Effects of vibrational excitation on the F + H2O → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH].

The reaction F + H2O → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F-(H2O) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on ab initio potential energy surfaces. Prior to photodetachment at hνUV = 4.80 eV, the overtone of the ionic hydrogen bond mode in the precursor F-(H2O), 2νIHB at 2885 cm-1, was excited using a tunable IR laser. Experiment and theory show that vibrational energy in the anion can be effectively carried away by the photoelectron upon a Franck-Condon photodetachment, and also show evidence for an increase of branching into the F + H2O reactant channel. The experimental results suggest a greater role for product rotational excitation than theory. Improved potential energy surfaces and longer wavepacket propagation times would be helpful to further examine the nature of the discrepancy.

Chemical dynamics simulations of the monohydrated OH(-)(H2O) + CH3I reaction. Atomic-level mechanisms and comparison with experiment.

Direct dynamics simulations, with B97-1/ECP/d theory, were performed to study the role of microsolvation for the OH(-)(H2O) + CH3I reaction. The SN2 reaction dominates at all reactant collision energies, but at higher collision energies proton transfer to form CH2I(-), and to a lesser extent CH2I(-) (H2O), becomes important. The SN2 reaction occurs by direct rebound and stripping mechanisms, and 28 different indirect atomistic mechanisms, with the latter dominating. Important components of the indirect mechanisms are the roundabout and formation of SN2 and proton transfer pre-reaction complexes and intermediates, including [CH3--I--OH](-). In contrast, for the unsolvated OH(-) + CH3I SN2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH(-)(H2O) + CH3I SN2 reaction are in good agreement with experiment with respect to reaction rate constant, product branching ratio, etc. Differences between simulation and experiment are present for the SN2 velocity scattering angle at high collision energies and the proton transfer probability at low collision energies. Equilibrium solvation by the H2O molecule is unimportant. The SN2 reaction is dominated by events in which H2O leaves the reactive system as CH3OH is formed or before CH3OH formation. Formation of solvated products is unimportant and participation of the (H2O)CH3OH---I(-) post-reaction complex for the SN2 reaction is negligible.

Spectroscopy and dynamics of the HOCO radical: insights into the OH + CO → H + CO2 reaction.

After more than forty years of scrutiny, crucial new details regarding the elementary reaction OH + CO → H + CO2 are still emerging from experimental and theoretical studies of the HOCO radical intermediate. In this perspective, previous studies of this elementary reaction and the structure and energetics of the HOCO radical will be briefly reviewed. Particular attention will be paid to the experimental techniques used in our laboratory to prepare excited HOCO radicals by both photodetachment and dissociative photodetachment of HOCO(-). These experiments directly probe the dynamics occurring on the ground and excited states of the HOCO radical, and are sensitive to both direct and tunneling-induced dissociation. Photoelectron-photofragment coincidence experiments on HOCO(-) in particular have been used to study tunneling from the HOCO well to form H + CO2 products. In addition, new experimental insights into the OH + CO entrance channel for the reaction will be presented. These studies have provided a number of constraints on the potential energy surface for this system from an energetic and dynamical perspective, and have helped spur a renewed effort to characterize the global potential energy surface and reaction dynamics of this fundamental chemical reaction. Outstanding questions and new directions for future work on the HOCO radical will be discussed.

Identification of atomic-level mechanisms for gas-phase X- + CH3Y SN2 reactions by combined experiments and simulations.

