Millimeter-Wave Absorption Free Jet Spectrum, Barriers to Internal Rotation, and Torsional Relaxation in...

The rotational spectrum of p-anisaldehyde in a supersonic expansion has been investigated in the frequency range 60-78 GHz. Transitions up to J = 57 measured for the anti and syn conformers have been used to determine complete sets of fourth-order centrifugal distortion constants. Methyl group internal rotation splittings have also been observed for some of the lines and have yielded the respective barriers for both conformers. A vibrational satellite observed for each of the two conformers has been assigned to the respective first excited methoxy torsional state. The jet conditions, in particular the stagnation pressure of the carrier gas, can be adjusted to control the degree of cooling achieved in the expansion. This possibility has been exploited in the present work to enhance the intensities of the observed vibrational satellites. Applying a two-dimensional flexible model to the methoxyl and aldehydic groups torsions, the potential energy and structural deformation parameters transferred from anisole and benzaldehyde have been found to be suitable to describe these motions in p-anisaldehyde. Mixing of anti and syn conformations has been found to be insignificant within the first 82 calculated torsional eigenstates and therefore less likely to explain the intermediate bands observed in the low-resolution microwave spectrum than the internal vibrational relaxation suggested by R. K. Bohn, M. S. Farag, C. M. Ott, J. Radhakrishnan, S. A. Sorenson, and N. S. True (1992, J. Mol. Struct. 268, 107-121). A qualitative discussion of relaxation among the torsional states and its effect on the rotational spectrum is given. Copyright 1997 Academic Press. Copyright 1997Academic Press