and anharmonicity constant in the ground electronic state are compared. These data are used to construct internal rotation potential function V(phi) = 1/2Sigma(n=1) V-n = (1 - cosnphi). The difficulties involved in the methods are considered for the examples of the benzaldehyde, benzoyl fluoride, and benzoyl chloride molecules. The possibility of using UV spectra for constructing several Deslandres tables for torsional vibrations and multiple recurrence of both the torsional frequency itself (omega(tt)) and its overtones up to v = 7 in several Deslandres tables allow the torsional frequency and the anharmonicity constant (x(tt)) to be determined more reliably than from Fourier spectroscopy data: It follows that UV spectra give more reliable V-n parameters of the V(phi) potential function and more reliable heights of barriers to internal rotation. This conclusion is confirmed by quantum-mechanical calculations performed by other authors for the three molecules specified above.