Table : Xylose thermal degradation relative standard enthalpies at 0 K (∆H • 0 K ) (i.e., zero-point corrected electronic energies). In particular, initial step of the three main pathways are reported, i.e., ring opening, ring contraction, and dehydration ones. Values in kJ mol -1 . Investigation of xylose pyrolysis initial reaction steps has been performed and results are reported in this Section. Xylose can undergo 3 different initial reactions, ring opening, ring contraction and dehydration reactions, as detailed in Figure . Zero-point corrected electronic energies computed at different levels of theory are gathered in Table . In particular the performances of M06-2X/6-311++G(d,p) (hereafter M06-2X), CBS-QB3 and G4 are compared to those of junChS. Among CBS-QB3 and G4, the latter provides smaller errors, as already reported in literature , showing a Root-mean-square deviation (RMSD) of 3.23 kJ mol -1 with respect to 4.65 kJ mol -1 . M06-2X shows overall satisfactory performances, with a RMSD of 11.33 kJ mol -1 . At the CBS-QB3 level of theory, it was not possible to locate the transition states TS2 and TS3 for the ring contraction reactions.