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

Gas-Phase Scattering of Transition Metal Atoms Fe, Ir, and Pt with CH4, O2, and CO2

Understanding the interactions between transition metal atoms and molecules is important for the study of various related chemical and physical processes. In this study, we have investigated collisions between iron (Fe), iridium (Ir), and platinum (Pt) and the small molecules CH4, O2, and CO2 using a crossed-beam and time-sliced ion velocity map imaging technique. Elastic collisions were observed in all cases, except for collisions of Pt with O2 and CO2. Collisions of Fe or Ir with CH4, O2, and CO2 show mainly long-range attractive potentials at large impact parameters leading to forward scattering, whereas sideways and backward scatterings indicate the formation of short-lived complexes with lifetimes comparable to their rotational periods. In collisions of Pt with O2 and CO2, Pt may react with the gases and become chemically bound to them, which can deplete the nonreactive scattering signal. The insights gained from this study provide a foundation for improved understanding of the complex interactions between transitional metal atoms and other molecules.

Broadband Rotational Spectroscopy in Uniform Supersonic Flows: Chirped Pulse/Uniform Flow for Reaction Dynamics and Low Temperature Kinetics

ConspectusThe study of gas-phase chemical reactions at very low temperatures first became possible with the development and implementation of the CRESU (French acronym for Reaction Kinetics in Uniform Supersonic Flows) technique. CRESU relies on a uniform supersonic flow produced by expansion of a gas through a Laval (convergent-divergent) nozzle to produce a wall-less reactor at temperatures from 10 to 200 K and densities of 1016-1018 cm-3 for the study of low temperature kinetics, with particular application to astrochemistry. In recent years, we have combined uniform flows with revolutionary advances in broadband rotational spectroscopy to yield an instrument that affords near-universal detection for novel applications in photodissociation, reaction dynamics, and kinetics. This combination of uniform supersonic flows with chirped-pulse Fourier-transform microwave spectroscopy (Chirped-Pulse/Uniform Flow, CPUF) permits detection of any species with a modest dipole moment, thermalized to the uniform temperature of the gas flow, with isomer, conformer, and vibrational state specificity. In addition, the use of broadband, high-resolution, and time-dependent (microsecond time scale) micro- and mm-wave spectroscopy makes it an ideal tool for characterizing both transient and stable molecules, as well as studying their spectroscopy and dynamics.In this Account, we review recent advances made using the CPUF technique, including studies of photodissociation, radical-radical reaction dynamics, and low temperature kinetics. These studies highlight both the strength of universal and multiplexed detection and the challenges of coupling it to a high-density collisional environment. Product branching and product evolution as a function of time have been measured for astrochemically relevant systems, relying on the detailed characterization of these flow conditions via experiments and fluid dynamics simulations. In the photodissociation of isoxazole, an unusual heterocyclic molecule with a very low-energy conical intersection, we have identified 7 products in 5 reaction channels and determined the product branching, pointing to both direct and indirect pathways. We have also approached the same system from separated NO and C3H3 reactants to explore a broader range of the potential energy surface, demonstrating the power of multichannel branching measurements for complex radical-radical reactions. We determined the product branching in the C3H2 isomers in the photodissociation of the propargyl radical and identified the importance of a hydrogen atom catalyzed isomerization to the lowest energy cyclic form. This then motivated a study of direct D-H exchange reaction in radicals, in which we demonstrate that it is an important and overlooked pathway for deuterium fractionation in astrochemical environments. Recently, we have shown the measurement of low temperature kinetics inside an extended Laval nozzle, after which a shock-free secondary expansion to low temperature and density affords an ideal environment for detection by rotational spectroscopy. These results highlight the power and potential of the CPUF approach, and future prospects will also be discussed in light of these developments.

Low temperature reaction kinetics inside an extended Laval nozzle: REMPI characterization and detection by broadband rotational spectroscopy

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF ("Chirped-Pulse/Uniform flow"). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20 K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20 K of 6.66 ± 0.47 × 10-11 cm3 molec-1 s-1.

