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 lit are discussed in some detail. The rates of internal rotation about the C—N bond in N,N-dimethylcarbamyl fluoride have been measured, and the relative signs of the long range couplings H—C—N—C—F 4J proton to fluorine are obtained from the exchange modified spectra as well as from the comparison of the 4J 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 tetrachioride. The intermolecular exchange of halogens in the dimethyltin dihalide mixtures is formulated in detail. A 15 site exchange process breaks down into 5 x 3 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. INTRODUCTION The study of chemical exchange rates through the modified transverse magnetization is well understood and the theory is highly developed (for example references 1—8). Some simple aspects of chemical exchange theory in nmr spectra for first order cases have been emphasized in a recent paper by Reeves and Shaw9. For intramolecular exchange in the first order limit, the determination of relative signs of coupling constants to non-exchanging nuclei influences the line shapes in a readily recognizable manner'°' Exchanges among many sites (Larmor frequencies) of arbitrary population, saturation effects and site-dependent relaxation times have been accommodated in this recent theory9. A single matrix diagonalization enables sub.tL.W.R. thanks the NRCC for financial support in the form of operating and capital equipment grants.