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 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 À H…O W HBs are the strongest (6.9 to 12.4 kcal mol À 1 ). The next ones are O M À H…O M HBs (6.5 to 11.6 kcal mol À 1 ). The O W À H…O W (0.2 to 10.9 kcal mol À 1 ) and O W À H…O M HBs (0.3 to 10.3 kcal mol À 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 À H…O M (5.0 to 6.0 kcal mol À 1 ) > O W À H…O M (1.5 to 6.0 kcal mol À 1 ) > O M À H…O W (3.8 to 5.6 kcal mol À 1 ) > O W À H…O W (1.2 to 5.0 kcal mol À 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 À H…O W (2.4 to 8.6 kcal mol À 1 ) > O M À H…O M (1.3 to 6.3 kcal mol À 1 ) > O W À H…O W (À 1.0 to 6.5 kcal mol À 1 ) > O W À H…O M (À 1.2 to 5.3 kcal mol À 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.

Hydration shell model for expeditious and reliable individual hydrogen bond energies in large water clusters

“…For details of the original MTA method, see ref. 62-68. As noted above, our MTAbased method has been applied to the estimation of both intra- [49][50][51][52][53][54][55] and inter-molecular [56][57][58][59][60][61] individual hydrogen bond energies in molecular systems and clusters.…”

“…They studied clusters of water (W n , n = 3 to 8), 56 benzene (Bz) n , 57 ammonia (NH 3 ) n , 58 hydrogen fluoride (HF) n 59 as well as mixed (HF) m (W) n 60 and methanol–water clusters. 61 The MP2(full)/aug-cc-pVTZ calculated HB energies in W n , for n = 3 to 8, lies between 0.3 and 11.6 kcal mol −1 . 56 Although this method provides reliable estimates of individual HB energy and cooperativity, its application to large molecular clusters is computationally challenging.…”

“…62–68. As noted above, our MTA-based method has been applied to the estimation of both intra- 49–55 and inter-molecular 56–61 individual hydrogen bond energies in molecular systems and clusters.…”

A Tug of War between the Self‐ and Cross‐associating Hydrogen Bonds in Neutral Ammonia‐Water Clusters: Energetic Insights by Molecular Tailoring Approach

“…This structural feature is denoted as AA(W D )!W A (DD) and was attributed to the formation of strong HBs in water clusters, studied in our earlier work. [70,71,76] Furthermore, it has been suggested that the strong HBs have large contribution of cooperativity. Indeed, the cooperativity ChemPhysChem contribution towards HB3 in (NH 3 ) 1 (H 2 O) 6 is the largest (6.10 kcal mol À 1 ) among all the other OÀ H…O HBs.…”

“…[63][64][65][66][67][68][69] This methodology was successfully extended by Deshmukh and co-workers, for estimating the individual HB energy and cooperativity in various molecular clusters viz. water (H 2 O) n , [70,71] ammonia (NH 3 ) n , [72] hydrogen fluoride (HF) n , [73] benzene (Bz) n , [74] and mixed (HF) m -(H 2 O) n [75] as well as methanol-water (CH 3 OH) m (H 2 O) n [76] clusters. The calculated HB energies in (H 2 O) n , (n = 3 to 8) clusters, lie in the wide range of 0.3 to 11.6 kcal mol À 1 at MP2/aug-cc-pVTZ level.…”