Effects of the position and manner of hydration on the stability of solvated N-methylacetamides and the strength of binding between N-methylacetamide and water clusters: a computational study.

This work mainly studies the effects of the position (there are two possible hydrated sites) and the manner (i.e., whether water acts as a proton donor or acceptor) of hydration by various numbers of water molecules on the stability of 14 solvated N-methylacetamide structures, NMA-(H(2)O)( n ) (n = 1-3), as well as the binding strength between the NMA and the water cluster, using molecular dynamics (MD) and B3LYP methods. Natural bond orbital (NBO) analysis is used to explore the origin of these effects. Some novel observations are obtained from the work. Our results show that monohydration at the carbonyl site favors stability and binding strength compared to monohydration at the amino site. Similarly, the preferred hydration at the carbonyl site is observed for dihydrated NMAs when the second water is added as a proton donor to the C=O group or the first water is H-bonded to the C=O group. However, unfavorable hydration at the C=O site occurs if the second water acts as a proton acceptor. Trihydration by a ring cluster of three water molecules at either the carbonyl site or the amino one yields relatively stable complexes, but significantly disfavors binding strength. The other trihydrated NMAs show similar behavior to dihydrated NMAs. In addition, our results show that the C=O and N-H frequencies can still be utilized to examine the H-bond effects of the water cluster.