Pressure Dependent Low Temperature Kinetics for CN + CH3CN: Competition between Chemical Reaction and van der Waals Complex Formation

International audienceThe gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CN⋯CH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CN⋯CN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed

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

A Kinetic Database For Astrochemistry (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources

Vibrational spectroscopy and density functional theory of transition-metal ion-benzene and dibenzene complexes in the gas phase

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A first principles theoretical determination of the rate constant for the dissociation of singlet ketene

State-of-the-art ab initio quantum chemical techniques have been employed in the determination of the reaction path and attendant energetics for the singlet dissociation of CH,CO. Variational RRKM calculations implementing these results provide first principles predictions for the dissociation kinetics which are in quantitative agreement with the corresponding experimental data. Rice-Ramsperger-Kassel-Marcus (RRKM) theory has been and continues to be widely used in the chemical kinetics community as a means of interpolating and extrapolating the temperature and/or pressure dependence of unimolecular reactions.' Accordingly, it is of fundamental importance to understand its limits of validity and to enhance the accuracy of its implementations. The singlet dissociation of ketene provides an excellent opportunity for a quantitative assessment of the validity of RRKM theory, or more precisely of its form which constitutes variationa transition state theory.2 Recent groundbreaking energy-resolved experimental studies of the product state distributions3-9 and of the rate constants in real time" have provided data which are among the most detailed, accurate, and wide ranging for any dissociation, thereby placing stringent limits on any RRKM modeling. Furthermore, the relatively small size of ketene facilitates accurate quantum chemical determinations of the reaction energetics. Finally, the singlet dissociation of ketene is exemplary of the class of unimolecular reactions for which there is no reverse barrier. For such reactions quantitative, nonempirical tests of the validity of RRKM theory have been very limited." An earlier theoretical study12 has demonstrated that RRKM models can simultaneously provide a quantitative description of the energy dependence of the rate constants and the product rovibrational distributions for the singlet dissociation of ketene. However, this investigation employed a simple model potential energy surface and thus, as in most RRKM studies, could not be taken as definitive evidence for the quantitative validity of the theory. Furthermore, subsequent variational RRKM calculations implementing a potential based on second-order MQller-Plesset perturbation theory (MP2)13 with a 6-31G* basis set14 yielded rate constants which were in error by as much as a factor of 4.15 Such discrepancies in the rate constant suggest corresponding errors in the product state distributions, which were not explicitly calculated, however. This communication describes a definitive, first principles RRKM calculation of the dissociation kinetics for the CH2C04'CH,+C0 reaction. Within RRKM theory, the microcanonical rate constant, k,, for dissociation at energy E and total angular momentum J, is given by' GJ k,J=-hPEJ . In this study, the evaluation of the reactant density of states, pEJ, is based on a high-level ab initio quartic force field,16 while the enumeration of the number of accessible states (Nil) at the transition state is based on a bond-length reaction coordinate variational RRKM formalism directly implementing high-level ab initio predictions for the CH2**C0 interaction potential.17 The scheme employed in the determination of the quartic force field for the reactants is described in more detail in a forthcoming publication.'6 The quadratic force field was determined at the coupled-cluster singles and doubles (CCSD)'* level of theory with a basis set nominally of quadruple-zeta quality including 2 sets of correlationoptimized polarization functions on all atoms. This basis set, which is denoted QZ(2d, 2~) here, was also augmented for various applications by higher-order polarization manifolds on all atoms to give a QZ(2d lf, 2p 1 d) set." The reference geometry employed in these CCSD/QZ(2d, 2p) computations was obtained from structural optimizations at the CCSD(T)/QZ(2d lf, 2pld) level, which includes a perturbative contribution from connected triple excitations.20 The cubic and quartic force constants were then determined via finite differences of analytic MP2lQZ(2d, 2p) second derivatives21 around the CCSD(T)IQZ(2dlf, 2p 1 d) optimized geometry. The resulting reference quartic force field was then employed in the computation of anharmonic fundamental vibrational frequencies by means of standard spectroscopic perturbation theory.z2 A fitting procedure involving very minor adjustments to the reference geometry (i.e., coordinate shifts of less than 0.007 A and 0.11") and to the quadratic part of the force field (variations typically less than 1%) then yielded agreement to within 3 cm-' between the calculated and experimental fundamentals of the isotopomers of ketene. The reactant number of available states was obtained through the direct summation over each of the vibrational quantum numbers of a step function in the total energy minus the state-dependent vibrational energy, i.e., 919

Exploring water dimer formation

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