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

In this work, we examine the strength of various types of individual hydrogen bond (HB) in mixed methanol-water MnWm, (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 O-M-H horizontal ellipsis O-W HBs are the strongest (6.9 to 12.4 kcal mol(-1)). The next ones are O-M-H horizontal ellipsis O-M HBs (6.5 to 11.6 kcal mol(-1)). The O-W-H horizontal ellipsis O-W (0.2 to 10.9 kcal mol(-1)) and O-W-H horizontal ellipsis O-M 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., O-M-H horizontal ellipsis O-M (5.0 to 6.0 kcal mol(-1))>O-W-H horizontal ellipsis O-M (1.5 to 6.0 kcal mol(-1))>O-M-H horizontal ellipsis O-W (3.8 to 5.6 kcal mol(-1))>O-W-H horizontal ellipsis O-W (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: O-M-H horizontal ellipsis O-W (2.4 to 8.6 kcal mol(-1))>O-M-H horizontal ellipsis O-M (1.3 to 6.3 kcal mol(-1))>O-W-H horizontal ellipsis O-W (-1.0 to 6.5 kcal mol(-1))>O-W-H horizontal ellipsis O-M (-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.