Gas-phase reactivity of CH3OH toward OH at interstellar temperatures (11.7-177.5 K): experimental and theoretical study

The reactivity of methanol (CH3OH) toward the hydroxyl (OH) radical was investigated in the temperature range 11.7-177.5 K using the CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique. In the present study, the temperature dependence of the rate coefficient for the OH + CH3OH reaction, k(T), has been revisited and additional experimental and computational data are reported. New kinetic measurements were performed to fill the existing gaps (<22 K, 22-42 K and 88-123 K), reporting k(T < 20 K) for the first time. The lowest temperature ever achieved by a pulsed CRESU has been obtained in this work (11.7 K). k(T) abruptly increases by almost 2 orders of magnitude from 177.5 K to around 100 K. At T < 100 K, this increase is less pronounced, reaching the capture limit at temperatures below 22 K. The pressure dependence of k(T) has been investigated for selected temperatures and gas densities (1.5 × 1016 to 4.3 × 1017 cm-3), combining our results with those previously reported. No dependence was observed within the experimental uncertainties below 110 K. The high- and low-pressure rate coefficients, kHPL(T) and kLPL(T), were also studied in detail using high-level quantum chemical and theoretical kinetic methodologies, closely reproducing the experimental data between 20 and 400 K. The results suggest that the experimental data are near the high pressure limit at the lowest temperatures, but that the reaction remains a fast and effective source of CH2OH and CH3O at the low pressures and temperatures prevalent in the interstellar medium.

A Review on Reactions of Polycyclic Aromatic Hydrocarbons with the Most Abundant Atmospheric Chemical Fragments: Theoretical and Experimental Data

Aerosols are ubiquitous in the atmosphere and have strong effects on climate and public health due to the importance of reactions of polycyclic aromatic hydrocarbon (PAH) compounds in air. Over the last decade, study of the reactions of PAHs and their derivatives in the atmosphere has become a key topic to find an effective way to decrease the impact of this spontaneous reaction and so reduce air pollution. This article aims to pool the majority of research on the reactions of PAHs with atmospheric agents such as oxygen, hydrogen and ozone and compare the theoretical and experimental results. In examining theoretical research, the number of aromatic rings is very important in calculating the rate constants and determining the main pathway of the reaction. So, while there are weak theoretical data, several papers issued in this field have concurred with key experimental results. For reactants with more than six aromatic rings, small basis sets have good conformity with experimental outcomes. Due to the abundance of OH in the atmosphere, much research has been done to find the best reaction pathway and calculate the associated rate constants experimentally and theoretically. In future, the opportunity exists for new researchers to detect the main intermediates, most important pathways, rate constants and the products of reactions with more than six aromatic rings and to detect PAHs in a dense atmosphere. Product identification will help to reduce air pollution.

An Exprimental and Computational Study on the Cl Atom Initiated Photo-Oxidization Reactions of Butenes in the Gas Phase

Temperature-dependent rate coefficients for the reactions of Cl atoms with trans-2-butene and isobutene were measured over the temperature range of 263-363 K using relative rate technique with reference to 1,3-butadiene, isoprene, and 1-pentene. The measured rate coefficients for the reactions of Cl atoms with isobutene and trans-2-butene are k_R1^298K= (3.43 ± 0.11) × 10^-10 and k_R2^298K = (3.20 ± 0.04) × 10^-10 cm³ molecule^-1 s^-1, respectively, at 298 K and 760 torr. Measured rate coefficients were used to fit the Arrhenius equations, which are obtained to be k_R1-Exp^269-363K = (4.99 ± 0.42) × 10^-11 exp[(584 ± 26)/T] and k_R2-Exp^269-363K = (1.11 ± 0.3) × 10^-10 exp[(291 ± 88)/T] cm³ molecule^-1 s^-1 for isobutene and trans-2-butene, respectively. To understand the reaction mechanism, estimate the contribution of each reaction site, and to complement our experimental results, computational studies were also performed. Canonical variational transition state theory with small curvature tunneling in combination with MP2/6-31G(d), MP2/6-31G(d,p), MP2/6-31+G(d,p), CCSD(T)/cc-pvdz, and QCISD(T)/cc-pvdz level of theories were used to calculate the temperature-dependent rate coefficients over the temperature range of 200-400 K. The effective lifetimes, thermodynamic parameters, and atmospheric implications of the test molecules were also estimated.

