Tunneling Enhancement of the Gas-Phase CH + CO2 Reaction at Low Temperature.

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero-point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants.

Experimental and theoretical studies of the N(2D) + H2 and D2 reactions.

This study reports the results of an experimental and theoretical investigation of the N(2D) + H2 and N(2D) + D2 reactions at room temperature and below. On the experimental side, a supersonic flow (Laval nozzle) reactor was employed to measure rate constants for these processes at temperatures as low as 127 K. N(2D) was produced indirectly by pulsed laser photolysis and these atoms were detected directly by pulsed laser induced fluorescence in the vacuum ultraviolet wavelength region. On the theoretical side, two different approaches were used to calculate rate constants for these reactions; a statistical quantum mechanical (SQM) method and a quasi-classical trajectory capture model including a semi-classical correction for tunneling (SC-Capture). This work is described in the context of previous studies, while the discrepancies between both experiment and theory, as well as between the theoretical results themselves are discussed.

Rate constants for the N(2D) + C2H2 reaction over the 50-296 K temperature range.

The reactions of metastable atomic nitrogen N(2D) are important processes in the gas-phase chemistry of several planetary atmospheres. Here we present a combined experimental and theoretical investigation of the N(2D) + acetylene reaction due to its potential significance for the photochemistry of Titan's atmosphere. Experimentally, a continuous supersonic flow reactor was used to study this reaction over the 50-296 K temperature range employing pulsed laser photolysis and vacuum ultraviolet laser induced fluorescence to produce and detect N(2D) atoms, respectively. The measured rate constants are substantially larger than those obtained in earlier work and remain constant as a function of temperature. Moreover, these results are supported by new electronic structure calculations, which indicate that this process is likely to be barrierless. The impact of the new rate constants on Titan's atmospheric chemistry is tested through simulations using a 1D coupled ion-neutral photochemical model.

Experimental and Theoretical Study of the O(1D) + HD Reaction.

This work addresses the kinetics and dynamics of the gas-phase reaction between O(1D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(1D) production and pulsed-laser-induced fluorescence for O(1D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction. In parallel, theoretical rate constants and branching ratios were calculated by using three different techniques; mean potential phase space theory (MPPST), the statistical quantum mechanical method (SQM), and ring polymer molecular dynamics (RPMD). Although the agreement between experimental and theoretical rate constants is reasonably good, with differences not exceeding 30% over the entire temperature range, the theoretical branching ratios derived by the MPPST and SQM methods are as much as 50% larger than the experimental ones. These results are presented in the context of earlier work, while the possible origins of the discrepancies between experiment and theory are discussed.

Conical intersection-regulated intermediates in bimolecular reactions: Insights from C(1D) + HD dynamics.

The importance of conical intersections (CIs) in electronically nonadiabatic processes is well known, but their influence on adiabatic dynamics has been underestimated. Here, through combined experimental and theoretical studies, we show that CIs induce a barrier and regulate conversion from a precursor metastable intermediate (CI-R) to a deep well one. This results in bond-selective activation, influencing the adiabatic dynamics markedly in the C(1D) + HD reaction. Theory is validated by experiment; quantum dynamics calculations on highly accurate ab initio potential energy surfaces yield rate coefficients and product branching ratios in excellent agreement with the experiment. Quasi-classical trajectory calculations reveal that the CI-R intermediate leads to unusual reaction mechanisms (designated as C─H activation complex conversion and cyclic complex), which are responsible for large branching ratios. We also reveal that CI-R intermediates exist in other reactive systems, and the dynamical effects uncovered here may have general significance.

A low temperature investigation of the N(2D) + CH4, C2H6 and C3H8 reactions.

The gas-phase reactions between metastable nitrogen atoms, N(2D) and saturated hydrocarbons CH4, C2H6 and C3H8 have been investigated using a supersonic flow reactor over the 296-75 K temperature range. N(2D) was generated as a product of the C(3P) + NO → N(2D) + CO reaction, with C(3P) atoms created in situ by pulsed laser photolysis of CBr4. The kinetics of N(2D) loss were followed by vacuum ultraviolet laser induced fluorescence. The measured rate constants for the N(2D) + CH4 reaction are in good agreement with earlier work and extend the available kinetic data for this process down to 127 K. The measured rate constants for the N(2D) + C2H6 and N(2D) + C3H6 reactions are in reasonable agreement with previous work at room temperature and extend the available kinetic data for these processes down to 75 K. The rate constants for all three reactions decrease as the temperature falls, indicating the presence of activation barriers for these processes. While the recommended values for the low temperature rate constants of the N(2D) + CH4 reaction are close to those reported here, the previous recommendations for the other saturated hydrocarbon reactions significantly overestimate the rate constants for these processes. The effects of the new rate constants on a coupled ion-neutral photochemical model of Titan's atmosphere are discussed.

Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical.

Diborenes, R-BB-R', are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B2H6, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner-Teller splitting in the X+ 2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron-boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron-boron bond length, RBB, from 1.514 to 1.606 Å.

