Molecular Conformation and Hydrogen Bond Formation in Liquid Ethylene Glycol

Abstract

The ethylene glycol (EG) molecule, HOCH2CH2OH, adopts a conformation where the central OCCO dihedral is exclusively gauche in the gaseous and crystalline states, but in the liquid state, for close to 20% of the molecules, the central OCCO adopts the energetically unfavorable trans conformation. Here we report calculations, based on ab initio molecular dynamics simulations, on the thermodynamics associated with hydrogen bond formation in the liquid state of EG between donor–acceptor pairs with different molecular conformations. We establish an operational, geometric definition of hydrogen bonds in liquid EG from an analysis of the proton NMR data and show that the key feature, irrespective of the conformation, is marked directionality with almost linear ∠HO···O angles. The free energy for hydrogen bond formation estimated as the potential of mean force for the reversible work associated with the passage from a hypothetical state where hydrogen bonding is absent and donor–acceptor pairs are randomly oriented to the hydrogen-bonded state where the pairs are oriented showed comparable magnitudes irrespective of the molecular conformation of either the donor or acceptor. The results suggest that the presence of the trans conformer in liquid EG would require an understanding of its role in the extended hydrogen-bonded network of the liquid.

Search for H-Bonded Motifs in Liquid Ethylene Glycol Using a Machine Learning Strategy

“…The fragments as described in the previous section were enumerated by considering EG molecules associated by H-bonding. The geometrical cutoff criteria, r OH···O ≤ 2.5 Å, r O···O ≤ 3.6 Å, and ∠HO···O ≤ 26°, defining H-bonds in liquid EG, had been previously established from an AIMD-aided analysis of 1 H NMR measurements . The dimensions of the input vectors for the H-bonded dimer, trimer, tetramer, and pentamer fragments are different, and hence the SOMs associated with the four are discussed and evaluated individually.…”

“…, defining H-bonds in liquid EG, had been previously established from an AIMD-aided analysis of 1 H NMR measurements. 30 The dimensions of the input vectors for the H-bonded dimer, trimer, tetramer, and pentamer fragments are different, and hence the SOMs associated with the four are discussed and evaluated individually. Dimer Fragments: SOM Analysis.…”

“…As the first step of this exercise, we enumerate H-bonded fragments of different lengthsdimers, trimers, tetramers, and pentamersusing the intermolecular H-bond geometrical cutoff criteria for liquid EG that had been previously established, r OH···O ≤ 2.5 Å, r O···O ≤ 3.6 Å, and ∠HO···O ≤ 26° . The fragments were enumerated for multiple frames 0.5 ps apart over the last 62 ps of the trajectory.…”

“…To enumerate the fragments, we label all EG molecules with a numeric index from 1 to 100. For counting all possible dimer fragments, we start with the EG molecule labeled 1, EG 1 , and pick all possible ways of associating another EG with EG 1 molecule in a particular frame by satisfying the geometric H-bond cutoff criteria for liquid EG previously established from 1 H NMR measurements and AIMD simulations, 30 The process is repeated for all 100 EG molecules in the simulation cell. The list was then sorted to avoid multiple counting of the same fragments, resulting in a unique list of dimer fragments present in a frame.…”

A Review on Combination of Ab Initio Molecular Dynamics and NMR Parameters Calculations

“…The example which plainly shows the high accuracy of aiMD calculations, even if applied in a complex system, is a recent work [ 62 ]. In this study, on the basis of data derived from CPMD, the H-H bond distances between ethylene glycol and water within their mixture without any other additives were calculated.…”