Analysing oxygen reduction electrocatalysis on transition metal doped niobium oxide(110).

A good oxygen reduction reaction (ORR) catalyst should be stable and active under electrochemical reaction conditions. Niobium pentaoxide (Nb2O5) is known to be stable under ORR conditions. However it has a large band gap, which makes conductivity a challenge during the reaction. In this work, we aim to understand if surface modification of the 110 facet of niobium pentaoxide with transition metal doping has any effect on its ORR activity and conductivity. While the problem of conductivity in the case of transition metal oxides (TMOs) can be partially rectified by transition metal doping, it has negligible influence on the ORR activity of the doped systems.

Opportunities and Challenges in Electrolytic Propylene Epoxidation.

Propylene oxide (PO) is an important chemical. So far, its synthesis protocol relies on expensive oxidants. In contrast, direct epoxidation of propylene (DEP) using molecular oxygen is considered ideal for PO synthesis. Unfortunately, DEP has not met industrial demands due to the low propylene conversion and high side-product selectivity for known catalysts. Instead of a thermal process using molecular oxygen, electrolytic propylene oxidation can synthesize PO at room temperature, using the atomic oxygen generated from water-splitting. Herein, using density functional theory, surface Pourbaix analysis, scaling relation analysis, and microkinetic modeling, we show that (i) propylene epoxidation is facile on weak-binding catalysts if reactive atomic oxygen preexists; (ii) electrolytic epoxidation is facile to provide atomic oxygen for epoxidation, while hydroperoxyl formation does not overwhelm the epoxidation process at the potential of interest; (iii) propylene dehydrogenation is a competing step that forms side products. Finally, we discuss the opportunities and challenges of this green PO synthesis method.

Electrochemical oxidation of molecular nitrogen to nitric acid - towards a molecular level understanding of the challenges.

Nitric acid is manufactured by oxidizing ammonia where the ammonia comes from an energy demanding and non-eco-friendly, Haber-Bosch process. Electrochemical oxidation of N2 to nitric acid using renewable electricity could be a promising alternative to bypass the ammonia route. In this work, we discuss the plausible reaction mechanisms of electrochemical N2 oxidation (N2OR) at the molecular level and its competition with the parasitic oxygen evolution reaction (OER). We suggest the design strategies for N2 oxidation electro-catalysts by first comparing the performance of two catalysts - TiO2(110) (poor OER catalyst) and IrO2(110) (good OER catalyst), towards dinitrogen oxidation and then establish trends/scaling relations to correlate OER and N2OR activities. The challenges associated with electrochemical N2OR are highlighted.

Scaling Relationships and Volcano Plots in Homogeneous Catalysis.

Scaling relations and volcano plots are widely used in heterogeneous catalysis. In this Perspective, we discuss the prospects and challenges associated with the application of similar concepts in homogeneous catalysis using examples from the literature that have appeared recently.

Is silver a mere terminal oxidant in palladium catalyzed C-H bond activation reactions?

In the contemporary practice of palladium catalysis, a molecular understanding of the role of vital additives used in such reactions continues to remain rather vague. Herein, we disclose an intriguing and a potentially general role for one of the most commonly used silver salt additives, discovered through rigorous computational investigations on four diverse Pd-catalyzed C-H bond activation reactions involving sp2 aryl C-H bonds. The catalytic pathways of different reactions such as phosphorylation, arylation, alkynylation, and oxidative cycloaddition are analyzed, with and without the explicit inclusion of the silver additive in the respective transition states and intermediates. Our results indicate that the pivotal role of silver salts is likely to manifest in the form of a Pd-Ag heterobimetallic species that facilitates intermetallic electronic communication. The Pd-Ag interaction is found to provide a consistently lower energetic span as compared to an analogous pathway devoid of such interaction. Identification of a lower energy pathway as well as enhanced catalytic efficiency due to Pd-Ag interaction could have broad practical implications in the mechanism of transition metal catalysis and the current perceptions on the same.

Hydrogen bond-aromaticity cooperativity in self-assembling 4-pyridone chains.

Self-assembling building blocks like the 4-pyridone can exhibit extraordinary H-bond-aromaticity coupling effects. Computed dissected nucleus independent chemical shifts (NICS(1)zz), natural bond orbital (NBO) charges, and energy decomposition analyses (EDA) for a series of hydrogen (H-) bonded 4-pyridone chains (4-py)n (n = 2 to 8) reveal that H-bonding interactions can polarize the 4-pyridone exocyclic C=O bonds and increase 4n+2 π-electron delocalization in the six-membered ring. The resulting H-bonded 4-pyridone units display enhanced π-aromatic character (both magnetically and energetically) and their corresponding N-H···O=C interactions are strengthened. These π-electron polarization effects do not depend on the relative orientations (co-planar or perpendicular) of the neighboring 4-pyridone units, but increase with the number of H-bonded units.

