Computational Measurement of Steric Effects:  the Size of Organic Substituents Computed by Ligand Repulsive Energies

Ligand repulsive energies, E R, have been demonstrated to provide reliable steric parameters for ligands in organometallic systems. E R values have now been computed for 167 different organic substituents. Three different fragments were employed for the calculation of the ligand repulsive energies: CH 3, CH 2 COOH, and Cr(CO) 5 . All compounds were modeled using molecular mechanics. Two different force fields were employed: Allinger's MMP2 and Rappé's Universal Force Field (UFF). Both molecular dynamics and stochastic mechanics were used to determine the lowest energy conformer for each species. Steric sizes were compared against standard steric measures in organic chemistry: Taft−Dubois steric parameter, E ‘ S, A -values, cone angles, θ, and solid angles, Ω S . Good correlations between E R and the model-based steric measures (θ and Ω S ) were found. Experimental-based measures, E ‘ S and A -values, showed a mix of steric and electronic effects. On the basis of these correlations, the use of CH 3, CH 2 COOH, and Cr(CO) 5 fragments for steric size quantification was critically examined.