How do X-ray diffraction and neutron diffraction reveal the structure of liquids?

To reveal the molecular structure of liquids, X-ray diffraction and neutron diffraction are used to measure the radial distribution function, denoted as g.1

In these experiments, a liquid sample is exposed to monochromatic radiation—a beam with a specific, single wavelength. As the radiation interacts with the molecules in the liquid, it scatters, and the resulting data is collected. Because liquids lack the long-range order of a crystal, this scattering data does not produce the sharp spots seen in crystallography; instead, it provides a measure of the probability of finding the center of one molecule at a specific distance r from the center of another.1

To interpret this raw scattering data, scientists apply a mathematical treatment known as a Fourier transform. This process converts the diffraction measurements into the radial distribution function, which reveals the local packing and preferred distances between molecules in the liquid state. By understanding these molecular distances, researchers can calculate the macroscopic properties of the substance.1