Molecular motion and methyl group rotation barriers from ¹³C nmr relaxation times. Norbornane and adamantane derivatives in solution

Abstract

H E L M U~ BEIERBECK. ROBERTMARTINO. and JOHN K . SAUNDERS. Can. J. Chem. 58, 102 (1980). The carbon-I3 relaxation time data for several norbornane and adamantane derivatives are presented. It is shown that the most probable principal axis of the rotational diffusion tensor is an axis which contains the centre of mass of the molecule and the heterosubstituent. The infl~lence of the C-C-H bond angle on the calculation of methyl group rotation barriers from relaxation time data and on the barrier is discussed. The analysis of I3C nmr spin lattice relaxation times ( T I ) provides insight into the dynamic behaviour of organic molecules in solution. including the overall diffusion constants for the molecules (1) and diffusion constants for rotation about single bonds (2-4). The simplest case of overall motion is isotropic motion. In such a case the diffusion constant can be obtained directly from TI data using eq.[I], since most models of molecular reorientation assume that the rotational diffusion constant R i about the ith axis is related to the correlation time about this axis (.rCi) by the expression (1/6Ri) = z ,~ ( 5 ) .The internal reorientation rate of a CH, group rotation can be calculated using eq.[2] (6). whereIn an earlier publication (8) we have shown that the values of R,,, for CH, gmups in steroids depend on the number of 1.3 diaxial CH,-hydrogen interactions to which the CH, group is subjected. In order to further explore the factors which influence R,,,,, we have studied several methylated bicyclo-[2,2,l]heptanes and adamantanes. the results of which are presented in Table 1. The TI values represent the dipole-dipole contributions since the nOe was 2.0 2 0.1 for all carbons in all molecules.

Molecular motion and methyl group rotation barriers from 13C nmr relaxation times. Podacarpic acid derivatives

“…The V , value for C-18 of compound 4, with a methyl group at C-4 is 15.6 kJ/mol, which can be compared to the value of 14.4 kJ/mol found in 4,4-dimethyl adamantan-Zone (6). Both com- pounds have only one 1,3 diaxial CH, grouphydrogen interaction.…”

“…They found that the use of Woessner's equation did not give satisfactory values for relatively concentrated solutions of borneol in CDC1,. Recently we have reported (6) the results for a number of norbornane and adamantane derivatives including borneol where satisfactory agreement was obtained between experimental values and those calculated using Woessner's equation for a symmetric top (1). The best correlation was obtained by assuming that the principal axis of the diffusion tensor passes through the heterosubstituent and the centre of mass of the molecule.…”

“…The values for the two types of experiments were within experimental error. Errors limits are estimated to be + 10% in T, values which give rise to estimated errors of + 15% in R and + 0.3 kJ/mol in V R (6).…”

“…Carbon-13 relaxation time data were obtained using the inversion recovery methods on a Bruker HX-90 as previously described (6). Sweep widths of 4000 Hz were generally employed.…”

Nuclear magnetic resonance studies of microemulsions and related systems: 2-butoxyethanol in water

“…No corrections for bulk magnetic susceptibility were made. The T, measurements were obtained by use of the standard inversion recovery technique as described previously (24). Results were treated using both an exponential three-parameter fit and a logarithmic plot.…”