Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

The reaction of CH3 + O2: experimental determination of the rate coefficients for the product channels at high temperatures

Abstract: The reaction of methyl radicals (CH 3 ) with molecular oxygen (O 2 ) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k 1 = k 1a + k 1b , and individual rate coefficients for the two high-temperature product channels, (1a) producing CH 3 O + O and (1b) producing CH 2 O + OH, were determined using ultra-lean mixtures of CH 3 I and O 2 in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k 1 but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a) . Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590–2430 K, k 1a = 6.08 × 10 7 T 1.54 exp (−14005/ T ) cm 3 mol −1 s −1 and k 1b = 68.6 T 2.86 exp (−4916/ T ) cm 3 mol −1 s −1 . The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k 1a and k 1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range.

The Proteinase-Binding Reaction of α2M

Abstract: First identified as a proteolytic enzyme inhibitor, a2-macroglobulin (azM) has assumed new importance because of the nature of its cellular receptor and its potential role as a transport protein for growth factors and other agents. Most of the receptor interactions and transport functions of a2M, however, depend on prior reaction with proteinases, and an understanding of proteinase binding remains at the heart of azM physiology. The general features of the proteinase reaction are understood, but a complete description has so far been elusive-a general rate equation has not been obtained and, in some cases, the number and the nature of the intermediates are unknown. The mechanism of binding of proteinases to azM is unusual; the active site of the proteinase remains free and, in this way, the mechanism is very different from that of the classical proteinase inhibitors such as the serpins’” or the plant and pancreatic inhibitors! However, the small protein inhibitors and a2M have one feature in common: they contain a reactive site, referred to as the “bait” region in azM, with a susceptible peptide bond that provides a target for the proteinase. In the case of the serpins or plant inhibitors, the reaction at this site stops before proteolysis is complete. A stable complex is formed that, in most cases, is a distorted tetrahedral inte~mediate.~ For azM, on the other hand, bait region cleavage is successful and the active site of the bound proteinase remains free. Proteinase binding occurs through amide bonds between its lysyl amino groups and a glutamyl residue of the unique thioester bond of a2M (equation 1) and through conformational changes that sterically encapsulate (“trap”) the proteinase. Although free to react with small substrates and inhibitors, the active site of the bound proteinase is prevented by the steric constraints of the trap from reaction with large protein substrates or protein inhibitors:

Surface Science Investigations of Oxidative Chemistry on Gold

Abstract: Because of gold's resistance to oxidation and corrosion, historically chemists have considered this metal inert. However, decades ago, researchers discovered that highly dispersed gold particles on metal oxides are highly chemically active, particularly in low-temperature CO oxidations. These seminal findings spurred considerable interest in investigations and applications of gold-based materials. Since the discovery of gold's chemical activity at the nanoscale, researchers found that bulk gold also has interesting catalytic properties. Thus, it is important to understand and contrast the intrinsic chemical properties of bulk gold with those of nanoparticle Au. Despite numerous studies, the structure and active site of supported Au nanoclusters and the active oxygen species remain elusive, and model studies under well-controlled conditions could help identify these species. The {111} facet has the lowest surface energy and is the most stable and prevalent configuration of most supported gold nanoparticles. Therefore, a molecular-level understanding of the physical properties and surface chemistry of Au(111) could provide mechanistic details regarding the nature of Au-based catalysts and lead to improved catalytic processes. This Account focuses on our current understanding of oxidative chemistry on well-defined gold single crystals, predominantly from recent investigations on Au(111) that we have performed using modern surface science techniques. Our model system strategy allows us to control reaction conditions, which assists in the identification of reaction intermediates, the determination of the elementary reaction steps, and the evaluation of reaction energetics for rate-limiting steps. We have employed temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), and Auger electron spectroscopy (AES) to evaluate surface oxidative chemistry. In some cases, we have combined these results with density functional theory (DFT) calculations. By controlling the reaction parameters that determine product selectivity, we have examined the chemical properties of bulk gold. Based on our investigations, the surface-bound oxygen atoms are metastable at low temperature. We also demonstrate that the oxygen atoms and formed hydroxyls are responsible for some of the distinct chemical behavior of gold and participate in surface reactions either as a Bronsted base or a nucleophilic base. We observe similar reaction patterns on gold surfaces to those on copper and silver surfaces, suggesting that the acid-base reactions that have been observed on copper and silver may also occur on gold. Our model chemical studies on gold surfaces have provided intrinsic fundamental insights into high surface area gold-based catalysts and the origin of the reactive oxygen species.

Nonclassical kinetics in three dimensions: Simulations of elementary A+B and A+A reactions.

Abstract: Monte Carlo simulations are employed to study the rate laws of A+A\ensuremath{\rightarrow}0 and A+B\ensuremath{\rightarrow}0 diffusion-limited elementary reactions in three dimensions (3D). Using reflective instead of cyclic boundary conditions we do observe the Zeldovich regime in 3D for the A+B reaction. The time and density for the crossover into the Zeldovich regime in 3D agree with the existing scaling laws and provide the hitherto missing scaling coefficient. We show that the behavior of the A+A reaction rates in 1D, 2D, and 3D and the early time behavior of the A+B reaction rate map the rate of distinct sites visited by a single random walker, giving nonclassical kinetics at early times in all cases. We also determine a simple scaling law for crossover to finite size effects, which depends only on the linear lattice length except when the crossover to finite size effects and the crossover to the Zeldovich regime are concomitant. \textcopyright{} 1996 The American Physical Society.