▪ Abstract Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observ...

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Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth

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

Stellar nucleosynthesis of heavy elements, followed by their subsequent release into the interstellar medium, enables the formation of stable carbon compounds in both gas and solid phases. Spectroscopic astronomical observations provide evidence that the same chemical pathways are widespread both in the Milky Way and in external galaxies. The physical and chemical conditions—including density, temperature, ultraviolet radiation and energetic particle flux—determine reaction pathways and the complexity of organic molecules in different space environments. Most of the organic carbon in space is in the form of poorly-defined macromolecular networks. Furthermore, it is also unknown how interstellar material evolves during the collapse of molecular clouds to form stars and planets. Meteorites provide important constraints for the formation of our Solar System and the origin of life. Organic carbon, though only a trace element in these extraterrestrial rock fragments, can be investigated in great detail with sensitive laboratory methods. Such studies have revealed that many molecules which are essential in terrestrial biochemistry are present in meteorites. To understand if those compounds necessarily had any implications for the origin of life on Earth is the objective of several current and future space missions. However, to address questions such as how simple organic molecules assembled into complex structures like membranes and cells, requires interdisciplinary collaborations involving various scientific disciplines. Introduction Life in the Universe is the consequence of the increasing complexity of chemical pathways which led to stable carbon compounds assembling into cells and higher organisms.

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A voyage from dark clouds to the early Earth

Kinetic modeling of hydrocarbon/nitric oxide interactions in a flow reactor

Rate constants for the reactions of CN with hydrocarbons at low and ultra-low temperatures

The pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for the study of the kinetics of free-radical reactions is applied to reactions of CH radicals both in the ultralow-temperature environment of a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus, as well as in a conventional heated cell. The combined techniques provide kinetic data over an extremely wide temperature range. Rate coefficients are reported for the reactions of CH with O2 (T = 13−708 K), NO (T = 13−708 K), and NH3 (T = 23−295 K). All three reactions remain very rapid down to the lowest temperatures at which measurements were made and display differing temperature dependences. The results are discussed in the light of previous work on these systems and of the reaction mechanisms.

Ultralow-Temperature Kinetics of CH(X2Π) Reactions: Rate Coefficients for Reactions with O2 and NO (T = 13−708 K), and with NH3 (T = 23−295 K)

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH4, C2H6, C2H4, C3H6, and C2H2. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)n exp(−θ/T), yielding: for CH4, A′(298) = 7.0 × 10−13 cm3 molecule−1 s−1, n = 2.3, and θ = −16 K; and for C2H6, A′(298) = 5.6 × 10−12 cm3 molecule−1 s−1, n = 1.8, and θ = −500 K. The rate constants for the reactions with C2H4, C3H6, and C2H2 all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)n with k(298) = 2.5 × 10−10 cm3 molecule−1 s−1, n = −0.24 for CN + C2H4; k(298) = 3.4 × 10−10 cm3 molecule−1 s−1, n = −0.19 for CN + C3H6; and k(298) = 2.9 × 10−10 cm3 molecule−1 s−1, n = −0.53 for CN + C2H2. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Rate constants have been determined for the collisional relaxation of CH(X2Π,ν=1) by CO and N2 over a wide range of temperatures. Experiments between 584 and 86 K have been performed in heated and cryogenically cooled cells, and rate constants from 295 to 23 K have been measured in a CRESU (cinetique de reaction en ecoulement supersonique uniforme) apparatus. In both series of experiments, pulsed laser photolysis was employed to generate CH radicals and the laser-induced fluorescence technique was used to observe the rate of removal of CH(ν=1). Vibrational relaxation is rapid, k298 = 9.7 × 10-11 cm3 molecule-1 s-1 for CO and k298 = 3.1 × 10-11 cm3 molecule-1 s-1 for N2, and the dependence of the rate constants on temperature is quite slight but complex. The results suggest that relaxation occurs via strongly bound complexes, and the implications of the results for the interpretation of the reactions between CH radicals and CO and N2 are discussed.

Rate Constants for the Relaxation of CH(X2Π,ν=1) by CO and N2 at Temperatures from 23 to 584 K

Rate constants have been determined for the reaction between CH radicals and N atoms at temperatures between 216 and 584 K. Discharge-flow methods were used to generate known steady-state concentrations of N atoms in a cell which could either be heated within an oven or cryogenically cooled. CH radicals were produced, in concentrations much smaller than those of the atomic radicals, by pulsed laser photolysis (PLP) of a small concentration of CHBr3 introduced into the gas flow, and their first-order kinetic decays were observed using the time-resolved laser-induced fluorescence (LIF) technique. The rate constants show only a slight dependence on temperature in the range covered in the present experiments and they are fitted by the functional form: (k1(T)/cm3 molecule–1 s–1=(1.66± 0.12)× 10–10(T/298)(–0.09 ± 0.2) where the errors are single standard deviations. If corrections are made to allow for the effects of electronic degeneracies and near degeneracies (Smith and Stewart, J. Chem. Soc. Faraday Trans., 1994, 90, 3221), rate constants, k′1(T), can be derived for just that fraction of collisions which occur on the potential-energy surfaces assumed to lead to reaction. The values of k′1(T)/cm3 molecule–1 s–1 fit the expression (3.4 ± 0.25)× 10–10(T/298)(0.04 ± 0.2), suggesting that the rate of reaction is determined by ‘capture’ on the long-range potentials which correlate adiabatically with reagents and products. It is proposed that the value k1(T)/cm3 molecule–1 s–1= 2.0 × 10–10 is used in chemical models of dense interstellar clouds.

Lee B. Herbert

Papers

Rate constants for the reactions of CN with hydrocarbons at low and ultra-low temperatures

Ultralow-Temperature Kinetics of CH(X2Π) Reactions: Rate Coefficients for Reactions with O2 and NO (T = 13−708 K), and with NH3 (T = 23−295 K)

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Rate Constants for the Relaxation of CH(X2Π,ν=1) by CO and N2 at Temperatures from 23 to 584 K

Kinetics of reactions between neutral free radicals. Rate constants for the reaction of CH radicals with N atoms between 216 and 584 K