Xylopyranose Ring-Opening by Single and Double Proton Transfers Under Pyrolysis Conditions

This study unveils a new transition state (TS) leading to the acyclic product via synchronous double proton transfer by automatedly exploring the potential energy surface of $\beta$-D-xylopyranose under pyrolysis conditions. Quantum chemistry methods with multi-path canonical variational transition state theory show that the standard activation enthalpy of the new TS (44.9 kcal ${\text{mol}}^{-1}$) is 1.5 kcal ${\text{mol}}^{-1}$ lower than that of the well-established channel; however, the latter's rate constant ($4.36\times 10^{-2}$–$9.96\times 10^1$ ${\text{s}}^{-1}$) is higher in the 673.15–873.15 K pyrolytic range by a factor of 5–8. This gap narrows to a factor of 2 within 320–400 K, signifying that the new TS can potentially impact the acyclic product production in this low-temperature regime. This is particularly relevant for $\beta$-D-xylopyranose trimers, as the interior unit bears different substituents at the C1 and C3 positions.