Abstract
The ^13^C nuclear relaxation behaviour of n‐alkyl chains in a variety of compounds can be described in terms of two relaxation times, one for tumbling (τ~c~) and one for each carbon nucleus for the internal motions (τ~c~). Relaxation data for the quaternary carbon nuclei of n‐octylbenzene and n‐octyl cyanide have been used to determine τ~c~) and their activation energies [E~a~(τ~c~)]. These values have enabled τ~e~ and their associated activation energies [E~a~(τ~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 τ~e~(298) = 15 ± 2 ps and E~a~(τ~c~) = 15 ± 1 kJ mol^−1^. Assuming that these are universal values for all n‐octyl chains, we have used them to calculate τ~e~ and E~a~(τ~e~) for other simple n‐octyl compounds, thus allowing τ~c~ and E.~a~(τ~c~) to be determined. It was found that E~a~(τ~c~) values correlate well with those for E~a~(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 τ~c~ and τ~e~ and their activation energies without making any assumptions about C‐6. The values of τ~c~(298) and E~a~(τ~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.