Relative Translational Energy Research Articles

Effect of solvation on the dynamics of H + CH3 association

The gas phase association of CH3 with the HAr2 cluster to form a vibrationally/rotationally excited CH 4 * molecule is used as a model to study microscopic solvation dynamics. A potential energy surface for the reactive system is constructed from a previously fitted H + CH3 ab initio potential and 12-6 Lennard-Jones Ar-Ar, Ar-C, and Ar-H potentials. Classical trajectory calculations performed with the chemical dynamics computer program VENUS are used to investigate the CH3 + HAr2 → CH 4 * + Ar2 reaction dynamics. Reaction is dominated by a mechanism in which the CH3 “strips” the H-atom from HAr2 during large impact parameter collisions. For a large initial relative translational energy the CH3 + HAr2 → CH 4 * + Ar2 cross section is the same as that for H + CH3 association, so that HAr2 acts like a “heavy” H-atom. However, at a low initial relative translational energy, the long-range Ar2—CH3 attractive potential apparently makes the CH3 + HAr2 association cross section larger than that for H + CH3. Partitioning of energy to the CH 4 * and Ar2 products is consistent with a stripping mechanism. The initial and final relative translational energies are nearly identical and the CH 4 * rotational energy is controlled by the initial CH3 rotational energy. The velocity and orbital tilt scattering angles, θ(v i ,v f ) and θ(l i ,l f ), respectively, are consistent with the stripping mechanism. On average only a small amount of the product energy is partitioned to Ar2 vibration/rotation and CH 4 * + Ar2 relative translation.

Dissociation dynamics of D2 on rigid and nonrigid Ni clusters

We present results of a theoretical study of the dissociation dynamics of D2 on NiN clusters with N=3–13. First, the semiempirical density functional based molecular dynamics/Monte Carlo corrected effective medium method was used to predict the interaction energies of various Ni clusters in this size range. We identified the most stable clusters and found that only N=9 possesses two nearly equally stable isomers with an energy difference of only 0.03 eV. Second, we performed classical trajectory simulations of the reactions in which the Ni atoms were allowed to move. The dissociation cross sections from calculations on the nonrigid clusters were nearly equal to those obtained with the rigid Ni cluster. A particularly intriguing result was that the dissociation cross sections calculated for the two isomers of Ni9 differed significantly, especially at low collision energy. Third, we showed that the dissociation probabilities as functions of the impact parameter and relative translational energy were fit with good accuracy as a simple function of a single variable E⊥, the component of translational energy perpendicular to the surface of the cluster. This indicated that the normal energy scaling of the dissociative sticking probability observed in some gas/surface systems may have an equivalent behavior in gas/cluster systems.

Quasiclassical trajectory study of the effect of reactant rotation on the reaction B + + HD → BH + (BD +) + D(H)

Quasiclassical trajectories were calculated for the lowest triplet potential energy surface of the title reaction at different rotational states of the HD molecule and at two values of the relative translational energy of reactants. The opacity function, reaction cross sections, rotational and vibrational excitation of products, intramolecular isotopic effect, symmetry of backward-forward scattering and lifetime of the BHD+ collision complex are discussed in connection with the initial conditions defined. Results are compared with similar systems taken from the literature.

The reaction Ne+H 2 + (v=0, 1, 2, 3, 4)?NeH++H: 3D potential energy surface and quasiclassical trajectory calculations

A three-dimensional potential energy surface for the endoergic reaction Ne+H2+→NeH++H in the2A′ ground state of the system NeH2+ has been calculated by quantum chemical ab initio methods (CEPA approximation). The calculated points on this surface were fitted to an analytic ansatz in terms of an extended LEPS functional form augmented by a correction function. The latter was expanded in polynomials in inverse powers of the internuclear distances. This analytic form was used for quasiclassical trajectory calculations of reactive cross sections. In agreement with experimental investigations a strong vibrational enhancement is observed, i.e. the reaction is markedly favored if the necessary reaction energy is supplied as vibrational energy of H2+ rather than as relative translational energy. Other properties of the reaction dynamics such as the backward to forward scattering ratio, the lifetime of the collision complex NeH2+, and final rotational and vibrational state distributions are also discussed on the basis of the quasiclassical trajectory calculations.