A low temperature investigation of the gas-phase N(2D) + NO reaction. Towards a viable source of N(2D) atoms for kinetic studies in astrochemistry.

The gas-phase reaction of metastable atomic nitrogen N(2D) with nitric oxide has been investigated over the 296-50 K temperature range using a supersonic flow reactor. As N(2D) could not be produced photolytically in the present work, these excited state atoms were generated instead through the C(3P) + NO → N(2D) + CO reaction while C(3P) atoms were created in situ by the 266 nm pulsed laser photolysis of CBr4 precursor molecules. The kinetics of N(2D) atoms were followed on-resonance by vacuum ultraviolet laser induced fluorescence at 116.7 nm. The measured rate constants for the N(2D) + NO reaction are in excellent agreement with most of the earlier work at room temperature and represent the only available kinetic data for this process below 296 K. The rate constants are seen to increase slightly as the temperature falls to 100 K with a more substantial increase at even lower temperature; a finding which is not reproduced by theoretical work. The prospects for using this chemical source of N(2D) atoms in future studies of a wide range of N(2D) atom reactions are discussed.

Rate Constants and H-Atom Product Yields for the Reactions of O(1D) Atoms with Ethane and Acetylene from 50 to 296 K.

The gas phase reactions of atomic oxygen in its first excited state with ethane and acetylene have been investigated in a continuous supersonic flow reactor over the temperature range from 50 to 296 K. O(1D) atoms were produced by the pulsed laser photolysis of ozone at 266 nm. Two different types of experiments, kinetics measurements and H-atom product yield determinations, were performed by detecting O(1D) atoms and H(2S) atoms, respectively, by vacuum ultraviolet laser-induced fluorescence. The measured rate constants are in agreement with previous work at room temperature, and little or no temperature dependence was observed as the temperature was decreased to 50 K. H-atom yields were found to be independent of temperature for the reaction of O(1D) with ethane. These product yields are discussed in the context of earlier dynamics measurements at higher temperature. Due to the influence of secondary reactions, no H-atom yields could be obtained for the reaction of O(1D) with acetylene.

Kinetics of the Gas-Phase O(1D) + CO2 and C(1D) + CO2 Reactions over the 50-296 K Range.

The kinetics of the reactions of CO2 with atomic oxygen and atomic carbon in their first excited singlet states have been studied at room temperature and below using the Laval nozzle reactor method. O(1D) and C(1D) atoms were created in situ by the 266 nm pulsed laser photolysis of O3 and CBr4 precursor molecules, respectively. While O(1D) atoms were detected directly by vacuum ultraviolet laser-induced fluorescence at 115 nm, C(1D) atoms were followed indirectly through a chemical tracer method. The measured rate constants for the O(1D) + CO2 reaction are found to be in excellent agreement with earlier work and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C(1D) + CO2 reaction below room temperature. Both reactions display rate constants that increase as the temperature falls, with a more substantial rate increase observed for the O(1D) + CO2 reaction. This finding could be due to the increasing lifetimes of the intermediate species involved at lower temperatures.

Experimental and theoretical investigation of the temperature dependent electronic quenching of O(1D) atoms in collisions with Kr.

The kinetics and dynamics of the collisional electronic quenching of O(1D) atoms by Kr have been investigated in a joint experimental and theoretical study. The kinetics of quenching were measured over the temperature range 50-296 K using the Laval nozzle method. O(1D) atoms were prepared by 266 nm photolysis of ozone, and the decay of the O(1D) concentration was monitored through vacuum ultraviolet fluorescence at 115.215 nm, from which the rate constant was determined. To interpret the experiments, a quantum close-coupling treatment of the quenching transition from the 1D state to the 3Pj fine-structure levels in collisions with Kr, and also Ar and Xe, was carried out. The relevant potential energy curves and spin-orbit coupling matrix elements were obtained in electronic structure calculations. We find reasonable agreement between computed temperature-dependent O(1D)-Rg (Rg = Ar, Kr, Xe) quenching rate constants and the present measurements for Kr and earlier measurements. In particular, the temperature dependence is well described.

A combined theoretical and experimental investigation of the kinetics and dynamics of the O(1D) + D2 reaction at low temperature.

The O(1D) + H2 reaction is a prototype for simple atom-diatom insertion type mechanisms considered to involve deep potential wells. While exact quantum mechanical methods can be applied to describe the dynamics, such calculations are challenging given the numerous bound quantum states involved. Consequently, efforts have been made to develop alternative theoretical strategies to portray accurately the reactive process. Here we report an experimental and theoretical investigation of the O(1D) + D2 reaction over the 50-296 K range. The calculations employ three conceptually different approaches - mean potential phase space theory, the statistical quantum mechanical method and ring polymer molecular dynamics. The calculated rate constants are in excellent agreement over the entire temperature range, exhibiting only weak temperature dependence. The agreement between experiment and theory is also very good, with discrepancies smaller than 26%. Taken together, the present and previous theoretical results validate the hypothesis that long-lived complex formation dominates the reaction dynamics at low temperature.

The interstellar chemistry of C3H and C3H2 isomers.

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 → H + c-C3H2 and H + t-C3H2 → H + c-C3H2 isomerization reactions.