Development of a pulsed uniform supersonic gas expansion system based on an aerodynamic chopper for gas phase reaction kinetic studies at ultra-low temperatures

A detailed description of a new pulsed supersonic uniform gas expansion system is presented together with the experimental validation of the setup by applying the CRESU (French acronym for Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique to the gas-phase reaction of OH radicals with 1-butene at ca. 23 K and 0.63 millibars of helium (carrier gas). The carrier gas flow, containing negligible mixing ratios of OH-precursor and 1-butene, is expanded from a high pressure reservoir (337 millibars) to a low pressure region (0.63 millibars) through a convergent-divergent nozzle (Laval type). The novelty of this experimental setup is that the uniform supersonic flow is pulsed by means of a Teflon-coated aerodynamic chopper provided with two symmetrical apertures. Under these operational conditions, the designed Laval nozzle achieves a temperature of (22.4 ± 1.4) K in the gas jet. The spatial characterization of the temperature and the total gas density within the pulsed uniform supersonic flow has also been performed by both aerodynamical and spectroscopic methods. The gas consumption with this technique is considerably reduced with respect to a continuous CRESU system. The kinetics of the OH+1-butene reaction was investigated by the pulsed laser photolysis/laser induced fluorescence technique. The rotation speed of the disk is temporally synchronized with the exit of the photolysis and the probe lasers. The rate coefficient (k(OH)) for the reaction under investigation was then obtained and compared with the only available data at this temperature.

Ab initio kinetics for the decomposition of hydroxybutyl and butoxy radicals of n-butanol

The decomposition kinetics of the hydroxybutyl and butoxy radicals (C4H9O) arising via H abstraction from n-butanol were studied theoretically with ab initio transition-state-theory-based master equation analyses. Stationary points on the C4H9O potential energy surface were calculated at either the RQCISD(T)/CBS//B3LYP/6-311++G(d,p) level or the RQCISD(T)/CBS//CASPT2/aug-cc-pVDZ level. Unimolecular pressure- and temperature-dependent rate coefficients were calculated over broad ranges of temperature (300-2500 K) and pressure (1.3 × 10(-3) to 10(2) atm) by solving the time-dependent multiple-well master equation. The "well merging" phenomenon was observed and analyzed for its influence on the branching ratios and rate coefficients. The theoretical predictions were compared with the available experimental and theoretical data and any discrepancies were analyzed. The predicted rate coefficients are represented with forms that may readily be used in combustion modeling of n-butanol.

A two transition state model for radical-molecule reactions: a case study of the addition of OH to C2H4

A two transition state model is applied to the study of the addition of hydroxyl radical to ethylene. This reaction serves as a prototypical example of a radical-molecule reaction with a negative activation energy in the high-pressure limit. The model incorporates variational treatments of both inner and outer transition states. The outer transition state is treated with a recently derived long-range transition state theory approach focusing on the longest-ranged term in the potential. High-level quantum chemical estimates are incorporated in a variational transition state theory treatment of the inner transition state. Anharmonic effects in the inner transition state region are explored with direct phase space integration. A two-dimensional master equation is employed in treating the pressure dependence of the addition process. An accurate treatment of the two separate transition state regions at the energy and angular momentum resolved level is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is predicted to occur at about 130 K, with significant effects from both transition states over the 10 to 400 K temperature range. Modest adjustment in the ab initio predicted inner saddle point energy yields theoretical predictions which are in quantitative agreement with the available experimental observations. The theoretically predicted capture rate is reproduced to within 10% by the expression [4.93 x 10(-12) (T/298)(-2.488) exp(-107.9/RT) + 3.33 x 10(-12) (T/298)(0.451) exp(117.6/RT); with R = 1.987 and T in K] cm3 molecules(-1) s(-1) over the 10-600 K range.

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.

