The use of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) technique coupled with pulsed laser photochemical kinetics methods has shown that reactions involving radicals can be very rapid at temperatures down to 10 K or below. The results have had a major impact in astrochemistry and planetology, as well as proving an exacting test for theory.¹ The technique has also been applied to the formation of transient complexes of interest both in atmospheric chemistry² and combustion.³ Until now, all of the chemical reactions studied in this way have taken place on attractive potential energy surfaces with no overall barrier to reaction. The F + H₂ → HF + H reaction does possess a substantial energetic barrier ( ≅ 800 K), and might therefore be expected to slow to a negligible rate at very low temperatures. In fact, this H-atom abstraction reaction does take place efficiently at low temperatures due entirely to tunneling. I will report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations by François Lique (Université du Havre) and Millard Alexander (University of Maryland). ⁴ It is thought that long chain cyanopolyyne molecules H(C₂)_nCN may play an important role in the formation of the orange haze layer in Titan’s atmosphere. The longest carbon chain molecule observed in interstellar space, HC₁₁N, is also a member of this series. I will present new results, obtained in collaboration with Jean-Claude Guillemin (Ecole de Chimie de Rennes) and Stephen Klippenstein (Argonne National Labs), on reactions of C₂H, CN⁵ and C₃N radicals (using a new LIF scheme by Hoshina and Endo⁶) which contribute to the low temperature formation of (cyano)polyynes.
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¹H. Sabbah, L. Biennier, I. R. Sims, Y. Georgievskii, S. J. Klippenstein, I. W. M. Smith, Science 317, 102 (2007). ²S. D. Le Picard, M. Tizniti, A. Canosa, I. R. Sims, I. W. M. Smith, Science 328, 1258 (2010). ³H. Sabbah, L. Biennier, S. J. Klippenstein, I. R. Sims, B. R. Rowe, J. Phys. Chem. Lett. 1, 2962 (2010). ⁴ M. Tizniti, S. D. Le Picard, F. Lique, C. Berteloite, A. Canosa, M. H. Alexander, I. R. Sims, Nature Chemistry 6, 141 (2014). ⁵S. Cheikh Sid Ely, S. B. Morales, J. C. Guillemin, S. J. Klippenstein, I. R. Sims, J. Phys. Chem. A 117, 12155 (2013). ⁶K. Hoshina, Y. Endo, J. Chem. Phys. 127, 184304 (2007).

Reactions of migrating H-atoms in parahydrogen (pH₂) matrices with trapped molecular species provide a relatively unexplored yet well-established experimental method to study the kinetics and mechanisms of atom tunneling reactions in the 1.5 to 5 K temperature range. My group has now completed a series of experimental studies on the kinetics of reactions of H-atoms with HCOOH, CH₃OH, and N₂O which all show a pronounced inverse temperature dependence over this small temperature range.¹ Conversely, the analogous H-atom reaction with NO displays a more standard Arrhenius behavior. In this talk, I will present a brief summary of these results with the objective of developing a predictive understanding of the kinetics of these H-atom tunneling reactions. I will also emphasize the advantages of following the kinetics using rapid scan FTIR.
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¹Fredrick M. Mutunga, Shelby E. Follett, and David T. Anderson, J. Chem. Phys. 139, 151104 (2013).

The transduction of protons between donor and acceptor sites, as mediated by the action of adjoining hydrogen bonds, represents one of the most ubiquitous of chemical transformations. While the basic mechanisms underlying such phenomena often can be ascribed to simple acid-base chemistry, the putative roles of selective nuclear and electronic displacements should not be discounted, especially when the presence of a sizeable potential barrier impedes classical hydron-migration pathways. The vibrational and isotopic specificity of hindered intramolecular proton transfer taking place within the ground (~X¹A₁) and the lowest-lying excited (ùB₂ (π^*π) electronic states of the prototypical tropolone (TrOH) system has been probed by implementing multiple-color variants of resonant four-wave mixing (RFWM) spectroscopy, with polarization-resolved detection allowing for the extraction of quantitative rotation-tunneling information. The marked dependence of unimolecular dynamics on the extent and the type of excitation deposited into TrOH internal degrees of freedom will be discussed. Experimentally observed trends and propensities for tunneling-mediated reactivity will be interpreted through use of accompanying quantum-chemical calculations.

