A classical trajectory study of the effect of reaction barrier height on the vibrational energy transfer efficiency in the Cl + HCl system

The effects of reaction barrier height and initial rotational excitation of the reactants on the overall rate of H atom exchange between atomic chlorine and HCl (v = 0) and on the 0 → 1 vibrational excitation of HCl via reactive and nonreactive collisions have been investigated using quasiclassical trajectory techniques. Two empirical LEPS potential energy surfaces were employed in the calculations having reaction barrier heights of 9.84 and 7.05 kcal mol^?1. Trajectory studies of planar collisions were carried out on each surface over a range of relative translational energies with the ground-state HCI collision partner given initial rotational excitation corresponding J = 0, 3, and 7. Initial molecular rotation was found to be relatively inefficient in promoting the H atom exchange; the computed rate coefficient for H atom exchange between Cl + HCl (v = 0, J = 7) was only 4 times larger than that for CI + HCI (v = 0, J = 0). The vibrational excitation rate coefficient exhibited a stronger dependence on initial molecular rotational excitation. The observed increase in the vibrational excitation rate coefficient with increasing initial molecular rotational excitation was due primarily to nonreactive intermolecular R → V energy transfer. The vibrational excitation rate coefficients increase with decreasing reaction barrier height.     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol^?1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m_A = 20 u, m_B = 1 u, m_C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r_BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.     

Thermal and state-selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(³P) + H₂, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(³P) with D₂ and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H₂ to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.     

Ab initio computation of vibrational relaxation rate coefficients for the collisions of CO2 with helium and neon atoms

Ab initio calculations of rate coefficients are reported for the vibrational relaxation of CO₂ molecules in collision with helium and neon atoms. Self consistent-field computations have been performed to parameterise simple three-dimensional potential energy functions which have been used in vibrational close-coupling, rotational infinite-order-sudden calculations of rate coefficients. Excellent agreement is obtained between the calculated and experimental rate coefficients for the deactivation of the (01₁0) vibrational level in the He + CO₂ system at temperatures of 300 K and above. The ab initio predictions of rate coefficients for relaxation of CO₂ vibrational levels such as (10⁰0) and (02⁰0) should be useful in computer simulations of CO₂ lasers.     

Vibrational spectroscopy investigation using ab initio and density functional theory on 3′-chloropropiophenone and 3′-nitropropiophenone

The FTIR and FT Raman spectra of 3′-chloropropiophenone and 3′-nitropropiophenone have been recorded in the regions 4000–400 and 3500–100 cm^?1 respectively. The optimized geometry, frequency and intensity of the vibrational bands of 3′-chloropropiophenone and 3′-nitropropiophenone were obtained by ab initio and DFT levels of theory with complete relaxation in the potential energy surface using 6-31G (d,p) basis set. A complete vibrational assignment aided by the theoretical harmonic frequency analysis has been proposed. The harmonic vibrational frequencies calculated have been compared with experimental FTIR and FT Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed simulated spectrograms.     

Is There an Entrance Complex for the F+NH3 Reaction?

Challenges associated with the theoretical and experimental kinetics of the F+NH₃→HF+NH₂ reaction suggest the need for a more‐precise potential surface. We have investigated the reactants and the products of the reaction, as well as the transition state and two complexes, with rather rigorous ab initio methods. The F????NH₃ complex existing in the entrance valley is predicted to lie 13.7 kcal mol^?1 below the reactants. A small classical barrier of 2.0 kcal mol^?1 separates this entrance well from products HF+NH₂. These results explain the observation by Persky of unprecedented inverse temperature dependence for the F+NH₃ rate constants. The strong hydrogen‐bonded complex FH????NH₂ exists in the exit valley, and with a binding energy of 9.9 kcal mol^?1 relative to separated products. The vibrational frequencies of all stationary points are predicted with the CCSD(T)/aug‐cc‐pVQZ method.     

On the lowest electronic states of the C2F radical

The adiabatic energy surfaces of the lowest three electronic states [²(²A′ and ²A′)] and ²Σ⁺[²A′] of the C₂F radical were investigated by the Hartree-Fock multiconfiguration self-consistent field (HF—MCSCF) ab initio method using a large set of atomic natural orbitals (ANO) and an extended configuration space, and the results were shown to be in agreement with the predictions of valence theory for this radical. The electronic ground state was found to have a bent equilibrium structure, hence contradicting the Walsh rule which predicts for the isoelectronic molecules a ² linear state. The three states were found to be nearly degenerate and the potential energy surfaces of the two lowest electronic states exhibit an avoided crossing at an energy ∼2000 cm⁻¹ above the ground-state minimum, lower than the highest vibrational fundamental. The strong adiabatic interaction which is responsible for the ordering of the electronic states and their equilibrium geometry involves not only the bending coordinate as normally found for Renner-Teller pairs of states, but also the C—C stretching coordinate, due to the near degeneracy of the ²Σ⁺ and the ² lowest electronic states at linear geometries. © 1996 John Wiley & Sons, Inc.     

