Product Internal Energy Distributions Research Articles

Dynamics of the Cl- + CH3I reaction on a high-level abinitio analytical potential energy surface.

We have developed a full-dimensional analytical abinitio potential energy surface (PES) for the Cl- + CH3I reaction using the Robosurfer program system. The energy points have been computed using a robust composite method defined as CCSD-F12b + BCCD(T) - BCCD with the aug-cc-pVTZ(-PP) basis set and have been fitted by the permutationally invariant polynomial approach. Quasi-classical trajectory simulations on the new PES reveal that two product channels are open in the collision energy (Ecoll) range of 1-80 kcal/mol, i.e., SN2 leading to I- + CH3Cl and iodine abstraction (above ∼45 kcal/mol) resulting in ICl- + CH3. Scattering angle, initial attack angle, product translational energy, and product internal energy distributions show that the SN2 reaction is indirect at low Ecoll and becomes direct-rebound-back-side (CH3-side) attack-type, as Ecoll increases. Iodine abstraction mainly proceeds with direct stripping mechanism with side-on/back-side attack preference. Comparison with crossed-beam experiments and previous direct dynamics simulations shows quantitative or qualitative agreement and also highlights possible theoretical and/or experimental issues motivating further research.

Full-dimensional automated potential energy surface development and dynamics for the OH + C2H6 reaction.

We develop a full-dimensional analytical potential energy surface (PES) for the OH + C2H6 reaction using the Robosurfer program system, which automatically (1) selects geometries from quasi-classical trajectories, (2) performs ab initio computations using a coupled-cluster singles, doubles, and perturbative triples-F12/triple-zeta-quality composite method, (3) fits the energies utilizing the permutationally invariant monomial symmetrization approach, and (4) iteratively improves the PES via steps (1)-(3). Quasi-classical trajectory simulations on the new PES reveal that hydrogen abstraction leading to H2O + C2H5 dominates in the collision energy range of 10-50 kcal/mol. The abstraction cross sections increase and the dominant mechanism shifts from rebound (small impact parameters and backward scattering) to stripping (larger impact parameters and forward scattering) with increasing collision energy as opacity functions and scattering angle distributions indicate. The abstraction reaction clearly favors side-on OH attack over O-side and the least-preferred H-side approach, whereas C2H6 behaves like a spherical object with only slight C-C-perpendicular side-on preference. The collision energy efficiently flows into the relative translation of the products, whereas product internal energy distributions show only little collision energy dependence. H2O/C2H5 vibrational distributions slightly/significantly violate zero-point energy and are nearly independent of collision energy, whereas the rotational distributions clearly blue-shift as the collision energy increases.

Final-State-Resolved Dynamics of the H3+ + CO → H2 +HCO+/HOC+ Reaction: A Quasi-Classical Trajectory Study.

The ion-molecule reaction H3+ + CO → H2 + HCO+/HOC+, which initiates the formation of crucial organic molecules, plays a key role in interstellar and circumstellar environments. In this work, the quasi-classical trajectory method is employed to study the reaction dynamics on a recently developed full-dimensional global potential energy surface (PES). The calculated product internal energy distributions and relative internal excited fractions agree reasonably well with the experimental measurements. For the two reaction channels, most of the available energy flows into the vibrational modes of HCO+ or HOC+ at low collision energies, followed by the translational mode and the rotational modes of HCO+ or HOC+. As the collision energy increases, the proportion of the product translational energy increases while the proportion of the product vibrational energy decreases. Furthermore, the CH and CO stretching modes and their combination bands are effectively excited for the product HCO+ while the bending mode is remarkably excited for the product HOC+.

Mode-Specific Quasiclassical Dynamics of the F- + CH3I SN2 and Proton-Transfer Reactions.

