Journal of Molecular Graphics and Modelling 54 (2014) 148–163 Contents lists available at ScienceDirect Journal of Molecular Graphics and Modelling j ourna l h om epa ge: www.elsevier.com/locate/JMGM DFT study on X - ·(H 2 O) n=1-10 (X = OH, NO 2 , NO 3 , CO 3 ) anionic water cluster M. Lalitha, L. Senthilkumar ∗ Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India a r t i c l e i n f o Article history: Accepted 21 October 2014 Available online 27 October 2014 Keywords: Anion Water cluster Solvation energy Energy decomposition analysis AIM NBO a b s t r a c t A theoretical study to understand the interaction between anion and the water molecules through the hydration (X - ·(H 2 O) n (X = OH, NO 2 , NO 3 , CO 3 ), where n = 1–10), using the density functional theory method with B3LYP functional and 6-311++G(d,p) basis set has been carried out systematically. In these hydrated clusters we notice three different cases of bond arrangements, namely, symmetrical double hydrogen bond, single hydrogen bond and inter-water hydrogen bond. All the complexes are dominated by the O H· · ·O hydrogen bond, in which the anion act as a proton acceptor, while the water molecule act as a proton donor. A linear correlation is obtained between the solvent stabilization energy and the size (n) of the hydrated cluster for all the anions. The weighted average interaction energy values, shows that the water molecules strongly bind with the OH - anion. Besides, the solvation of the OH - anion requires less number of water molecules when compared with the other anions. Energy decomposition analysis (EDA) shows the strong dominance of the electrostatic energy component within the interaction energy. The total NPA charges on the anions indicate an increase in the solvation due to hydration. From AIM analysis, excellent linear inverse correlation is observed for both the electron density and Laplacian of the electron density with respect to the hydrogen bond length. Natural bonding orbital analysis (NBO) predicts large charge transfer between the OH - anion and the water molecules. © 2014 Elsevier Inc. All rights reserved. 1. Introduction Understanding the chemistry of the hydrated anions are highly important because of its relevance in both the basic and the applied sciences (hydrogen bonding, solvation and nucleation) [1–3]. Anions like OH - , O 2 - , HO 2 - , NO 2 - , CO 3 - , NO 3 - , HCO 4 - , NO 3 - (HNO 3 ) 2 , etc., are related with the important problems like the origin of ice crystals in the Antarctic stratosphere, the evo- lution of the positive and the negative cluster ions in both the stratosphere and the troposphere [4,5]. Existence of several domi- nant series of hydrated ions in the troposphere has been confirmed by few research groups [6–8] using the mass spectrometric anal- ysis. Besides, both positive and negative charged water clusters are known to exist at all levels of the atmosphere including the troposphere, stratosphere and ionosphere. Study on the microhydration of small anionic species is of great interest, because, the hydrated complexes formed between the anion and the water molecules can provide insight on the role of anions in the chemical and the atmospheric processes. ∗ Corresponding author. Tel.: +91 9443702753. E-mail address: lsenthilkumar@buc.edu.in (L. Senthilkumar). Likewise, the microhydration can also address issues regarding structure-energetics relations, the existence of anion on the surface or in the interior of water clusters, which subsequently can improve our understanding of the molecular level interactions between the solvent water molecules and the negatively charged ions in the aqueous solution. However, the anion solvation is more complicated since most of the anion-water interactions are gen- erally weaker and energetically comparable with the water–water interactions. As a consequence, a delicate balance between the anion-water and the water-water interactions is crucial in deter- mining the bulk vs. surface solvation, i.e., structures with the anion on the “surface” of water clusters [9]. Similarly, the difficulty in predicting the structures of the hydrated anions due to number of geometries is also inevitable [10]. Furthermore, extensive experi- mental as well as theoretical [11–15] study limits the anionic water clusters in the order of three to five water molecules, since the sen- sitivity of the negative clusters typically extends to four hydrates at room temperature [16]. However, the recent literatures related to hydration of anion have shown only few theoretical works on the hydration of the hydroperoxide and the hydroxyl ions. Like, Anick [17] explored the structure of gas phase (HOO - )(H 2 O) n clusters at B3LYP/6-311++G** level of theory, and found that (HOO - ) enhances the hydrogen bonding network, whereas HOOH disrupts http://dx.doi.org/10.1016/j.jmgm.2014.10.012 1093-3263/© 2014 Elsevier Inc. All rights reserved.