Rovibrational distributions of CH(A 2Δ,B 2Σ−) produced in energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) atoms to CH3 radical

Energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) to CH3 radical have been studied by observing emission spectra from excited fragments in the flowing afterglow. CH3 radicals were generated by the F+CH4 reaction. The CH(A 2Δ–X 2Πr:v′=0−2) and CH(B 2Σ−–X 2Πr:v′=0) emission systems were observed in the Ar(3P2) reaction, while only CH(A–X:v′=0,1) emission system was found in the Kr(3P2) and Xe(3P2) reactions. The nascent rovibrational distributions of CH(A:v′=0–2) were N0:N1:N2 =100(T0 =3400±400 K):28±5(T1 =1700±400 K):4±1(T2 =700±300 K) in the Ar(3P2) reaction and 100(T0 =1000±250 K):<5(T1 <800 K):0 in the Kr(3P2) and Xe(3P2) reactions. The rotational distribution of CH(B:v′=0) in the Ar(3P2) reaction was reproduced by a single Boltzmann temperature of 2800±300 K. The average fractions of total available energies channeled into vibration and rotation of CH(A,B) were less than 15% for all cases, suggesting that most of the available energies was deposited as relative translational energy of products and/or rovibrational energy of H2. The observed rovibrational distributions of CH(A) were colder than those predicted from statistical theories including and excluding the conservation of total angular momentum. The best agreement between the observed and statistical distributions was obtained for the mechanism giving CH(A,B) in two-body dissociation steps by assuming that 78–92% of the total available energy is released as kinetic energy in the first step, Rg(3P2)+CH3→CH*3+Rg, then the rest remains in the precursor CH*3 state as an internal energy.

Energy disposal in gas‐phase nucleophilic displacement reactions

AbstractThe partitioning of reaction exothermicity into relative translational energy of the products of gas‐phase SN2 (F− + CH3Cl) and nucleophilic aromatic substitution (F− + C6H5Cl) reactions has been investigated using kinetic energy release Fourier transform ion cyclotron resonance spectroscopy. The chloride product ion is observed to be highly translationally excited for the SN2 reaction, indicating a cold internal energy distribution for the products. For the chlorobenzene reaction the products are not generated with large translational energies. The results are compared with a statistical model. Ion‐intensity profiles for the CH3Cl reaction deviate significantly from the statistical model whereas the chlorobenzene results are consistent with this model. The kinetic energy release for the CH3C1 reaction is compared with energy‐disposal results for the photodissociation and dissociative electron‐attachment processes of halomethanes. In all three cases a node in the molecular orbital between the carbon atom and the departing halogen results in a repulsive energy release. Ion‐retention curves for the nucleophilic aromatic substitution reaction are consistent with the existence of a long‐lived ion‐dipole complex on the exit channel for this reaction.

Reactions in clusters

A new class of cluster-catalyzed reactions is proposed and investigated using extensive molecular-dynamics simulations. These reactions involve the collision between reactants which are embedded in inert gas clusters, whose role is that of a local heat bath (i.e., extended third body). The concept is demonstrated for the reactions [Na4Cl3]+Arq+Cl− (for q=12 and 32) and [Na14Cl12]+2Ar30+Cl− for several initial temperatures, relative translational energies between the reactants, and impact parameters. For associative reactions, i.e., Cl− attachment accompanied by the release of Ar atoms, the dynamics of the reactions, the dependencies of the isomeric structures of the product alkali-halide clusters, and of reaction pathways and branching ratios on reactant size and reaction conditions are systematically studied.

The 248 nm photodissociation of DOCl: laser excitation fluorescence spectra of OD

The dynamics of the 248 nm excimer-laser-induced photodissociation of DOCl has been investigated by the observation of the OD fragment by laser excitation fluorescence. The OD Π(A′) lambda doublet state is strongly favoured in the dissociation and there is a small preference for the 2Π 3 2 spin—orbit state. The observed OD rotational distribution differs for the two lambda doublets, and for the Π(A′) component of the 2Π 3 2 spin—orbit state the distribution is Gaussian. The results are interpreted in terms of a fast dissociation with most of the energy released into relative translational energy. The rotational distribution can be explained with a Franck—Condon type model.

