Determination of a High Potential Barrier Hindering Internal Rotation from the Analysis of the Ground State Spectrum: The Case of Ethyl Cyanide

Microwave spectroscopy is a very accurate method to determine the potential barrier restricting the rotation of a methyl group [1 —3] . The finite barrier hindering internal rotation normally splits each rotational line into two components. This splitting depends strongly on the height of the barrier. Generally the barrier is determined from the splittings in the ground torsional state. But for a high barrier, these splittings may be too small to be resolved. In this case the barrier may be determined from the larger splittings of rotational lines in an excited torsional state. However the analysis may be much more difficult because of the possible interaction of the torsion with the other vibrations. In this respect ethyl cyanide, CH 3 CHXN is a very interesting molecule: no internal rotation splitting could be observed up to now in the ground torsional state and an interaction between the CCN-bend in plane {vls = 206.9 ± 0 . 5 cm -1 [4]) and the first excited torsional state (j'23 = 210.5 cm -1 [5]) affects significantly the internal rotation fine structure [4] . Dreizler and coworkers [6, 7] developed a simplified rotational Hamiltonian with five degrees of freedom (three of overall rotation, one of internal rotation, and one of vibration) and suceeded in explaining the excited states fine structure [4]. This Hamiltonian was successfully used for a lot of other molecules [8] where an interaction between torsion