A Two Transition State Model for Radical−Molecule Reactions: Applications to Isomeric Branching in the OH−Isoprene Reaction

A two transition state model is applied to the prediction of the isomeric branching in the addition of hydroxyl radical to isoprene. The outer transition state is treated with phase space theory fitted to long-range transition state theory calculations on an electrostatic potential energy surface. High-level quantum chemical estimates are applied to the treatment of the inner transition state. A one-dimensional master equation based on an analytic reduction from two-dimensions for a particular statistical assumption about the rotational part of the energy transfer kernel is employed in the calculation of the pressure dependence of the addition process. We find that an accurate treatment of the two separate transition state regions, at the energy and angular momentum resolved level, is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is shown to occur at about 275 K, with significant effects from both transition states over the 30-500 K temperature range. Modest adjustments in the ab initio predicted inner saddle point energies yield predictions that are in quantitative agreement with the available high-pressure limit experimental observations and qualitative agreement with those in the falloff regime. The theoretically predicted capture rate is reproduced to within 10% by the expression [1.71 x 10(-10)(T/298)(-2.58) exp(-608.6/RT) + 5.47 x 10(-11)(T/298)-1.78 exp(-97.3/RT); with R = 1.987 and T in K] cm3 molecule(-1) s(-1) over the 30-500 K range. A 300 K branching ratio of 0.67:0.02:0.02:0.29 was determined for formation of the four possible OH-isoprene adduct isomers 1, 2, 3, and 4, respectively, and was found to be relatively insensitive to temperature. An Arrhenius activation energy of -0.77 kcal/mol was determined for the high-pressure addition rate constants around 300 K.

Strange Kinetics of the C(2)H(6) + CN Reaction Explained

In this paper, we employ state of the art quantum chemical and transition state theory methods in making a priori kinetic predictions for the abstraction reaction of CN with ethane. This reaction, which has been studied experimentally over an exceptionally broad range of temperature (25-1140 K), exhibits an unusually strong minimum in the rate constant near 200 K. The present theoretical predictions, which are based on a careful consideration of the two distinct transition state regimes, quantitatively reproduce the measured rate constant over the full range of temperature, with no adjustable parameters. At low temperatures, the rate-determining step for such radical-molecule reactions involves the formation of a weakly bound van der Waals complex. At higher temperatures, the passage over a subthreshold saddle point on the potential energy surface, related to the formation and dissolution of chemical bonds, becomes the rate-determining step. The calculations illustrate the changing importance of the two transition states with increasing temperature and also clearly demonstrate the need for including accurate treatments of both transition states. The present two transition state model is an extension of that employed in our previous work on the C2H4 + OH reaction [J. Phys. Chem. A 2005, 109, 6031]. It incorporates direct ab initio evaluations of the potential in classical phase space integral based calculations of the fully coupled anharmonic transition state partition functions for both transition states. Comparisons with more standard rigid-rotor harmonic oscillator representations for the "inner" transition state illustrate the importance of variational, anharmonic, and nonrigid effects. The effects of tunneling through the "inner" saddle point and of dynamical correlations between the two transition states are also discussed. A study of the kinetic isotope effect provides a further test for the present two transition state model.

Kinetics in Cold Laval Nozzle Expansions: From Atmospheric Chemistry to Oxidation of Biomolecules in the Gas Phase

New developments and recent applications of pulsed and miniaturised Laval nozzle technology allowing many gas-phase molecular processes to be studied at very low temperatures are highlighted. In the present Minireview we focus on molecular energy transfer and reactions of molecular radicals (e.g. OH) with neutral molecules. We show that with the combination of pulsed laser photolysis and sensitive laser-induced fluorescence detection a large number of fast reactions of radicals with more or less complex neutral molecules can be measured in Laval nozzle expansions nowadays. It is also demonstrated that collisional energy transfer of neutral molecules can be measured via kinetically controlled selective fluorescence (KCSF) excitation down to 58 Kelvin. Finally, we show that even the primary steps in the oxidation of biomolecules or biomolecular building blocks initiated by OH radicals can be followed at low temperatures. The temperature dependence of the measured rate constants is the key for an understanding of the underlying molecular mechanisms and the Laval nozzle expansion provides a unique environment for these measurements. The experimental finding that many reactions between radicals and neutral species can be rapid at low temperatures are discussed in terms of pre-reactive complexes formed in the overall complex forming bimolecular reactions.

