C(sp(2))-C(sp(3)) rotational barriers in simple amides: H2N-C(=O)-R(R = methyl, ethyl, i-propyl, tert-butyl)

Potential energy surfaces for rotation about the C(sp(2))-C(sp(3)) bond are reported for acetamide, propanamide, 2-methylpropanamide, and 2,2-dimethyl propanamide at different levels of ab initio electronic structure theory wi th correlation effects included. In all cases, fully optimized geometries o f rotational minima are consistent with gas phase electron diffraction data and crystal structure data. The experimental barrier height for methyl rot ation in acetamide is reproduced to within 0.1 kcal/mol. This study yields a set of improved criteria for the construction of rotational potentials fo r the C-a-C bond which are used to obtain improved MM3 torsional parameters . In addition, we find that the use of higher levels of theory leads to sig nificantly different results than those obtained in prior Hartree-Fock stud ies on acetamide and 2-methylpropanamide.