We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.

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The UMIST database for astrochemistry 2012

A photochemical model of Titan's atmosphere and ionosphere

There is now ample evidence from an assortment of experiments, especially those involving the CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) technique, that a variety of neutral–neutral reactions possess no activation energy barrier and are quite rapid at very low temperatures. These reactions include both radical–radical systems and, more surprisingly, systems involving an atom or a radical and one ‘stable’ species. Generalizing from the small but growing number of systems studied in the laboratory, we estimate reaction rate coefficients for a larger number of such reactions and include these estimates in a new network of gas-phase reactions for use in low-temperature interstellar chemistry. Designated osu.2003, the new network is available on the World Wide Web and will be continually updated. A table of new results for molecular abundances in the dark cloud TMC-1 (CP) is provided and compared with results from an older (new standard model; nsm) network.

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Rapid neutral–neutral reactions at low temperatures: a new network and first results for TMC‐1

. The available database concerning rate constants for gas-phase reactions of the hydroxyl (OH) radical with alkanes through early 2003 is presented over the entire temperature range for which measurements have been made (~180-2000 K). Measurements made using relative rate methods are re-evaluated using recent rate data for the reference compound (generally recommendations from this review). In general, whenever more than one study has been carried out over an overlapping temperature range, recommended rate constants or temperature-dependent rate expressions are presented. The recommended 298 K rate constants, temperature-dependent parameters, and temperature ranges over which these recommendations are applicable are listed in Table 1.

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Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

Using the pulsed laser-photolysis (PLP) laser-induced fluorescence (LIF) method, rate constants have been measured at low total pressure for the reactions between OH and CO (297≥T/K≥80) and OD+CO (295≥T/K≥178). Calculations have been carried out based on transition-state and RRKM theories and allowing approximately for quantum mechanical tunneling through the transition state (TS2) which separates HOCO (DOCO) from H(D)+CO 2 . They indicate (a) that tunneling is important in both these systems, (b) that the barrier at TS2 is higher than previously thought and closer to the ab initio value, and (c) that the observed kinetic isotope effect can be reproduced quite well. The possible role of hydrogen-bonded complexes in facilitating these reactions is discussed briefly

Reaction between hydroxyl (deuteroxyl) radicals and carbon monoxide at temperatures down to 80 K: experiment and theory

Two, quite different, experimental studies have been carried out on the reaction between OH and CO at low total pressures. In the first, rate constants have been determined at 82 and 106 K. At the lower temperature, measurements were made at 2 and 5 Torr total pressure, yielding k=(1.0 ± 0.12)× 10–13 cm3 molecule–1 s–1 and (0.91 ± 0.1)× 10–13 cm3 molecule–1 s–1, respectively. At 106 K and 4 Torr, k=(0.98 ± 0.08)× 10–13 cm3 molecule–1 s–1. Theoretical considerations show that the reaction must be in its low-pressure limit, yielding H + CO2, and that the vibrational ground-state adiabatic barrier to formation of HOCO must be <200 cm–1, significantly lower than estimated previously.In the second series of experiments, a tunable diode laser has been used to observe transient absorptions on transitions in the ν3 infrared bands of the CO2 product of the reaction, when it is initiated by flash photolysis at room temperature. There is no excitation of the ν3 mode, and the overall vibrational distribution corresponds to an averaged vibrational energy yield of only 6%. It is concluded that energy is released largely as repulsion following passage through a transition state in which the OCO angle is ca. 171° and the O—C, C—O bond distances are very similar to those in isolated CO2.