Is hydrogen abstraction an important pathway in the reaction of alkenes with the OH radical?

In this paper, we report the first temperature dependent kinetic measurements of the reaction of C(> or =5) 1-alkenes with OH radicals and also report the first room temperature measurements for 1-alkenes between C(8-11). The rate coefficients at room temperature and between 75 and 100 Torr for the reaction of OH with 1-pentene, 1-octene, 1-nonene, 1-decene and 1-undecene are (2.74 +/- 0.38) x 10(-11), (3.62 +/- 0.68) x 10(-11), (4.20 +/- 0.41) x 10(-11), (7.00 +/- 0.96) x 10(-11) and (12.44 +/- 1.62) x 10(-11) cm(3) molecule(-1) s(-1), respectively, at 298 K. No pressure dependence is observed, suggesting that these reactions are at their high pressure limit. Inspection of the variation of rate coefficient with temperature over the temperature range 262-300 K shows a weak negative temperature dependence, suggesting that adduct formation is taking place, however, a curved Arrhenius plot is clearly seen for 1-decene and theoretical analysis suggests that abstraction by OH is a non-negligible channel. An estimation of products formed from OH oxidation suggests that abstraction will lead to products that have a lower photochemical ozone creation potential than those formed via addition.

Long-range transition state theory

The implementation of variational transition state theory (VTST) for long-range asymptotic potential forms is considered, with particular emphasis on the energy and total angular momentum resolved (microJ-VTST) implementation. A long-range transition state approximation yields a remarkably simple and universal description of the kinetics of reactions governed by long-range interactions. The resulting (microJ-VTST) implementation is shown to yield capture-rate coefficients that compare favorably with those from trajectory simulations (deviating by less than 10%) for a wide variety of neutral and ionic long-range potential forms. Simple analytic results are derived for many of these cases. A brief comparison with a variety of low-temperature experimental studies illustrates the power of this approach as an analysis tool. The present VTST approach allows for a simple analysis of the applicability conditions for some related theoretical approaches. It also provides an estimate of the temperature or energy at which the "long-range transition state" moves to such short separations that short-range effects, such as chemical bonding, steric repulsion, and electronic state selectivity, must be considered.

Reactions at Very Low Temperatures: Gas Kinetics at a New Frontier

Advances in experimental techniques, especially the development of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method, allow many gas-phase molecular processes to be studied at very low temperatures. This Review focuses on the reactions of molecular and atomic radicals with neutral molecules. Rate constants for almost 50 such reactions have been measured at temperatures as low as 13 K by using the CRESU method. The surprising demonstration that so many reactions between electrically neutral species can be extremely rapid at these very low temperatures has excited interest both from theoreticians and from those seeking to understand the chemistry that gives rise to the 135 or so molecules that are present in low-temperature molecular clouds in the interstellar medium. Theoretical treatments of these reactions are based on the idea that a reaction occurs when the long-range potential between the reagent species brings them into close contact. The astrochemical context, theoretical studies, and the determination of the rate constants of these low-temperature reactions are critically discussed.

The temperature-dependence of rapid low temperature reactions: experiment, understanding and prediction

Despite the success of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method in measuring rate coefficients for neutral-neutral reactions of radicals down close to the very low temperatures prevalent in dense interstellar clouds (ISCs), there are still many reactions of potential importance in the chemistry of these objects for which there have been no measurements of low temperature rate coefficients. One important class of reactions is that between atomic and molecular free radicals and unsaturated hydrocarbons; that is, alkynes and alkenes. Based on semi-empirical arguments and correlations of 'room temperature' rate coefficients, k(298 K), for reactions of this type with the difference between the ionisation energy of the alkyne/alkene and the electron affinity of the radical, we suggest which reactions between the radicals, C(3P), O(3P), N(4S), CH, C2H and CN, and carbon chain molecules (Cn) and cyanopolyynes (HC2nCN and NCC2nCN) are likely to be fast at the temperature of dense ISCs. These reactions and rate coefficients have been incorporated into a purely gas-phase model (osu2005) of ISC chemistry. The results of these calculations are presented and discussed.

