In the present computational study a complete reaction mechanism for the sugar's mutarotation promoted by tautomeric catalysts in vacuum is outlined. The rate determining step of the process consists in the breaking of the C–O bond of the sugar ring which occurs by a concerted double proton transfer between the substrate and the catalyst. After ring opening, the –CHO group of the substrate rotates around the C2–C3 bond without dissociation of the substrate–catalyst adduct. The activation energy of this step is generally less than 8–9 kcal mol⁻¹. Alternative reaction paths which require the dissociation of the substrate–catalyst complex are less favourable for both probability and energetic reasons. When the mutarotation is catalysed by the 2-pyridone/2-hydroxypyridine (PD/HP) tautomeric couple, the process may be promoted by either PD or HP derived from the dissociation of the (PD)₂, (HP)₂ or PD–HP dimers. According to B3LYP and MP4 (SDQ) calculations the HP-promoted reaction path should be faster than the PD-promoted one. When the process is catalysed by the formamide/formamidic acid (F/FA) tautomeric couple, the reaction path promoted by FA, which is derived from the dissociation of the F–FA dimer, should be the most favourable, according to B3LYP, MP2 and MP4(SDQ) calculations. Solvent effects in benzene, calculated by the PCM method, did not affect to a relevant extent the mechanism outlined in vacuum. The present study suggests, differing from the hypothesis formulated in many experimental studies, that the sugar's mutarotation process is more efficiently catalysed by the less stable tautomer or catalyst dimer rather than by the more stable one of both the PD/HP and F/FA couples.

The present computational study investigates the adsorption of N₂ by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu⁺ ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N₂ and couples of Cu⁺ sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu⁺ ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu–N–N–Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu–N–N–Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N–N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.