Ab Initio Studies on N-Methylacetamide

Abstract

The potential energy surface of N-methylacetamide (NMA) is investigated ab initio at the Hartree–Fock/6-31G* level with full geometry optimisation with or without MP2 correlation and thermal corrections in order to characterise its stationary points and the Intrinsic Reaction Coordinate (IRC) of trans-cis isomerisation. The cis and trans minima and the two non-equivalent Transition States (TS) of the isomerisation with opposite pyramidalisation of the amidic group are determined. The TS result considerably destablised (∼16 and 21 kcal mol-1) with respect to the most stable equilibrium structure. The lowest energy transition state is the up form (both substituents at N bent toward the carbonyl oxygen), while the down form (with the N lone pair pointing toward the carbonyl oxygen) is higher in energy. A saddle point of index 2 connects the two transition states, ∼8 kcal mol-1 above the TS up, the absence of intermediates being verified. Adding the correlation corrections does not change the energy gaps appreciably. The IRC at HF/6-31G* level has been followed in the most favourable case: correlate deformation of peptide unit and decoupling of methyl groups with respect to the reaction coordinate are shown. Additional TS, depending on methyl groups orientation, confirm the substantial free rotation around the phiv; and ψ torsional angles.