An accurate multireference configuration interaction calculation of the potential energy surface for the F+H2→HF+H reaction

Dynamical resonances in the fluorine atom reaction with the hydrogen molecule

[Reaction: see text]. The concept of transition state has played a crucial role in the field of chemical kinetics and reaction dynamics. Resonances in the transition state region are important in many chemical reactions at reaction energies near the thresholds. Detecting and characterizing isolated reaction resonances, however, have been a major challenge in both experiment and theory. In this Account, we review the most recent developments in the study of reaction resonances in the benchmark F + H 2 --> HF + H reaction. Crossed molecular beam scattering experiments on the F + H 2 reaction have been carried out recently using the high-resolution, highly sensitive H-atom Rydberg tagging technique with HF rovibrational states almost fully resolved. Pronounced forward scattering for the HF (nu' = 2) product has been observed at the collision energy of 0.52 kcal/mol in the F + H 2 (j = 0) reaction. Quantum dynamical calculations based on two new potential energy surfaces, the Xu-Xie-Zhang (XXZ) surface and the Fu-Xu-Zhang (FXZ) surface, show that the observed forward scattering of HF (nu' = 2) in the F + H 2 reaction is caused by two Feshbach resonances (the ground resonance and first excited resonance). More interestingly, the pronounced forward scattering of HF (nu' = 2) at 0.52 kcal/mol is enhanced considerably by the constructive interference between the two resonances. In order to probe the resonance potential more accurately, the isotope substituted F + HD --> HF + D reaction has been studied using the D-atom Rydberg tagging technique. A remarkable and fast changing dynamical picture has been mapped out in the collision energy range of 0.3-1.2 kcal/mol for this reaction. Quantum dynamical calculations based on the XXZ surface suggest that the ground resonance on this potential is too high in comparison with the experimental results of the F + HD reaction. However, quantum scattering calculations on the FXZ surface can reproduce nearly quantitatively the resonance picture of the F + HD reaction observed in the experiment. It is clear that the dynamics of the F + HD reaction below the threshold was dominated by the ground resonance state. Furthermore, the forward scattering HF (nu' = 3) channel from the F + H 2 ( j = 0) reaction was investigated and was attributed mainly to a slow-down mechanism over the centrifugal exit barrier, with small contributions from a shape resonance mechanism in a narrow collision energy range. A striking effect of the reagent rotational excitation on resonance was also observed in F + H 2 ( j = 1), in comparison with F + H 2 ( j = 0). From these concerted experimental and theoretical studies, a clear physical picture of the reaction resonances in this benchmark reaction has emerged, providing a textbook example of dynamical resonances in elementary chemical reactions.

HF(v' = 3) forward scattering in the F + H2 reaction: shape resonance and slow-down mechanism

Crossed molecular beam experiments and accurate quantum dynamics calculations have been carried out to address the long standing and intriguing issue of the forward scattering observed in the F + H(2) --> HF(v' = 3) + H reaction. Our study reveals that forward scattering in the reaction channel is not caused by Feshbach or dynamical resonances as in the F + H(2) --> HF(v' = 2) + H reaction. It is caused predominantly by the slow-down mechanism over the centrifugal barrier in the exit channel, with some small contribution from the shape resonance mechanism in a very small collision energy regime slightly above the HF(v' = 3) threshold. Our analysis also shows that forward scattering caused by dynamical resonances can very likely be accompanied by forward scattering in a different product vibrational state caused by a slow-down mechanism.

Quantum mechanical limits to the control of atom-diatom chemical reactions through the polarisation of the reactants

This article considers the extent to which one can control the reactivity of atom-diatom systems through reactant polarisation. Three different limits for reactivity manipulation are defined: "absolute" limits that do not depend on the reaction dynamics but can only be obtained for particular combinations of quantum numbers, "unconstrained" limits that depend on dynamics but not on constraints imposed by any particular experimental setup, and "constrained" limits that depend on dynamics and also on the constraints imposed by a particular experimental setup. Methods for calculation of these limits are presented and applied to the benchmark F + H2 reaction. The variations of the maximum and minimum reactivity one can obtain are analysed in terms of reaction mechanisms and steric constraints. Tables listing the minimum and maximum values of angular momentum polarisation moments of rank up to 4, and integer and half-integer quantum numbers up to 5, are also presented.

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.

Breakdown of the Born-Oppenheimer Approximation in the F+ o -D 2 → DF + D Reaction

The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P(3/2)) and excited F*(2P(1/2)) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is approximately 1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.

