Multi-site chemical exchange by nmr

Abstract

The application of a recent multi-site chemical exchange formulation of the Bloch equations is made to an intramolecular and an intermolecular process. The experimental precautions to achieve a good complete line shape fit are discussed in some detail. The rates of internal rotation about the C—N bond in N, N-dimethylcar-bamyl fluoride have been measured, and the relative signs of the long range couplings H—C—N—C—F J proton to fluorine are obtained from the exchange modified spectra as well as from the comparison of the J measured in the fast and slow exchange limits. A 4-site rate matrix is defined with one adjustable parameter for the spectral fit. The measurements are especially difficult because of the small chemical shift varying between 2.2 Hz and 1.4 Hz over the range of interest. The long range coupling methyl to fluorine are 0.8 ± 0.05 Hz trans and 0.3 + 0.05 Hz cis and do not vary with temperature within 0.05 Hz for a 16.5 % solution in carbon tetrachloride. The intermolecular exchange of halogens in the dimethyltin dihalide mixtures is formulated in detail. A 15 site exchange process breaks down into 5x3 exclusive exchange groups and providing line positions and intensities are known with the line width in the absence of exchange, the whole spectrum can be reproduced using only 2-site rate matrix elements. Representative experimental and computed spectra are presented. The relationship of the matrix elements to chemical rate constants is derived but determinations of these constants are not yet complete. © 1972, Walter de Gruyter. All rights reserved.