Interplay of Hydrogen, Pnicogen, and Chalcogen Bonding in X(H 2 O) n =1-5 (X = NO, NO + , and NO - ) Complexes: Energetics Insights via a Molecular Tailoring Approach.

Nitric oxide (NO) and its redox congeners (NO + and NO - ), designated as X, play vital roles in various atmospheric and biological events. Understanding the interaction between X and water is inevitable to explain the different reactions that occur during these events. The present study is a unified attempt to explore the noncovalent interactions in microhydrated networks of X using the MP2/aug-cc-pVTZ//MP2/6-311++G(d,p) level of theory. The interactions between X and water have been probed by the molecular electrostatic potential (MESP) by exploiting the features of the most positive ( V max ) and most negative potential ( V min ) sites. The individual energy and cooperativity contributions of various types of noncovalent interactions present in X(H 2 O) n =1-5 complexes are estimated with the help of a molecular tailoring-based approach (MTA-based). The MTA-based analysis reveals that among various possible interactions in NO(H 2 O) n complexes, the water···water hydrogen bonds (HBs) are the strongest. Neutral NO can form hydrogen and pnicogen bonds (PBs) with water depending on the orientation; however, such HBs and PBs are the weakest. On the other hand, in the NO + (H 2 O) n complexes, the NO + ···water interactions that occur through PBs are the strongest; the next one is the chalcogen bonding (CB), and the water···water HBs are the weakest. In the case of the NO - (H 2 O) n complexes, the HB interactions via both N and O atoms of NO - and water molecules are the strongest ones. The strength of water···water HB interactions is also seen to increase with the increase in the number of water molecules in NO - (H 2 O) n . The present study exemplifies the applicability of MTA-based calculations for quantifying various types of individual noncovalent interactions and their interplay in microhydrated networks of NO and its related ions.