Entrance Valley Research Articles

ChemInform Abstract: Control of Chemical Reactivity by Transition‐State and Beyond

AbstractReview: 138 refs.

Two-center three-electron bonding in ClNH3 revealed via helium droplet infrared laser Stark spectroscopy: Entrance channel complex along the Cl + NH3 → ClNH2 + H reaction.

Pyrolytic dissociation of Cl2 is employed to dope helium droplets with single Cl atoms. Sequential addition of NH3 to Cl-doped droplets leads to the formation of a complex residing in the entry valley to the substitution reaction Cl + NH3 → ClNH2 + H. Infrared Stark spectroscopy in the NH stretching region reveals symmetric and antisymmetric vibrations of a C3v symmetric top. Frequency shifts from NH3 and dipole moment measurements are consistent with a ClNH3 complex containing a relatively strong two-center three-electron (2c-3e) bond. The nature of the 2c-3e bonding in ClNH3 is explored computationally and found to be consistent with the complexation-induced blue shifts observed experimentally. Computations of interconversion pathways reveal nearly barrierless routes to the formation of this complex, consistent with the absence in experimental spectra of two other complexes, NH3Cl and Cl-HNH2, which are predicted in the entry valley to the hydrogen abstraction reaction Cl + NH3 → HCl + NH2.

Dynamics of Complex-Forming Bimolecular Reactions: A Comparative Theoretical Study of the Reactions of H Atoms with O2((3)Σg(-)) and O2((1)Δg).

The atomic-level mechanism of the reaction of H atoms with triplet and singlet molecular oxygen, H((2)S) + O2((3)Σg(-)) → O((3)P) + OH((2)Πg) ( R1 ) and H((2)S) + O2((1)Δg) → O((3)P) + OH((2)Πg) ( R2 ) is analyzed in terms of the topology of the potential energy surfaces (PES) of the two reactions. Both PES exhibit a deep potential well corresponding to the ground and first excited electronic state of HO2. The ground-state reaction is endothermic with no barrier on either side of the well; the excited-state reaction is exothermic with a barrier in the entrance valley of the PES. The differences of the PES are manifested in properties such as the excitation functions, which show reaction R1 to be much slower and the effect of rotational excitation on reactivity, which speeds up reaction R1 and has little effect on R2 . Numerous common dynamics features arise from the presence of the deep potential well on the PES. Such are the significant role of isomerization (for example, 90% of reactive collisions in R2 involve at least one H atom transfer from one of the O atoms to the other in reaction R2 ), which is shown to give rise to a significant rotational excitation of the product OH radicals. Common is the significant sideways scattering of the products that originates from collisions in propeller-type arrangements induced by the presence of two bands of acceptance around the O2 molecule. The HO2 complex in both reactions proves to behave nonstatistically, with signatures of the dynamics in lifetime distributions, angular distributions, opacity functions, and product quantum-state distributions.

Ab initio study of the C + HNC, N + C2H, H + C2N and H + CNC reactions

The mechanisms of the C+HNC, N+C2H, H+C2N and H+CNC reactions have been investigated using ab initio and DFT calculations. The long-range interactions and species involved in these systems on the triplet potential energy surface were optimized at the M06-2X/cc-pVQZ, MRCI+Q/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ levels. The calculations show that these reactions proceed without a barrier in the entrance valley, and are mostly characterized by transition states which are submerged with respect to the reagent level. As a result, these processes are potentially very important for interstellar chemistry.

Perspective: Vibrational-induced steric effects in bimolecular reactions.