A Time-Dependent Quantum Dynamical Study of the H + HBr Reaction

Time-dependent wave packet calculations were carried out to study the exchange and abstraction processes in the title reaction on the Kurosaki-Takayanagi potential energy surface (Kurosaki, Y.; Takayanagi, T. J. Chem. Phys. 2003, 119, 7838). Total reaction probabilities and integral cross sections were calculated for the reactant HBr initially in the ground state, first rotationally excited state, and first vibrationally excited state for both the exchange and abstraction reactions. At low collision energy, only the abstraction reaction occurs because of its low barrier height. Once the collision energy exceeds the barrier height of the exchange reaction, the exchange process quickly becomes the dominant process presumably due to its larger acceptance cone. It is found that initial vibrational excitation of HBr enhances both processes, while initial rotational excitation of HBr from j(0) = 0 to 1 has essentially no effect on both processes. For the abstraction reaction, the theoretical cross section at E(c) = 1.6 eV is 1.06 A(2), which is smaller than the experimental result of 3 +/- 1 A(2) by a factor of 2-3. On the other hand, the theoretical rate constant is larger than the experimental results by about a factor of 2 in the temperature region between 220 and 550 K. It is also found that the present quantum rate constant is larger than the TST result by a factor of 2 at 200 K. However, the agreement between the present quantum rate constant and the TST result improves as the temperature increases.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

A diabatic representation is convenient in the study of electronically nonadiabatic chemical reactions because the diabatic energies and couplings are smooth functions of the nuclear coordinates and the couplings are scalar quantities. A method called the fourfold way was devised in our group to generate diabatic representations for spin-free electronic states. One drawback of diabatic states computed from the spin-free Hamiltonian, called a valence diabatic representation, for systems in which spin-orbit coupling cannot be ignored is that the couplings between the states are not zero in asymptotic regions, leading to difficulties in the calculation of reaction probabilities and other properties by semiclassical dynamics methods. Here we report an extension of the fourfold way to construct diabatic representations suitable for spin-coupled systems. In this article we formulate the method for the case of even-electron systems that yield pairs of fragments with doublet spin multiplicity. For this type of system, we introduce the further simplification of calculating the triplet diabatic energies in terms of the singlet diabatic energies via Slater's rules and assuming constant ratios of Coulomb to exchange integrals. Furthermore, the valence diabatic couplings in the triplet manifold are taken equal to the singlet ones. An important feature of the method is the introduction of scaling functions, as they allow one to deal with multibond reactions without having to include high-energy diabatic states. The global transformation matrix to the new diabatic representation, called the spin-valence diabatic representation, is constructed as the product of channel-specific transformation matrices, each one taken as the product of an asymptotic transformation matrix and a scaling function that depends on ratios of the spin-orbit splitting and the valence splittings. Thus the underlying basis functions are recoupled into suitable diabatic basis functions in a manner that provides a multibond generalization of the switch between Hund's cases in diatomic spectroscopy. The spin-orbit matrix elements in this representation are taken equal to their atomic values times a scaling function that depends on the internuclear distances. The spin-valence diabatic potential energy matrix is suitable for semiclassical dynamics simulations. Diagonalization of this matrix produces the spin-coupled adiabatic energies. For the sake of illustration, diabatic potential energy matrices are constructed along bond-fission coordinates for the HBr and the BrCH(2)Cl molecules. Comparison of the spin-coupled adiabatic energies obtained from the spin-valence diabatics with those obtained by ab initio calculations with geometry-dependent spin-orbit matrix elements shows that the new method is sufficiently accurate for practical purposes. The method formulated here should be most useful for systems with a large number of atoms, especially heavy atoms, and/or a large number of spin-coupled electronic states.

Exact quantum calculations of the kinetic isotope effect: cross sections and rate constants for the F+HD reaction and role of tunneling

In this paper we present integral cross sections (in the 5-220 meV collision energy range) and rate constants (in the 100-300 K range of temperature) for the F+HD reaction leading to HF+D and DF+H. The exact quantum reactive scattering calculations were carried out using the hyperquantization algorithm on an improved potential energy surface which incorporates the effects of open shell and fine structure of the fluorine atom in the entrance channel. The results reproduce satisfactorily molecular beam scattering experiments as well as chemical kinetics data for both the HF and DF channels. In particular, the agreement of the rate coefficients and the vibrational branching ratios with experimental measurements is improved with respect to previous studies. At thermal and subthermal energies, the rates are greatly influenced by tunneling through the reaction barrier. Therefore exchange of deuterium is shown to be penalized with respect to exchange of hydrogen, and the isotopic branching exhibits a strong dependence on translational energy. Also, it is found that rotational excitation of the reactant HD molecule enhances the production of HF and decreases the reactivity at the D end, obtaining insight on the reaction stereodynamics.

