Unexpected Crossover in the kinetics of mutarotation in the supercooled region: the role of H-bonds

Intra- and intermolecular studies on the molten L-sorbose have been carried out at variable temperature conditions to determine the crosover temperature (T(c)). In addition, isothermal time-dependent FTIR and Raman measurements were performed to probe the pace of mutarotation and activation energy of this reaction in the studied saccharide, which varied from 53–62 kJ/mol up to 177–192 kJ/mol below and above T(c), respectively. To explain the change in activation barrier for the mutarotation a complementary analysis using difference FTIR spectra collected around T(c) = 365 K in the hydroxyl region has been done. It was found that the alteration of kinetic parameters and molecular dynamics around T(c) are strictly related to the variation in the strength of H-bonds which above T(c) are significantly weaken, increasing the freedom of rotation of functional groups and movement of individual molecules. That phenomenon most likely affects the proton transfer, underlying molecular mechanism of mutarotation, which may lead to the significant increase in activation barrier. The new insight into a molecular aspect of the mutarotation around T(c) has created an opportunity to better understanding the relationship between physics of condensed matter and the potential role of H-bonds dynamics on the progress of the chemical reaction in highly viscous systems.