Mode-specific quasiclassical trajectory computations are performed for the F- + CH3I( v k = 0, 1) SN2 and proton-transfer reactions at nine different collision energies in the range of 1.0-35.3 kcal/mol using a full-dimensional high-level ab initio analytical potential energy surface with ground-state and excited CI stretching ( v3), CH3 rocking ( v6), CH3 umbrella ( v2), CH3 deformation ( v5), CH symmetric stretching ( v1), and CH asymmetric stretching ( v4) initial vibrational modes. Millions of trajectories provide statistically definitive mode-specific cross sections, opacity functions, scattering angle distributions, and product internal energy distributions. The excitation functions reveal slight vibrational SN2 inversion inhibition/enhancement at low/high collision energies ( Ecoll), whereas large decaying-with- Ecoll vibrational enhancement effects for the SN2 retention (double inversion) and proton-transfer channels. The most efficient vibrational enhancement is found by exciting the CI stretching (high Ecoll) for SN2 inversion and the CH stretching modes (low Ecoll) for double inversion and proton transfer. Mode-specific effects do not show up in the scattering angle distributions and do blue-shift the hot/cold SN2/proton-transfer product internal energies.

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products.

The 205-230 nm photodissociation of vibrationally excited CO2 at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO2 molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO2. Photodissociation was observed from vibrationally excited CO2 with internal energies up to about 20 000 cm-1, and CO(X1Σ+), O(3P), and O(1D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO2 vibrational excitation made this investigation feasible. The internal energies of heated CO2 molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X1Σ+) products in v″ = 0. At 230 nm, CO2 needs to have at least 4000 cm-1 of rovibrational energy to absorb the UV radiation and produce CO(X1Σ+) + O(3P). CO2 internal energies in excess of 16 000 cm-1 were confirmed by observing O(1D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A1B2 and B1A2 states, explaining the nearly isotropic angular distributions of the products. CO(X1Σ+) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X1Σ+), O(3P), and O(1D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO2 temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO2 (and the corresponding available energy) increases. The KER distributions of O(1D) and O(3P) show broad internal energy distributions in the CO(X1Σ+) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO2 VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a3B2 and b3A2, and the ground electronic state are involved in the dynamical pathways that lead to product formation.

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.

Collision Dynamics of O(3P) + DMMP Using a Specific Reaction Parameters Potential Form

Starting from previous benchmark CBS-QB3 electronic structure calculations (Conforti, P. F.; Braunstein, M.; Dodd, J. A. J. Phys. Chem. A 2009, 113, 13752), we develop two global potential energy surfaces for O((3)P) + DMMP collisions, using the specific reaction parameters approach. Each surface is simultaneously fit along the three major reaction pathways: hydrogen abstraction, hydrogen elimination, and methyl elimination. We then use these surfaces in classical dynamics simulations and compute reactive cross sections from 4 to 10 km s(-1) collision velocity. We examine the energy disposal and angular distributions of the reactive and nonreactive products. We find that for reactive collisions, an unusually large amount of the initial collision energy is transformed into internal energy. We analyze the nonreactive and reactive product internal energy distributions, many of which fit Boltzmann temperatures up to ~2000 K.

Communication: New insight into the barrier governing CO2 formation from OH + CO

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO(2) has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO(2) from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system.

Photodissociation dynamics of acetylene via the C̃ Π1u electronic state

Photodissociation of acetylene has been studied using the H-atom Rydberg tagging time-of-flight technique at two excitation wavelengths (148.35 and 151.82 nm) in the vacuum ultraviolet region. Product translational energy distributions have been obtained from the H-atom time-of-flight spectra. Experimental results indicate that the C(2)H product is mainly populated in the A state. Clear trans-bend nu(2) and C-C stretch nu(3) vibrational progressions of the C(2)H(A) product in the product internal energy distribution were observed. The anisotropy parameter obtained from experiment is clearly translational energy dependent for both photolysis wavelengths. The anisotropy parameters at the two photolysis wavelengths were also found to be significantly different from each other, suggesting different dissociation dynamics for the two photolysis wavelengths.

Three Reaction Pathways in the H + HCO → H2 + CO Reaction

We report quasiclassical trajectory calculations of the H + HCO --> H(2) + CO reaction using a recent global potential energy surface. Three microscopic pathways are identified for this reaction. One is a direct abstraction, and the others both proceed via initial formation of a long-lived complex in the H(2)CO well followed by reaction (a) over the molecular saddle point transition state or (b) via a roaming pathway previously identified for the unimolecular photodissociation reaction [Townsend et al., Science 2004, 306, 1158]. Cross sections and product internal energy distributions for each pathway are calculated for five initial collision energies.