Multichannel Radical-Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy

Chirped-pulse rotational spectroscopy in a quasi-uniform flow has been used to investigate the reaction dynamics of a multichannel radical-radical reaction of relevance to planetary atmospheres and combustion. In this work, the NO + propargyl (C₃H₃) reaction was found to yield six product channels containing eight detected species. These products and their branching fractions (%), are as follows: HCN (50), HCNO (18), CH₂CN (12), CH₃CN (7.4), HC₃N (6.2), HNC (2.3), CH₂CO (1.3), HCO (1.8). The results are discussed in light of previous unimolecular photodissociation studies of isoxazole and prior potential energy surface calculations of the NO + C₃H₃ system. The results also show that the product branching is strongly influenced by the excess energy of the reactant radicals. The implications of the title reaction to the planetary atmospheres, particularly to Titan, are discussed.

Capture Theory Models: An overview of their development, experimental verification, and applications to ion-molecule reactions

Since Arrhenius first proposed an equation to account for the behaviour of thermally activated reactions in 1889, significant progress has been made in our understanding of chemical reactivity. A number of capture theory models have been developed over the past several decades to predict the rate coefficients for reactions between ions and molecules-ranging from the Langevin equation (for reactions between ions and non-polar molecules) to more recent fully quantum theories (for reactions at ultra-cold temperatures). A number of different capture theory methods are discussed, with the key assumptions underpinning each approach clearly set out. The strengths and limitations of these capture theory methods are examined through detailed comparisons between low-temperature experimental measurements and capture theory predictions. Guidance is provided on the selection of an appropriate capture theory method for a given class of ion-molecule reaction and set of experimental conditions-identifying when a capture-based model is likely to provide an accurate prediction. Finally, the impact of capture theories on fields such as astrochemical modelling is noted, with some potential future directions of capture-based approaches outlined.

Isotope-selective chemistry in the Be+(2S1/2) + HOD → BeOD+/BeOH+ + H/D reaction

Low temperature reactions between laser-cooled Be+(2S1/2) ions and partially deuterated water (HOD) molecules have been investigated using an ion trap and interpreted with zero-point corrected quasi-classical trajectory calculations on a highly accurate global potential energy surface for the ground electronic state. Both product channels have been observed for the first time, and the branching to BeOD+ + H is found to be 0.58 ± 0.14. The experimental observation is reproduced by both quasi-classical trajectory and statistical calculations. Theoretical analyses reveal that the branching to the two product channels is largely due to the availability of open states in each channel.

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.

Kinetic Studies of Atomic Carbon, C[2p2 (3PJ)], with Thiols by Time-Resolved Atomic Resonance Absorption Spectroscopy in the Vacuum Ultraviolet

Reaction rates of atomic carbon in its electronic ground state, C[2p2 (3PJ)], with a range of large sulfur-containing molecules have been investigated using time-resolved atomic spectroscopy in the vacuum ultraviolet following pulsed irradiation. Absolute rate data for the collisional removal at 300 K of C(23PJ) by the gases 1-propanethiol, 1-butanethiol, 1-pentanethiol, 2-propanethiol, 2-methyl-1-propanethiol, 2-methyl-2-propanethiol and dimethyl sulfide are reported. All processes proceed at rates of the order of the collision numbers, supporting an overall mechanism of C-atom insertion into the S–H bond following initial addition, which is energetically favourable, a mechanism demonstrated analogously hitherto with H2S and where H-atom abstraction would also be endothermic.

High-resolution collision energy control through ion position modulation in atom-ion hybrid systems

We demonstrate an ion shuttling technique for high-resolution control of atom-ion collision energy by translating an ion held within a radio-frequency trap through a magneto-optical atom trap. The technique is demonstrated both experimentally and through numerical simulations, with the experimental results indicating control of ion kinetic energies from 0.05 to 1 K with a fractional resolution of ∼10 and the simulations demonstrating that kinetic energy control up to 120 K with a maximum predicted resolution of ∼100 is possible, offering order-of-magnitude improvements over most alternative techniques. Finally, we perform a proof-of-principle chemistry experiment using this technique and outline how the method may be refined in the future and applied to the study of molecular ion chemistry.

