Force fields and vibrational absorption intensities in aromatic molecules

Abstract

A general quadratic force field, using the overlay technique developed by Schachtschneider and Snyder, was determined for the in-plane vibrations of Benzene and fifteen Fluorine substituted Benzenes by the cyclic refinement of the initial guesses for the forceconstants, known as the perturbation method. It was necessary to assume a Kekule type C-C/C-C interaction relation (Ri Ri+1) = -(Ri Ri+2) = (Ri Ri+3), as first introduced by Schererand Overend. Removal of this constraint gave rise to linear relationships between the force constants, and satisfactory convergence onto one force field was not obtained. The appropriate force constantsof the derived field are compared with those obtained by Duinker for the in-plane vibrations of Benzene. Although the model used is similar to his, the transferability condition is a severe extra constraint, and the extent of agreement is better than anticipated. Reassignments of the fundamentals were made for most of the molecules, and spectra were re-examined, where necessary, to obtain Raman polarisation data, and vapour phase infra red band contours.Biphenyl is known to be planar in the crystal and twisted in solution. The force field for Biphenyl and two deuterated analogues was refined to minimise Assuming that the force field does not change with geometry, the vibrational frequencies were calculated for angles of twist and an estimate obtainedfor the dihedral angle of the molecule in solution. Similar calculations were tried for difluoro Biphenyl, but asthere is insufficient frequency data available, the results were inconclusiveThe combination hands arising from the out-of-plane deformations of the C-H bonds of eight Fluorine substituted Benzenes were investigated, to measure the absolute infra red intensities, and, by a least squares analysis, to derive values for bond moments and bond moment derivatives. Regrettably, studies were limited to 1,4 difluoro Benzene, and, because of experimental difficulties, the intensities measured were so inaccurate that it was impossible to obtain reliable values for the parameters. A valuable new technique is currently being developed by other workers. In a solution spectrum, bands arising from a deformation are observed to shift in frequency, and to broaden when is added to the solution. The effects of this phenomenon were investigated for all eight molecules.