Quantum-state resolved products via vacuum ultraviolet photostimulated desorption from geologic calcite

We report the results of a photostimulated desorption (PSD) study of neutral CO products from room temperature geologic calcite utilizing fluences<300 μJ/cm 2 of 157-nm excimer laser radiation. At this wavelength, we are coincident with an intense absorption peak (absorption coefficient=4.39×10 5 cm −1) such that most excitation occurs near or at the crystal surface. Detailed product state distributions are obtained using (2+1) resonance enhanced multiphoton ionization. The CO translational energy can be characterized by a temperature, T=105±10 K, significantly lower than that of the substrate. The CO rotational and vibration state distributions appears thermalized to the surface. This maybe contrasted with our previous study of CO PSD from room temperature calcite done using 193-nm and 213-nm excitation. Bulk UV absorption decreases by two orders of magnitude at these wavelengths and excitation occurs throughout numerous bulk crystal layers. In those studies, CO translational energy could be characterized by a temperature of T=110 K with product internal energy distributions which showed both thermal and non-thermal components. This seemed to indicate a rapid, scattered release from the surface. Broadly similar PSD results over a range of excitation wavelengths and crystal absorptivities confirms a final surface state decay mechanism in the production of product CO. Subtle variations may reflect state differences in desorption for excitations occurring near or within the band gap and those occurring more deeply within the `charge-transfer' band.

Wavelength dependence of the BaO * product chemiluminescence on the N 2O reactant orientation in the Ba + N 2O → BaO * + N 2 reaction

Coarse wavelength analysis of chemiluminescence from the crossed-beam reaction of Ba with oriented N 2O reveals a distinct dependence of the BaO ★ final product internal energy distribution on the initial collision geometry. Favorable (Ba approaches the “O” end of N 2O) and unfavorable (Ba approaches the “N” end of N 2O) orientations are compared at two collision energies. Reactions taking place in the unfavorable orientation produce BaO ★ with relatively higher internal energy than reactions via the favorable orientation. The steric effect depends strongly on the wavelength and it is more pronounced at a higher collision energy.

Role of the entrance channel on the product internal energy distribution in the reaction: (Xe-Cl 2) * → XeCl * + Cl

The Xe-Cl 2 van der Waals complex formed in a supersonic expansion is excited by a two-photon process at 297.5 nm. The XeCl ∗ fragment obtained is vibrationally cold. This result is discussed and compared with those of the Xe ∗ + Cl 2 reaction and laser-assisted collision.

Comparison of reagent translation and vibration on the dynamics of the endothermic reaction Sr+HF

The endothermic reaction Sr+HF(v=0)→SrF+H has been studied as a function of collision energy (4–14 kcal/mole) using a crossed beam geometry in which a seeded HF beam intersects a thermal Sr beam. At the same total energy this reaction is compared to the Sr+HF(v=1, J=1) reaction carried out under beam-gas conditions with a pulsed HF laser as the excitation source. In both cases the SrF products are detected by laser induced fluorescence. Using the Ba+HF→BaF+H reaction as an internal reference, the cross section of the Sr+HF(v=1) reaction is found to be 1–10 times greater than for the Sr+HF(v=0) reaction when the same total energy is supplied as reagent translation. Product internal energy distribution and total relative cross sections as a function of collision energy are also measured. Phase space calculations are able to reproduce most of these results. This agreement and other arguments suggest that the Sr+HF reaction often proceeds via multiple encounters which scramble the reagent energy modes during the course of a reactive collision.