Photo-induced radical chemistry is crucial to atmospheric processes, namely: oxidation, particulate matter formation, and climate change. The combination of transient absorption and pulsed cavity ring-down spectroscopies is used to study weak electronic or overtone transitions of trace gas-phase species and investigate the kinetic and photochemical properties of important transient atmospheric radicals.

Figure We investigate the photolytic production of two radical intermediates in the reaction of OH with propene, one from addition of the hydroxyl radical to the terminal carbon and the other from addition to the center carbon. In a collision-free environment, we photodissociate a mixture of 1-bromo-2-propanol and 2-bromo-1-propanol at 193 nm to produce these radical intermediates. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling bromine atoms, yielding the distribution of kinetic energies of the nascent C₃H₆OH radicals + Br. Resolving the velocity distributions of Br(²P_1/2) and Br(²P_3/2) separately with 2+1 REMPI allows us to determine the total (vibrational + rotational) internal energy distribution in the nascent radicals. Using an impulsive model to estimate the rotational energy imparted to the nascent C₃H₆OH radicals, we predict the percentage of radicals having vibrational energy above and below the lowest dissociation barrier, that to OH + propene; it accurately predicts the measured velocity distribution of the stable C₃H₆OH radicals. In addition, we use photofragment translational spectroscopy to detect several dissociation products of the unstable C₃H₆OH radicals: OH + propene, methyl + acetaldehyde, and ethyl + formaldehyde. We also use the angular momenta of the unstable radicals to estimate the energy partitioned to relative kinetic energy when they dissociate to OH + propene, which agrees very well with the data.

CO₂ is known as a strong contributor to the greenhouse effect, and its concentration in the atmosphere increases annually. Photodissociation of CO₂ is considered an important photochemical sink of CO₂ molecules which could ultimately limit the increase of CO₂ concentration in the atmosphere. Since CO₂ molecules have negligibly small absorption from the visible region down to about 200 nm, photodissociation studies of CO₂ in the vacuum ultraviolet (VUV) region below 200 nm are of great importance in understanding the photochemical decomposition processes of CO₂ molecules. State-to-state photodissociation of CO₂ has been investigated by employing two independent VUV lasers and the time-slice velocity-map-imaging-photoion (VMI-PI) method. The spin-allowed photoproduct channels, CO(X¹Σ⁺) + O(¹D), CO(X¹Σ⁺) + O(¹S), CO(a³Π) + O(³P_J), and C(³P_J) + O₂(X³Σ_g⁻), and the spin-forbidden photoproduct channel, CO(X¹Σ⁺) + O(³P_J), were directly observed from the time-slice VMI-PI images. The angular anisotropic parameters were evaluated, allowing us to estimate the lifetimes for the formations of these dissociation channels. To the authors’ knowledge, the current CO₂ photodissociation studies show for the first time that all of the energetically available photodissociation channels are formed in the CO₂ photoexcitation energy of interest.

High frequency pure rotational transitions of the simplest Criegee intermediate, methylene peroxide (CH2OO), have been measured up to 1 THz. The data provide the most accurate spectroscopic parameters for the ground vibrational state. The molecule was produced in a flow cell with a DC discharge of CH₂I₂, O₂ and Ar. Doppler limited measurements were made in the frequency ranges 220-320, 580-680 and 970-1080 GHz at room temperature. A total of 211 transitions were measured and added to four transitions measured in the microwave to obtain a fit up to J^max = 49 and K^max = 14 of the a-dipole spectrum.

