All the structures of the neutral Ala, Ala–(H2O)n=1–5 and ZAla–(H2O)n=2–5 complexes have been optimized at X3LYP/6-311++G(d,p) level of theory . The binding energy of Ala–(H2O)n=1–5 complexes is calculated using the relationship: BE = Ecomplex _ (EAla + EH2O) and the values are calculated. The values of Gibbs energy and enthalpy of ZAla–(H2O)n=2–5 complexes are presented relative to the corresponding Ala–(H2O)n=2–5 complexes. The frequencies of hydrogen bond bridging modes, O--H and N--H have also been calculated. During the stepwise addition of water molecules it has been observed that the stable structure of ZAla–(H2O) complex does not exist in gas phase. We found that at least two water molecules are needed to stabilize the structure of ZAla in gas phase. It is also observed that the hydration process enhances the values of valence angle due to the redistribution and localization of charges among the water molecules. The Gibbs energies, interaction enthalpies and entropies of the complexes were also determined. The computed values of Gibbs energy and enthalpy are found to minimum for the ZAla–(H2O)5 complex which is contrast to similar reports. Atom in molecule (AIM) and natural bond orbital (NBO) methods have been used for hydrogen bond strength comparison and electrostatic charge changes during the hydrogen bond evolution.

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