AB Initio Quantum Mechanics Studies of Amide-Water Clusters

Abstract

The solvation of trans amides was investigated by the use of full gradient optimization ab initio quantum mechanical calculations techniques (HONDO 7) [1]. The complexes have been determined at the Hartree Fock (HF) level with 4–31 G* basis set and at the second order Möller-Plesset perturbation (MP2) level. Three NMA-water clusters were investigated (Figure 1): trans-NMA with one molecule of water at the CO group and one at the NH group (complex (A)) [2]; trans-NMA with two molecules of water forming a ring cluster at the amide oxygen (complex (B)); trans-NMA with two molecules of water at the amide oxygen forming hydrogen bonds along the direction of the lone-pair electrons (complex (C)) [3]. The $C = O \cdots H(W)$ hydrogen bond lengths (Table 1), electron density population analysis and molecular orbital analysis demonstrate the importance of cooperative hydrogen bond interactions of the bound molecules of H₂O. Calculations at the HF level indicate that complex (B) with a nonplanar amide bond and a chiral center at the amide nitrogen is more stable than complex (C). The stability of complexes is reversed at the MP2 level. Orbital analysis suggests that there is considerable solute occupied space reorganization caused by the rearrangement of the water solvent molecules.