It was found that the binding energy values calculated by B3LYP differ significantly from the same calculated at M06-2X (Table 1). This might be due to the contribution of long-range dispersion interaction which is known to play an important role in stabilizing weak H-bonded complexes. Therefore, geometry optimization, frequency calculation and single point counterpoise correction have been carried out using ωB97X-D functional and 6-311++G** basis set. Further, counterpoise corrected single point energy calculation has been performed using M06-2X functional and 6-311++G** basis set for geometries optimized at ωB97X-D/6-311++G** level of theory given in Table 1. Here, the stabilization energy obtained at this level was close to what was predicted by ωB97X-D/6-311++G** level of theory. Therefore throughout the manuscript stabilization energy calculated at ωB97X-D/6-311++G** level has been used for study the molecular cluster. Natural bond orbital and atoms in molecules calculations have been carried out using ωB97X-D functional and 6-311++G** to corroborate with the energetic and geometric findings. All the geometry optimization, normal mode frequency and natural bond orbital (NBO)⁶⁰calculation have been carried out using Gaussian16⁶¹ suite of program where as topological analysis was performed using AIM 2000 software package.⁶²Throughout the manuscript, all the binding energy values mentioned are counterpoise corrected values calculated at ωB97X-D/6-311++G** level of theory, unless otherwise mentioned.

In this work, we have investigated the intermolecular C-H···O H-bond formed between two CHD molecules and how they undergo modifications with increasing size of CHD clusters from dimer to hexamer. In order to do so, we have focused on the geometric, energetic and electronic features of the C-H—O H-bonds and subsequently tried to quantify the extent of cooperativity (both positive and negative) exerted by them.