DENSITY-FUNCTIONAL STUDIES ON N-METHYLACETAMIDE WATER COMPLEXES

Density functional theory (DFT) calculations were performed to study t he conformations, hydrogen-bonding effects, and the stabilities of dif ferent methyl group orientations of isolated trans- and cis-N-methylac etamide (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 cal culations. The isolated trans-NMA structures obtained from these two D FT methods are consistent with experimental data from gas-phase electr on 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 c ooperative effect was found for cis-NMA-water complexes. The trans-NMA structures with the methyl orientations Phi = Psi = 0 degrees and Phi = 180 degrees and Psi = 0 degrees are the most stable conformations o f isolated trans-NMA, and the structure with Phi = 180 degrees and Psi = 0 degrees corresponds to the energy minimum state of trans-NMA in h ydrated situations. The methyl group orientation of Phi = Psi = 180 de grees corresponds to the most stable structure both for gas-phase cis- NMA and the cis-NMA-water complexes.