Product energy correlations in the dissociation of overtone excited nitric oxide dimer

The photodissociation spectra, predissociative lifetimes, fragment internal state and translational energy distributions, and product energy and vector correlations of overtone excited nitric oxide dimer, (NO){sub 2}, have been measured using pulsed molecular beam and Doppler profile analysis techniques. The v{sub 1} + v{sub 5} and 2v{sub 5} excited NO dimer are 34 {plus minus} 6 and 20 {plus minus} 3 ps, respectively. At least 97% of the dissociations produce NO(v = 1) + NO(v = 0); no product in the (v = 0) + (v = 0) channel was detected. For both modes, about 85% of the energy available after vibrational energy distribution goes into relative translational energy of the fragments. Only 3% of this available energy appears in fragment rotational energy. The propensities of the three spin-orbit correlated channels were measured for both modes. For v{sub 1}+v{sub 5} excitation 77 {plus minus} 7% of the fragments are formed along the {sup 2}II{sub 1/2}-{sup 2}II{sub 3/2} channel and 21 {plus minus} 5% via {sup 2}II{sub 1/2}-{sup 2}II{sub 1/2}. In contrast, 39 {plus minus} 6% of the dissociations following 2v{sub 5} excitation proceed via the {sup 2}II{sub 1/2}-{sup 2}II{sub 1/2} channel. The only nonzero vector correlation observed is the fragment recoilmore » anisotropy, which depends on the rotational transition excited in the parent. The results are discussed in terms of the dissociation and energy partitioning mechanisms.« less

Vibrational coherence effects in the pump–probe studies of photochemical predissociation

We examine how the results of pump–probe experiments with short pulses are affected by the coherence of the excitation process, in the particular case when the pump causes the predissociation of the molecule. To document and understand this process we perform exact quantum mechanical calculations and examine simple models. The exact calculations are performed on a model of CH3ONO photodissociation. We examine the effect of coherence when the probe excites the transition complex and when it excites the NO molecule produced by photodissociation. In both cases the vibrational coherence leads to oscillations in the dependence of the two pulse laser-induced fluorescence (LIF) signal on the delay time between the pulses. In the first case the effect is due to the coherent excitation of the predissociation resonances. In the second the important factor is the coherence of the NO vibrational state produced by photodissociation. The physical factors affecting this process are discussed in detail. One of the interesting findings is that the relative translational energy distribution affects in an important way the manner in which vibrational coherence modifies the two pulse LIF signal.

The C ( 3P J) + NO(X 2Π r)→ CN (X 2Σ +) + O ( 3P J) reaction dynamics studied at 0.06 and 0.23 eV relative translational energy in pulsed crossed supersonic molecular beams

The C ( 3P J ) + NO (X 2Π r)→CN (X 2Σ +) + O ( 3P J ) reaction is studied with pulsed, crossed, supersonic, molec translational energies of 0.06 and 0.23 eV. The rovibrational distributions of the CN (X 2Σ +) product are determined by laser-induced fluorescence. The experimental evidence strongly suggests that the reaction proceeds via a pseudo-direct mechanism. The rovibrational distributions differ from the prior outcome. At lower collision energy, the fraction of available energy channelled into vibration and rotation is 〈ƒ V〉 = 0.30 and 〈ƒ R〉 = 0.15, respectively. At higher collision energy, experiments show that the additional translational energy is converted largely into CN product rotation (55%) and vibration (28%). Evidence is also found for forward scattering. The agreement with theoretical results is discussed.

Energy transfer in kiloelectronvolt collisional activation determined from relative yields and translational energies of fragment ions formed in collision activated decomposion of CH +4 ions

Relative yields of product ions from collision activated dissociation of 3 or 8 KeV CH + 4 ions with He, Ar or CH 4 target gas have been measured under conditions in which scattering losses are observed to be negligible. Scatter profiles indicate that scattering is more extensive for the higher energy requirement product ions C + and CH + (and H +), which explains why previous comparable spectra show smaller relative yields of these ions than do the present spectra. The efficiencies of the CAD reactions are also larger than previously observed and approach 40% for He target gas. Parent ion translational energy losses associated with each fragment ion type have also been measured and these values, coupled with the relative yield values, allow derivation of energy transfer functions in an approximate form. These functions, which vary with target gas and with collision energy, are associated with mean energy transfers of up to about 13 eV and show significant energy tailing into the 20–50 eV range, this being due in part to target particle excitation occurring concurrently with excitation of the projectile ion.

