Carbon-13 spin-lattice relaxation study of the molecular dynamics of 10-methylnonadecane

Abstract

Carbon-13 spin-lattice relaxation times (T1) have been measured for the 11 magnetically distinct carbons of 10-methylnonadecane over the temperature range ca. 10-90°C. Effective correlation times, τeff, have been determined from the T1 data and used to discuss the motional features of the C19 alkane chain. The τeff values for the methylene carbons of 10-methylnonadecane follow a temperature dependence defined by "apparent" activation energies that range from 5.1 to 5.5 kcal/mol. The methyl carbons' τeff values have a temperature dependence associated with lower activation parameters, and the reorientation rates of these carbons have been shown to be related to the potential barriers to methyl rotation. Collectively, the temperature dependence of the τeff data and the comparative values of τeff along the chain are consistent with a model of molecular motion in which overall and various internal reorientational modes contribute to the carbon relaxation in a 19-carbon alkane.