The catalytic hydrogenolysis of compounds derived from guaiacol on the Cu (111) surface: mechanisms from DFT studies.

Pyrolysis oils have inferior properties compared to liquid hydrocarbon fuels, owing to the presence of oxygenated compounds such as guaiacol, C6H4(OH)(OCH3). The catalytic hydro-deoxygenation (HDO) of phenolic compounds derived from guaiacol, i.e. catechol, phenol and anisole were investigated over the Cu (111) surface to unravel the elementary steps involved in the process of bio-oil upgrade. The phenolic compounds adsorb through their π systems to the surface, where steric effects of the methoxy group reduce the stability of anisole on the surface. To produce benzene, hydroxyl removal from catechol and phenol occurs in a stepwise fashion, where dehydroxylation of catechol is more challenging than phenol. Thermodynamically, catechol is the preferred oxygenated product, but it is the most challenging to transform to benzene, requiring an energy barrier of 1.8 eV to be overcome, which is similar to the HDO of anisole with an activation energy of 1.7 eV but more difficult than the HDO of phenol with an activation energy of 1.2 eV. The rate limiting steps in the HDO reactions are catechol dehydroxylation, anisole demethoxylation and phenol dehydroxylation. Our results show that ortho substituents impede C-O bond cleavage, as seen for catechol, whereas in the absence of an ortho substituent -OH cleavage is easier than -OCH3 cleavage to form benzene.

CO2 activation and dissociation on the low miller index surfaces of pure and Ni-coated iron metal: a DFT study.

We have used spin polarized density functional theory calculations to perform extensive mechanistic studies of CO2 dissociation into CO and O on the clean Fe(100), (110) and (111) surfaces and on the same surfaces coated by a monolayer of nickel. CO2 chemisorbs on all three bare facets and binds more strongly to the stepped (111) surface than on the open flat (100) and close-packed (110) surfaces, with adsorption energies of -88.7 kJ mol-1, -70.8 kJ mol-1 and -116.8 kJ mol-1 on the (100), (110) and (111) facets, respectively. Compared to the bare Fe surfaces, we found weaker binding of the CO2 molecules on the Ni-deposited surfaces, where the adsorption energies are calculated at +47.2 kJ mol-1, -29.5 kJ mol-1 and -65.0 kJ mol-1 on the Ni-deposited (100), (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO2 dissociation into CO and O on the bare and Ni-deposited surfaces. Generally, we found that the dissociative adsorption states are thermodynamically preferred over molecular adsorption, with the dissociation most favoured thermodynamically on the close-packed (110) facet. The trends in activation energy barriers were observed to follow that of the trends in surface work functions; consequently, the increased surface work functions observed on the Ni-deposited surfaces resulted in increased dissociation barriers and vice versa. These results suggest that measures to lower the surface work function will kinetically promote the dissociation of CO2 into CO and O, although the instability of the activated CO2 on the Ni-covered surfaces will probably result in CO2 desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface.

A quantum mechanistic investigation into the unusual reactions of nitrilimine and nitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone.

CONTEXT: The theoretical study investigates the [3 + 2] cycloaddition (32CA) reactions between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone and benzonitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone. Nitrilimines and nitrile oxides are dipoles used in the synthesis of several heterocyclic compounds, including spiropyrazoline oxindoles and isoxazolines. The derivatives of these compounds are found with different biological activities, such as ion channel blockers, anti-inflammatory and anticancer agents as well as antimalarial. Conceptual density functional theory (CDFT) analysis, along with the activation energies of the 32CA reaction between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone, demonstrates concordance with the empirical findings. The 32CA reaction is found to proceed through a very polar single-step asynchronous mechanism. While deductions from the activation energies of the 32CA reaction between benzonitrile oxide and 2,3,4,5-tetraphenylcyclopentadienone are found to lead to the experimental product, the parr function analysis could not explain the observed chemo- and regioselectivity. This 32CA reaction is also found to proceed through a one-step asynchronous mechanism, though with a non-polar character. The modulation of substituents positioned at the reactive sites of the reactants is found to influence the kinetics, thermodynamics, and CDFT parameters of the two 32CA reactions, consequently impacting the observed selectivities. METHODS: The 32CA reactions between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone and benzonitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone have been explored theoretically using the density functional theory method at the hybrid ωB97X-D coupled with the split valence triple-ξ (TZ) basis set as implemented in the Gaussian 09. Solvent effects were taken into account by full optimization of the gas phase geometries through the polarizable continuum model developed within the self-consistent reaction field.

A quantum mechanistic insight into the chemo- and regio-selective [3 + 2]-cycloaddition reaction of aryl hetaryl thioketones with diazoalkanes and nitrile oxide derivatives.

In this quantum mechanistic study, density functional theory computations at the B3LYP hybrid level of theory, in addition to triple zeta basis set 6-311G (d, p), were utilized to investigate the chemoselectivities and regioselectivities of the [3 + 2] cycloaddition reaction of phenyl (2-thienyl) thioketone (B1) derivatives with nitrile oxide (B2) and diazopropane derivatives (B3). From the computations obtained, the reactions of nitrile oxide and diazopropane derivatives with phenyl (2-thienyl) thioketone proceed through an asynchronous one-step mechanism. The initial [3 + 2] cycloaddition reaction of B1 and B3 is followed by a nitrogen extrusion which is also highly asynchronous. Despite the steric and electronic effects of the substituent on the energetics, the reaction center is selectively observed at the thiocarbonyl site of B1. A study of the Parr functions at the different reaction sites in B1 indicates the addition of B2 and B3 via the atomic centers with the largest Mulliken atomic spin densities. These results show that the thiocarbonyl site is the most reactive center compared to the other ethylene groups on B1, irrespective of the three atom components used. The global electron density transfer results are in agreement with the selectivity and activation barriers observed in the reaction. Our results agree well with experimental observations.