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My research is focused on how the dynamical and structural prop- erties of liquids change with temperature (T) and pressure (p). Specifically, I have developed methods for calculating the derivatives of dynamical timescales with respect to T and p from molecular dynamics simulations at a single temperature and pressure.[1−8] This method provides previously unobtainable mechanistic insight via decomposition of activation energies into contributions from molecular interactions. My work has enhanced our understanding of diffusion,[1-2] reorientation,[1,3,7] viscosity,[5] and hydrogen bond exchanges[8] in liquid water, while also revealing more about the nature of activation energies.

A feature of this method is that it determines the analytical derivatives at a particular T and p rather than being calculated numerically using a range of temperatures and pressures as is done in an Arrhenius analysis. This allows activation energies to be extracted even for dynamical timescales that are non-Arrhenius (e.g., liquid water diffusion), or where the temperature ranges needed for an Arrhenius analysis are not accessible (e.g., near a phase transition).

 

We have illustrated how derivatives of the diffusion coefficient, reorientation times, and liquid structure may be used to predict the temperature dependence of these quantities deeply into the supercooled regime. For example, from room temperature simulations we used our method to predict the diffusion coefficient of water down to 125 K and found it to be in agreement with experimental measurements,[9] as shown in Figure 1.[7]

A century ago Tolman showed that the activation energy can be understood as the excess energy needed to surmount the barrier (as opposed to the common view of the height of the barrier).[10] The activation energy decompositions obtained from our method, then provide an accounting of the excess energy of a particular type (electrostatic, kinetic, etc.) required to surmount such a barrier. In every dynamical process in water that we have studied, we have found that the the Lennard-Jones and electrostatic activation energy contributions are in competition with one another, with the latter dominating. This is consistent with the energetic changes involved in breaking a hydrogen bond, as illustrated schematically in Figure 2.[8]

 

We are developing this work in a number of directions, including ongoing collaborations with Chris Mundy, Greg Schenter, Damien Laage, and Elise Diboué-Dijon. In the future, this method should allow insight into activation energies in other cases where an Arrhenius analysis is difficult, for instance a system at phase coexistence. We are also extending the decompositions to the level of individual molecular interactions (e.g., the effect of particular residues on the reorientation activation energy of a water molecule in a protein hydration shell).

References: [1] Piskulich, Z. A., Mesele, O. O. & Thompson, W. H. J. Chem. Phys. 147, 134103 (2017). [2] Piskulich, Z. A., Mesele, O. O. & Thompson, W. H. J. Chem. Phys. 148, 134105 (2018). [3] Piskulich, Z. A. & Thompson, W. H., J. Chem. Phys. 149, 164504 (2018). [4] Piskulich, Z. A., Mesele, O. O., & Thompson, W. H. J. Phys. Chem. A. 123, 7185-7194 (2019). [5] Mendis, C. H., Piskulich, Z. A. & Thompson, W. H. J. Phys. Chem. B. 123, 5857-5865 (2019). [6] Piskulich, Z.A. & Thompson, W.H. J. Chem. Phys. Commun., 152, 011102 (2020). [7] Piskulich, Z.A. & Thompson, W.H. J. Chem. Phys., 152, 074505 (2020). [8] Piskulich, Z.A., Laage, D., & Thompson, W.H. J. Chem. Phys. submitted (2020). [9] Xu, Y., Petrik, N.G., Smith, R.S., Kay, B.D. & Kimmel, G.A. Proc. Natl. Acad. Sci. 113, 14921 (2016). [10] Tolman, R. C. J. Am. Chem. Soc. 42, 2506–2528 (1920).

During my second year of support from the NSF Graduate Research Fellowship Program (GRFP) I have made strides in my academic progress, my professional development, and my research. I should note, I also took my NSF GROW trip this year to Paris to work with Damien Laage; however, the COVID-19 crisis occurred within the first two weeks of this experience. 

My research focuses on developing new simulation and analysis techniques for understanding how molecular motions influence chemical reactions (e.g., catalysis). Throughout my time on the NSF fellowship I have been developing techniques for understanding the temperature dependence of these motions, as well as that of molecular structure. The developed technique allows not only for the calculation of activation energies, which control the temperature dependence of these quantities, but also the temperature dependence of these activation energies. With this information we have shown that it is possible to use it to predict the motions and structures of complex liquids (e.g. water) over large temperature ranges. Additionally, the developed methods provide mechanistic insight, unavailable by any other method, into how these motions and structures originate from specific molecular interactions.

Intellectual Merit: There however remains a scientific debate about the specific structures and motions that water undergoes when supercooled, and whether these are apparent at room temperature. In our work over the past year, we have developed the above techniques for predicting the temperature dependence of both molecular motions (diffusion,reorientation) as well as liquid structure in order to help resolve this debate. Thus far, we have shown that the motions of water can be predicted over wide range of temperatures reaching deeply into the supercooled regime (down to 125 K in some cases, about 150 K below the freezing point) from room temperature. We have similarly shown that the structure supercooled water can be predicted down to 235 K from room temperature, and are currently developing techniques for predicting even lower temperatures. Taken together, this indicates that the strange behavior observed and debated over in the scientific literature must have origins that are present in the intermolecular interactions present at room temperature.

Broader Impacts: It has been estimated that most of the water in the universe exists as a supercooled liquid, or a liquid that has been cooled slowly enough that it remains liquid-like instead of freezing at temperatures well below its freezing point. Indeed on earth, certain species of insects have developed specific proteins that suppress freezing in water to remain viable in extreme environments. By better understanding supercooled water, we can better understand these processes as well.

Over the past year, I have participated in multiple professional development experiences and scientific conferences. As the vice-president of the Chemistry graduate student organization at KU I helped develop a weekly professional development seminar series, as well as an Alumni careers panel. I also helped organize the The Physics and Chemistry of Liquids Gordon Research Seminar which was held in Holderness, New Hampshire, August 2019. I attended both the aforementioned conference along with the associated Gordon Research Conference as well as the Pacific Conference on Spectroscopy and Dynamics, where I presented a contributed talk on my research. The research presented at that conference was also included in a Viewpoint article in the Journal of Physical Chemistry. During my time, I have also been involved in mentoring two other graduate students as well as two undergraduate students.