For the traditional model of gas-phase X(-) + CH3Y SN2 reactions, C3v ion-dipole pre- and postreaction complexes X(-)---CH3Y and XCH3---Y(-), separated by a central barrier, are formed. Statistical intramolecular dynamics are assumed for these complexes, so that their unimolecular rate constants are given by RRKM theory. Both previous simulations and experiments have shown that the dynamics of these complexes are not statistical and of interest is how these nonstatistical dynamics affect the SN2 rate constant. This work also found there was a transition from an indirect, nonstatistical, complex forming mechanism, to a direct mechanism, as either the vibrational and/or relative translational energy of the reactants was increased. The current Account reviews recent collaborative studies involving molecular beam ion-imaging experiments and direct (on-the-fly) dynamics simulations of the SN2 reactions for which Cl(-), F(-), and OH(-) react with CH3I. Also considered are reactions of the microsolvated anions OH(-)(H2O) and OH(-)(H2O)2 with CH3I. These studies have provided a detailed understanding of the atomistic mechanisms for these SN2 reactions. Overall, the atomistic dynamics for the Cl(-) + CH3I SN2 reaction follows those found in previous studies. The reaction is indirect, complex forming at low reactant collision energies, and then there is a transition to direct reaction between 0.2 and 0.4 eV. The direct reaction may occur by rebound mechanism, in which the ClCH3 product rebounds backward from the I(-) product or a stripping mechanism in which Cl(-) strips CH3 from the I atom and scatters in the forward direction. A similar indirect to direct mechanistic transition was observed in previous work for the Cl(-) + CH3Cl and Cl(-) + CH3Br SN2 reactions. At the high collision energy of 1.9 eV, a new indirect mechanism, called the roundabout, was discovered. For the F(-) + CH3I reaction, there is not a transition from indirect to direct reaction as Erel is increased. The indirect mechanism, with prereaction complex formation, is important at all the Erel investigated, contributing up ∼60% of the reaction. The remaining direct reaction occurs by the rebound and stripping mechanisms. Though the potential energy curve for the OH(-) + CH3I reaction is similar to that for F(-) + CH3I, the two reactions have different dynamics. They are akin, in that for both there is not a transition from an indirect to direct reaction. However, for F(-) + CH3I indirect reaction dominates at all Erel, but it is less important for OH(-) + CH3I and becomes negligible as Erel is increased. Stripping is a minor channel for F(-) + CH3I, but accounts for more than 60% of the OH(-) + CH3I reaction at high Erel. Adding one or two H2O molecules to OH(-) alters the reaction dynamics from that for unsolvated OH(-). Adding one H2O molecule enhances indirect reaction at low Erel, and changes the reaction mechanism from primarily stripping to rebound at high Erel. With two H2O molecules the dynamics is indirect and isotropic at all collision energies.

Imaging dynamics on the F + H2O -> HF + OH potential energy surfaces from wells to barriers.

The study of gas-phase reaction dynamics has advanced to a point where four-atom reactions are the proving ground for detailed comparisons between experiment and theory. Here, a combined experimental and theoretical study of the dissociation dynamics of the tetra-atomic FH2O system is presented, providing snapshots of the F + H2O → HF + OH reaction. Photoelectron-photofragment coincidence measurements of the dissociative photodetachment (DPD) of the F(-)(H2O) anion revealed various dissociation pathways along different electronic states. A distinct photoelectron spectrum of stable FH-OH complexes was also measured and attributed to long-lived Feshbach resonances. Comparison to full-dimensional quantum calculations confirms the sensitivity of the DPD measurements to the subtle dynamics on the low-lying FH2O potential energy surfaces over a wide range of nuclear configurations and energies.

High resolution spatial map imaging of a gaseous target.

Electrostatic ion imaging with the velocity map imaging mode is a widely used method in atomic and molecular physics and physical chemistry. In contrast, the spatial map imaging (SMI) mode has received very little attention, despite the fact that it has been proposed earlier [A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997)]. Here, we present a detailed parametric characterization of SMI both by simulation and experiment. One-, two- and three-dimensional imaging modes are described. The influence of different parameters on the imaging process is described by means of a Taylor expansion. To experimentally quantify elements of the Taylor expansion and to infer the spatial resolution of our spectrometer, photoionization of toluene with a focused laser beam has been carried out. A spatial resolution of better than 4 µm out of a focal volume of several mm in diameter has been achieved. Our results will be useful for applications of SMI to the characterization of laser beams, the overlap control of multiple particle or light beams, and the determination of absolute collision cross sections.

