The out-of-plane vibrations of the fluorinated benzenes

Abstract

From the fundamental frequencies of the out-of-plane vibrations of Benzene and nineteen Fluorine substituted Benzenes, a number of force fields have been calculated using the perturbation method. Of these fields, only a 23 parameter force field fitted to the frequencies of allthe molecules, which reproduces the observed frequencies to within 19cm with an average error of 5.85cm, is found to be acceptable. This field is in agreement with Scherer's similar field for the Chlorine substituted Benzene and those force constants which overlap with Benzene are in agreement with the values calculated by Whiffen. The fact, that the diagonal force constants for the C-H and C-F out-of-plane angle bends decrease with increasing Fluorine substitution at the ringposition adjacent to the moving substituent, is discussed. Reassignments of the fundamentals were made for most of the molecules and, for some of them, the gas phase far infrared spectra were recorded for the first time. The a fundamental of Hexafluoro Benzeneis discussed in some detail with particular reference to the use of time correlation functions in making vibrational assignments. Also, the absolute infrared intensities of the out-of-plane vibrations of 1,4 difluoro Benzene and 1,3,5 trifluoro Benzene have been measured. The results were interpreted assuming that the rehybridization moment of +0.3 D.rad which Steele and Wheatley suggested is acting during both C-H and C-F out-of-plane angle bends of Benzene and Hexafluoro Benzene respectively, acts also in these molecules. Therefore, bond moments of +0.29 D (C-H) and -0.92 D (C-F) in 1,4 difluoro Benzene and +0.42 D (C-H) and -1.05 D (C-F) in 1,3,5 trifluoro Benzene, which are in reasonable agreement with currently accepted values, were calculated. In analysing the results, the electro-optical theory of Gribov was used.