Non-linear least squares program for the analysis of 13C spin–lattice relaxation data based on a symmetric top model. Description of the method and application

Abstract

A non-linear least squares program has been developed to analyse data for the dipole–dipole relaxation time of carbon (T₁^DD) based on the symmetric top model of molecular rotational diffusion. The program employs four parameters in the simulation, i.e. two rotational diffusion constants and two angles related to the axis of molecular motion. The internal rotation of methyl groups is also incorporated. The application of the program to methylbenzofurans indicated the reliability and wide applicability of the method. The factors affecting the methyl rotation barrier are also examined with the aid of MNDO MO calculations, and a novel inter-relation is found with the bond length of C–CH₃.