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

The Atmospheric Chemistry of Alkoxy Radicals

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

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 kinetics of the reactions of CH3O and C2H5O radicals with NO2 have been studied using pulsed laser photolysis to create the radicals from the corresponding alkyl nitrite and by time-resolved, laser-induced fluorescence to observe the decay of the radical concentration. At 295 K, rate constants for CH3O + NO2 have been measured in the presence of Ar (6–100 Torr), He (30–100 Torr) and CF4(30–125 Torr). The association reaction to CH3ONO2 appears to predominate, and rate constants in the limit of low and high pressure are deduced: (k°/10–29 cm6 molecule–2 s–1)= 2.8 ± 0.6 (M = Ar); 1.6 ± 0.6 (M = He); 3.4 ± 1.0 (M = CF4), and (k∞/10–11 cm3 molecule–1 s–1)= 2.0 ± 0.4, where the quoted errors correspond to two standard deviations. A limited number of measurements is also reported for this reaction at 390 K. The rate of the reaction of C2H5O with NO2 at 295 K is found to be the same in the presence of 15 and 100 Torr of Ar, and the measurements yield a rate constant: (k∞10–11 cm3 molecule–1 s–1)= 2.8 ± 0.3.

Rate constants for the reactions of CH3O and C2H5O with NO2 over a range of temperature and total pressure

Infrared—ultraviolet double resonance measurements on the relaxation of rotational energy in the (31, 214151) Fermi resonance states of C2H2

A tunable diode laser has been used to observe transient absorptions on transitions in the ν3 infrared bands of CO2 when it is formed as a product of the flash-initiated reaction between OH and CO. The absence of any population in excited states with ν3 > 0 and the limited extent of excitation in the Fermi-linked ν1 and ν2 modes indicate that energy is released largely as repulsion following passage through a transition state in which the OCO atoms are close to their final structure in isolated CO2.

Michael J. Frost

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

Rate constants for the reactions of CH3O and C2H5O with NO2 over a range of temperature and total pressure

Infrared—ultraviolet double resonance measurements on the relaxation of rotational energy in the (31, 214151) Fermi resonance states of C2H2

Faraday communications. Vibrational-state distribution of CO2 produced in the reaction between OH radicals and CO