The QCT calculation of the rate constants for the N(4S)+O2(X3Σ g ? ) →NO(X2Π)+O(3P) reaction

A quasi-classical trajectory (QCT) calculation with the fourth-order explicit symplectic algorithm for the N(⁴S) + O₂(X³Σ_g⁻) → NO(X²Π) + O(³P) reaction has been performed by employing the ground and first-excited potential energy surfaces (PESs). Since the translational temperature considered is up to 5000 K, the larger relative translational energy and the higher vibrational and rotational level of O₂ molecule have been taken into account. The affect of the relative translational energy, the vibrational and rotational level of O₂ molecule in the reaction cross-sections of the ground and first-excited PESs has been discussed in a extensive range. And we exhibit the dependence of microscopic rate constants on the vibrational and rotational level of O₂ molecule at T = 4000 K. The thermal rate constants at the translational temperature betweem 300 and 5000 K have been evaluated and the corresponding Arrhenius curve has been fitted for reaction (1). It is found by comparison that the thermal rate constants determined in this work have a better agreement with the experimental data and provide a more valid theoretical reference.     

The microscopic reaction dynamics and branching ratio in the F + HCOOH and F + H2CO 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 H₂CO 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 + H₂CO 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 HF2 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.     

Inelastic collisions of molecular hydrogen: a comparison of results from quantum and classical mechanics

Full-dimensional quantum and classical calculations have been carried out for inelastic (nonreactive) energy transfer in H2+H2 on the ab initio potential energy surface of Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)]. State-to-state cross sections are determined and compared for transitions from H2(0,j(ab))+H2(1,j(cd)). While there is excellent agreement for transitions involving small Deltaj, for larger Deltaj and for vibrational relaxation, significant differences are observed which exhibit no systematic trends. Reasons for this disagreement are discussed.     

Velocity Mapping Studies of Vibrational Energy Disposal Following Methyl Iodide Photodissociation

In this paper previous results are compared for two different types of velocity mapping studies which probe vibrational energy disposal following the A-band photodissociation of methyl iodide, CH₃I + hv → CH₃ (v) + 1(²P_3/2), 1*(²P_1/2). Full three-dimensional state-specific speed and angular distributions of the nascent fragments have been recorded for the photoelectrons, iodine atoms, and methyl radicals, using state- and mass-selective (2+1) resonance-enhanced multi-photon ionization (REMPI)/time-of-flight spectrometry. Two sources of information on the vibrational energy disposal are available from velocity mapping: (a) the photoelectron images, which give information on the initial stages of vibrational excitation in electronically excited CH₃I, and (b) methyl radical images, which indicate the final energy disposal channels. Even though the two signals are believed to probe very different time-scales of the dissociation process, good agreement between the two is found for the vibrational energy disposal trends. Several trends found in the data for methyl iodide photodissociation indicate that readjustment of the ab initio multi-dimensional potential energy surfaces calculated for this molecule appears to be needed.     

一维精确量子散射研究——H+ClH(u)在反应势能面上的振动去激

HCl化学激光中存在振动激发的HCl及游离的H和Cl,故HCl在H原子和Cl原子碰撞下振动弛豫速率过程的研究很重要。不久前我们报道了Cl原子对HCl碰撞去激的一维精确量子散射研究,本文用类似方法,讨论H原子对激发态的HCl的碰撞去激。     

Role of van der Waals resonances in the vibrational relaxation of HF by collisions with H atoms

Vibrational relaxation of HF(v) in collisions with H atoms can occur by three pathways: inelastic scattering with and without H atom exchange, and, for v>or=3, the HF+H-->F+H2 reaction. Fully quantum, reactive scattering calculations on the Stark-Werner FH2 potential energy surface reveal narrow peaks in the energy dependence of the integral cross sections for each of these processes. By means of an adiabatic-bender analysis, we show that each of these peaks corresponds to the position of quasibound HF-H vibrational states trapped in the weak van der Waals well. The width of these resonances indicates that the lifetime of the quasibound states is up to 30 periods of the HF-H van der Waals vibration.     

Infrared chemiluminescence in the reactions of atomic fluorine with ethanol,propanol-(2) and some deuterated analogs

In the reactive systems F+C₂H₅OH, F+C₂D₅OD, F+C₂H₅OD, F+(CH₃)₂CHOH, F+(CD₃)₂CHOH, and F+(CD₃)₂CDOH the infrared emission spectra were recorded from HF and/or DF in the fundamental region. Hydrogen abstraction takes place from CH and OH bonds. Vibrational relaxation was suppressed and rotational relaxation took place only to a minor extent. HF(DF) excitation reaches the thermodynamic limit within error limits in all cases. The vibrational distributions of HF for the systems F+(CD₃)₂CHOH, F+(CD₃)₂CDOH show no populati inversion. The vibrational distribution of HF for all other systems and all the DF vibrational distributions obtained show population inversion. Inform theory was used to describe the results of those reaction channels that could be studied separately because of isotopic substitution. The results are c to the systems F + methanol and deuterated analogs investigated before in our laboratory, and to the F+CH₄, F+CD₄, and F+H₂O₂ reactio     

Quantum mechanical scattering calculations on a spline-fitted ab initio surface: the He + H+2 (ν = 0, 1, 2) → HeH+ + H reaction

All-channel time-dependent quantum mechanical reaction probabilities are reported for the collinear He + H⁺₂(ν = 0, 1, 2) → HeH⁺ + H reaction at a total energy of 1.2 eV on previously reported diatomics-in-molecule (DIM) and spline fitted ab initio (SAI) surfaces. These results are in agreement with the previous quasiclassical trajectory results in that there is vibrational enhancement of the reaction probability on the SAI surface but not on the DIM surface. This agreement lends support to our previously drawn conclusion that small differences in the potential-energy surface can lead to substantially different dynamic results.     