Kinetic and Product Study of the Reactions of C(1D) with CH4 and C2H6 at Low Temperature.

The reactions of atomic carbon in its first excited 1D state with both CH4 and C2H6 have been investigated using a continuous supersonic flow reactor over the 50-296 K temperature range. C(1D) atoms were generated in situ by the pulsed laser photolysis of CBr4 at 266 nm. To follow the reaction kinetics, product H atoms were detected by vacuum ultraviolet laser-induced fluorescence at 121.567 nm. Absolute H-atom yields for both reactions were determined by comparison with the H-atom signal generated by the reference C(1D) + H2 reaction. Although the rate constant for the C(1D) + CH4 reaction is in excellent agreement with earlier work at room temperature, this process displays a surprising reactivity increase below 100 K. In contrast, the reactivity of the C(1D) + C2H6 system decreases as the temperature falls, obeying a capture-type rate law. The H-atom product yields of the C(1D) + CH4 reaction agree with the results of earlier crossed-beam experiments at higher collision energy. Although no previous data is available on the product channels of the C(1D) + C2H6 reaction, comparison with earlier work involving the same singlet C3H6 potential energy surface allows us to draw conclusions from the measured H-atom yields.

Quantum Tunneling Enhancement of the C + H2O and C + D2O Reactions at Low Temperature.

Recent studies of neutral gas-phase reactions characterized by barriers show that certain complex forming processes involving light atoms are enhanced by quantum mechanical tunneling at low temperature. Here, we performed kinetic experiments on the activated C((3)P) + H2O reaction, observing a surprising reactivity increase below 100 K, an effect that is only partially reproduced when water is replaced by its deuterated analogue. Product measurements of H- and D-atom formation allowed us to quantify the contribution of complex stabilization to the total rate while confirming the lower tunneling efficiency of deuterium. This result, which is validated through statistical calculations of the intermediate complexes and transition states has important consequences for simulated interstellar water abundances and suggests that tunneling mechanisms could be ubiquitous in cold dense clouds.

Reacción de reducción de Cr(VI) a Cr(III) por iones sulfito en presencia de iones fosfato / Reduction reaction of Cr(VI) to Cr(III) by sulfite ions in the presence of phosphate ions

La sinovitis crónica, complicación frecuente en artritis reumatoide y hemofi lia, es tratada con éxito mediante inyecciones intrarticulares de dispersiones radiactivas (Radiosinoviortesis). En Cuba está en uso una formulación de Fosfato Crómico marcado con . A fin de aumentar el escalado, perfeccionar la tecnología y obtener formulaciones con otros radionúclidos, se estudia la reacción de obtención del precipitado, basada en la reducción de Cr(VI) a Cr(III) con iones sulfito en presencia de iones fosfato. Si la relación / es 18.5 veces, la precipitación tiene lugar próximo a las dos horas a temperatura ambiente y el pH crece al inicio y permanece luego constante a 7.63 por acción buffer del sulfito. Se estableció que la reducción de Cr(VI) a Cr(III) a ese pH sigue una reacción de segundo orden. La formación de especies insolubles de cromo y fósforo favorece la reducción. A 70 ºC la precipitación ocurre de inmediato. Por el método de los indicadores radiactivos se establece que la relación Cr/P = 1.6 en el precipitado, tanto a temperatura ambiente como a 70 ºC. En este caso, cada componente de la razón y la masa disminuyen en un 8 %, lo que indica probablemente se trate de un solo producto. El calor acelera la formación del precipitado y lo disuelve en parte. Si / = 1.5, el precipitado se forma de inmediato a temperatura ambiente alrededor de pH 5. Bajo las condiciones utilizadas, las especies de cromo en el precipitado aumentan con el pH. Se sugieren posibles composiciones para los productos obtenidos.

Chronic sinovitis, a frequent complication in rheumatoid arthritis and hemophilia, is successfully treated by means of intrarticular injections of radioactive dispersions (Radiosynoviorthesis). In Cuba, a formulation of chromic phosphate labeled with is used. In order to scale up the process, improve the technology and obtain formulations with other radionuclides, the reaction, based on the reduction of Cr(VI) to Cr(III) with sulphite ions in presence of ions phosphate is studied. If the relationship / is 18.5 times, the precipitation takes place at room temperature roughly at two hours and the pH increases at the beginning and then remains constant at 7.63 because of the buffer action of the sulphite. It was established that the reduction of Cr(VI) to Cr(III) at this pH is a second order reaction. The formation of insoluble species of chromium and phosphor, favors the reduction. At 70 ºC the precipitation occurs immediately. Using the method of radioactive indicators, it was established that the relationship Cr/P in the precipitate, at room temperature and at 70 ºC is 1.6. In this case, each component of the relationship and the mass diminish in 8 %, which indicates we are probably in presence of the same product. The heat accelerates the formation of the precipitate and partially dissolves it. If / = 1.5, and the pH value around 5, the precipitate is formed immediately at room temperature. Under the used experimental conditions, the chromium species in the precipitate increase as the pH increases. Possible compositions for the obtained products are suggested.