Absolute infrared absorption intensities in some aromatic systems

Abstract

The TT electron rehybridization accompanying deformation of an aromatic substituent out of the plane of the ring has been invoked in the past to explain differences between the effective CH dipoles of benzene as deduced from intensity studies of in-plane and out-of-plane vibrations. As such rehybridisation is likely to make a considerable contribution to inter-molecular forces between aromatic systems an attempt has been made to verify the rehybridlzation theory. The magnitude of the rehybridization moment should be Independent of the substituents on the aromatic ring. As analysis of the vapour phase band intensities of hexafluorobenzene has established that the difference between the effective CP dipoles as deduced from the in-plane and out-of-plane vibrations is 0.3 D. in the sense compatible with the fluorine atom being at the negative end of the dipole. This rehybridisation moment of 0.3 D. is equal to that deduced for benzene. Refinement of the analysis is achieved by the determination of Coriolis coupling coefficients and absolute intensities from band shape calculations. A computer program has been written for this purpose. The intensities of all Infrared active fundamental bands of hexafluorobenzene have been measured in solution in carbon disulphide, cyclohexene and benzene. The Intensity changes are satisfactorily correlated on the basis of dielectric theories except for the A fundamental of hexafluorobenzene in benzene. Benzene interacts with many week electron acceptors to form weakly bound complexes. The weak complexes formed between benzene and hexafluoro-benzene and benzene and boron tribromide have been investigated. A high pressure system has been developed to study the pressure and temperature dependence of the infrared absorption intensities of the fundamental vibrations of benzene.