On the influence of water on THz vibrational spectral features of molecular crystals

TL;DR

This study combines computational and experimental approaches to understand how water influences terahertz spectral features of lactose molecular crystals, revealing complex phase behavior and the active role of hydration water in vibrational modes.

Contribution

It demonstrates the importance of periodic DFT simulations for interpreting THz spectra and clarifies water's dual role in affecting vibrational features in lactose crystals.

Findings

Water affects lactose THz spectra both directly and indirectly.

Lactose exhibits multiple crystal phases, including hydrated forms.

Hydration water facilitates THz vibrational modes.

Abstract

The nanoscale structure of molecular assemblies plays a major role in many ($\mu$)-biological mechanisms. Molecular crystals are one of the most simple of these assemblies and are widely used in a variety of applications from pharmaceuticals and agrochemicals, to nutraceuticals and cosmetics. The collective vibrations in such molecular crystals can be probed using terahertz spectroscopy, providing unique characteristic spectral fingerprints. However, the association of the spectral features to the crystal conformation, crystal phase and its environment is a difficult task. We present a combined computationalexperimental study on the incorporation of water in lactose molecular crystals, and show how simulations can be used to associate spectral features in the THz region to crystal conformations and phases. Using periodic DFT simulations of lactose molecular crystals, the role of water in the observed lactose THz spectrum is clarified, presenting both direct and indirect contributions. A specific experimental setup is built to allow the controlled heating and corresponding dehydration of the sample, providing the monitoring of the crystal phase transformation dynamics. Besides the observation that lactose phases and phase transformation appear to be more complex than previously thoughtincluding several crystal forms in a single phase and a non-negligible water content in the so-called anhydrous phasewe draw two main conclusions from this study. Firstly, THz modes are spread over more than one molecule and require periodic computation rather than a gas-phase one. Secondly, hydration water does not only play a perturbative role but also participates in the facilitation of the THz vibrations.