Hydrogen Interactions with a Pd4Cluster: Triplet and Singlet States and Transition Probability

Catalytic performance of Pdn (n = 1, 2, 3, 4 and 6) clusters supported on TiO2-V for the formation of dimethyl oxalate via the CO catalytic coupling reaction: a theoretical study

The formation of dimethyl oxalate (DMO) via CO catalytic coupling on a series of catalysts including Pdn (n = 1, 2, 3, 4 and 6) clusters loaded on TiO2-V has been explored by density functional theory (DFT) calculation. The results show that different Pdn clusters have a remarkable influence on DMO formation. The Pd1/TiO2-V catalyst is not suitable for the CO catalytic coupling reaction since CO is easily bound to the O atom on the surface of TiO2-V leading to the formation of CO2. The activity of four catalysts complies with the following order of Pd4/TiO2-V > Pd6/TiO2-V > Pd2/TiO2-V > Pd3/TiO2-V by comparing the activation energy barriers of the rate-determining steps in the optimal paths. Charge analysis implies that less charge is transferred from the Pd4/TiO2-V and Pd6/TiO2-V catalysts to CO than on the other catalysts, which leads to the relatively weak adsorption of CO, and therefore CO has a greater tendency to react with other species on the surface. In addition, Pd6/TiO2-V also exhibits relatively higher selectivity toward DMO than the other three catalysts. Therefore, Pd6 is regarded as a suitable cluster, which is supported on TiO2-V demonstrating high catalytic activity and selectivity to DMO.

Potential nonadiabatic reactions: ring-opening 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives to aromatic reactive intermediates

Potential singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives were explored using density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods. Since these ring openings involve relatively high energy species that lead to relatively stable aromatic species, a good scenario for potential nonadiabatic events, we posited that the reaction paths of these ring openings might come close to or cross excited state surfaces. At the DFT level of theory, all reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths. 6-Methylidenebicyclo[3.1.0]hex-3-en-2-one and a closely related derivative (4-methylidenebicyclo[3.1.0]hex-2-en-6-one) were explored at the CASSCF level of theory; CASSCF results were qualitatively similar to DFT results and yielded spin-orbit couplings of 1.1-1.4 cm(-1) at the singlet-triplet crossing points.

Size effect of Pd clusters on hydrogen adsorption

The size effect of Pd clusters on hydrogen adsorption was investigated by density functional theory calculations. From molecular dynamics simulations, we found that the hydrogen molecules were dissociatively adsorbed on the Pd clusters; the most stable adsorption site was the hollow site. We also found that the adsorption energy increased as the size of Pd clusters decreased. These results were in good agreement with experimental findings. By analyzing the electronic structure, we found that the d-band center shifted toward the Fermi energy as the size of the Pd cluster decreased. This shift resulted in stronger hydrogen bonding.

Decomposition of anthranil. Single pulse shock-tube experiments, potential energy surfaces and multiwell transition-state calculations. The role of intersystem crossing

The thermal decomposition of anthranil diluted in argon was studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 825-1000 K and overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products: aniline and cyclopentadiene carbonitrile (accompanied by carbon monoxide) and four minor products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, pyridine, CH(2)=CHCN, HCN and CHC-CN, and comprised only a few percents of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that lead to the formation of cyclopentadiene carbonitrile and of phenylnitrene/phenylimine that are the precursors of aniline. They form aniline by reactions with traces of water impurities. To produce cyclopentadiene carbonitrile, two main processes must take place: CO elimination and ring contraction from a six- to a five-membered ring. It was shown that this can occur via two parallel pathways where CO elimination takes place prior to or following ring contraction. Singlet potential energy surfaces for all the elementary reactions that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine were obtained. Their rate constants were calculated on the basis of the results of the quantum chemical calculations using transition-state theory. A kinetic scheme containing these reactions was constructed and multiwell calculations were performed to evaluate the mole percent of the products as a function of temperature. A very serious disagreement between the experimental results and the results of calculations showed that the singlet PESs could not account for the observed experimental rates. No other singlet PESs that lead to the formation of these products could be found. In view of this observation, attempts to find pathways that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine on triplet surfaces were made. Such surfaces were found, and singlet <--> triplet intersystem crossing probabilities and crossing rate constants were calculated as well as the rate constants of all the elementary steps on the triplet surfaces. A reaction scheme was constructed and multiwell calculations were performed, including also the pathways on the singlet surfaces, to evaluate the mole percent of the products as a function of temperature. The agreement between the experimental results and these calculations was quite satisfactory.