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Carbon-13 NMR relaxation studies of some dicyclohexyl compounds at high pressure

✍ Scribed by Duncan G. Gillies; Stephen J. Matthews; Leslie H. Sutcliffe

Publisher
John Wiley and Sons
Year
1991
Tongue
English
Weight
622 KB
Volume
29
Category
Article
ISSN
0749-1581

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✦ Synopsis

Abstract

Four dicyclohexyl compounds were investigated, one of which is a traction fluid and the remainder have related structures in which the cyclohexyl rings are separated by two or three carbon atoms. Carbon‐13 spin‐lattice relaxation times have been measured at 25.16 MHz at pressures up to 200 MPa over the temperature range 280‐329 K. High‐pressure relaxation data have been successfully interpreted in terms of the ‘model‐free’ approximation. A pressure‐dependent rotational coupling parameter, κ, was evaluated from a modified Stokes‐Einstein‐Debye relationship using experimentally determined pressure‐dependent rotational correlation times. The smallest value of κ was found to be associated with the most rigid and least‐extended molecule, showing that molecular shape has a significant effect on the rotational‐translational coupling. The values obtained for the activation volumes derived from methyl group data suggest that barriers to internal motion are governed by intramolecular effects. It has been demonstrated that high‐pressure ^13^C relaxation measurements can provide a powerful means of studying the relationship between molecular rigidity and the macroscopic behaviour of functional fluids.

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