Use of carbon-13 and hydrogen-1 nuclear magnetic resonance to probe internal motion in dimethyl sulfoxide

Abstract

/sup 13/C NMR and /sup 1/H NMR have been used to study the internal rotation of the methyl groups of Me/sub 2/SO in Me/sub 2/SO--H/sub 2/O mixtures. The sensitivity of various NMR methods for studying internal rotation is discussed and it is found that measurements which are proportional to the internal rotation parameter chi/sup CH/ are optimal if the rate of internal rotation is 1 to 10 times that of overall molecular rotation. Alternatively, the ratio T/sub 1//sup /sup 1/H//T/sub 1//sup /sup 13/C/, which is proportional to the ratio of the parameters chi/sup CH//chi/sup HH/, is most sensitive to the effect of internal motion for rates in the range 10 to 100 times the rate of overall molecular motion. The latter method is found to be fairly sensitive for the case of internal methyl rotation with the ratio changing by more than a factor of 2 in going from the no rotation to the very fast rotation case. In using this approach, care must be taken in evaluating only the intramolecular contribution to T/sub 1//sup /sup 1/H/. This has been accomplished in the present case by the use of ternary D/sub 2/O--Me/sub 2/SO-d/sub 6/--Me/sub 2/SO mixtures. The results of thesemore » studies indicate that the rate of internal methyl rotation is slow compared with the rate of overall molecular rotation.« less