NMR RELAXATION IN LONG-CHAIN QUATERNARY AMMONIUM-IONS - THE ROLE OF TORSIONAL FLEXIBILITY

The conformational dynamics of alkyl chains is usually analysed within the framework of the rotational isomeric state (RIS) approximation, where only the conformers corresponding to the configurational potential minima are considered, and torsional fluctuations about the minima are ignored. In order to test the validity of this procedure, C-13 NMR T1 relaxation times have been measured in the series of symmetric tetra-n-alkylammonium ions (C(N)H2N+1)4N+, with N = 8, 10, 12, 16 and 18, and compared with theoretical results predicted by the RIS model. Spin relaxation rates of the terminal carbon atoms show a tendency to level off for increasing chain length, in contrast with the monotonic increase predicted by the RIS model. The experimental behaviour reveals that deviations from RIS results appear for chains with more than ten carbon atoms.