The concept of preferred collision geometry in a bimolecular reaction is at the heart of reaction dynamics. Exemplified by a series of crossed molecular beam studies on the reactions of a C-H stretch-excited CHD3(v1 = 1) with F, Cl, and O((3)P) atoms, two types of steric control of chemical reactivity will be highlighted. A passive control is governed in a reaction with strong anisotropic entry valley that can significantly steer the incoming trajectories. This disorientation effect is illustrated by the F and O((3)P) + CHD3(v1 = 1) reactions. In the former case, the long-range anisotropic interaction acts like an optical "negative" lens by deflecting the trajectories away from the favored transition-state geometry, and thus inhibiting the bond rupture of the stretch-excited CHD3. On the contrary, the interaction between O((3)P) and CHD3(v1 = 1) behaves as a "positive" lens by funneling the large impact-parameter collisions into the cone of acceptance, and thereby enhances the reactivity. As for reactions with relatively weak anisotropic interactions in the entry valley, an active control can be performed by exploiting the polarization property of the infrared excitation laser to polarize the reactants in space, as demonstrated in the reaction of Cl with a pre-aligned CHD3(v1 = 1) reactant. A simpler case, the end-on versus side-on collisions, will be elucidated for demonstrating a means to disentangle the impact-parameter averaging. A few general remarks about some closely related issues, such as mode-, bond-selectivity, and Polanyi's rules, are made.

New Potential Energy Surface Features for the Li + HF → LiF + H Reaction

The existing potential energy surfaces for the Li + HF system have been challenged by the experiments of Loesch, Stienkemeier, and co-workers. Here a very accurate potential energy surface has been obtained with rather rigorous theoretical methods. Methods up to full CCSDT have been pursued with basis sets as large as core correlated quintuple ζ. Reported here are the reactants, products, two transition states, and three intermediate complexes for this reaction. These reveal one previously undiscovered equilibrium geometry. The stationary point relative energies are very sensitive to level of theory. The reaction has a classical endothermicity of 2.6 kcal mol(-1). The complex Li···HF in the entrance valley lies 6.1 kcal/mol below the reactants. The expected transition state Li···H···F is bent with an angle of 72.2° and lies 4.5 kcal/mol above the reactants. The latter predicted classical barrier should be no more than one kcal/mol above the exact barrier. Not one but two product complexes lie 1.6 and 2.2 kcal/mol above reactants, respectively. Between the two product complexes, a second transition state, very broad, is found. The vibrational frequencies and zero-point vibrational energies (ZPVE) of all stationary points are reported, and significantly affect the relative energies.

State-Selected Reaction of Muonium with Vibrationally Excited H2

We report a new advance in the study of muonium (Mu) reactivity; specifically, we report the rate constant for the Mu + H2(vibrational quantum number n = 1) reaction determined by measurements at 300 K and by converged quantum mechanical calculations. Comparisons are made with earlier results for D + H2 (n = 1) and with the corresponding thermal reaction rates. The measurements are a sensitive probe of the high-curvature region in the entrance valley of the potential energy surface (PES) and thus provide a qualitatively different probe of the PES than that provided by any previous experiment.

Vibrationally resolved transition state spectroscopy of the F + H2 and F + CH4 reactions

The transition state regions of the F + para-H2, F + normal-H2, F + CH4 and F + CD4 reactions have been studied by slow electron velocity-map imaging (SEVI) spectroscopy of the anionic precursor clusters para-FH2-, normal-FH2-, FCH4 and FCD4. The F + H2 results improve on previously published photoelectron spectra, resolving a narrow peak that appears in the same position in the para-FH2 and normal-FH2- spectra, and suggesting that additional theoretical treatment is necessary to fully describe and assign the experimental results. A small peak in the para-FH2- results is also identified, matching simulations of a product resonance in the v' = 3 vibrational level. SEVI spectra of the 2P3/2 bands of FCH4- and FCD4- show extended structure from transitions to the entrance valley van der Waals region and the reactant side of the F + CH4 transition state region. Much of this structure is attributed to bending or hindered rotation of the methane moiety and may be a spectroscopic signature of reactive resonances.

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

Challenges associated with the theoretical and experimental kinetics of the F+NH(3)→HF+NH(2) 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·····NH(3) 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+NH(2). These results explain the observation by Persky of unprecedented inverse temperature dependence for the F+NH(3) rate constants. The strong hydrogen-bonded complex FH·····NH(2) 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.