Dissociative chemisorption of D2 on a Ni13 cluster

Selected characteristic results of a quasiclassical trajectory study of dissociative adsorption of a D2 molecule on a Ni13 cluster are presented. These include detailed probabilities as functions of the impact parameter and of the relative translational energy of D2, and cross sections as functions of this energy. The roles of the initial rovibrational state of the D2 molecule and of the initial temperature of the cluster are examined. The effect of freezing the cluster into a rigid geometry is tested.

Born–Oppenheimer dynamics using density-functional theory: Equilibrium and fragmentation of small sodium clusters

The properties of small neutral and positively charged sodium clusters and the fragmentation dynamics of Na++4 are investigated using a simulation technique which combines classical molecular dynamics on the electronic Born–Oppenheimer ground-state potential surface with electronic structure calculations via the local spin-density functional method. Results for the optimal energies and structures of Nan and Na+n (n≤4) are in quantitative agreement with previous studies and experimental data. Fission of Na++4 on its ground state Born–Oppenheimer potential-energy surface, following sudden ionization of selected configurations of an Na+4 (or Na4) cluster, whose vibrational energy content corresponds to 300 K, is found to occur on a picosecond time scale. The preferred fission channel is found to be Na+3+Na+, with an interfragment relative translational kinetic energy of ∼2 eV, and a vibrationally excited Na+3. The dynamics of the fragmentation process is analyzed.

Hydrogen exchange reaction H + D2 in crossed beams

Despite its fundamental importance as the prototypical bimolecular reaction, the hydrogen exchange reaction still remains a challenging and open problem, both experimentally and theoretically. Theory has now developed to a stage much superior to that of experiment. Nowhere is this more true than, for example, in the determination of differential scattering cross-sections, state-to-state specific with respect to the vibrational and rotational degrees of freedom of the molecular products. In this paper we describe a new experimental approach to such measurements, and present first results from crossed-beam studies of the H + D2 reaction (at relative translational energies of 1.29 and 0.54 eV) using the novel technique of hydrogen Rydberg atom time-of-flight spectroscopy to monitor the velocity and angular distributions of the D atom product.

Trajectory studies of SN2 nucleophilic substitution. I. Dynamics of Cl−+CH3Cl reactive collisions

Classical trajectories were used to study the dynamics of the Cl−+CH3Cl→Cl−---CH3Cl association and the Cl−+CH3Cl→ClCH3+Cl− substitution reactions. Substantial deviations are found between the underlying microscopic dynamics of the reactions and the assumptions of statistical rate theories. The energy dependence of the trajectory rate constant for the majority of Cl−---CH3Cl→Cl−+CH3Cl dissociation is in accord with a model in which only the Cl−---C stretch and the two Cl−---CH3Cl bend modes are active degrees of freedom. At 300 K the trajectory rate constant for Cl−+CH3Cl→Cl−---CH3Cl association is approximately forty percent smaller than that of microcanonical variational transition state theory, with the difference increasing with an increase in temperature. For thermal conditions substitution occurs by an indirect mechanism in which the reactive system is initially trapped in the Cl−---CH3Cl potential well. The cross section for this process decreases dramatically as the reactant relative translational energy is increased. The effect of rotational energy is less precipitous. Exciting the C–Cl stretch normal mode of CH3Cl opens up a direct substitution mechanism without trapping in either of the two potential wells. There is a significant decrease in the cross section for this direct substitution when CH3Cl is rotationally excited.

Trajectory study of Si4 formation and decay and of exchange and abstraction reactions in Si+Si3 collisions

