Unraveling the Atomic Structure of Molecules in Solution

What is it about?

In homogeneous catalysis, solar photochemistry, and battery engineering, chemistry and chemical dynamics occur in the solution phase. To unravel the structural details of small molecule solvation, we present an ansatz for total scattering studies of dilute solutions.

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Photo by BoliviaInteligente on Unsplash

Photo by BoliviaInteligente on Unsplash

Why is it important?

Solvation is a ubiquitous phenomenon that dictates the behavior of molecules in the condensed phase. Typically, spectroscopy can reveal the solvent dependence of a chemical system, and remains the central tool for quantitative kinetic studies; for example, studies of the solvent dependence of ultrafast photoinduced electron transfer typically rely on optical transient absorption (OTA) spectroscopy. However, underpinning this solvent dependence is the so-called "solvation structure". That is, in principle, dissolution both affects the intramolecular structure of the solute (relative to its bulk or gas-phase structure), and creates a "cage" of solvent molecules ordered locally about the dissolved particle. Inasmuch as the atomic structure is coupled to the (adiabatic) potential energy surface, elucidating the solvation structure is critical for understanding chemical dynamics in solution. For crystalline systems, the mature field of X-ray crystallography routinely provides 3-dimensional structural information with sub-atomic resolution. However, in disordered systems, such as liquids, the sharp point-like Bragg scattering gives way to broad, isotropic, diffuse scattering. While this reduces the 3-dimensional information to 1-dimension, the diffuse scattering intensity still encodes structural information in the form of the pair distribution function (PDF)—essentially a weighted histogram of all interatomic distances in the sample. With advances in hard X-ray science at synchrotron light sources, it is now also routinely possible to collect high-resolution total scattering data on disordered as well as crystalline materials. While the PDF represents an ill-posed structure determination problem, since 3-dimensional information is projected down to 1-dimension, when combined with physically-constrained atomistic models, it can provide structural details of molecules in condensed matter with resolution approaching crystallography. To solve this ill-posed problem, a forward-modeling ansatz capturing the essential structural information encoded by the PDF is required. In this paper, we develop such an ansatz, synthesizing concepts developed in both small/wide-angle X-ray scattering (SAXS/WAXS) studies of macromolecules in solution, as well as the burgeoning field of ultrafast time-resolved X-ray solution scattering (TR-XSS), the latter enabled by the advent of the X-ray free-electron laser (XFEL). We anticipate that this ansatz will serve as the basis for the development of quantitative, atomistic, structural models of molecules in the solution phase.

Perspectives