State-to-state chemiluminescence in reactions of Mn atoms with S2Cl2

A combined experimental and time-dependent density functional theory (TDDFT) investigation of the title reaction is presented. Both 'hot' and 'cold' laser-ablated Mn atom beams have been employed to determine the translational excitation functions for production of MnCl*(c(5)Σ(+), d(5)Π, e(5)Δ, e(5)Σ(+), A(7)Π). Analysis in terms of the multiple line-of-centres approach shows that the 'hot' results are dominated by reactions of the second metastable state of Mn, z(8)P(J), all with very low thresholds; while the first metastable state, a(6)D(J), and the ground state, a(6)S, are the precursors in the 'cold' results, all with significant excess barriers. The post-threshold behaviour of most z(8)P(J) and a(6)D(J) reaction channels implies that the transition states shift forward with increasing collision energy. The TDDFT calculations suggest that, while Mn*(z(8)P(J), a(6)D(J)) insertion into the S-Cl bond is facile, the observed chemiluminescence channels mostly derive from abstraction in a preferred linear Mn-Cl-S configuration, and that the low z(8)P(J) thresholds originate from attractive but excited reagent potentials which either reach a seam of interactions in the product valley or (in the c(5)Σ(+) case) lead to an octet potential very close in energy to the product sextet. The excess barriers in the Mn*(a(6)D(J)) and Mn(a(6)S) reactions appear for the most part to derive from exit channel mixing with lower-lying product potentials. The observed transition state shifts are consistent with the system being forced to ride up the repulsive wall of the entrance valley as collision energy increases, the location of that wall being different for the z(8)P(J) and a(6)D(J) cases.

Role of the C−H Stretch Mode Excitation in the Dynamics of the Cl + CHD3 Reaction: A Quasi-classical Trajectory Calculation

To analyze the effect of the C-H stretch mode excitation on the dynamics of the Cl + CHD3 gas-phase abstraction reaction, an exhaustive state-to-state dynamics study was performed. This reaction can evolve along two channels: H-abstraction, CD3 + ClH, and D-abstraction, CHD2 + ClD. On an analytical potential energy surface constructed previously by our group, named PES-2005, quasi-classical trajectory calculations were performed at a collision energy of 0.18 eV, including corrections to avoid zero-point energy leakage along the trajectories. First, strong coupling between different vibrational modes in the entry valley was observed; i.e., the reaction is vibrationally nonadiabatic. Second, for the ground-state CHD3(nu=0) reaction, the diatomic fragments appeared in their ground states, and the H- and D-abstraction reactions showed similar reactivities. However, when the reactivity per atom is considered, the H is three times more reactive than the D atom. Third, when the C-H stretch mode is excited by one quantum, CHD3(nu1=1), the H-abstraction is strongly favored, and the C-H stretch excitation is maintained in the product CHD2(nu1=1) + ClD channel; i.e., the reaction shows mode selectivity, reproducing the experimental evidence, and also the reactivity of the vibrational ground state is increased, in agreement with experiment. Fourth, the state-to-state angular distributions of the CD3 and CHD2 products showed the products to be practically sideways for the reactant ground state, while the C-H excitation yielded a more forward scattering, reproducing the experimental data. The role of the zero-point energy correction was also analyzed, and we find that the dynamics results are very sensitive on how the ZPE issue is treated. Finally, a comparison is made with the similar H + CHD3(nu1=0,1) and Cl + CH4(nu1=0,1) reactions.

