Professor Benjamin Schwartz and graduate student Devon Widmer co-authored the paper featured in a recent issue of the prestigious journal Nature Chemistry.
Widmer (pictured right), a sixth year graduate student, science writer, and recipient of the UCLA 2018 Distinguished Teaching Award for Teaching Assistants, was first author of the paper titled “Solvents Can Control Solute Molecular Identity”.
Schwartz, a professor of physical chemistry and Senior Editor of the Journal of Physical Chemistry, was the senior author. His group is one of the few pursing both experimental and theoretical approaches to solving problems in chemical reaction dynamics in condensed phases.
For more information about the Schwartz group’s research, visit their website.
About the research (by Devon Widmer):
The solvent is often considered a medium to hold the solute; however, for some condensed-phase systems, the solvent can actually become an integral part of the chemical identity of the solute.
Oftentimes, a solute in solvent can be approximated as a jellybean in a jar of marbles. If we shake the jar, marbles jostle the jellybean, dictating its trajectory and potentially even distorting its shape as it gets squished. For condensed-phase systems where the solvent does not strongly interact with the solute, this simplified picture is a reasonable approximation. For instance, our molecular dynamics simulations of the sodium dimer (Na2) molecule in liquid argon (Ar) reveal that the nonpolar Ar atoms act essentially like marbles. A solvent cage of Ar atoms traps the Na2, compressing its bonding electron density and thus increasing the molecule’s vibrational frequency. Furthermore, like the marbles squishing the jellybean out of shape, collisions between Ar atoms and the Na2 bonding electron density give rise to distortions with relatively large instantaneous dipole moments. This has the surprising effect of making the otherwise completely symmetric Na2 infrared active.
Real solvent molecules, however, are not hard, rigid, uniform marbles. Instead, they often are complex molecules with complex properties, and the jellybean in a marble jar analogy quickly falls apart for condensed-phase systems where the solute and solvent interact, even when the interaction is only about as strong as a hydrogen bond. In our paper, available from Nature Chemistry, we show using molecular dynamics simulations of Na2 in liquid tetrahydrofuran (THF) that, when such local interactions are present, solvent molecules actually integrate themselves as part of the solute, dictating not only the bond dynamics but also the molecular identity of the solute.
The weak interactions between the sodium atoms of Na2 and the oxygen atoms of the THF molecules lead to several stable coordination states, each with a different number of solvent molecules interacting with each Na atom. Although these interactions would not be conventionally considered “bonds,” the discrete THF-Na atom coordination states must surmount free energy barriers of approximately 8 kBT, essentially undergoing a chemical reactions, to interconvert. Furthermore, each coordination state has its own bond length and dynamics as well as different infrared and UV-Visible absorption spectra. Instead, we can easily explain the properties of the system as an average of multiple coordination states in equilibrium. Thus, there is no way to explain this system as a simple Na2 “jellybean” in a jar of THF “marbles;” the overall properties simply wouldn’t fit the observed dynamics and spectroscopy. Instead, for systems where the solute and solvent interact even as weakly as a hydrogen bond, the local solvent environment must be carefully considered to truly understand the condensed phase system. Maybe a better analogy for this system would be a jellybean in a jar of gooey gummy bears that stick to, pull and modulate the jellybean motion in unique ways.
How significant are these local solvent effects? After all, chemists have long been aware that the properties of the solvent can influence solute chemical reactivity, such as stabilizing products and determining reaction rates. However, we also know that many of the biological processes necessary for life, such as the folding of a protein into its biologically functional form, are controlled by delicate equilibria. The majority of chemical reactions, particularly those relevant to biological systems, take place in solution. In many of these scenarios, the solvent, far from being a mere spectator, may play an integral role in the dynamics and even chemical identity of the solute. Currently, our group is continuing to explore these effects in new theoretical systems, and we hope our paper will inspire experimentalists to probe for these chemical identity effects. At the very least, we should all tread carefully when working with condensed phase systems as the solvent may be sneakily integrating itself as part of the very chemical identity of the solute.