The microscopic reaction dynamics and branching ratio in the F + HCOOH and F + H 2CO reactions

Infrared chemiluminescence under conditions of arrested relaxation has been applied to the study of the hydrogen and deuterium abstraction reactions of HCOOH, DCOOH and H 2CO with F atoms. Two distinctly different modes of product excitation are observed, depending upon whether the reaction proceeds via the formyl or carboxyl hydrogen. Reaction at the formyl hydrogen (or deuterium) causes substantial inversion in the diatomic product internal energy distributions. The F + H 2CO and F + DCOOH reactions respectively channel 56% and 54% of the available energy into vibration in the product diatomic when they occur at the formyl site. In both cases the product energy distributions are qualitatively similar to those observed in direct reactions of triatomic systems on repulsive energy surfaces. In contrast to these, reaction at the carboxyl hydrogen of DCOOH gives an HF† product vibrational distribution having a Boltzmann equilibrium shape at a temperature of 4300 K. The ratio of HF to DF product from the F + DCOOH study shows that reaction occurs at the carboxyl hydrogen approximately twice as often as at the formyl site. Comparison with triatomic reactions involving the same mass-combinations implies that abstraction of the formyl hydrogen occurs via single-collision, direct encounters, whereas reaction at the carboxyl site involves a long-lived complex in which extensive randomisation of the reaction exoergicity among all the product vibrational modes can occur.

The effect of reagent translation on product internal energy distributions: Laser fluorescence study of Al+O2

The effect of incident translational energy on the product internal state distribution has been investigated for the reaction Al+O2→AlO+O. The incident energy is varied through the use of time-of-flight velocity selection of an Al atomic beam, and dye laser fluoresence excitation spectra are reported for AlO produced in a ’’beam-gas’’ arrangement under single-collision conditions. Relative cross sections and effective rotational temperatures are derived for the observed vibrational levels. On the average about 20% of the available energy appears as product vibration and 30% as product rotation. It is found that the AlO internal excitation increases with increasing incident kinetic energy but that ∼80% of the additional kinetic energy is disposed of as product translation and rotation. Comparison of the present results with previous theoretical work suggests that the Al+O2 reaction proceeds on an attractive surface with mixed energy release channeled primarily into product translation and rotation. The observation of a translational energy threshold for the production of AlO in the v=2 level has been employed to derive a lower bound to the AlO dissociation energy D0° (AlO) ?121.0±0.6 kcal/mole. With the help of previous work it is concluded that this lower bound is an equality and a value of 121–122 kcal/mole is recommended for the dissociation energy of aluminum monoxide.

Trajectories in Ion‐Molecule Reactions

AbstractAn appraisal of the classical trajectory method as a tool for studying ion‐molecule reactions is presented. The method has recently been extended to incorporate nonadiabatic processes such as charge transfer. This approach has been applied to two fundamental ion‐molecule reactions, Ar+ + H2 and H+ + H (and isotopic variants). Important conclusions concerning the dynamics of these reactions are discussed. Detailed comparisons with the results of molecular beam experiments are included. The agreement between the computer simulations and experiment is indicative of a very promising future for the trajectory method. Provided that reliable potential energy surfaces can be constructed, the method should be capable of accurate prediction of most reaction attributes, e.g. reaction rates, product internal energy distributions, etc. In addition, it can provide unambiguous information about reaction mechanisms that cannot be obtained by experimental means, and it can be used to assess the validity of models which employ simplified forces or dynamics. Possible directions are suggested for future work in this area.

Monte Carlo trajectories: Dynamics of the reaction F+D2 on a semiempirical valence-bond potential energy surface

A semiempirical valence-bond calculation was carried out for the potential energy surface of H2F treating explicitly seven valence electrons (2pF51sHa1sHb). The integrals were evaluated from diatomic potential energy curves using the Cashion-Herschbach method. Using this potential energy surface, the chemical reaction F+D2→FD+D was studied by Monte Carlo calculations of quasiclassical trajectories. Cross-section calculations were carried out for initial conditions in these ranges: relative translational energy, 1.56–19.3 kcal/mole; rotational angular momentum (in units of ℏ), 0–5; vibrational quantum number, 0–1. In addition, distributions of internal energy and scattering angles for the molecular product were calculated. The results were compared with those of previous theoretical studies and the comparison indicates that subtle features (not well understood) of the potential surface may be important for obtaining correct results. The results were also compared with molecular beam, chemical laser, and infrared chemiluminescence results. The angular distributions are not accurate but the product internal energy distributions are in fairly good agreement with experiment. For the reaction F+H2→FH+H, a limited number of trajectories were carried out for initial vibrational quantum numbers 0–3.