C-13 NMR-STUDY OF THE OVERALL AND INTERNAL MOTIONS OF A SEMIRIGID HYDROCARBON

An extensive investigation has been made of the 13C NMR relaxation parameters of 2,4‐dicyclohexyl‐2‐methylpentane neat and in solution. Spin‐lattice relaxation times and nuclear Overhauser enhancements were measured as a function of temperature, concentration and radiofrequency. The results for all the carbons, except the methyls, can be interpreted by using a reduced spectral density which suggests that there is an isotropically rotating semi‐rigid backbone structure whose internal librations and vibrations reduce the amplitude of the spectral density. However, interpretation of the methyl group data required the addition of a discrete correlation time characteristic of the internal motion about a C3 axis. From dilution studies, the validity of using the two‐correlation time spectral density with S2 = 0 when the rate of overall motion approaches that of the internal motion was established, for the first time. Copyright © 1990 John Wiley & Sons, Ltd.