Empirical Force-Field Calculations on a Model System for Trimesityl Derivatives of Group IIIa, IVa, and Va Elements. Investigation of Stereoisomerization Pathways

Stereoisomerization in the class of compounds (Mes)₃Z (Mes = mesityl; Z = B, CH, N, etc.) has been investigated using the technique of empirical force-field calculations. The study was made feasible by employing a simplified model based on trimesitylmethane in which the only distinction among molecules in this class was the difference in the preferred length of the Z-Mes bond. Calculated energies for the idealized transition states point to the two-ring flip mechanism as the pathway of lowest energy over a wide range of structures. Calculated energies for this mechanism follow a trend which parallels experimental values. Activation energies are predicted for (Mes)₃N, (Mes)₃SnH, and (Mes)₃Bi of 25-27, 5-7, and 3-5 kcal/mol, respectively. Results for the higher energy mechanisms are discussed in terms of structural changes accompanying the change in bond length to the central atom. The three-ring flip is found to be consistently higher in energy than the two-ring flip in the region of interest.