Optical Control of Reactions between Water and Laser-Cooled Be+ Ions

We investigate reactions between laser-cooled Be⁺ ions and room-temperature water molecules using an integrated ion trap and high-resolution time-of-flight mass spectrometer. This system allows simultaneous measurement of individual reaction rates that are resolved by reaction product. The rate coefficient of the Be⁺(²S_1/2) + H₂O → BeOH⁺ + H reaction is measured for the first time and is found to be approximately two times smaller than predicted by an ion-dipole capture model. Zero-point-corrected quasi-classical trajectory calculations on a highly accurate potential energy surface for the ground electronic state reveal that the reaction is capture-dominated, but a submerged barrier in the product channel lowers the reactivity. Furthermore, laser excitation of the ions from the ²S_1/2 ground state to the ²P_3/2 state opens new reaction channels, and we report the rate and branching ratio of the Be⁺(²P_3/2) + H₂O → BeOH⁺ + H and H₂O⁺ + Be reactions. The excited-state reactions are nonadiabatic in nature.

Low-temperature kinetics and dynamics with Coulomb crystals

Coulomb crystals-as a source of translationally cold, highly localized ions-are being increasingly utilized in the investigation of ion-molecule reaction dynamics in the cold regime. To develop a fundamental understanding of ion-molecule reactions, and to challenge existing models that describe the rates, product branching ratios, and temperature dependence of such processes, investigators need to exercise full control over the experimental reaction parameters. This requires not only state selection of the reactants, but also control over the collision process (e.g., the collisional energy and angular momentum) and state-selective product detection. The combination of Coulomb crystals in ion traps with cold neutral-molecule sources is enabling the measurement of state-selective reaction rates in a diverse range of systems. With the development of appropriate product detection techniques, we are moving toward the ultimate goal of examining low-energy, state-to-state ion-molecule reaction dynamics.

A combined experimental and theoretical study of reactions between the hydroxyl radical and oxygenated hydrocarbons relevant to astrochemical environments

Rate coefficients for the reactions of the hydroxyl radical with acetone and dimethyl ether increase dramatically at very low temperatures.

Reaction of OH and NO at Low Temperatures in the Presence of Water: the Role of Clusters

There has been a lot of speculation about the role of water in gas phase reactions involving neutrals, radicals and ions. The reaction of NO and OH has attracted a lot of attention in the past due to its relevance for ozone chemistry in the atmosphere. In the present contribution we report low temperature measurements of the recombination of OH and NO at low temperatures in Laval nozzle expansions between 300 K and 60 K. We find an increase of the bimolecular rate constant in the presence of water of up to 40%. This effect has been attributed to water molecules acting either as an efficient collider releasing energy from the intermediate (in collisions) or – which is more likely for the present experimental conditions – as a cluster partner of the reaction intermediate HONO that also dissipates energy via cluster dissociation, which can in turn both stabilize the reaction intermediate, decrease back reaction to OH and NO, and enh ance finally the overall reaction to the products. The supersaturation of water vapor in the cold Laval nozzle expansion strongly favors the formation of clusters in the nozzle throat; their exact concentration is, however, difficult to estimate due to non-equilibrium conditions. The possible role of clusters in the recombination of OH and NO is investigated using ab initio molecular dynamics calculations. Beyond the reaction intermediate HONO and intramolecular proton transfer events also transient HOON was observed in the theoretical study.