The dynamics of formation of vibrationally excited HF in reactions of O (21 D2) atoms with partially fluorinated alkanes

The nascent vibrational energy distributions of the HF? formed in the reactions of a series of partially fluorinated alkanes (R_FH; R_F = CH₂F, CHF₂, CF₃, C₂F₅, C₃F₇, and C₇F₁₅) with electronically excited oxygen atoms O(2¹D₂) have been determined by measuring the appearance times of stimulated emissions from various vibration–rotation transitions in a grating-tuned optical cavity. The vibrational energy contents of the HF formed in these reactions were found to be considerably greater than statistically expected. These reactions are believed to occur via vibrationally excited short-lived α;-fluorinated alcohols (R_FOH?), formed by insertion of the O(2¹D₂) atoms into C? H bonds. The observation of nonstatistical energy partitioning in the above reactions is in clear contrast to the result obtained from the O(2¹D₂) + CF₃CH₃ reaction that produces the β-fluorinated alcohol CF₃CH₂OH, from which the HF product carries a near statistical vibrational energy distribution. A mechanism for HF? formation in these very exothermic reactions is presented.     

Quantum mechanical study of electronic transitions in collinear atom—diatom collisions

Quantum mechanical calculations are reported for model, nonreactive, collinear collision systems composed of the H₂ diatom and the halogen atom X = F, Cl, Br or I. The model involves two electronic potential energy surfaces, obtained in a diatomics-in-molecules formulation, that correspond asymptotically to the two spin-orbit states of X. On each surface the calculations include as many vibrational states of H₂ as are asymptotically allowed, up to a limiting number of five. The first two collision systems, FH₂ and ClH₂, are characterized by electronic splittings much smaller than any vibrational spacing included in the diatom spectrum, and as a result they show a high degree of vibrational elasticity with essentially all transition activity testricted to spin—orbit switching in the halogen. This pattern is broken for BrH₂ collisions, where the near-equality between electronic and vibrational quanta apparently leads to a resonant exchange of energy between the two modes. The greater spinorbit splitting in iodine (～ 2 vibrational quanta) results in largely elastic behavior in IH₂ collisions for both vibrational and electronic transitions. A modified Massey criterion is exhibited for some of the FH₂ and BrH₂ transitions.     

Electronic and Vibrational Coherence Dynamics in a Cyanine Dye Studied Using a Few‐Cycle Pulsed Laser

We report the relaxation times of electronic and vibrational coherence in the cyanine dye 1,1′,3,3,3′,3′‐hexamethyl‐4,4′,5,5′‐dibenzo‐2,2′‐indotricarbocyanine, measured using a 7.1 fs pulsed laser. The vibrational phase relaxation times are found to be between 380 and 680 fs in the ground and lowest excited singlet states. The vibrational dephasing times of the 294, 446, and 736 cm^?1 modes are relatively long among the six modes associated with excited‐state wave packets. The slower relaxations are explained in terms of a coupled triplet of vibrational modes, which preserves coherence by forming a tightly bound group to satisfy the condition of circa conservation of vibrational energy. Using data from the negative‐time range (i.e., when the probe pulse precedes the pump pulse), the electronic phase relaxation time is found to be 31±1 fs. The dynamic vibrational mode in the excited state (1171 cm^?1), detected in the positive‐time range, is also studied from the negative‐time traces under the same experimental conditions.     

An experimental test of the Bernstein—Levine theory of branching ratios

The first test of the information-theoretic approach to branching ratios has been made for the reaction: F + HD å HF2 (V′, R′) + D å DF3(V′, R′) + H.The vibrational (V′) and rotational (R′) product energy-distributions for both branches of this reaction have been obtained by the infrared chemiluminescence technique, and have been used in the calculation of an information-theoretic branching ratio, Γ_HF/DF = 1.41 ± 0.18. This is in excellent agreement with the experimentally measured branching ratio of 1.45. However, results from classical trajectory calculations raise a question as to the significance of this agreement. Classical trajectory calculations (on various energy-surfaces) predict an increase in Γ with reagent J. The information-theoretic analysis applied to the product energy-distributions from these trajectory calculations leads to a qualitatively different result. As a possible alternative to the information-theoretic view, simple kinematic features are noted which could account for Γ > 1, as well as for the significant differences in product energy-distribution. On this alternative view, the two features are not indisolubly linked — the extent to which they appear in conjunction will depend on the nature of the energy surface.