Reaction of Anthracene with CH Radicals: An Experimental Study of the Kinetics between 58 and 470 K

The rate coefficient of the reaction of the methylidine radical CH with anthracene has been studied over a wide temperature range (58-470 K) in a dedicated "Cinétique de Réaction en Ecoulement Supersonique Uniforme" (Reaction Kinetics in Uniform Supersonic Flow) apparatus. The reaction exhibits a slight positive temperature dependence, which can be fitted to the expression k(T) = (3.32 +/- 1.00) x 10(-10)(T/298)((0.46+/-0.14)) cm3 molecule(-1) s(-1). Even at the lowest temperature, the reaction remains very fast indicating that the kinetics are probably driven by a capture process.

Temperature Dependence and Kinetic Isotope Effects for the OH + HBr Reaction and H/D Isotopic Variants at Low Temperatures (53−135 K) Measured Using a Pulsed Supersonic Laval Nozzle Flow Reactor

The reactions of OH + HBr and all isotopic variants have been measured in a pulsed supersonic Laval nozzle flow reactor between 53 and 135 K, using a pulsed DC discharge to create the radical species and laser induced fluorescence on the A 2sigma <-- X 2pi (v' = 1 <-- v'' = 0) transition. All reactions are found to possess an inverse temperature dependence, in accord with previous work, and are fit to the form k = A(T/298)(-n), with k1 (OH + HBr) = (10.84 +/- 0.31) x 10(-12) (T/298)(-0.67+/-0.02) cm3/s, k2 (OD + HBr) = (6.43 +/- 2.60) x 10(-12) (T/298)(-1.19+/-0.26) cm3/s, k3 (OH + DBr) = (5.89 +/- 1.93) x 10(-12) (T/298)(-0.76+/-0.22) cm3/s, and k4 (OD + DBr) = (4.71 +/- 1.56) x 10(-12) (T/298)(-1.09+/-0.21) cm3/s. A global fit of k vs T over the temperature range 23-360 K, including the new OH + HBr data, yields kT = (1.06 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.11) cm3/s, and (0.96 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.03) exp((-2.88+/-1.82 K)/T) cm3/s, in accord with previous fits. In addition, the primary and secondary kinetic isotope effects are found to be independent of temperature within experimental error over the range investigated and take on the value of (kH/kD)(AVG) = 1.64 for the primary effect and (kH/kD)(AVG) = 0.87 for the secondary effect. These results are discussed within the context of current experimental and theoretical work.

The reaction of anthracene with OH radicals: An experimental study of the kinetics between 58 and 470K

The first direct measurement of the reaction rate constant of a polycyclic aromatic hydrocarbon in the gas phase in the temperature range 58-470 K is reported. The reaction is OH+ anthracene and the experiment has been performed in a continuous flow Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus, which had to be modified for this purpose. Pulsed laser photolysis of H(2)O(2) has been used to generate OH radicals and laser-induced fluorescence to observe the kinetic decay of the radicals and hence determine the rate coefficients. The reaction is found to be fast, and the rate constant increases monotonically as the temperature is lowered. The rate coefficients match the expression k(cm(3) molecules(-1) s(-1))=1.12 x 10(-10)(T/300)(-0.46).

Reaction kinetics at very low temperatures: laboratory studies and interstellar chemistry

Studies of gas-phase processes at temperatures down to 10 K have recently blossomed, largely through application of the CRESU (cinétique de réaction en ecoulement supersonique uniforme) technique. The results are of considerable relevance to the synthesis of molecules in dense interstellar clouds, demonstrating that the models developed to explain the observed molecular abundances must be expanded to include reactions between electrically neutral species. In addition, the experimental results have stimulated theoretical efforts to describe the factors that control the rates of such low-temperature reactions. In this Account, the CRESU method is described and the relevance of the results discussed.