Veronica M. Bierbaum

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.

The interstellar chemistry of PAH cations

Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+and C16H10+) in the gas phase. We find that these PAH cations are very reactive, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.

The tandem flowing afterglow-shift-drift

Abstract The design of a new tandem flowing afterglow-SIFT-drift instrument which provides high sensitivity, resolution, and chemical versatility is described. The performance of the instrument is evaluated in terms of (a) the intensities and variety of ions which can be generated, mass-selected, and injected; (b) the efficiency of the dual annulus SIFT injector as a venturi inlet; and (c) the reliability of the kinetic data.

Hydrogenation and Charge States of Polycyclic Aromatic Hydrocarbons in Diffuse Clouds. II. Results

We have modeled the states of hydrogenation and charge of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds for molecules ranging from benzene up to species containing 200 carbon atoms. It is found that the hydrogenation state of PAHs strongly depends on the size of the molecule. Small PAHs with fewer than about 15-20 carbon atoms are destroyed in most environments. Intermediate-size PAHs in the range of 20-30 carbon atoms are stripped of most of their peripheral hydrogen atoms, but may be able to survive in the interstellar medium because of the relative stability of their carbon skeleton upon UV photon absorption. Larger PAHs primarily have normal hydrogen coverage (i.e., with each peripheral carbon atom bearing a single hydrogen), with competition between this form and PAHs containing an additional hydrogen. Very large PAHs may be fully hydrogenated, with every peripheral carbon atom bearing two hydrogen atoms. Our finding that extremely dehydrogenated PAH neutrals or positively charged CmH, with m ranging from 15 to 30 and n ≤ 2, can survive in the interstellar medium contrasts with previous work, where it was generally assumed that PAHs losing their hydrogen coverage were quickly destroyed. A mechanism is proposed for the selective growth of these small dehydrogenated PAHs in diffuse clouds with respect to larger PAHs. Finally, our results are compared to previous studies on the hydrogenation and charge states of PAHs.

Ion Chemistry in the Interstellar Medium

We present an overview of the interstellar medium, including physical and chemical conditions, spectroscopic observations, and current challenges in characterizing interstellar chemistry. Laboratory studies of ion-atom reactions, including experimental approaches and instrumentation, are described. We also tabulate and discuss comprehensive summaries of ion-neutral reactions involving hydrogen, nitrogen, and oxygen atoms that have been studied since Sablier and Rolandos 1993 review.

Hydrogenation and Charge States of PAHs in Diffuse Clouds. I. Development of a Model

A model of the hydrogenation and charge states of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds is presented. The main physical and chemical processes included in the model are ionization and photodissociation in the interstellar UV field, electron recombination with PAH cations, and chemistry between PAH cations and major interstellar species present in the diffuse medium, such as H2, H, O, and N atoms. A statistical model of photodissociation is presented, which is a simplified version of the Rice-Ramsperger-Kassel-Marcus theory. The predictions of this new approach have been successfully compared to experimental results for small PAH cations, justifying the application of the model to larger PAHs for which no experimental data are available. This simplified statistical theory has also been used to estimate the importance of the dissociative recombination channel in the reaction between PAH cations and electrons. Recent experimental results obtained on the chemistry between PAH cations and H2, H, O, and N atoms are discussed and included in the model. Finally, a discussion is presented on other important processes which may affect the PAH distribution in the interstellar medium, such as electron attachment, photodetachment, photofragmentation with carbon loss, double ionization, and chemistry between PAH cations and minor species present in diffuse clouds. The results obtained with this model for compact PAHs ranging from benzene to species bearing up to 200 carbon atoms are discussed in a separate paper.

Kinetics and Mechanism of the Formation of Water Cluster Ions from O2+ and H2O

The reaction sequence leading from O2+ to H3O+· H2O was examined in He, Ar, N2, and O2 carrier gases in a flowing afterglow system. The rate constants for the reactions were measured and the kinetic analysis for their determination is presented. For M=N2, two new steps involving the formation and reaction of O2+· N2 were proposed and examined. The rate constants are discussed and compared with other experimental values.

