Elementary reaction

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

Sequential clustering reactions of Si+ with silane: A theoretical study of the reaction mechanisms

Abstract: The mechanisms of the sequential clustering reactions of Si+ with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including the effects of electron correlation and zero‐point corrections. In accordance with the experimental results of Mandich, Reents, and Jarrold, the reactions lead to the formation of Si2H+2, SiH+4, Si4H+6, and finally Si5H+10. The study of the detailed reaction pathways including the necessary transition structures leads to the assignment of specific isomeric products in each reaction step. The specific isomers assigned are H2Si–Si+, H3Si–SiH–Si+, (H3Si)2Si–Si+, and (H3Si)3Si–SiH+. The bottleneck in the reaction sequence is due to the formation of the branched product Si5H+10 where elimination of H2 is not easily possible due to the saturation of the bonding involving the central silicon atom. Isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism. Quantitative comparisons are made be...

Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

Abstract: Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH3 → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a) , (2b) , (2c) , (3a) , (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k1(He) = (0.8 ± 0.4) × 10−10exp(−(66.0 ± 3.4) kJ mol−1/RT) cm3 molecule−1 s−1. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10−18 cm2 molecule−1 at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a) , (2b) , (2c) , (3a) , (3b) are: k2 = (3.6 ± 0.8) × 10−11 cm3 molecule−1 s−1 (298 K); k3 = (9.3 ± 2.3) × 10−11 cm3 molecule−1 s−1(298 and 588 K).

Kinetic consequences of the principle of microscopic reversibility

Abstract: The principle of microscopic reversibility was formulated for elementary reactions at equilibrium. It follows from the principle that for any system at equilibrium, and for any elementary reaction whether at equilibrium or not, the favoured reaction path in one direction must be the reverse of that in the opposite direction, and that the ratio of rate constants is the equilibrium constant. The same is true for non-chain reactions occurring under steady-state conditions provided that the alternative paths are equivalent as far as kinetic order is concerned. For reactions occurring under non-steady-state conditions, for chain reactions even in the steady state, and for non-chain reactions in which the alternative paths are not kinetically equivalent, the preferred reaction path in one direction may be different from that in the other; in such cases the ratio of the overall rate constants is not equal to the equilibrium constant. The principle of microscopic reversibility can never be used to prove the mechanism of a reaction when that for the reverse reaction has been established; it can only be used to eliminate kinetically equivalent mechanisms which are not the exact reverse of that for the reaction in the opposite direction.

Accurate rate constants for decomposition of aqueous nitrous acid.

Abstract: Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ⇆ NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined.

The reaction of 2,5-dimethylfuran with hydrogen atoms – An experimental and theoretical study

Abstract: The kinetics of the reaction of hydrogen atoms with 2,5-dimethylfuran (25DMF), a promising liquid transport biofuel, was experimentally studied in a shock tube at temperatures between 970 and 1240 K and pressures of 1.6 and 4.8 bar. The hydrogen atoms were produced by pyrolysis of ethyl iodide and monitored by atom resonance absorption spectrometry. From the hydrogen atom concentration–time profiles, overall rate coefficients for the reaction H + 25DMF → products (R1) were inferred. The results can be expressed by the Arrhenius equation k 1 = 4.4 × 10 −11 exp(−1180 K/ T ) cm −3 s −1 with an estimated uncertainty of ±30%. A significant pressure dependence was not observed. The results were analyzed in terms of statistical rate theory with molecular and transition state data from quantum chemical calculations. Three different compound methods were used to characterize the potential energy surface: CBS-QB3, CBS-APNO, and G3. It is found that reaction (R1) mainly (>75%) proceeds via an addition–elimination mechanism to yield 2-methylfuran + CH 3 . Kinetic parameters for the most important competing channels of the net reaction (R1) were calculated.