New Analytical Potential Energy Surface for the F(2P) + CH4 Hydrogen Abstraction Reaction: Kinetics and Dynamics

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

Potential Energy Surface for the F(2P3/2,2P1/2) + CH4 Hydrogen Abstraction Reaction. Kinetics and Dynamics Study

A modified and recalibrated potential energy surface (PES) is reported for the gas-phase F(2P(3/2),2P(1/2)) + CH4 reaction and its deuterated analogue. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to the updated experimental and theoretical stationary point properties and experimental thermal rate constants. To take into account the two spin-orbit electronic states of the fluorine atom, two versions of the surface were constructed, the PES-SO and PES-NOSO surfaces, which differ in the choice of the zero reference level of the reactants. On both surfaces, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. While the PES-SO surface overestimates the experimental rate constants, the PES-NOSO surface shows a better agreement, reproducing the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. The kinetic isotope effects calculated showed good agreement with the sparse experimental data at 283 and 298 K. Finally, on the PES-NOSO surface, other dynamical features, such as the coupling between the reaction coordinate and the vibrational modes, were analyzed. It was found qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. These kinetics and dynamics results seem to indicate that a single, adiabatic PES is adequate to describe this reaction.

Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processes

AbstractThe Okavango wetland in northern Botswana is one of the world's largest inland deltas. The delta is a dynamic environment with shifting channel routes, causing growth and decay of flanking wetlands, and giving birth to islands. Primary island nuclei are formed by fluvial processes and bioengineering, and subsequently grow into secondary larger islands of irregular shape by clastic and chemical sedimentation, and later by coalescence. This article presents classifications and quantitative estimations of channels, wetlands and islands of the Okavango Delta. Islands were classified dependent on composition, pattern of composition, shape and juxtaposition. 90 per cent of all islands in the entire wetland were identified, with a classification accuracy of 60 to 85 per cent. Smaller islands of the nucleus types dominate the upper parts of the delta, whereas larger secondary islands are more common in the distal part, a reflection of the age of the islands. Islands in the entry valley of the delta, the Panhandle, are larger in the top end – the primary region of recent clastic sedimentation. The overall size distribution of islands in the delta, however, shows no clumps, indicating that island growth is a uniform process over time and space. The total area flooded at least every decade is approximately 14 000 km2, of which 9000 km2 is classified as actual wetland. Channel meandering decreases from the Panhandle to the distal part of the delta, with the abandoned Thaoge channel as an exception. Occurrence of fluvially formed islands in the distal delta indicates that the water flow and area of inundation must once have been much larger. Copyright © 2004 John Wiley & Sons, Ltd.

Narrow Subthreshold Quantum Mechanical Resonances in the Li + HF → H + LiF Reaction

State-to-state, state-specific, and cumulative reaction probabilities are presented for the bimolecular scattering process Li + HF → H + LiF in the ground electronic state. Calculations were performed for zero total angular momentum at total energies from 0.26 to 0.50 eV (relative to HF at its classical equilibrium bond distance and infinitely far from Li). The energy dependence of the state-to-state, initial-state-selected, and cumulative reaction probabilities for LiFH in the low-energy regime displays a pronounced resonance structure due to quasibound states associated with a Li...FH van der Waals well in the entrance valley of the potential energy surface. The lifetimes of the long-lived resonances are obtained by fitting the calculated eigenphase sum to the multichannel Breit-Wigner formula. The final rotational state distributions of the LiF product fragment resulting from decay of the resonance state complexes are presented for two resonances. Quantum numbers are assigned to the resonances using bound-state and quasibound-state calculations in the Li...FH van der Waals well, and possible decay mechanisms are discussed. The lifetimes show a systematic dependence on the translational vibrational quantum number.

Wave packet dynamics along bifurcating reaction paths

The problem of bifurcating reaction paths is revisited by wave packet (WP) dynamics. The pitchfork model connecting five stationary points—a reactive, two transition structures and two enantiomeric products—is characterized by a Valley Ridge inflection point (VRI) where WP could leave the standard intrinsic reaction path. We question the role of such a VRI point to determine whether the mechanism is sequential or concerted. WP simulations on two-dimensional minimum energy surfaces are carried out in the benchmark case of the methoxy radical isomerization H3CO→H2COH. The ab initio potential energy surface (PES) is fitted to an analytical model which is bent to analyze the incidence of geometrical parameters on the WP behavior. For each of these generated PES, the WP width in the entrance valley is the main factor which conditions the behavior on the unstable ridge. The WP evolution is also analyzed in terms of nonadiabatic transitions among adiabatic channels along the reaction coordinate. Finally, the location of VRI points according to an invariant definition is discussed.

