13C NMR relaxation study of intramolecular local dynamics of n-alkane

The intramolecular local dynamics in n-alkane of moderate chain length is investigated by C-13 NMR relaxation study. For this purpose a simplified jump model of segmental motions for these molecules is proposed that takes into account the cooperativities between internal rotations about several different C - C bonds. In this model it is assumed that the conformational interconversions occurring in the inner part of the chain involve only the cooperative coupled transitions of C-C bonds while those occurring in the terminal part including ethyl and propyl groups involve both the coupled and single transitions. information of the rate constants for these cooperative conformational transitions can be extracted by comparing the calculated T-1's with those observed over the temperature range of 228 - 308K. The calculations can be performed according to the method proposed by Wittebort and Szabo and n-undecane is taken as an example to demonstrate detailed procedure of such calculations. The activation energies, ranging from 14 to 20 kJ/mol, can be obtained from the Arrhenius plot of thus calculated rate constants. (C) 1999 Elsevier Science B.V. All rights reserved.