Spin-orbit relaxation of Cl(P1∕22) and F(P1∕22) in a gas of H2

The authors present quantum scattering calculations of rate coefficients for the spin-orbit relaxation of F(2P1/2) atoms in a gas of H2 molecules and Cl(2P1/2) atoms in a gas of H2 and D2 molecules. Their calculation of the thermally averaged rate coefficient for the electronic relaxation of chlorine in H2 agrees very well with an experimental measurement at room temperature. It is found that the spin-orbit relaxation of chlorine atoms in collisions with hydrogen molecules in the rotationally excited state j=2 is dominated by the near-resonant electronic-to-rotational energy transfer accompanied by rotational excitation of the molecules. The rate of the spin-orbit relaxation in collisions with D2 molecules increases to a great extent with the rotational excitation of the molecules. They have found that the H2/D2 isotope effect in the relaxation of Cl(2P1/2) is very sensitive to temperature due to the significant role of molecular rotations in the nonadiabatic transitions. Their calculation yields a rate ratio of 10 for the electronic relaxation in H2 and D2 at room temperature, in qualitative agreement with the experimental measurement of the isotope ratio of about 5. The isotope effect becomes less significant at higher temperatures.

State-to-State Dynamics of Elementary Bimolecular Reactions

The study of state-to-state dynamics of elementary bimolecular reactions has provided remarkable insights into chemical reactivity at the most fundamental level. This review covers exciting developments in this important field in the past decade. I focus on recent studies of quantum-state-resolved molecular-beam reactive-scattering studies of elementary chemical reactions, from triatomic to polyatomic systems. Researchers have made great advances in the fundamental understanding of many elementary chemical reactions through state-to-state dynamics studies. The strong interaction between theory and experiment has significantly enhanced our understanding of the dynamics of these reactions. I hope this review provides a glimpse of this exciting research field to both experts and beginners.

Overlapping resonances and Regge oscillations in the state-to-state integral cross sections of the F+H2 reaction

A Regge pole analysis is employed to explain the oscillatory patterns observed in numerical simulations of integral cross section for the F+H(2)(v=0,j=0)-->HF(v(')=2,j(')=0)+H reaction in the translational collision energy range 25-50 meV. In this range the integral cross section for the transition, affected by two overlapping resonances, shows nearly sinusoidal oscillations below 38 meV and a more structured oscillatory pattern at larger energies. The two types of oscillations are related to the two Regge trajectories which (pseudo) cross near the energy where the resonances are aligned. Simple estimates are given for the periods of the oscillations.

Interacting resonances in the F+H2 reaction revisited: Complex terms, Riemann surfaces, and angular distributions

We study the effect of overlapping resonances on the angular distributions of the reaction F+H2(v=0,j=0)-->HF(v=2,j=0)+H in the collision energy range from 5 to 65 meV, i.e., under the reaction barrier. Reactive scattering calculations were performed using the hyperquantization algorithm on the potential energy surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. The positions of the Regge and complex energy poles are obtained by Pade reconstruction of the scattering matrix element. The Sturmian theory is invoked to relate the Regge and complex energy terms. For two interacting resonances, a two-sheet Riemann surface is contracted and inverted. The semiclassical complex angular momentum analysis is used to decompose the scattering amplitude into the direct and resonance contributions.

Probing Feshbach resonances in F+H2(j=1)→HF+H: Dynamical effect of single quantum H2-rotation

Full quantum state resolved scattering of the F atom reaction with H(2)(j=0) and H(2)(j=1) was investigated at the collision energies of 0.19 and 0.56 kcalmol. Dramatic difference between the dynamics for the F+H(2)(j=0,1) reactions at both collision energies have been observed. Forward scattering HF(v(')=2) products have been observed unambiguously for the F+H(2)(j=1) reaction at low collision energies, which was attributed to the Feshbach resonances. This study provides a unique case of reaction resonances involving a rotationally excited reagent.

