Ethylene Glycol in the Liquid State: Structure, Conformation and Dynamics - A Study Using Ab Initio Molecular Dynamics Simulations
Ethylene glycol (EG) is an important organic molecule with unique properties and a wide range of applications. The EG molecule shares a structural motif with many biomolecules and serves as a part of the training set for parameterizing the molecular mechanics program. It is one of the simplest molecules capable of intra- and intermolecular hydrogen bonding while at the same time exhibiting conformational degrees of freedom. In this thesis ab initio Car-Parrinello molecular dynamics simulations were used to probe the structure, conformation, and dynamics of the ethylene glycol molecule in the liquid state. The three key issues that the thesis wished to address were (i) Can simulations reproduce the results of the Raman study on the molecular conformation of the EG molecule in the liquid, and how are the two - conformation and vibration spectra - related? (ii) How is the hydrogen bond in the strongly associated liquid EG defined, and how does the hydrogen bond geometry and thermodynamics of hydrogen bond formation vary with the conformation of the EG molecule? and (iii) Can hydrogen-bonded motifs or recurrent patterns of hydrogen-bonded association in liquid EG be identified from a search of the ab initio MD trajectories? In summing up, the thesis has addressed these issues with varying degrees of success - some completely, some partially, and some not at all. One of the key successes was that the AIMD simulations reported in the thesis are indeed able to reproduce the experimental observations that in the liquid EG molecules with the OCCO dihedral in both trans (20%) and gauche conformations are present. It may be recalled that classical MD simulations failed to do so as the results depend on the choice of the force field. It has been possible to get the more complete picture of the conformation of EG molecules - the distribution of the three dihedral angles - in the liquid and hence been able to relate the molecular conformation with the vibrational spectra of the liquid. From the trajectories of the AIMD simulations it has been possible to define a geometrical criteria of intermolecular hydrogen bonds for both the trans and gauche conformations in liquid EG and the associated thermodynamics of hydrogen bond formation. The results showed that the free energy for hydrogen bond formation is very similar for both the trans and gauche conformations. This result leaves the question as to how the large energy difference between the trans and gauche conformers reported for the isolated EG molecule is compensated when EG molecules condense to form the liquid, an open, unanswered question. The search for hydrogen bonded motifs using a machine learning strategy to analyze AIMD trajectories was successful in identifying hydrogen-bonded dimer and trimer fragments that were long-lived and also appeared in the structure of the more significant tetramer and pentamer fragments. One of the important questions that the present simulations have been unable to address is why the energetically unfavorable trans conformer is present in the liquid. The thermodynamics of hydrogen bond formation shows no preference for the trans conformer over the gauche and the search for hydrogen-bonded motif was unsuccessful in uncovering any significant role for the trans geometry in the hydrogen bonding network of liquid EG. The presence of the trans conformer in liquid EG and its specific role in the liquid remains an open question.