Topic
Elementary reaction
About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.
Papers published on a yearly basis
Papers
More filters
••
TL;DR: In this article, the authors proposed a coherent control approach to control the branching ratio of a reaction by modifying the amplitude and phase of the laser pulses, which can be seen as an analog of Young's two-slit experiment.
Abstract: The traditional goals of chemical kinetics are to measure the rates of chemical reactions and to understand their mechanisms on a molecular level. With the advent of lasers and molecular beams, it has become possible to study the reactions of molecules in individual quantum states and to explain their behavior in terms of single collisions governed by well-defined potential energy surfaces. Likewise, numerical methods have been refined to the point that it is possible to predict diverse properties of elementary reactions from first principles. In recent years, the challenge has shifted from measuring and calculating rates of reactions to devising methods of controlling their outcome. The idea of controlling the yield and product distribution of a reaction is, of course, a very old one. The field of catalysis, for example, is devoted to finding means of enhancing the natural yield of a reaction. Similarly, temperature and pressure have been used for decades in the chemical industry to alter reaction rates. With the development of narrow-band lasers, it has become possible to excite selectively a single molecular mode. Provided energy transfer is slow as compared to reaction, such mode-selective preparation of a molecule can be used to alter the outcome of its reaction.1-3 For example, if the OH bond in HOD is vibrationally excited, that bond becomes more reactive, and collisions with Cl atoms preferentially yield HCl rather than DCl.4 A more general, photochemical method for altering reaction pathways exploits the phase property of lasers and has become known as coherent control, or phase control.5 Two main schemes of coherently controlling physical and chemical processes have been developed over the past decade, based on similar concepts but differing in the properties of the electromagnetic fields that are used. The first, introduced by Tannor and Rice,6 uses an ultrashort pulse of laser light to create a coherent superposition of energy-resolved eigenstates, namely, a wave packet. Such carefully phased superposition states are, in general, nonstationary. By altering the amplitudes and phases of the light pulses, it is possible to modify the content of the superposition state and thus control the motion of the wave packet. Typically, a vibrational or electronic wave packet is generated at time t ) 0 by a short laser pulse. At some later time, when the wave packet has evolved to a desired configuration, a second pulse triggers the reaction. An experimental example of this method is control of the electronic branching ratio of Na atoms in the photodissociation of NaI, which was achieved by varying the timing between two transform-limited ultrashort pulses.7 Optimal control theory, introduced by Rabitz and co-workers8 and subsequently developed by several groups,5 employs a feedback loop to optimize the spectral content and temporal shape of a pulse in order to maximize the yield of a given product. Experimental demonstrations of the control of branching ratios by means of optimally tailored laser pulses were reported by Bardeen et al.9 and by Assion et al.10 The use of the intensity property of short pulses to control reactions was also reported.11 A second method was introduced by Brumer and Shapiro12 and is the subject of the present paper. In this approach, two long laser pulses (in principle, they could be continuous beams) excite an atom or a molecule from an initial state to a final state. The frequencies of the lasers are chosen so that the target absorbs either m photons from the first laser or n photons from the second laser to reach the same final state.13 That is, the laser frequencies satisfy the relation mωm ) nωn. An important property of the laser beams is that they have a well-defined phase relation; that is, the phases of the two electromagnetic fields differ by a controllable amount, φ. As we will show, by varying the relative phase of the two beams, it is possible to control the branching ratio of the reaction. It is useful to think of the latter method as an analog of Young’s two-slit experiment.14 In that experiment, particles emerging from two slits create a pattern on a screen. If only one slit is open, the result is a diffraction pattern produced by that slit. If both slits are open, Robert J. Gordon obtained his doctorate from Dudley Herschbach at Harvard University in 1970. After postdoctoral studies at Caltech and the Naval Research Laboratory, he came to the University of Illinois at Chicago, where he is a professor of chemistry. Prof. Gordon’s research interests include experimental studies of the spectroscopy and reaction dynamics of small molecules.
96 citations
••
TL;DR: In this paper, a theoretical study of the gas-phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations is presented.
Abstract: This work reports a theoretical study of the gas-phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data ( , S°, ) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition-state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed us to validate the theoretical ...
96 citations
••
TL;DR: In this paper, the authors present reaction pathways and kinetics for cyclopentadienyl radical association with H, OH, HO2, O, and O2 in the temperature range 900−1300 K and atmospheric pressure.
Abstract: Reaction pathways and kinetics for cyclopentadienyl radical association with H, OH, HO2, O, and O2 are presented in the temperature range 900−1300 K and atmospheric pressure. Thermochemical data for reactants, intermediate, and product species are evaluated from literature data and from group additivity with hydrogen bond increments. High-pressure limit rate constants for the radical combination reactions and decomposition of the energized adducts are estimated. Pressure-dependent rate constants for each channel in the reaction systems are calculated using bimolecular quantum Rice Ramsperger Kassel, QRRK, for k(E) with a modified strong collision approach for falloff. A submechanism of important cyclopentadienyl radical reactions is assembled and tested in an elementary reaction model for combustion of benzene, where the cyclopentadienyl radical is a key intermediate in the stepwise (C6 → C5 → C4) decomposition. Modeling results are compared with limited literature data on species profiles for appropriate...
96 citations
••
TL;DR: The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained and it is confirmed that the C-O and C-H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and Hydroxymethyl and atom hydrogen, respectively, have higher energy barriers.
Abstract: The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory−generalized gradient approximation (DFT−GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O−H bond breaking in CH3OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH3O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C−O and C−H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.
95 citations
••
TL;DR: A kinetic model for hydrocracking of an industrial feedstock, fully incorporating the carbenium ion chemistry, was developed in this paper, where individual hydrocarbons in the reaction network were relumped into 8 lumps per carbon number: n-alkane, mono-, di- and tri-branched alkanes, mono, di-, tri-and tetraring cycloalkanes.
Abstract: A kinetic model for hydrocracking of an industrial feedstock, fully incorporating the carbenium ion chemistry, was developed. Individual hydrocarbons in the reaction network were relumped into 8 lumps per carbon number: n-alkane, mono-, di- and tri-branched alkanes, mono-, di-, tri- and tetraring cycloalkanes. The rate coefficient of a reaction in the relumped network resulted from the product of the rate coefficients of the elementary reaction steps with lumping coefficients. The former were obtained from regressions on gas-phase hydrocracking data of model components. Lumping coefficients were calculated based on the assignment of all (cyclo)alkanes and the corresponding carbenium ions to structural classes comprising species with identical thermodynamic properties. The simulation of an industrial reactor with vacuum gas-oil feed revealed the relative unimportance of transfer limitations for hydrocracking of saturated hydrocarbons and the model's ability to detail the influence of process conditions on the product composition.
95 citations