Quantum state-resolved reactive scattering of F+CH4→HF(v,J)+CH3: Nascent HF(v,J) product state distributions

Non-equilibrium dynamics at the gas-liquid interface: State-resolved studies of NO evaporation from a benzyl alcohol liquid microjet

First measurements of internal quantum-state distributions for nitric oxide (NO) evaporating from liquid benzyl alcohol are presented over a broad range of temperatures, performed by liquid-microjet techniques in an essentially collision-free regime, with rotational/spin-orbit populations in the ²Π_1/2,3/2 manifolds measured by laser-induced fluorescence. The observed rotational distributions exhibit highly linear (i.e., thermal) Boltzmann plots but notably reflect rotational temperatures (T_rot) as much as 30 K lower than the liquid temperature (T_jet). A comparable lack of equilibrium behavior is also noted in the electronic degrees of freedom but with populations corresponding to spin-orbit temperatures (T_SO) consistently higher than T_rot by ∼15 K. These results unambiguously demonstrate evaporation into a non-equilibrium distribution, which, by detailed-balance considerations, predict quantum-state-dependent sticking coefficients for incident collisions of NO at the gas-liquid interface. Comparison and parallels with previous experimental studies of NO thermal desorption and molecular-beam scattering in other systems are discussed, which suggests the emergence of a self-consistent picture for the non-equilibrium dynamics.

Benchmark ab initio stationary-point characterization of the complex potential energy surface of the multi-channel Cl + CH3NH2 reaction

We characterize the exothermic low/submerged-barrier hydrogen-abstraction (HCl + CH2NH2/CH3NH) as well as, for the first time, the endothermic high-barrier amino-substitution (CH3Cl + NH2), methyl-substitution (NH2Cl + CH3), and hydrogen-substitution (CH2ClNH2/CH3NHCl + H) pathways of the Cl + CH3NH2 reaction using an accurate composite ab initio approach. The computations reveal a CH3NH2Cl complex in the entrance channel, nine transition states corresponding to different abstractions, Walden-inversion substitution, and configuration-retaining front-side attack substitution pathways, as well as nine post-reaction complexes. The global minima of the electronic and vibrationally adiabatic potential energy surfaces correspond to the pre-reaction CH3NH2Cl and post-reaction CH2NH2HCl complexes, respectively. The benchmark composite energies of the stationary points are obtained by considering basis-set effects up to the correlation-consistent polarized valence quadruple-zeta basis augmented with diffuse functions (aug-cc-pVQZ) using the explicitly-correlated coupled-cluster singles, doubles, and perturbative triples CCSD(T)-F12b method, post-(T) correlation up to CCSDT(Q) including full triples and perturbative quadruples, core correlation, and scalar relativistic and spin-orbit effects, as well as harmonic zero-point energy corrections.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.

Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν)

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1. The νb (ν4) vibrational mode denotes the bending (umbrella) motion of the CH3D reactant (CH2D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward-backward peakings in the Ec-evolution of the two dominant pair-correlated angular distributions at lower Ec suggest a resonance-mediated, time-delay mechanism. As Ec increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH3D(νb = 1) promotes the reaction rate by two- to three-fold up to Ec = 2.1 kcal mol-1. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH2D(00) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.

Natural reaction channels in H + CHD3 → H2 + CD3

Natural reaction channels control the mode-specific chemistry of methane and its isotopomeres.

Role of an ethyl radical and the problem of HF(v) bimodal vibrational distribution in the F(2P) + C2H6 → HF(v) + C2H5 reaction

A theoretical study of the dynamics of the F(²P) + C₂H₆ hydrogen abstraction reaction was presented using quasi-classical trajectories propagated on an ab initio fitted global potential energy surface, PES-2018.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

Quasiclassical trajectory calculations of correlated product distributions for the F + CHD3(v1 = 0, 1) reactions using an ab initio potential energy surface

