ISSN:
0749-1581
Keywords:
Molecular motion
;
Carbon-13 relaxation
;
Octyl chains
;
Polydecene
;
Model lubricants
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 13C nuclear relaxation behaviour of n-alkyl chains in a variety of compounds can be described in terms of two relaxation times, one for tumbling (τc) and one for each carbon nucleus for the internal motions (τc). Relaxation data for the quaternary carbon nuclei of n-octylbenzene and n-octyl cyanide have been used to determine τc) and their activation energies [Ea(τc)]. These values have enabled τe and their associated activation energies [Ea(τe)] to be calculated for each carbon in the alkyl chains of the two compounds. In these compounds the values for C-6 appear to be invariant with τe(298) = 15 ± 2 ps and Ea(τc) = 15 ± 1 kJ mol-1. Assuming that these are universal values for all n-octyl chains, we have used them to calculate τe and Ea(τe) for other simple n-octyl compounds, thus allowing τc and E.a(τc) to be determined. It was found that Ea(τc) values correlate well with those for Ea(n/T) obtained from viscosity measurements. Di-n-octyl ether shows motional properties similar to those of the straightchain compounds.Simultaneous fitting of relaxation rate and NOE enhancement data measured over a range of radiofrequencies and temperatures permits a wealth of motional properties to be discerned when there is a suitable frequency dispersion. Both tri-n-octylamine and polydecene tumble sufficiently slowly to allow both relaxation rate and NOE data to be used to evaluate τc and τe and their activation energies without making any assumptions about C-6. The values of τc(298) and Ea(τc) obtained from C-6 for tri-n-octylamine are 314 ± 12 ps and 18.5 ± 0.4 kJ mol-1, respectively, and the corresponding values for polydecene are 576 ± 41 ps and 16.1 ± 1.3 kJ mol-1, respectively.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260290314
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