A semiclassical model for orientation effects in electron transfer reactions

An approximate solution to the single-particle Schrödinger equation with an oblate spheroidal potential well of finite depth is presented. The electronic matrix element HBA for thermal electron transfer is calculated using these wave functions, and is compared with values of HBA obtained using the exact solution of the same Schrödinger equation. The present method yields accurate results for HBA, within the oblate spheroidal potential well model, and is useful for examining the orientational effects of the two centers on the rate of electron transfer

Theoretical kinetic estimates for the recombination of hydrogen atoms with propargyl and allyl radicals

Ab initio quantum chemical simulations are coupled with variational transition state theory in estimating rate constants for the H+C{sub 3}H{sub 3} and H+C{sub 3}H{sub 5} recombination reactions. The energy of interaction between the H atom and each of the radicals is evaluated at the CAS+1+2 level for the range of separations and relative orientations spanning the transition state region. An analytic representation of these interaction energies is then implemented in variable reaction coordinate transition state theory calculations of the high pressure limit recombination rate constant for temperatures ranging from 200 to 2000 K. For the propargyl reaction the overall addition rate is separated into contributions correlating with the initial formation of allene and propyne. These theoretical results are compared with the available experimental data as well as with corresponding theoretical estimates for the H+C{sub 2}H{sub 3} and H+C{sub 2}H{sub 5} reactions. The H+propargyl and H+allyl total recombination rates are remarkably similar, with both being greater than the H+vinyl and H+ethyl rates, due to the presence of twice as many addition channels

Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of NCNO into NC and NO

A recently described method for implementing RRKM theory for unimolecular reactions with highly flexible transition states is applied to the calculation of energy and angular momentum resolved rate constants and rotational–vibrational energy distributions for the reaction NCNO-->h nu NCNO*-->NCNO(vib. hot)-->NC+NO. The dissociation rate results are compared to the recent experimental results of Khundkar et al., and the vibrational and rotational distribution results are compared to the experimental values of Nadler et al. Comparison is also made with phase space theory calculations. The calculated rotational distributions at energies below the vibrational threshold of the products are the same as those of PST. At energies (2348, 2875 cm^−1) above this threshold energy the rovibrational distribution is in better agreement with the data than is that of PST. The need for obtaining more accurate ab initio potential energy surfaces is noted, particularly for treating reactions at still higher energies

Kinetics of 1-butyl and 2-butyl radical reactions with molecular oxygen : Experiment and theory

The reaction of O-2 with butyl radicals is a key early step in the oxidation of n-butane, which is a prototypical alkane fuel with combustion properties that mimic those of many larger alkanes. Current combustion mechanisms employ kinetic descriptions for such radical oxidations that are based on fairly limited information. The present work employs a combination of experiment and theory to probe the kinetics of O-2 reacting with both 1- and 2-butyl radicals. The experiments employ laser photolysis to generate butyl radicals and thereby initiate the reaction kinetics. Photoionization mass spectrometric observations of the time-dependent butyl radical concentration yield rate coefficients for the overall reaction. The experiments cover temperatures ranging from 200 to 500 K and He bath gas pressures ranging from 0.3 to 6 Torr. Ab initio transition state theory (TST) based master equation calculations are used to predict the kinetics over a broad range of conditions. The calculations consider both the barrierless R + O-2 entrance channel, treated with direct CASPT2 variable reaction coordinate TST, and the decomposition of the RO2 complex to HO2 + alkenes, treated with CCSD(T)/CBS based TST. Theory and experiment are in good agreement, with maximum discrepancies of about 30%, suggesting the appropriateness of the theory based predictions for conditions of greater relevance to combustion. The kinetic description arising from this work should be of considerable utility to combustion modeling of n-butane, as well as of other related saturated hydrocarbons. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

Photodissociation transition states characterized by chirped pulse millimeter wave spectroscopy.

The 193-nm photolysis of CH2CHCN illustrates the capability of chirped-pulse Fourier transform millimeter-wave spectroscopy to characterize transition states. We investigate the HCN, HNC photofragments in highly excited vibrational states using both frequency and intensity information. Measured relative intensities of J = 1-0 rotational transition lines yield vibrational-level population distributions (VPD). These VPDs encode the properties of the parent molecule transition state at which the fragment molecule was born. A Poisson distribution formalism, based on the generalized Franck-Condon principle, is proposed as a framework for extracting information about the transition-state structure from the observed VPD. We employ the isotopologue CH2CDCN to disentangle the unimolecular 3-center DCN elimination mechanism from other pathways to HCN. Our experimental results reveal a previously unknown transition state that we tentatively associate with the HCN eliminated via a secondary, bimolecular reaction

Review of important reactions for the nitrogen chemistry in the interstellar medium

Predictions of astrochemical models depend strongly on the reaction rate coefficients used in the simulations. We reviewed a number of key reactions for the chemistry of nitrogen-bearing species in the dense interstellar medium and proposed new reaction rate coefficients for those reactions. The details of the reviews are given in the form of a datasheet associated with each reaction. The new recommended rate coefficients are given with an uncertainty and a temperature range of validity and will be included in KIDA (http://kida.obs.u-bordeaux1.fr).Comment: 39 pages, not published in refereed journal, datasheets are given in KID

State-of-the-Art Quantum Chemistry Meets Variable Reaction Coordinate Transition State Theory to Solve the Puzzling Case of the H2S + Cl System

The atmospheric reaction of H$_2$S with Cl has been reinvestigated to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core-valence correlation, spin-orbit, and scalar relativistic effects as well as diagonal Born-Oppenheimer corrections. This highly accurate composite scheme has also been paralleled by less expensive yet promising computational approaches. Moving to kinetics, variational transition state theory and its variable reaction coordinate extension for barrierless steps have been exploited, thus obtaining a reaction rate constant (8.16 x 10$^{-11}$ cm$^3$ molecule$^{-1}$ s$^{-1}$ at 300 K and 1 atm) in remarkable agreement with the experimental counterpart. Therefore, contrary to previous claims, there is no need to invoke any failure of the transition state theory, provided that sufficiently accurate quantum-chemical computations are performed. The investigation of the puzzling case of the H$_2$S + Cl system allowed us to present a robust approach for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest.Comment: 49 pages, 7 figures, published online in JCT