Theories of reactive scattering

This paper is an overview of the theory of reactive scattering, with emphasis on fully quantum mechanical theories that have been developed to describe simple chemical reactions, especially atom-diatom reactions. We also describe related quasiclassical trajectory applications, and in all of this review the emphasis is on methods and applications concerned with state-resolved reaction dynamics. The review first provides an overview of the development of the theory, including a discussion of computational methods based on coupled channel calculations, variational methods, and wave packet methods. Choices of coordinates, including the use of hyperspherical coordinates are discussed, as are basis set and discrete variational representations. The review also summarizes a number of applications that have been performed, especially the two most comprehensively studied systems, H+H2 and F+H2, along with brief discussions of a large number of other systems, including other hydrogen atom transfer reactions, insertion reactions, electronically nonadiabatic reactions, and reactions involving four or more atoms. For each reaction we describe the method used and important new physical insight extracted from the results.

Local angular momentum-local impact parameter analysis: derivation and properties of the fundamental identity, with applications to the F+H2, H+D2, and Cl+HCl chemical reactions

The technique of local angular momentum-local impact parameter (LAM-LIP) analysis has recently been shown to provide valuable dynamical information on the angular scattering of chemical reactions under semiclassical conditions. The LAM-LIP technique exploits a nearside-farside (NF) decomposition of the scattering amplitude, which is assumed to be a Legendre partial wave series. In this paper, we derive the "fundamental NF LAM identity," which relates the full LAM to the NF LAMs (there is a similar identity for the LIP case). Two derivations are presented. The first uses complex variable techniques, while the second exploits an analogy between the motion of the scattering amplitude in the Argand plane with changing angle and the classical mechanical motion of a particle in a plane with changing time. Alternative forms of the fundamental LAM-LIP identity are described, one of which gives rise to a CLAM-CLIP plot, where CLAM denotes (Cross section) x LAM and CLIP denotes (Cross section) x LIP. Applications of the NF LAM theory, together with CLAM plots, are reported for state-to-state transitions of the benchmark reactions F+H2-->FH+H, H+D2-->HD+D, and Cl+HCl-->ClH+Cl, using as input both numerical and parametrized scattering matrix elements. We use the fundamental LAM identity to explain the important empirical observation that a NF cross section analysis and a NF LAM analysis provide consistent (and complementary) information on the dynamics of chemical reactions.

A crossed-beam study of the F+HD→HF+D reaction: The resonance-mediated channel

This is the last report of our extensive studies on the title reaction. Presented here are the state-to-state differential cross section determinations at 11 collision energies, ranging from 1.30 to 4.53 kcal/mol. Together with previously reported results at six lower energies (0.4-1.18 kcal/mol), this perhaps represents one of the most comprehensive set of data from a single investigation for any chemical reaction. The information contents of this set of data are examined in detail, from which the dynamical consequences of reactive resonances are elucidated. Qualitative interpretations of some of the major findings are proposed. Observations that need further theoretical investigations for better physical understanding are pointed out.

Mechanism and control of the F+H2 reaction at low and ultralow collision energies

This article uses theoretical methods to study the dependence on stereodynamical factors of the mechanism and reactivity of the F+H2 reaction at low and ultralow collision energies. The impact of polarization of the H2 reactant on total and state-to-state integral and differential cross sections is analyzed. This leads to detailed pictures of the reaction mechanism in the cold and ultracold regimes, accounting, in particular, for distinctions associated with the various product states and scattering angles. The extent to which selection of reactant polarization allows for external control of the reactivity and reaction mechanism is assessed. This reveals that even the simplest of reactant polarization schemes allows for fine, product state-selective control of differential and (for reactions involving more than a single, zero orbital angular momentum partial wave) integral cross sections.

Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.

Recent advances in crossed-beam studies of bimolecular reactions

A critical overview of the recent progress in crossed-beam reactive scattering is presented. This review is not intended to be an exhaustive nor a comprehensive one, but rather a critical assessment of what we have been learning about bimolecular reaction dynamics using crossed molecular beams since year 2000. Particular emphasis is placed on the information content encoded in the product angular distribution-the trait of a typical molecular beam scattering experiment-and how the information can help in answering fundamental questions about chemical reactivity. We will start with simple reactions by highlighting a few benchmark three-atom reactions, and then move on progressively to the more complex chemical systems and with more sophisticated types of measurements. Understanding what cause the experimental observations is more than computationally simulating the results. The give and take between experiment and theory in unraveling the physical picture of the underlying dynamics is illustrated throughout this review.

