MILLIMETER-WAVE ABSORPTION FREE JET SPECTRUM, BARRIERS TO INTERNAL-ROTATION, AND TORSIONAL RELAXATION IN P-ANISALDEHYDE

The rotational spectrum of p-anisaldehyde in a supersonic expansion ha s been investigated in the frequency range 60-78 GHz. Transitions up t o J = 57 measured for the anti and syn conformers have been used to de termine complete sets of fourth-order centrifugal distortion constants . Methyl group internal rotation splittings have also been observed fo r some of the lines and have yielded the respective barriers for both conformers. A vibrational satellite observed for each of the two confo rmers has been assigned to the respective first excited methoxy torsio nal state, The jet conditions, in particular the stagnation pressure o f the carrier gas, can be adjusted to control the degree of cooling ac hieved in the expansion. This possibility has been exploited in the pr esent work to enhance the intensities of the observed vibrational sate llites. Applying a two-dimensional flexible model to the methoxyl and aldehydic groups torsions, the potential energy and structural deforma tion parameters transferred from anisole and benzaldehyde have been fo und to be suitable to describe these motions in p-anisaldehyde. Mixing of anti and syn conformations has been found to be insignificant with in the first 82 calculated torsional eigenstates and therefore less li kely 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. Sorenso n, 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. (C) 1997 Academic Press.