C(sp(2))-C(aryl) bond rotation barrier in N-methylbenzamide

The potential energy surfaces at different levels of ab initio electronic s tructure theory with correlation effects included are reported for rotation about the C(sp(2))-C(aryl) bond in N-methylbenzamide. The results reveal a minimum at a C=C-C=O dihedral angle of +/-28 degrees with barrier heights (MP2/aug-cc-pVTZ//BLYP/DZVP2/A2) of 0.48 kcal/mol at 0 degrees and 2.80 kca l/mol at 90 degrees. Fully optimized geometries are in good agreement with crystal structure data, and potential energy surfaces are consistent with t he experimental dihedral angle distribution. The results are used to assign MM3 force field parameters to allow calculation on N-methylbenzamide and o ther benzamide derivatives.