Structure and Dynamic Stereochemistry of Trimesitylmethane. II. Empirical Force Field Calculations

Abstract

The static and dynamic stereochemistry of trimesitylmethane (1) has been investigated utilizing the approach of full relaxation empirical force field calculations. 1 adopts a propeller geometry (C3) in the ground state. Geometric parameters are indicative of a molecule accommodating some degree of strain due to the presence of the o-methyl substituents. Specifically, elongation of the central C-C bond to 1.55 Å and enlargement of the central C-C-C angle to 117.7° was found. The dihedral angle subtended by the plane of a given mesityl ring and a plane containing the C3 axis and passing through the central bond to that ring is 40.7°. Support for the calculated geometry of 1 derives from a comparison with X-ray structural data for dimesityl-1-(2,4,6-trimethoxyphenyl) methane and trimesitylborane. Calculated activation energies for the idealized one-, two-, and three-ring flip stereoisomerization mechanisms are 47, 31, and 80 kcal/mol, respectively. Calculations were performed which describe in some detail the pathway corresponding to the lowest energy isomerization (i.e., two-ring flip) mechanism. The calculated activation energy of 20 kcal/mol for this process compares favorably with the value (21.9 kcal/mol) found experimentally. © 1974, American Chemical Society. All rights reserved.