Criegee radicals are important atmospheric intermediates formed from ozonolysis of alkenes. It potentially contributes to the atmospheric oxidation cycle mainly by generating OH radicals through unimolecular decomposition. In this work, we focus on studying the unimolecular decomposition reaction of the smallest Criegee intermediate (CH₂OO), which was generated by reacting CH₂I with O₂. While generating the CH₂OO molecule by reacting CH₂I with O₂, significant amounts of the OH radical were observed by laser-induced fluorescence. The addition of molecules known to react with CH₂OO increased the observed decay rates of the OH signal. Using the OH signals as a proxy for the CH₂OO concentration, the rate constant for the reaction of hexafluoroacetone with CH₂OO was determined. The rate constant for the reaction of SO₂ with CH₂OO showed no pressure dependence over the range of 50 to 200 Torr. This work provides the direct experimental evidence for the unimolecular decomposition of CH₂OO, and possible mechanisms of CH₂OO have been investigated by this multidimensional study.

Effects of reactant rotation, which can disentangle the stereodynamical properties of chemical reactivity, are of great importance in understanding and controlling the steric effects in chemical reactions. Here, we report the rotational quantum-state control of a late-barrier reaction of chlorine atom with CH₄ and CHD₃ in a crossed molecular beam experiment. Experimental results demonstrate that, in both reactions, the more detailed product translational and angular distributions are essentially the same for different rotational states of the vibrationally excited CH₄ and CHD₃ reactants. Yet, the integral cross sections show strong dependence on the reactant rotational excitation, suggesting that the reactivity diversity arises from the anisotropic interactions enroute to the reaction barrier. More detailed analysis indicates that the effects of reactant rotation do not derive from the rotational-energy effects or long-range forces, rather are the result of short-range forces in the transition state region. Exactly how the transition-state properties, e.g., the barrier location and the tightness of barrier, influence the rotational reactivity diversity, however, remains unclear. Further investigations are on-going to gain deeper insights.

Rotational energy transfer and depolarization rates have been determined for fine-structure-selected rotational levels of the vibrationless ground state of CN (~X ²Σ⁺) radicals. Collisions with Ar, He and the photolytic precursor, CH₃COCN have been studied with polarized transient frequency modulation (FM) absorption spectroscopy. Transient FM signals were recorded as a function of probe laser detuning across Doppler-broadened lines of the Ã− ~X (1−0) band, monitoring the saturation recovery kinetics after a tunable dye laser pulse selectively bleached the probed CN rotational level. The refilling kinetics of a hole in an otherwise thermalized state distribution is identical to the hypothetical decay of the corresponding isolated level in an empty manifold, even to the extent of displaying hyperfine quantum beats in the hole alignment. The Doppler-resolved kinetics reflect a competition between the speed-dependent rotational energy transfer rates, which tend to cool the velocity distribution of the hole, and velocity-changing collisions, which tend to rethermalize the velocity distribution. The observations are of relevance to speed-dependent effects in pressure broadening, but measured under Doppler-limited pressure conditions. Elastic depolarization contributes significantly to the observed signals at low rotational states, negligibly so at high J. A strongly J-dependent contribution to the relaxation kinetics due to small amounts of the photolytic precursor cannot be neglected when extracting the rare-gas-dependent rate coefficients from the observed kinetics. Some qualitative differences are thereby found with previously published energy transfer studies on He or Ar + CN (~X, v=2) by Fei, et al. [J. Chem. Phys. 100, 1190 (1994); Chem. Phys. Lett. 232, 547 (1995)].
Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.

In an effort to understand how the Polanyi rules can be extended from radical-diatomic molecule reactions to radical-polyatomic molecule reactions, members of the bimolecular gas-phase dynamics community have often used H-atom abstraction from methane (or one of its isotopologues) by X (where X=H, F or Cl) as a model system. Each of these model reactions can be separately characterized by both the height and location of the potential energy barrier along the reaction coordinate. Currently, we are working to understand the gas-phase dynamics of H-atom abstraction from CH₄ when X=Br. In this iteration of the model system, the abstraction barrier is located very late along the reaction coordinate and is quite high by comparison to other studied systems. This leads to some surprising dynamical effects in the X=Br system that we have not seen in other systems studied thus far.

