Computation and analysis of NMR spin-spin interactions

The investigations described in this thesis are concerned with the development of an improved and generalized description of torsion angles and to the electronegativity of the substituents attached to the HA-C-C-C-HB fragment. The necessary theoretical background: the Newton-Raphson optimization technique and the quantum mechanical description of nuclear spin-spin coupling by the Fermi contact mechanism is presented. The results of theoretical calculations on simple substitued ethanes are analyzed, in order to deduce the essential trends in the torsion-angle dependency of vicinal coupling constants. The extended Karplus equation, to be used in analyses of experimental and theoretical coupling constant data is presented and mathematical aspects which are assiciated with the simultaneous optimization of both Fourier coefficients and electronegativity values in such an equation are addressed. The formalism is applied to coupling constant data calculated for substituted ethanes with the aid of a reparametrized version of the Extended Hueckel method. Alternatively a sum-over-states procedure within the Complete Neglect of Differential Overlap approximation is used. A set of experimental average coupling constants in mono-and 1,1-disubstituted ethanes are parametrized in order to obtain information concerning the torsion-angle-independent Fourier coefficients, as well as the relative electronegativities of a large number of common chemical groups. An attempt is made to fully optimize the extended Karplus equation using a data set of experimental coupling constants versus torsion angles calculated with the aid of Molecular Mechanics. Finally, the applicability of simple localized orbital pictures to the NMR coupling phenomenon is investigated as a preliminary study to possible ab initio calculations. (author). 224 refs.; 8 figs.; 36 tabs