The dynamics of H2 formation on a graphite surface at low temperature

The catalytic formation of H2 on carbonaceous grains in interstellar conditions is studied theoretically. The grain is modelled by a coronene molecule. The coronene–H–H interaction is described at the DFT level. The dynamics study is limited to the collinear case, with two degrees of freedom (Cor–H and H–H) handled by a wavepacket description. The collision energy range extends between 0.3 meV and 0.5 eV. The main results are: (i) the collision time has the same order of magnitude as the lattice relaxation time; (ii) the vibrational excitation is very large; (iii) the reaction probability is very sensitive to the small potential barrier in the entrance valley predicted by the DFT calculation.

Potential energy surface for the CH3+HBr→CH4+Br hydrogen abstraction reaction: Thermal and state-selected rate constants, and kinetic isotope effects

The gas-phase hydrogen abstraction title reaction was carefully investigated. First, ab initio molecular orbital theory was used to study the stationary points along the reaction path: reactants, hydrogen-bonded complex, saddle point, and products. Optimized geometries and harmonic vibrational frequencies were calculated at the second-order Møller–Plesset perturbation theory level, and then single-point calculations were performed at a higher level of calculation: coupled-cluster with triple-zeta basis set. The effects of the level of calculation, zero-point energy (ZPE), thermal corrections [TC (298.15 K)], spin–orbit coupling, and basis set superposition error (BSSE) on the energy changes were analyzed. It was concluded that at room temperature (i.e., with ZPE and TC), when the BSSE was included, the complex disappears and the activation enthalpy is +0.39 kcal mol−1 above the reactants. Second, an analytical potential energy surface was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using experimental and theoretical stationary point properties and the tendency of the kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range. In both cases, we found a direct dependence on temperature and, therefore, positive activation energies. The influence of the tunneling factor was very small due to the flattening of the surface in the entrance valley. This surface was also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate constants. It was found that excitation of the BrH stretching mode enhances the forward reaction, whereas the excitation of the CH3 umbrella mode has the opposite effect.

Temperature dependence of fast neutral–neutral reactions: a triatomic model study

Some neutral–neutral reactions are known to proceed rapidly at low temperatures as a consequence of strong inverse temperature power-law dependences of the rate constants. Previous calculations, based on capture approximations, failed to account for these experimental data. In this article, short-range effects (subsequent to capture) are investigated using a simple planar atom–diatom toy model based on pairwise atomic interactions. Reaction rate constants have been estimated in the temperature range of 25–300 K using a quasi-classical trajectory Monte Carlo approach. It is shown that a small short-range barrier in the entrance valley may significantly influence the reactivity. In particular, our crude triatomic model can reproduce a strong inverse temperature dependence of the rate constant in good agreement with experimental evidence for more complex systems. These predictions are interpreted using a 3-D representation of the effective potential surfaces, illustrating the crucial importance of vector correlations between partial angular momenta. Thus, the strong inverse power-law temperature dependence of the rate constant may be attributed to the population of higher rotational states of the reactants with increasing temperature.

Intermolecular potential energy surfaces and bound states in F–HF

Semiempirical potential energy surfaces for F–HF are constructed, based on recent Ne–HF, Ne–F, and Ne–Ne potentials. The electrostatic forces due to the quadrupole of the F atom are included. The potentials are presented in diabatic and adiabatic representations, with and without spin–orbit coupling. Fully coupled bound-state calculations are carried out, and the resulting energy levels and wave functions are analyzed. The well depth is 317 cm−1, and the ground state is bound by 174 cm−1. The complex is a promising candidate for spectroscopic observation, which would provide detailed information on the potential energy surfaces in the entrance and exit valleys of the F+HF reaction.