Coordinate transformation methods to calculate state-to-state reaction probabilities with wave packet treatments

A procedure for the transformation from reactant to product Jacobi coordinates is proposed, which is designed for the extraction of state-to-state reaction probabilities using a time-dependent method in a body-fixed frame. The method consists of several steps which involve a negligible extra computational time as compared with the propagation. Several intermediate coordinates are used, in which the efficiency depends on the masses of the atoms involved in the reaction. A detailed study of the relative efficiency of using reactant and product Jacobi coordinates is presented for several systems, and simple arguments are found depending on the masses of the atoms involved in the reaction. It is found that the proposed method is, in general, more efficient than the use of product Jacobi coordinates, specially for nonzero total angular momentum. State-to-state reaction probabilities are obtained for Li+FH-->LiF+H and F+HO-->FH+O collisions for several total angular momenta.

A global 12-dimensional ab initio potential energy surface and dynamical studies for the SiH4+H→SiH3+H2 reaction

A global 12-dimensional ab initio interpolated potential energy surface (PES) for the SiH(4)+H-->SiH(3)+H(2) reaction is presented. The ab initio calculations are based on the unrestricted quadratic configuration interaction treatment with all single and double excitations together with the cc-pVTZ basis set, and the modified Shepard interpolation method of Collins and co-workers [K. C. Thompson et al., J. Chem. Phys. 108, 8302 (1998); M. A. Collins, Theor. Chem. Acc. 108, 313 (2002); R. P. A. Bettens and M. A. Collins, J. Chem. Phys. 111, 816 (1999)] is applied. Using this PES, classical trajectory and variational transition state theory calculations have been carried out, and the computed rate constants are in good agreement with the available experimental data.

A crossed-beam study of the F+HD→DF+H reaction: The direct scattering channel

State-to-state differential cross sections of the title reaction are presented at four collision energies, ranging from 1.18 to 4.0 kcal /mol. Product angular distributions are predominantly backscattered at low energies and shift toward sideways (peaking near 150 degrees ) at higher energies. Experimental evidence for contributions from migratory trajectories was found in the more detailed angle-specific internal state distributions. The dynamics of this reaction is mostly governed by classical mechanics, and several major findings can qualitatively be rationalized. These "classical" behaviors serve as "references" and are to be contrasted to the attributes observed for the other isotopic product channel, HF+D, in a forthcoming paper.

Multireference configuration interaction calculations for the F(P2)+HCl→HF+Cl(P2) reaction: A correlation scaled ground state (1A′2) potential energy surface

This paper presents a new ground state (1 (2)A(')) electronic potential energy surface for the F((2)P)+HCl-->HF+Cl((2)P) reaction. The ab initio calculations are done at the multireference configuration interaction+Davidson correction (MRCI+Q) level of theory by complete basis set extrapolation of the aug-cc-pVnZ (n=2,3,4) energies. Due to low-lying charge transfer states in the transition state region, the molecular orbitals are obtained by six-state dynamically weighted multichannel self-consistent field methods. Additional perturbative refinement of the energies is achieved by implementing simple one-parameter correlation energy scaling to reproduce the experimental exothermicity (DeltaE=-33.06 kcalmol) for the reaction. Ab initio points are fitted to an analytical function based on sum of two- and three-body contributions, yielding a rms deviation of <0.3 kcalmol for all geometries below 10 kcalmol above the barrier. Of particular relevance to nonadiabatic dynamics, the calculations show significant multireference character in the transition state region, which is located 3.8 kcalmol with respect to F+HCl reactants and features a strongly bent F-H-Cl transition state geometry (theta approximately 123.5 degrees ). Finally, the surface also exhibits two conical intersection seams that are energetically accessible at low collision energies. These seams arise naturally from allowed crossings in the C(infinityv) linear configuration that become avoided in C(s) bent configurations of both the reactant and product, and should be a hallmark of all X-H-Y atom transfer reaction dynamics between ((2)P) halogen atoms.