We report quasiclassical trajectory (QCT) calculations of the correlated product distributions and branching ratios of the reactions F+CHD(3)(v(1)=0,1)-->HF(v)+CD(3)(v) and DF(v)+CHD(2)(v) using a recently published ab initio-based full-dimensional global potential energy surface [G. Czako et al., J. Chem. Phys. 130, 084301 (2009)]. Harmonic normal mode analysis is done for the methyl products to determine the classical actions of each normal mode and then standard histogram binning and Gaussian binning (GB) methods are employed to obtain quantum state-resolved probabilities of the products. QCT calculations have been performed for both the vibrationally ground state and the CH stretching excited F+CHD(3)(v(1)=0,1) reactions at eight different collision energies in the 0.5-7.0 kcal/mol range. HF and DF vibrationally state-resolved rotational and angular distributions, CD(3) and CHD(2) mode-specific vibrational distributions, and correlated vibrationally state-specific distributions for the product pairs have been obtained and the correlated results were compared to the experiment. We find that the use of GB can be advantageous especially in the threshold regions. The CH stretching excitation in the reactant does not change the CD(3) vibrational distributions significantly, whereas the HF molecules become vibrationally and rotationally hotter. On the other hand in the case of the DF+CHD(2) channel the initially excited CH stretch appears mainly "intact" in the CHD(2) product and the DF distributions are virtually the same as formed from the ground state CHD(3) reaction. The computed results qualitatively agree with recent crossed molecular beam experiment [W. Zhang et al., Science 325, 303 (2009)] that (a) CHD(2)(v(1)=1) is the most populated product state of the F+CHD(3)(v(1)=1) reaction and this reaction produces much less CHD(2)(v=0) compared to the reaction F+CHD(3)(v=0); (b) the cross section ratio of CHD(2)(v(1)=1):CHD(2)(v=0) formed from the reactions F+CHD(3)(v(1)=1):F+CHD(3)(v=0) is less than 1 and shows little collision energy dependency; (c) the reactant CH stretch excitation increases the DF:HF ratio at low collision energies; (d) the correlated vibrational and angular distributions for DF(v)+CHD(2)(v(1)=0,1) from the ground state and stretch-excited reactions, respectively, are almost identical.

Quantum state resolved scattering dynamics of F+HCl→HF(v,J)+Cl

State-to-state reaction dynamics of the reaction F+HCl-->HF(v,J)+Cl have been studied under single-collision conditions using an intense discharge F atom source in crossed supersonic molecular beams at Ecom=4.3(1.3) kcal/mol. Nascent HF product is monitored by shot-noise limited direct infrared laser absorption, providing quantum state distributions as well as additional information on kinetic energy release from high resolution Dopplerimetry. The vibrational distributions are highly inverted, with 34(4)%, 44(2)%, and 8(1)% of the total population in vHF=1, 2, and 3, respectively, consistent with predominant energy release into the newly formed bond. However, there is a small [14(1)%] but significant formation channel into the vHF=0 ground state, which is directly detectable for the first time via direct absorption methods. Of particular dynamical interest, both the HF(v=2,J) and HF(v=1,J) populations exhibit strongly bimodal J distributions. These results differ significantly from previous flow and arrested-relaxation studies and may signal the presence of microscopic branching in the reaction dynamics.

New analytical potential energy surface for the F(2P)+CH4 hydrogen abstraction reaction: kinetics and dynamics

A new potential energy surface for the gas-phase F(2P)+CH4 reaction and its deuterated analogues is reported, and its kinetics and dynamics are studied exhaustively. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce the topology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they show good agreement with the sparse experimental data. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found that the FH(nu',j') rovibrational distributions agree with experiment. Second, the excitation function presents an oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributions present reasonable agreement with experiment, and as the FH(nu') vibrational state increases the scattering angle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabatic potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment (always qualitative and sometimes quantitative) lends confidence to the new surface.

Product pair correlation in bimolecular reactions

The vast majority of chemical reactions involve polyatomic species as reactants and/or products. The added degree of complexity offers opportunities to address dynamical questions other than those already encountered in a typical atom + diatom reaction. Product pair correlation is one of them. This article introduces the basic concept, outlines the experimental approach we developed and then highlights some of the applications to bimolecular reaction dynamics. Particular emphasis is placed on the information contents and unique insights gained from this type of measurements, which otherwise would have been lost by the conventional approach.

A crossed molecular beam study on the dynamics of F atom reaction with SiH4

In this report, the dynamics of the F+SiH4 reaction has been studied using the universal crossed molecular beam method. Angular resolved time-of-flight spectra have been measured for all reaction products in a single set of experiments. Two different reaction channels have been observed: HF+SiH3 and SiH3F+H. Product angular distributions as well as energy distributions were determined for these two product channels. Experimental results show that the HF product is forward scattered relative to the F atom beam direction, while the SiH3F product is backward scattered relative the F atom beam direction, suggesting that two reaction channels proceed with distinctive reaction dynamics. The relative branching ratios of the two channels have also been estimated.