Aggregation-induced chemical reactions: acid dissociation in growing water clusters

Understanding chemical reactivity at ultracold conditions, thus enabling molecular syntheses via interstellar and atmospheric processes, is a key issue in cryochemistry. In particular, acid dissociation and proton transfer reactions are ubiquitous in aqueous microsolvation environments. Here, the full dissociation of a HCl molecule upon stepwise solvation by a small number of water molecules at low temperatures, as relevant to helium nanodroplet isolation (HENDI) spectroscopy, is analyzed in mechanistic detail. It is found that upon successive aggregation of HCl with H(2)O molecules, a series of cyclic heteromolecular structures, up to and including HCl(H(2)O)(3), are initially obtained before a precursor state for dissociation, HCl(H(2)O)(3)···H(2)O, is observed upon addition of a fourth water molecule. The latter partially aggregated structure can be viewed as an "activated species", which readily leads to dissociation of HCl and to the formation of a solvent-shared ion pair, H(3)O(+)(H(2)O)(3)Cl(-). Overall, the process is mostly downhill in potential energy, and, in addition, small remaining barriers are overcome by using kinetic energy released as a result of forming hydrogen bonds due to aggregation. The associated barrier is not ruled by thermal equilibrium but is generated by athermal non-equilibrium dynamics. These "aggregation-induced chemical reactions" are expected to be of broad relevance to chemistry at ultralow temperature much beyond HENDI spectroscopy.

Elementary reactions and their role in gas-phase prebiotic chemistry

The formation of complex organic molecules in a reactor filled with gaseous mixtures possibly reproducing the primitive terrestrial atmosphere and ocean demonstrated more than 50 years ago that inorganic synthesis of prebiotic molecules is possible, provided that some form of energy is provided to the system. After that groundbreaking experiment, gas-phase prebiotic molecules have been observed in a wide variety of extraterrestrial objects (including interstellar clouds, comets and planetary atmospheres) where the physical conditions vary widely. A thorough characterization of the chemical evolution of those objects relies on a multi-disciplinary approach: 1) observations allow us to identify the molecules and their number densities as they are nowadays; 2) the chemistry which lies behind their formation starting from atoms and simple molecules is accounted for by complex reaction networks; 3) for a realistic modeling of such networks, a number of experimental parameters are needed and, therefore, the relevant molecular processes should be fully characterized in laboratory experiments. A survey of the available literature reveals, however, that much information is still lacking if it is true that only a small percentage of the elementary reactions considered in the models have been characterized in laboratory experiments. New experimental approaches to characterize the relevant elementary reactions in laboratory are presented and the implications of the results are discussed.

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 Rolando's 1993 review.

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.

Ions in space

We review the detection history, observation, distribution, and reactivity of molecular ions in extraterrestrial space, with particular (though not exclusive) reference to interstellar monocations. The diversity of interstellar ion chemistry is highlighted with reaction examples, drawn from the authors' own laboratories and elsewhere, and attempt to provide an overview of this broad and increasingly divergent field. Emphasis is given to the role of ions in the synthesis of molecules, including their ability to catalyze the transformation of neutral molecules.

Bimolecular reactions of molecular dications: reactivity paradigms and bond-forming processes

The bimolecular reactivity of molecular dications in the gas phase is reviewed from an experimental point of view. Recent research has demonstrated that in addition to the ubiquitous occurrence of electron transfer in the reactions of gaseous dications with neutral molecules, bond-forming reactions play a much larger role than anticipated before. Thus, quite a number of hydrogen-containing dications show proton transfer to neutral reagents as an abundant or even as the major pathway, and also the nature of the neutral reagent itself is decisive for the amount of proton transfer which takes place. Further, several hydrocarbon dications C(m)H(n)(2+) of medium size (m = 6-14, n = 6-10) undergo bond-forming reactions with unsaturated hydrocarbons such as acetylene or benzene, thereby offering new routes for the formation of larger aromatic compounds under extreme conditions such as interstellar environments. Likewise, recent results on the bimolecular reactivity of multiply charged metal ions have revealed the occurrence of a number of new bond-forming reactions which open promising prospects for further research.