C-13 NMR RELAXATION STUDY OF THE OVERALL AND INTERNAL MOTIONS IN COMPOUNDS CONTAINING NORMAL-OCTYL CHAINS

The C-13 nuclear relaxation behaviour of n-alkyl chains in a variety of compounds can be described in terms of two relaxation times, one for tumbling (tau-c) and one for each carbon nucleus for the internal motions (tau-e). Relaxation data for the quaternary carbon nuclei of n-octylbenzene and n-octyl cyanide have been used to determine tau-c and their activation energies [E(a)(tau-c)]. These values have enabled tau-e and their associated activation energies [E(a)(tau-e)] 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 tau-e(298) = 15 +/- 2 ps and E(a)(tau-e) = 15 +/- 1 kJ mol-1. Assuming that these are universal values for all n-octyl chains, we have used them to calculate tau-e and E(a)(tau-e) for other simple n-octyl compounds, thus allowing tau-c and E(a)(tau-c) to be determined. It was found that E(a)(tau-c) values correlate well with those for E(a)(eta/T) obtained from viscosity measurements. Di-n-octyl ether shows motional properties similar to those of the straight-chain 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 tau-c and tau-e and their activation energies without making any assumptions about C-6. The values of tau-c(298) and E(a)(tau-c) 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.