Reactions of H, N, and O Atoms with Carbon Chain Anions of Interstellar Interest: An Experimental Study

The molecular ions HC and HC have recently been identified both in the laboratory and in astronomical environments. In addition, carbon chain anions have been proposed as potential carriers of the diffuse interstellar bands. Using a selected ion flow tube apparatus, we have determined the rate constants for the reactions of C and HC with H, N, and O atoms in order to characterize their interstellar chemistry. Reactions between the carbon chain anions and H and O occur readily, with rate constants ranging from mid to high 10-10 cm3 s -1. H atom reactions proceed primarily by associative detachment, and O atom reactions proceed both by associative detachment and by extrusion of CO. Reactions between the carbon chains and N atoms are slower, but the reaction products that are observed could provide a negative ion pathway for the synthesis of observed interstellar species.

Gas-phase hydrogen-deuterium exchange reactions of hydroxide and hydroxide-d ions with weakly acidic neutrals

Rate constants for hydrogen-deuterium exchange reactions between HO/sup -/ and DO/sup -/ and a series of weakly acidic neutrals, both organic and inorganic, have been measured in the gas phase by using the selected ion flow tube (SIFT) technique. The reaction efficiencies are discussed in terms of the initial ion-dipole complex energies, the relative acidities of the neutrals, and the change in solvation energy accompanying proton transfer; the effect of these energies on transition-state properties profoundly influences the outcome of the reactions. Exchange occurs rapidly between hydroxide and most aromatic and vinyl compounds but is relatively inefficient for hydrogen. The efficiency for exchange with ammonia is intermediate. Ethylene, dimethyl ether, and methane do not exhibit exchange.

Thermochemistry of the benzyl and allyl radicals and ions

Abstract We have studied the thermochemistry of the resonantly stabilized radicals, C6H5CH2 and CH2CHCH2 and their corresponding cations and anions. A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction: C6H5CH3 + CH3O− ⇌C6H5CH2− + CH3OH C6H5CH3 + CD3O− ⇌C6H5CH2− + CD3OH C6D5CD3 + CH3O− ⇌C6D5CD2− + CH3OD C6D5CD3 + CD3O− ⇌C6D5CD2− + CD3OD CH2CHCH3 + HO− ⇌CH2CHCH2− + H2O The ratio of the rate constants gives a free energy change for each reaction and use of the established gas phase acidity of CH3OH or H2O provides values for the acidities. We calculate the entropy changes, ΔacidS300(C6H5CH3) and ΔacidS300(CH2CHCH3), to extract values for ΔacidH300(C6H5CH3) and ΔacidH300(CH2CHCH3). Earlier photoelectron experiments have provided ionization potentials and electron affinities for the benzyl and allyl radicals. Use of the IPs and EAs, together with the enthalpies of deprotonation, provides values for the C H bond enthalpies at 300 K and the C H bond energies at 0 K. These bond energies are used to compute the heats of formation of the radicals and ions as well as the cation hydride affinities, HA (all values in kcal mol−1): C6H5CH2−H CH2CHCH2−H ΔacidG300(R−H) 374.9 ± 0.2 383.8 ± 0.1 ΔacidH300(R−H) 382.3 ± 0.5 391.1 ± 0.3 DH300(R−H) 89.8 ± 0.6 88.8 ± 0.4 D0(R−H) 88.1 ± 0.6 87.4 ± 0.4 ΔfH0(R) 54.1 ± 0.6 44.4 ± 0.5 ΔfH300(R) 49.7 ± 0.6 41.4 ± 0.4 ΔfH0(R−) 33.1 ± 0.6 33.1 ± 0.5 ΔfH300(R−) 28.7 ± 0.5 30.1 ± 0.4 ΔfH0(R+) 221.3 ± 0.6 231.6 ± 0.7 ΔfH300(R+) 216.8 ± 0.6 228.9 ± 0.6 HA0(R+) 237.9 ± 0.7 254.7 ± 0.7 HA300(R+) 239.5 ± 0.6 258.9 ± 0.7 In addition, we find ΔacidG300(C6D5CD3) = 377.0 ± 0.3 kcal mol−1.