Carbon‐13 NMR relaxation studies of some dicyclohexyl compounds

Abstract

AbstractFour dicyclohexyl compounds were studied, one of which is a traction fluid and the remainder have related structures in which the cyclohexane rings are separated by two or three carbon atoms. Carbon‐13 NMR T1 and NOE data were obtained for the four compounds at 22.50 and 100.62 MHz over a wide temperature range. The data were fitted simultaneously to either a reduced Lorentzian spectral density or to a ‘model‐free’ two‐correlation time spectral density. For the first time it has been possible to observe the tracking from the full ‘model‐free’ two‐correlation time spectral density to the reduced Lorentzian spectral density. From the correlation times and activation energies derived from these fittings, it can be concluded that all the compounds have semi‐rigid structures. Computer modelling of the structures and molecular mechanics calculations of the rotational barriers gave results in agreement with those from the NMR data. Similarly, it was found that the friction coefficients of the compounds correlate with the NMR and molecular mechanics conclusions, namely that the increased rigidity produced by the introduction of methyl groups into the linkage between the cyclohexane rings leads to a more effective traction fluids. Hence it seems to be feasible to design new traction fluids in the laboratory.