Ab initio molecular orbital calculations of solvent clusters of trans-N-methylacetamide: structure, ring cluster formation and out-of-plane deformation

Abstract

The solvation of trans amides has been investigated by the use of full gradient optimization ab initio quantum mechanical calculation techniques. The complexes have been determined at the Hartree–Fock (HF) level with a 4-31G*/4-31G** basis set and at the second-order Møller–Plesset perturbation (MP2) level. Three NMA–water clusters were investigated: trans-NMA with two molecules of water forming a ring cluster at the amide oxygen; trans-NMA with two molecules of water at the amide oxygen forming hydrogen bonds along the direction of the lone-pair electrons; trans-NMA with one molecule of water at the CO group and one at the NH group. In addition, 4-31G* basis set calculations for trans-NMA with two molecules of acetonitrile were performed. The CO⋯H(W) hydrogen bond lengths, electron-density population analysis and molecular-orbital analysis of trans-NMA with two molecules of water at the amide oxygen demonstrate the importance of concurrent water–water and water–(carbonyl) oxygen hydrogen-bond interactions. The complex of trans-NMA with two molecules of water forming a ring cluster at the amide oxygen indicates the formation of a non-planar amide bond and the generation of a chiral centre at the amide nitrogen; this structure has a 5 % Boltzmann distribution at room temperature at the MP2 level. Vibrational-frequency analysis shows that its hydrogen-bonded water molecules are vibrationally coupled. Orbital analysis suggests that there is a considerable solute-occupied space reorganization caused by the rearrangement of the water solvent molecules. Comparisons are made with previous theoretical studies of amide–water interactions and experimental spectroscopic, X-ray and neutron-diffraction data on the hydration of amides, peptides and proteins.