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Chemical applications of pulsed and steady-state nuclear magnetic resonance Shaw, Keith Newman

Abstract

A general matrix formulation for the effects of chemical exchange processes in high resolution nuclear magnetic resonance (NMR) has been developed which allows a concise description and efficient numerical calculation of exchange modified lineshapes for an arbitrary number of spin-sites. A complete description of all rate processes is contained in a single matrix, and both first- and second-order spin systems (as described in terms of a spin density matrix) are accommodated through specific forms for a spin-site frequency matrix. The hindered rotation about the N-C bond in substituted amides has been studied on a comparative basis using free energies of activation derived from first-order rate constants obtained by complete analyses of digital lineshape data. A versatile FORTRAN computer program has been.used for routine iterative lineshape fitting and it has been shown that, even with the precision now attainable using this technique, the most reliable kinetic parameter is the free energy of activation. Huckel H-MO and semi-empirical SCF-LCAO-MO calculations have been used in a description of the electronic factors determining the barriers to hindered rotation and the charge distributions in the amides studied experimentally. Application of the Fourier transform in high resolution NMR has also been considered in detail, with particular emphasis upon quantitative lineshape studies using the data available from simple pulsed NMR experiments. The advantages of the pulse method with digital data acquisition have been examined, and the numerical computations and corrective factors involved have been incorporated into a general and efficient computer program.

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