Energetic Ordering of Hydrogen Bond Strengths in Methanol-Water Clusters: Insights via Molecular Tailoring Approach.

Abstract

In this work, we examine the strength of various types of individual hydrogen bond (HB) in mixed methanol-water Mn Wm , (n+m=2 to 7) clusters, with an aim to understand the relative order of their strength, using our recently proposed molecular tailoring-based approach (MTA). Among all the types of HB, it is observed that the OM -H…OW HBs are the strongest (6.9 to 12.4 kcal mol-1 ). The next ones are OM -H…OM HBs (6.5 to 11.6 kcal mol-1 ). The OW -H…OW (0.2 to 10.9 kcal mol-1 ) and OW -H…OM HBs (0.3 to 10.3 kcal mol-1 ) are the weakest ones. This energetic ordering of HBs is seen to be different from the respective HB energies in the dimer i. e., OM -H…OM (5.0 to 6.0 kcal mol-1 )>OW -H…OM (1.5 to 6.0 kcal mol-1 )>OM -H…OW (3.8 to 5.6 kcal mol-1 )>OW -H…OW (1.2 to 5.0 kcal mol-1 ). The plausible reason for the difference in the HB energy ordering may be attributed to the increase or decrease in HB strengths due to the formation of cooperative or anti-cooperative HB networks. For instance, the cooperativity contribution towards the different types of HB follows: OM -H…OW (2.4 to 8.6 kcal mol-1 )>OM -H…OM (1.3 to 6.3 kcal mol-1 )>OW -H…OW (-1.0 to 6.5 kcal mol-1 )>OW -H…OM (-1.2 to 5.3 kcal mol-1 ). This ordering of cooperativity contribution is similar to the HB energy ordering obtained by the MTA-based method. It is emphasized here that, the interplay between the cooperative and anti-cooperative contributions are indispensable for the correct energetic ordering of these HBs.