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Abstract
Intra- and intermolecular studies on the molten L-sorbose have been carried out at variable temperature conditions to determine the crosover temperature (Tc). 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 Tc, respectively. To explain the change in activation barrier for the mutarotation a complementary analysis using difference FTIR spectra collected around Tc = 365 K in the hydroxyl region has been done. It was found that the alteration of kinetic parameters and molecular dynamics around Tc are strictly related to the variation in the strength of H-bonds which above Tc 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 Tc 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.
Highlights
The phenomenon of crossover temperature (Tc) in molecular dynamics, that in common view marks the onset of the cooperative dynamics, has been studied intensively in the past
Inter- and intramolecular dynamics of L-sorbose has been investigated in a wide temperature range
We found that at around T = 365 K there is a splitting of structural (α) and γ-relaxations which are accompanied by the change in intramolecular dynamics
Summary
The phenomenon of crossover temperature (Tc) in molecular dynamics, that in common view marks the onset of the cooperative dynamics, has been studied intensively in the past. The progress and kinetics of this reaction have been studied since 1846, when Dubrunfaut[19] discovered it, still, new data and experimental finding are coming out that contribute to revisiting current models describing mutarotation In this context one can remind very recent studies on pure supercooled saccharides demonstrating that the progress of mutarotation in solid state is much different than the kinetics of the reaction carried out in various solvents[20,21,22,23,24,25,26]. It is well-manifested in the two times higher activation barrier and sigmoidal shape of the kinetic curves[21]. Combination of the data obtained from various techniques allowed to assign unexpected change in the kinetics of mutarotation around Tc to the variation in dynamics and strength of H-bonds
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