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 reorientation. 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.

Dynamic aspects of the stereochemistry of phenothiazines in solution. Part 2. Segmental motion and conformational analysis of the side-chain in promazines

“…Obviously many factors other than libration motion contribute to the relaxation of the aromatic carbons, for instance the number of hydrogens in the aposition, or the particular conformation of the side-chain which brings the methylene protons closer to the peri-position (C-1), or even the structure and size of the aggregates present in solution. Therefore we argue that TI values of promazines are not the best experimental parameters for verifying the proposed 12 model of librational motion in tricyclic systems.…”

Dynamics of non-rigid molecules. - I. — 13C T 1 NMR study of the reorientational and ring-puckering motions of cyclopentene in the liquid state

“…The dipolar relaxation time T' has for example been related by London [12,13] to characteristic correlation times of overall isotropic reorientation and of exchange between two equivalent conformations. A similar treatment has been given by Baldo et al [14] for '8-butterfly-like molecule having the CH vector in one of the librating planes. The latter work allows the more general case of an asymmetric top molecule to be treated.…”

“…Results in agreement with those given by Baldo et al (Eq. (23) of reference [14]) are also obtained in the limit where the CH vector lies in the librating plane. Table II. -Values of'the coefficients (oi and am(i) of equation 9ajter Table I of reference [24].…”

“…At this stage it is necessary to introduce an explicit model for the conformational change. As in most previous work [12][13][14][15][16], we suppose that the relaxation vector CH performs instantaneous jumps between two equivalent positions. If the residence time at each site is called te, the following expression is obtained P(rJ.i PJ is the a priori probability for the CH vector in site i to have polar angles ai = 0 and 3i = 0 in the xyz frame.…”

Motional behavior of "asperlin" in solution. A 13C spin-lattice relaxation study

“…Some theoretical expressions (14)(15)(16)(17)(18) , According to these models, the rates of jumping or, equivalently, the lifetimes of the various conformations determine the relaxation rates. However, the fact that low amplitude diffusion is ineffective (15,16) in causing relaxation suggests that a jump model involving relatively few conformations should be generally suitable for describing relaxation.…”