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 M n W m , (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…O W HBs are the strongest (6.9 to 12.4 kcal mol -1 ). The next ones are O M -H…O M HBs (6.5 to 11.6 kcal mol -1 ). The O W -H…O W (0.2 to 10.9 kcal mol -1 ) and O W -H…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…O M (5.0 to 6.0 kcal mol -1 )>O W -H…O M (1.5 to 6.0 kcal mol -1 )>O M -H…O W (3.8 to 5.6 kcal mol -1 )>O W -H…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…O W (2.4 to 8.6 kcal mol -1 )>O M -H…O M (1.3 to 6.3 kcal mol -1 )>O W -H…O W (-1.0 to 6.5 kcal mol -1 )>O W -H…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.