X-ray Diffraction and Solid-State NMR Investigation of the Single-Crystal to Single-Crystal Dehydration of Thiamine Hydrochloride Monohydrate

The dehydration of a thiamine hydrochloride (vitamin B<sub>1</sub>) hydrate, commonly referred to as the monohydrate, was investigated by solid-state NMR (SSNMR) and X-ray diffraction (XRD) techniques. The hydrate can be classified as a nonstoichiometric solvate since the water content depends on the water vapor pressure of the surrounding atmosphere and essentially maintains the three-dimensional molecular arrangement upon dehydration. Thus, we were able to determine the crystal structures of both the hydrate and the isomorphic desolvate with one single crystal. The loss of water leads to a shrinkage of the unit cell volume of about 5% and to a slight increase in the free volume. This is also accompanied by an increase in molecular motion as is demonstrated by SSNMR <sup>1</sup>H and <sup>13</sup>C <i>T</i><sub>1</sub> measurements. The largest change in <i>T</i><sub>1</sub> was observed for the carbons of the hydroxyethyl functional group that is hydrogen bonded to the water molecules in the hydrate. This investigation confirms that an increase in free volume results in an increase in molecular mobility and demonstrates the impact of molecular interactions on the mobility of specific molecular entities.