Direct dynamics simulations of the product channels and atomistic mechanisms for the OH(-) + CH3I reaction. Comparison with experiment.

Electronic structure and direct dynamics calculations were used to study the potential energy surface and atomic-level dynamics for the OH(-) + CH3I reactions. The results are compared with crossed molecular beam, ion imaging experiments. The DFT/B97-1/ECP/d level of theory gives reaction energetics in good agreement with experiment and higher level calculations, and it was used for the direct dynamics simulations that were performed for reactant collision energies of 2.0, 1.0, 0.5, and 0.05 eV. Five different pathways are observed in the simulations, forming CH3OH + I(-), CH2I(-) + H2O, CH2 + I(-) + H2O, IOH(-) + CH3, and [CH3--I--OH](-). The SN2 first pathway and the proton-transfer second pathway dominate the reaction dynamics. Though the reaction energetics favor the SN2 pathway, the proton-transfer pathway is more important except for the lowest collision energy. The relative ion yield determined from the simulations is in overall good agreement with experiment. Both the SN2 and proton-transfer pathways occur via direct rebound, direct stripping, and indirect mechanisms. Except for the highest collision energy, 70-90% of the indirect reaction for the SN2 pathway occurs via formation of the hydrogen-bonded OH(-)---HCH2I prereaction complex. For the proton-transfer pathway the indirect reaction is more complex with the roundabout mechanism and formation of the OH(-)---HCH2I and CH2I(-)---HOH complexes contributing to the reaction. The majority of the SN2 reaction is direct at 2.0, 1.0, and 0.5 eV, dominated by stripping. At 0.05 eV the two direct mechanisms and the indirect mechanisms have nearly equal contributions. The majority of the proton-transfer pathway is direct stripping at 2.0, 1.0, and 0.5 eV, but the majority of the reaction is indirect at 0.05 eV. The product relative translational energy distributions are in good agreement with experiment for both the SN2 and proton-transfer pathways. For both, direct reaction preferentially transfers the product energy to relative translation, whereas transfer to product vibration is more important for the indirect reactions. For the proton-transfer reactions the velocity scattering angle distribution is peaked in the forward direction and in quite good agreement with experiment. However, for the SN2 reaction, the experimental scattering is isotropic in nature whereas forward scattering dominates the simulation distributions. The implication is that the simulations give too much stripping, which leads to forward scattering. The dynamics for the OH(-) + CH3I SN2 pathway are similar to those found previously for the F(-) + CH3I SN2 reaction.

Indirect dynamics in a highly exoergic substitution reaction.

The highly exoergic nucleophilic substitution reaction F(-) + CH3I shows reaction dynamics strikingly different from that of substitution reactions of larger halogen anions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH3F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl(-) + CH3I [Science 2008, 319, 183-186] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747-2752]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F(-)-HCH2I hydrogen-bonded complex, a direct rebound, and a direct stripping reaction. The indirect mechanism is found to contribute about one-half of the overall substitution reaction rate at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, because the barrier for the F(-)-HCH2I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.

Internal state thermometry of cold trapped molecular anions.

Photodetachment spectroscopy of OH(-) and H(3)O(2)(-) anions has been performed in a cryogenic 22-pole radiofrequency multipole trap. Measurements of the detachment cross section as a function of laser frequency near threshold have been analysed. Using this bound-free spectroscopy approach we could demonstrate rotational and vibrational cooling of the trapped anions by the buffer gas in the multipole trap. Below 50 K the OH(-) rotational temperature shows deviations from the buffer gas temperature, and possible causes for this are discussed. For H(3)O(2)(-) vibrational cooling of the lowest vibrational quantum states into the vibrational ground state is observed. Its photodetachment cross section near threshold is modelled with a Franck-Condon model, with a detachment threshold that is lower, but still in agreement with the expected threshold for this system.