Ozonolysis reactions of alkenes are important oxidation pathways of alkenes in the atmosphere, and they are also significant sources of tropospheric hydroxyl radicals. In this work, ozone reactions with trans- and cis-2-butene are studied using cavity ring-down spectroscopy (CRDS). Vinoxy (CH₂CHO) radical, a proposed co-product of OH from dissociation of Criegee intermediates following the primary ozonolysis of 2-butenes, is directly observed. The vinoxy formation is found to decrease with increasing pressure. Formaldehyde (HCHO), a side-product in the ozonolysis reactions, is also monitored. Chemical kinetic modeling has been performed to illustrate the reaction mechanisms and to quantify the reactive intermediate and product yields.

Methylene, CH₂, possesses a ground ~X³B₁ ground electronic state and an excited ã¹A₁ state only 3150cm⁻¹ higher in energy. The collision-induced singlet-triplet crossing in the gaseous mixtures is important in determining overall reaction rates and chemical behavior. Accidental near-degeneracies between rotational levels of the singlet state and the vibrationally excited triplet state result in a few gateway rotational levels that mediate collision-induced intersystem crossing. The mixed states can be recognized and quantified by deperturbation, knowing the zero-order singlet and triplet energy levels. Hyperfine structure can be used as alternative indicator of singlet-triplet mixing. Non-zero mixing will induce hyperfine splittings intermediate between the unresolved hyperfine structure of pure singlet and the resolvable ( ≈ 50MHz) splittings of pure triplet, arising from the (I·S) interaction in the ortho states, where nuclear spin I=1¹. Collision-induced intersystem crossing rates from the (010) state are comparable to those for (000)², yet the identities and characters of the presumed gateway states are unknown. A new spectrometer is under construction to investigate triplet mixing rotational levels of ã¹A₁(010) by sub-Doppler measurements of perturbation-induced hyperfine splittings. Their observation will permit the identification of gateway states and quantification of the degree of triplet contamination of the singlet wavefunction. Progress in the measurements and the analysis of rotational energy transfer in (010) will be reported. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.
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¹C.-H. Chang, G. E. Hall, T. J. Sears, J. Chem. Phys 133, 144310(2010) ²G. E. Hall, A. V. Komissarov, and T. J. Sears, J. Phys. Chem. A 108 7922-7927 (2004)

We present the results of quantum dynamics studies of photodissociation of BrCN⁻ following electronic excitation to states that dissociate to Br⁻ + CN and Br* + CN⁻. The electronic structure of BrCN⁻ was evaluated with MRCI-SO/aug-cc-pVTZ at a fixed CN distance of 1.18 Å. The calculations were used to evaluate the two-dimensional potential surfaces for the ground and excited states as functions of the Br-CN(center of mass) distances, R, and angles, θ, between CN and R. A diabatic model developed for the two relevant excited states shows a dramatic change in the electronic character of the states near the BrCN⁻ geometry when θ ≤ π/2. The quantum dynamics studies on the bare BrCN⁻ were carried out by exciting wave packets of six vibrational states of BrCN⁻ that are thermally populated at 150K onto each of the two excited states. Upon excitations of the wave packets onto the state accessible in the visible region, 51% Br⁻ and 49% CN⁻ photoproducts were calculated. Similar calculations in the UV region yielded 56% Br⁻ and 44% CN⁻ photoproducts. Experimentally, when BrCN⁻ is excited in the visible region, 60% Br⁻ and 40% CN⁻ photoproducts are obtained while 80% Br⁻ and 20% CN⁻ yields were obtained in the UV region. Further dynamics studies of BrCN⁻ solvated in argon will be carried out with varying Ar_n(BrCN⁻) cluster sizes.