Glycosidic C-O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

Mechanistic insights into glycosidic bond activation in cellulose pyrolysis were obtained via first principles density functional theory calculations that explain the peculiar similarity in kinetics for different stereochemical glycosidic bonds (β vs α) and establish the role of the three-dimensional hydroxyl environment around the reaction center in activation dynamics. The reported activating mechanism of the α-isomer was shown to require an initial formation of a transient C1-O2-C2 epoxide, that subsequently undergoes transformation to levoglucosan. Density functional theory results from maltose, a model compound for the α-isomer, show that the intramolecular C2 hydroxyl group favorably interacts with lone pair electrons on the ether oxygen atom of an α-glycosidic bond in a manner similar to the hydroxymethyl (C6 hydroxyl) group interacting with the lone pair electrons on the ether oxygen atom of a β glycosidic bond. This mechanism has an activation energy of 52.4 kcal/mol, which is similar to the barriers reported for non-catalytic transglycosylation mechanism (~50 kcal/mol). Subsequent constrained ab initio molecular dynamics (AIMD) simulations revealed that vicinal hydroxyl groups in the condensed environment of a reacting carbohydrate melt anchor transition states via two-to-three hydrogen bonds and lead to lower free energy barriers (~32-37 kcal mol-1) in agreement with previous experiments.

Glycosidic C-O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

Mechanistic insights into glycosidic bond activation in cellulose pyrolysis were obtained via first principles density functional theory calculations that explain the peculiar similarity in kinetics for different stereochemical glycosidic bonds (β vs α) and establish the role of the three-dimensional hydroxyl environment around the reaction center in activation dynamics. The reported activating mechanism of the α-isomer was shown to require an initial formation of a transient C1-O2-C2 epoxide, that subsequently undergoes transformation to levoglucosan. Density functional theory results from maltose, a model compound for the α-isomer, show that the intramolecular C2 hydroxyl group favorably interacts with lone pair electrons on the ether oxygen atom of an α-glycosidic bond in a manner similar to the hydroxymethyl (C6 hydroxyl) group interacting with the lone pair electrons on the ether oxygen atom of a β glycosidic bond. This mechanism has an activation energy of 52.4 kcal/mol, which is similar to the barriers reported for non-catalytic transglycosylation mechanism (~50 kcal/mol). Subsequent constrained ab initio molecular dynamics (AIMD) simulations revealed that vicinal hydroxyl groups in the condensed environment of a reacting carbohydrate melt anchor transition states via two-to-three hydrogen bonds and lead to lower free energy barriers (~32-37 kcal mol-1) in agreement with previous experiments.

Correction: The effects of the position of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on their physicochemical properties

Correction for ‘The effects of the position of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on their physicochemical properties’ by Kazuki Yoshii et al., Phys. Chem. Chem. Phys., 2020, DOI: 10.1039/d0cp02662j.

The effects of the position of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on their physicochemical properties

A joint computational and experimental approach uncovered that the position effect of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on the physicochemical properties.

Effects of the ether oxygen atom in alkyl side chains on the physical properties of piperidinium ionic liquids

Various types of piperidinium ionic liquids equipped with an oxygen atom-containing alkyl side chain on the positively charged nitrogen atom were systematically synthesized and their physical properties investigated.

A theoretical analysis of the kinetics of the reaction of atomic bromine with tetrahydrofuran

The kinetic behaviour for the reaction of atomic bromine with tetrahydrofuran has been analysed using the information from quantum chemical calculations. Structures and energy profiles were first obtained using density functional theory (DFT) employing the Dunning’s basis sets of triple-zeta quality, and then for an accurate energetic description, single-point calculations were carried out at the coupled-cluster with single and double excitations (CCSD) and the fourth-order Møller–Plesset (MP4(SDQ)) levels of theory. The rate coefficients and the equilibrium constants for the potential reaction channels were obtained from the statistical rate theories and statistical thermodynamics, respectively, using the results of quantum chemical calculations; and the results were compared with our recently published experimental data. In terms of reaction mechanism, this reaction was found to be analogous to the reactions of the Br atom with 1,4-dioxane and with methanol, where the reaction proceeds via an addition–elimination mechanism. The dominant reaction channel involved coordination of the approaching Br atom to one of the hydrogen atoms adjacent to the ether oxygen atom, i.e., β-hydrogen abstraction is uncompetitive. Although the complexes formed by direct coordination of the Br atom to the ether oxygen atom appeared in the reaction mechanism, we were not able to link them specifically to any reaction. The density functional theory predicted an activation energy and enthalpy of reaction that were much smaller than the experimental values, which led to an overestimation of the theoretical rate coefficients. The source of this discrepancy could be attributed to the overbinding of the transition states and of the tetrahydrofuranyl radical by DFT. Single-point calculations at the DFT structures using the CCSD and MP4(SDQ) methods yielded an accurate energetic description of the reaction of tetrahydrofuran with bromine, resulting in rate coefficients that showed excellent agreement with the experimental values.

Protonen on Tour – DFT-Studie zur H+-Wanderung in [1.1.1]- und [2.2.2]-Cryptanden / Protons on Tour – DFT-study of H+-Migration in [1.1.1]- and [2.2.2]-Cryptands

DFT-calculations (RB3LYP/LANL2DZp) show that the migration of a proton inside [1.1.1]- and [2.2.2]-cryptand from one nitrogen atom to the other follows different paths. While the proton in [H⊂1.1.1]-cryptand moves via an ether oxygen atom (activation energy: 19.2 kcal/mol), the proton in [H⊂2.2.2]-cryptand moves directly from one nitrogenatom to the other (activation energy: 16.1 kcal/mol). Our calculations rule out the application of doubly protonated [2.2.2]-cryptands as anion hosts.