Density Functional Studies on N-Methylacetamide−Water Complexes

Density functional theory (DFT) calculations were performed to study the conformations, hydrogen-bonding effects, and the stabilities of different methyl group orientations of isolated trans - and cis - N -methylacetamide (NMA) and of 12 NMA−water complexes. The DFT functionals B3LYP, BLYP, and the basis set 6-311++G(d,p) were used all through the calculations. The isolated trans -NMA structures obtained from these two DFT methods are consistent with experimental data from gas-phase electron diffraction and comparable to MP2 calculations at a much lower cost. Cooperative effects of hydrogen bonding were found in the trans -NMA·2H 2 O complexes, in which the two water molecules attach to the N H and C O groups of trans -NMA, respectively. No substantial H-bonding cooperative effect was found for cis -NMA−water complexes. The trans -NMA structures with the methyl orientations Φ = Ψ = 0° and Φ = 180° and Ψ = 0° are the most stable conformations of isolated trans -NMA, and the structure with Φ = 180° and Ψ = 0° corresponds to the energy minimum state of trans -NMA in hydrated situations. The methyl group orientation of Φ = Ψ = 180° corresponds to the most stable structure both for gas-phase cis -NMA and the cis -NMA−water complexes.