Energetics of Tetrel, Pnicogen, and Hydrogen Bonds in Microhydrated Clusters of CO and NO.

In this study, the noncovalent interactions present in microhydrated clusters of the isoelectronic molecules viz. CO and NO were investigated by evaluating the energy of individual noncovalent interactions and cooperative contributions using the molecular tailoring approach-based (MTA-based) method. The molecular electrostatic potential (MESP) analysis revealed that CO acts as a better electron acceptor due to a more pronounced electron-deficient region on its C-atom, compared to the central N-atom of NO. The energies of the individual tetrel bonds (TBs), pnicogen bonds (PBs), and hydrogen bonds (HBs) observed in CO…water and NO…water in the dimeric clusters calculated using the MTA-based method align well with the MESP results. As the number of water molecules increases (n = 1-5), the most stable configurations reveal that CO and NO preferentially interact with cyclic water clusters, indicating that water…water HBs dominate energetically over CO…water and NO…water interactions in larger clusters. This is clearly evident from the higher values of water…water HB energies (4.72-9.67 kcal/mol in CO(HO) and 4.50-9.35 kcal/mol in NO(HO)) calculated at the MP2/aug-cc-pVTZ level as compared to the CO…water and NO…water interactions (range of 0.31-4.05 kcal/mol and 0.04-3.28 kcal/mol, respectively). Based on the calculated energies and cooperative contributions by the MTA-based method, the order of interaction strength in these microhydrated clusters follows: HB in water…water > TB in CO…water > PB in NO…water > HOH…N of (NO) HB > HOH…O of (CO) HB > HOH…O of (NO) HB. We wish to emphasize here that the present study is the first systematic attempt to establish an energetic hierarchy among various HBs, TBs, and PBs, thereby providing deeper insight into the microhydration networks of two atmospherically relevant isoelectronic molecules. These findings are expected to be crucial for elucidating the subtle interplay of noncovalent interactions in atmospheric and related environments.