Nuclear magnetic resonance relaxation study of strychnine in solution. Molecular dynamics models for the interpretation of 13C T1 data and relaxation mechanisms of the quaternary carbons.

Abstract

Nuclear magnetic resonance relaxation times (T1) of 13C nuclei in strychnine have been measured in CDCl3 at 2.35T (25MHz) to investigate the molecular dynamics and the structural information included in the relaxation times. The T1 data of the proton-bearing carbons have been treated by a computer-aided method of analysis based on several models of molecular motion. The calculation supported an anisotropic nature of the molecular motion, although it is small. The T1 data of quaternary carbons are discussed after estimating the dipole-dipole parts for these carbons : these parts are dependent on the number of α-protons attached to the adjacent atoms and amount to 50-60% of the total relaxation rates. Contributions of the relaxation mechanisms other than the dipole-dipole one are estimated for the quaternary carbons from the T1 data including those at 11.74T (125MHz) reported elsewhere. Anisotropies in the chemical shift are also determined for the quaternary carbons. The anisotropy is relatively small for the carbonyl carbon attached to a nitrogen atom, indicating an electronegativity effect of the adjacent atom.