Retrospective investigation of the effects and efficiency of self-ligating and conventional brackets.

INTRODUCTION: The purpose of this retrospective cohort study was to assess the effects and efficiency of self-ligating brackets compared with conventional brackets. A secondary purpose was to identify the pretreatment factors associated with the choice of self-ligating or conventional brackets. METHODS: The subjects were treated by 2 private practitioners who used both self-ligating and conventional brackets in their practices. The self-ligating subjects were consecutively identified (treatment completed between January 2011 and April 2012), and then an age- and sex-matched control group was chosen from the same office. The outcome measures were changes in arch dimensions, changes in mandibular incisor inclinations, final peer assessment rating (PAR) scores, percentages of PAR reduction, overall treatment times, total number of visits, and number of emergency visits. All cast and cephalometric measurements were performed on digital records in a blinded manner. Two calibrated assessors measured the PAR scores. RESULTS: The final sample comprised 74 patients from clinician 1 and 34 patients from clinician 2. The practitioners had significant differences for several treatment parameters; therefore, the data from the 2 clinicians were analyzed separately. For clinician 1, no significant differences were observed between the self-ligating and conventional groups, other than increased arch length in the self-ligating group. The self-ligation patients treated by clinician 2 demonstrated significant increases in transverse dimensions, lower percentages of reduction in PAR scores, shorter treatment times, fewer visits, and more wire-sliding emergencies than the conventional bracket group. CONCLUSIONS: Although some significant findings were observed, the small sample and the lack of consistent findings between the 2 clinicians made it difficult to draw strong conclusions.

Non-innocent additives in a palladium(II)-catalyzed C-H bond activation reaction: insights into multimetallic active catalysts.

The role of a widely employed additive (AgOAc) in a palladium acetate-catalyzed ortho-C-H bond activation reaction has been examined using the M06 density functional theory. A new hetero-bimetallic Pd-(µ-OAc)3-Ag is identified as the most likely active species. This finding could have far-reaching implications with respect to the notion of the active species in palladium catalysis in the presence of other metal salt additives.

Microsolvated transition state models for improved insight into chemical properties and reaction mechanisms.

Over the years, several methods have been developed to effectively represent the chemical behavior of solutes in solvents. The environmental effects arising due to solvation can generally be achieved either through inclusion of discrete solvent molecules or by inscribing into a cavity in a homogeneous and continuum dielectric medium. In both these approaches of computational origin, the perturbations on the solute induced by the surrounding solvent are at the focus of the problem. While the rigor and method of inclusion of solvent effects vary, such solvation models have found widespread applications, as evident from modern chemical literature. A hybrid method, commonly referred to as cluster-continuum model (CCM), brings together the key advantages of discrete and continuum models. In this perspective, we intend to highlight the latent potential of CCM toward obtaining accurate estimates on a number of properties as well as reactions of contemporary significance. The objective has generally been achieved by choosing illustrative examples from the literature, besides expending efforts to bring out the complementary advantages of CCM as compared to continuum or discrete solvation models. The majority of examples emanate from the prevalent applications of CCM to organic reactions, although a handful of interesting organometallic reactions have also been discussed. In addition, increasingly accurate computations of properties like pK(a) and solvation of ions obtained using the CCM protocol are also presented.

Mechanism of cooperative catalysis in a Lewis acid promoted nickel-catalyzed dual C-H activation reaction.

The mechanism of cooperativity offered by AlMe(3) in a Ni-catalyzed dehydrogenative cycloaddition between substituted formamides and an alkyne is investigated by using DFT(SMD(toluene)/M06/6-31G**) methods. The preferred pathway is identified to involve dual C-H activation, with first a higher barrier formyl C(sp(2))-H oxidative insertion followed by benzylic methyl C(sp(3))-H activation. The cooperativity is traced to be of kinetic origin as evidenced by stabilized transition states when AlMe(3) is bound to the formyl group, particularly in the oxidative insertion step.

Palladium(II)-catalyzed direct alkoxylation of arenes: evidence for solvent-assisted concerted metalation deprotonation.

Density functional theory investigations on the mechanism of palladium acetate catalyzed direct alkoxylation of N-methoxybenzamide in methanol reveal that the key steps involve solvent-assisted N-H as well as C-H bond activations. The transition state for the critical palladium-carbon bond formation through a concerted metalation deprotonation (CMD) process leading to a palladacycle intermediate has been found to be more stable in the methanol-assisted pathway as compared to an unassisted route.