A detailed theoretical study of hydrogen-bond formation in different polyethylene glycol + water complex and dipropylene glycol + water have been performed by Hartree Fock (HF) method, second-order M？ller-Plesset perturbation theory (MP2), and density functional theory (DFT) using 6-31++G(d,p) basis set. B3LYP DFT-D, WB97XD, M06, and M06-2X functionals have been used to describe highly dispersive hydrogen-bond formation appropriately. Geometrical parameters, interaction energies, deformation energies, deviation of potential energy curves of hydrogen bonded O–H from that of free O–H, frontier orbitals, and charge transfer have been studied to analyze stability and nature of hydrogen bond formation of various glycol and water complexes. It is found that WB97XD is best among all the applied DFT functionals to describe hydrogen bond interaction, and intermolecular hydrogen bonds have higher covalent character and accordingly higher strength when glycol acts as proton donor for glycol + 1 water complex system. 1. Introduction Polyethylene glycol and its derivatives are applied extensively as drag delivering medium in medical industry [1] and gas hydrate inhibitor in petroleum industry [2, 3]. Experimental study of ethylene glycol molecule and ethylene glycol aqueous solution has been performed using nuclear magnetic resonance (NMR) spectroscopy [4, 5], infrared spectroscopy (IR) [6–9], ultraviolet (UV) spectroscopy [9], Raman Spectroscopy [10], X-ray, and Neutron diffraction techniques [11]. Quantum chemical-based study on different conformers of ethylene glycol has revealed that the gauche form is the most stable conformer in aqueous solution [12, 13]. Hydrogen-bond, an attractive proton donor-acceptor interaction between donor (bonded combination of hydrogen with other electronegative atom) and acceptor (electron-rich region) [14, 15], plays crucial role in determining microscopic and macroscopic behaviour of glycols and water system. Since aqueous solution of glycols are used as gas hydrate inhibitor during drilling practice in petroleum industry, detailed scientific understanding of hydrogen-bond interaction between glycol and water is essential to utilize glycols more efficiently as gas hydrate inhibitor. Quantum chemical calculation is very effective to investigate the hydrogen-bond interaction and its impact on the performance of gas hydrate inhibitors. The effect of microsolvation on ethylene glycol has been studied using density functional theory considering the contribution of many body energies by Chaudhari and Lee [16]. A polymer reference
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