Energy and structure of the transition states in the reaction OH + CO → H + CO2

The pulsed laser photolysis (PLP)–laser-induced fluorescence (LIF) method has been used to study the kinetics at low temperatures of the reactions of the OH free radical with HCl (138–298 K), CH4(178–298 K) and C2H6(138–298 K). In deriving rate constants for the reactions of OH with CH4 and C2H6, small but significant corrections were made to allow for the fast secondary reactions between OH and the alkyl radicals produced in the primary reactions. The results are compared with predictions based on extrapolations of kinetic data obtained at higher temperatures, and limitations of the experimental method at low temperatures are considered. The rate constants for OH + CH4 and OH + C2H6 are in reasonable agreement with values calculated using modified Arrhenius expressions which have been fitted to data obtained at higher temperatures, although our rate constant for OH + CH4 at 216 K is significantly higher than a recent determination at 223 K by Vaghjiani and Ravishankara [Nature (London), 1991, 350, 406]. However, the rate constants for OH + HCl at lower temperatures deviate significantly from such predictions.

Kinetics of elementary reactions at low temperatures: rate constants for the reactions of OH with HCl (298 T/K 138), CH4(298 T/K 178) and C2H6(298 T/K 138)

Using pulsed laser photolysis (PLP) and laser-induced fluorescence (LIF) we have studied the formation of O2(X3Σg-) in high vibrational levels when O3 is photolysed at 266 nm. Experiments have been performed in both Ar and N2 diluents. In the former, O2(X3Σg-) in high vibrational levels is formed primarily as a product of the reaction between O(1D) atoms and O3: O(1D)+O3→2 O2(X3Σg-, v). In a large excess of N2, the O(1D) atoms are quenched and one only observes the O2(X3Σg-) molecules created by direct photolysis in the ‘triplet channel’: O3+hν (λ=266 nm)→O2(X3Σg-, v)+O(3P). Employing LIF in the (0, v) and (2, v) bands of the O2 (B 3Σu-–X3Σg-) system, we have characterised the nascent vibrational distributions from both these processes for 18⩽v⩽23. In addition, by observing how the population in specific vibrational levels changes with time, we have determined rate constants for vibrational relaxation of O2(X3Σg-, v=21 and 22) with He, O2, N2, CO2, N2O and CH4. The results are compared with those obtained in other studies for relaxation from the same and other levels of O2(X3Σg-) and the implications of the results for atmospheric chemistry are discussed.

Formation and relaxation of O2(X3Σg-) in high vibrational levels (18⩽⩽23) in the photolysis of O3 at 266 nm

Rate coefficients are reported for the removal of H2O in selected vibrational states in collisions with H atoms. Pulses of tunable infrared radiation have been employed to excite H2O molecules to the specific vibrational levels ∣04〉-, ∣13〉-, ∣03〉-, ∣12〉- and ∣02〉-2〉, where the notation is that used by Sinha etal. (A. Sinha, M. C. Hsaio and F. F. Crim, J.Chem. Phys., 1991, 94, 4928), in the presence of known concentrations of both H atoms and H2O. OH radicals formed either in v=0 or v=1 by the reaction of H atoms with the vibrationally excited H2O are observed by laser-induced fluorescence. Kinetic data are derived from the variation of LIF intensity with delay time between the infrared pump and ultraviolet probe lasers. Rate coefficients are reported for three kinds of process: (i) the removal of excited H2O molecules from the initially excited level by the sum of reaction and relaxation with H atoms, (ii) the relaxation of excited H2O molecules from the initially excited level by collisions with unexcited H2O, and (iii) the relaxation of OH(v=1) by H atoms and H2O. The results are compared with previous experimental data, especially that on selective vibrational enhancement of endothermic chemical reactions, and with the results of theoretical calculations on the H+H2O reaction.

Paul Sharkey

Papers

Reaction between hydroxyl (deuteroxyl) radicals and carbon monoxide at temperatures down to 80 K: experiment and theory

Energy and structure of the transition states in the reaction OH + CO → H + CO2

Kinetics of elementary reactions at low temperatures: rate constants for the reactions of OH with HCl (298 T/K 138), CH4(298 T/K 178) and C2H6(298 T/K 138)

Formation and relaxation of O2(X3Σg-) in high vibrational levels (18⩽⩽23) in the photolysis of O3 at 266 nm

Rate coefficients for the reaction and relaxation of H2O in specific vibrational states with H atoms and H2O