The formation and subsequent decay of Si4 complexes as well as the direct exchange and abstraction processes in Si+Si3 collisions have been studied using quasiclassical trajectories on a new global Si4 potential energy surface fitted to available experimental and ab initio data, and on Bolding and Andersen’s (BA) recently formulated silicon potential for arbitrary cluster sizes. Cross sections for Si4 formation, σf(Et), were computed as a function of initial relative translational energy Et over the range 0.01 to 4.0 eV, with the Si3 internal energy described by the Boltzmann distribution at 800 K. The cross section was found to peak sharply near Et=0, as expected, and to fall off linearly at high energy. An analytical expression for kf(T), the thermal rate constant for Si4 formation, was found by averaging σf(Et) over the Maxwell–Boltzmann distribution for Et. The analytical values of kf(T) lie between 6×1014 and 8×1014 cm3/mol s for the range 800–1500 K, and are in excellent accord with trajectory calculations of kf at 800 and 1200 K. Unimolecular dissociation rate constants for Si4, kd, were calculated as a function of Et over the 0.4 to 4.0 eV. The values of kd are well described by the RRK expression, with a value of 4.67 for the effective number of vibrational modes. Averaging the dissociation rate constant over the Maxwell–Boltzmann distribution yields an average Si4 lifetime of 413 ps at 800 K, which is not long enough for a stabilizing collision to occur at pressures characteristic of low-pressure CVD experiments. The direct exchange reaction is found to be unimportant for Et less than 1 eV, since for lower relative energies essentially all reactions proceed indirectly via Si4 complex formation. Direct atomic abstraction is energetically forbidden, on average, for Et less than 0.9 eV, and is unlikely for Et less than 2 eV. At higher energies, the end-atom exchange and abstraction channels, which are statistically favored over the apex-atom channels, are dynamically favored as well. When exchange or abstraction proceeds indirectly via an Si4 intermediate, the distinction between apex-atom, end-atom, and no-reaction channels is lost. Both the direct and indirect pathways leave a large fraction of the energy and angular momentum in the reaction products. Cross sections for Si4 formation on the BA surface are smaller than those on the global Si4 surface due to the cutoff function in the BA two-body potential terms; Si4 dissociation rates for total energies between 1.3 and 2 eV above threshold agree to within a factor of 2.3 or better with corresponding values for the Si4 surface.

One-color photolysis–ionization study of HN3: The N2 fragment internal energy distribution and μ-v-J correlations

A one-color photolysis–ionization study of the photodissociation of hydrazoic acid near 283 nm has been carried out. The N2 and NH(a 1Δ) products have both been detected in a state-specific manner by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The N2 fragments were observed in the v=0 vibrational mainfold and were found to be highly rotationally excited, with an average rotational excitation of 0.79 eV. The μ-J correlation parameter 𝒜(2)0 for N2 was determined to be positive and approximately equal to 0.5, indicating that the angular momentum vector J tends to be parallel with the transition dipole μ. Relatively little rotational energy was found in the NH product. From the observed polarization dependence of the NH REMPI mass peak profiles, we conclude that the recoil anisotropy parameter β is approximately −0.5 for low-J values and becomes positive by J=10. The observed width of these mass peaks provides information of the fragment recoil energy, and we estimate from the NH mass peaks that approximately 1.4±0.4 eV of the available energy appears as product relative translational energy. These experimental results are interpreted in terms of our understanding of the lowest excited singlet electronic state of this molecule.

Criteria for applicability of the impulse approach to collisions.

Using an exact formulation of impulse approach (IA) to atom-diatom collisions, we assess its internal consistency. By comparing the cross sections in the forward and reverse directions for the vibrational-rotational inelastic processes, using the half-on-the-shell (post and prior) models of the two-body {ital t} matrix, we show that in both cases the IA leads to a violation of the semidetailed balance (SDB) condition for small scattering angles. An off-shell model for the two-body {ital t} matrix, which preserves SDB, is shown to have other serious shortcomings. The cross sections are studied quantitatively as a function of the relative translational energy and the mass of the incident particle, and criteria discussed for the applicability of IA.

A low-energy quasiclassical trajectory study of O(3P)+OH(2Π) →O2(3Σ−g)+H(2S). I. Cross sections and reaction dynamics

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent DMBE III (double many body expansion) potential-energy surface by Varandas et al. (1988). The dynamics of complex formation were studied in detail and two different types of complexes, referred to as close and distant complexes, were observed. Corresponding cross sections and reaction cross sections are reported as a function of diatom rotational quantum number and atom–diatom relative translational energy in the range 0.25–2.0 kcal/mol. The cross sections decrease with increasing translational energy. The rotational-state dependence of the reaction cross section is found to be complicated. One interesting observation is that the cross section for the rotational ground state is much smaller than for other rotational states. The features in the cross sections are related to the shape of the potential-energy surface and the reaction dynamics.