Reactive scattering dynamics in atom+polyatomic systems: F+C2H6-->HF(v,J)+C2H5

State-to-state scattering dynamics of F+C2H6-->HF(v,J)+C2H5 have been investigated at Ecom=3.2(6) kcalmol under single-collision conditions, via detection of nascent rovibrationally resolved HF(v,J) product states with high-resolution infrared laser absorption methods. State-resolved Doppler absorption profiles are recorded for multiple HF(v,J) transitions originating in the v=0,1,2,3 manifold, analyzed to yield absolute column-integrated densities via known HF transition moments, and converted into nascent probabilities via density-to-flux analysis. The spectral resolution of the probe laser also permits Doppler study of translational energy release into quantum-state-resolved HF fragments, which reveals a remarkable linear correlation between (i) HF(v,J) translational recoil and (ii) the remaining energy available, Eavail=Etot-E(HF(v,J)). The dynamics are interpreted in the context of a simple impulsive model based on conservation of linearangular momentum that yields predictions in good agreement with experiment. Deviations from the model indicate only minor excitation of ethyl vibrations, in contrast with a picture of extensive intramolecular vibrational energy flow but consistent with Franck-Condon excitation of the methylene CH2 bending mode. The results suggest a relatively simple dynamical picture for exothermic atom+polyatomic scattering, i.e., that of early barrier dynamics in atom+diatom systems but modified by impulsive recoil coupling at the transition state between translationalrotational degrees of freedom.

Observation of Feshbach resonances in the F + H2 --> HF + H reaction

Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 --> HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v' = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v' = 3)-H' vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.

Barriers of Hydrogen Abstraction vs Halogen Exchange: An Experimental Manifestation of Charge-Shift Bonding

This paper shows that the differences between the barriers of the halogen exchange reactions, in the H + XH systems, and the hydrogen abstraction reactions, in the X + HX systems (X = F, Cl, Br), measure the covalent-ionic resonance energies of the corresponding X-H bonds. These processes are investigated using CCSD(T) calculations as well as the breathing-orbital valence bond (BOVB) method. Thus, the VB analysis shows that (i) at the level of covalent structures the barriers are the same for the two series and (ii) the higher barriers for halogen exchange processes originate solely from the less efficient mixing of the ionic structures into the respective covalent structures. The barrier differences, in the HXH vs XHX series, which decrease as X is varied from F to I, can be estimated as one-quarter of the covalent-ionic resonance energy of the H-X bond. The largest difference (22 kcal/mol) is calculated for X = F in accord with the finding that the H-F bond possesses the largest covalent-ionic resonance energy, 87 kcal/mol, which constitutes the major part of the bonding energy. The H-F bond belongs to the class of "charge-shift" bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D. L.; Hiberty, P. C. Chem. Eur. J. 2005, 21, 6358), which are all typified by dominant covalent-ionic resonance energies. Since the barrier difference between the two series is an experimental measure of the resonance energy quantity, in the particular case of X = F, the unusually high barrier for the fluorine exchange reaction emerges as an experimental manifestation of charge-shift bonding.

Theoretical Study of the Reaction of H Atoms with Vibrationally Highly Excited HF Molecules

Vibrationally highly excited molecules react extremely fast with atoms and probably with radicals. The phenomenon can be utilized for selectively enhancing the rate of reactions of specific bonds. On the basis of quasiclassical trajectory calculations, the paper analyzes mechanistic details of a prototype reaction, H + HF(v). At vibrational quantum numbers v above 2, the reaction exhibits capture-type behavior, that is, the reactive cross section diverges as the relative translational energy of the partners decreases, both for the abstraction and for the exchange channel. The mechanism of the reaction for both channels is different at low and at high translational energy. At low vibrational energy, the reaction is activated, which is switched to capture-type at high excitation. The reason is an attractive potential that acts on the attacking H atom when the HF molecule is stretched. In contrast to the 6-SEC potential surface of Mielke et al., the switch cannot be observed on the Stark-Werner potential surface, due to a small artificial barrier at high H-HF separation, preventing the reactants from obeying the attractive potential and also proving the importance of the latter. The exchange reaction can be observed even when the total energy available for the partners is below the exchange barrier, because at low translational energies the product F atom of a successful abstraction step can re-abstract that H atom from the intermediate product H2 molecule that was originally the attacker.

Interactions of 2P Atoms with Closed-Shell Diatomic Molecules: Alternative Diabatic Representations for the Electronic Anisotropy

The matrices of electrostatic and spin-orbit Hamiltonians for the system of a 2P atom interacting with a closed shell diatomic molecule in uncoupled, coupled, and complex-valued representations for electronic diabatic basis functions are rederived, and the unitary transformations connecting them are given explicitly. The links to previous derivations are established and existing inconsistencies are identified and eliminated. It is proven that the block-diagonalization of a 6 x 6 matrix of the electronic Hamiltonian is a result of using the basis functions with well-defined properties with respect to time reversal. Consideration of time-reversal symmetry also enforces phase consistency relevant for applications to multisurface reactive scattering and photodetachment spectroscopy calculations, as well as for perspective studies of inelastic effects in cold and ultracold environments. These and further developments are briefly sketched.