MOLECULAR POLARISABILITY AND CONFORMATIONAL ANALYSIS

Abstract

The work in this thesis, entitled "Molecular Polarisability and Conformational Analysis", involves firstly the setting up of the Kerr effect apparatus for the measurement of electric birefringence - a property related to polarisability. Certain parts of the requisite equipment had to be orderecd from overseas while others such as the photomultiplier system, the Kerr cell, the power supply for the compensator unit, etc, were built or assembled in our own university laboratories. Following its satisfactory testing and calibration, the apparatus was used for several problems in conformational analysis, the first group of molecules studied comprised four chloralides. Dipole moment and Kerr effect studies revealed the decree of non-planarity of the ring atoms of the compound which were also shown to exist as trans geometrical isomers with respect to the bulky subsituents at two of the ring carbon atoms. Optical rotation and spectroscopic investigations on these compounds were also carried out. The high carbonyl frequencies observed for those chloralides indicated the ring strain in these molecules. The findings on chloralide itself have been published (Journal of the Chemical Society, (B), 1968, 1369-1373 ), It is anticipated that the work on the chloralides of lactic, malic ancl citric acids would constitute the basis of another paper. The second group of molecules investigated involved several substituted ethanes. Conformational questions related to rotational isomerism centred on parameters like dihedral angles, isomeric ratios, energy differences and barriers between the different rotamers, The conformational analysis data available from our techniques here were supplemented by extensive infrared and proton N.M.R. studies during a 6 month study-leave at the University of East Anglia and Raman measurements in Oxford. The work on the first member (2,3-dicyano-2,3-dimethylbutane) han been accepted for publication by the Journal of the Chemical Society. This compound exists as a mixture of trans (82%) and gauche (dihedral angle of 85°) conformers in carbon tetrachloride with an energy difference of 1.31 kcal/mole. Other ethane-like molecules include 2, 3-dimethyl-2, 3-diphenylbutane, 2, 3-dimethyl-2, 3-di-p-halogenophenylbutanes (halogen = bromine, chlorine and fluorine), 1,2-phenyl tetrachloroethane, the symmetrically substituted tetrabromo and tetrachloro ethanes and meso-3,4-dicyano-3 ,4.-diphenylhexane. The spectroscopic and Kerr investigations together with recent dipole moment data shows that most of these substituted ethanes exist in the trans form in the solid state and as a mixture of trans and gauche conformers in solution, usually with the latter form predominating. The energy differences erences between the conformers are approximately zero. Some aspects of this work have been submitted for publication in the J.C.S. (sent March, 1969). The last group of compounds studied are three ?- and ?-lactones. It is shown that the bond polarisabilities used in the chlornlides are very satisfactorily applicable to (?-propiolactone and indicate non-planar ring structures for ?-butyrolactone and ?-valerolactone.