CONFORMATIONAL AND SUBSTITUENT DEPENDENCE OF NMR COUPLING CONSTANTS.

Abstract

Indirect nuclear spin-spin coupling constants in high resolution nuclear magnetic resonance (NMR) spectroscopy provide the most powerful method for determining conformations and structures of molecules in solution. The research described in this dissertation establishes the conformational and substituent dependencies of several new types of nuclear spin-spin coupling for use in structural studies. It was also of interest to determine the electronic factors which lead to the observed experimental trends. Geminal, vicinal, and four-bond carbon-carbon nuclear magnetic resonance coupling constants were studied experimentally with fifty carbon-13 enriched alicyclic compounds. Because of the small magnitudes of many of these coupling constants it ~vas necessary to modify existing Fourier transform NMR equipment to perform J-resolved two-dimensional NMR experiments. With the greater spectral resolution it was possible to determine 56 intercarbon geminal (²J(cc)), 107 vicinal (³J(cc)) and 26 four-bond (⁴J(cc)) coupling constants. These data represent the largest collection of ²J(cc), the only known data for ⁴J(cc), and the most accurate compilation of vicinal ¹³C-¹³C coupling constants. In combination with molecular orbital methods, the relevant conformational and substituent trends in each of these series were elucidated. Because of the great amount of interest in the importance of bridgehead interactions in "strained" molecules, a study is presented of coupling transmitted via nonbonded interactions between the bridgehead carbons in a series of l-substituted-bicycloalkanes. The two-dimensional Fourier transform methods were again used to obtain the experimental coupling constants and molecular orbital methods were used to determine the importance of the intercarbon bridgehead interactions on ¹H-¹H, ¹³c-¹H, ¹³C-¹³C, ¹³c-¹⁹F , and ¹H-¹⁹F coupling constants. In all cases the nonbonded interactions increased dramatically as the separation between the bridgehead carbons decreased. Further understanding of mechanisms of "through space" coupling were based on NMP. studies of the long range coupling constants (⁵J(HF) and ⁴J(HF)) in N-methyl-8-fluoroquinolinium halides. The X-ray structure of N-methyl-8-fluoroquinolinium chloride was determined in an attempt to calculate the observed ⁵J(HF) and ⁴J(HF). A selective population inversion mIR method was used to show that the sign of ⁵J(HF) in N-ethyl-8- fluoroquinolinium iodide is positive.