AB-INITIO MOLECULAR-ORBITAL CALCULATIONS OF SOLVENT CLUSTERS OF TRANS-N-METHYLACETAMIDE - STRUCTURE, RING CLUSTER FORMATION AND OUT-OF-PLANE DEFORMATION

The solvation of trans amides has been investigated by the use of full gradient optimization ab initio quantum mechanical calculation techni ques. The complexes have been determined at the Hartree-Fock (HF) leve l with a 4-31G/4-31G** basis set and at the second-order Moller-Pless et perturbation (MP2) level. Three NMA-water clusters were investigate d: 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 oxyg en forming hydrogen bonds along the direction of the lone-pair electro ns; trans-NMA with one molecule of water at the CO group and one at th e NH group. In addition, 4-31G basis set calculations for trans-NMA w ith two molecules of acetonitrile were performed. The C=O ... H(W) hyd rogen bond lengths, electron-density population analysis and molecular -orbital analysis of trans-NMA with two molecules of water at the amid e oxygen demonstrate the importance of concurrent water-water and wate r-(carbonyl) oxygen hydrogen-bond interactions. The complex of trans-N MA with two molecules of water forming a ring cluster at the amide oxy gen indicates the formation of a non-planar amide bond and the generat ion of a chiral centre at the amide nitrogen; this structure has a 5% Boltzmann distribution at room temperature at the MP2 level. Vibration al-frequency analysis shows that its hydrogen-bonded water molecules a re vibrationally coupled. Orbital analysis suggests that there is a co nsiderable solute-occupied space reorganization caused by the rearrang ement of the water solvent molecules. Comparisons are made with previo us theoretical studies of amide-water interactions and experimental sp ectroscopic, X-ray and neutron-diffraction data on the hydration of am ides, peptides and proteins.