Librational effects on carbon-13 N.M.R. spin-lattice relaxation times of tricyclic condensed molecules

Abstract

A general method is described for calculating the effect of internal librational motion on the correlation times in tricyclic molecules having a folded structure about the central hetero-ring. The relaxation equations have been derived for isotropic as well as anisotropic models of overall molecular re-orientation. In the case of the isotropic model of motion, the parameter which determines the effective relaxation times is the librational amplitude; for the anisotropic model a somewhat more complicated correlation function is found. The present treatment provides a tool for interpreting in a quantitative way the spin-lattice relaxation times of hydrogen bearing carbons in pharmacologically important tricyclic systems where internal conformational flexibility could be exclusively monitored by using such a non conventional N.M.R. spectroscopic approach. Application of the method and its limitations are discussed briefly.