J. M. Lucas

The He + H2+ reaction: a dynamical test on potential energy surfaces for a system exhibiting a pronounced resonance pattern

Quantum mechanical calculations on three potential energy surfaces for the prototype ion-molecule reaction He + H2+ → HeH+ + H have been performed in order to test the influence of their accuracies on reaction probabilities and cross sections. The ab initio points of McLaughlin and Thompson (1979) fitted by two different functional forms, and a fit of a new set of ab initio points have been used. Dynamical results, in particular the rich resonance pattern, illustrate the dependence both on the nature of the potential energy surface, and on the type of functional form used to fit the same ab initio data.

Ab initio dynamics of the He + H+ 2 → HeH+ +H reaction: a new potential energy surface and quantum mechanical cross-sections

The reaction He + H+ 2(v,j = 0) → HeH+(v′ = 0, j′) for v = 0, 1,2 and 3 and for scattering energies near the threshold (0.95–1.15 eV) has been studied by calculating ab initio points at MRCI level and ‘exact’ integral quantum reactive cross-sections. More than 1400 nuclear geometries have been chosen to cover the most important regions for the dynamics, an extended set of points being taken directly on a hyperspherical coordinate grid. A many-body expansion with a large number of terms permits an accurate analytical representation of the potential energy surface with a root-mean-square deviation <12meV. The hyperquantization algorithm has been extended to obtain quantum mechanical integral cross-sections which are compared with previous calculations and with experimental results.

Lifetime of reactive scattering resonances: Q-matrix analysis and angular momentum dependence for the F+H2 reaction by the hyperquantization algorithm

We report a study on the behavior with total angular momentum J of several resonances occurring at collision energies below or slightly above the reaction barrier in the F+H2-->HF+H reaction. Resonance positions and widths are extracted from exact time-independent quantum mechanical calculations using the hyperquantization algorithm and Smiths Q-matrix formalism which exploits complete S-matrix information. The results confirm previous work but provide much greater insight. Identification of quasi-bound states responsible for the resonances based on adiabatic models for the long-range atom-molecule interactions both in the entrance and exit channels, is successful except for the feature occurring at the lowest energy, which is found to overlap with an exit-channel resonance for J approximately 7. The two features are analyzed as overlapping resonances and their excellent Lorentzian fits, with well-behaved J-dependences of positions and widths, support the interpretation of the low-energy feature as a resonance to be associated to the triatomic transition state of the reaction. Resonance role on the reactive observables (integral cross sections and angular distributions) is investigated. The mechanism leading to forward scattering in the reactive differential cross section is commented, while the effects on rate constants, as well as the sensitivity of the resonance pattern to modification of the potential energy surface, are fully discussed elsewhere.

A generalized formulation of ion-π electron interactions: role of the nonelectrostatic component and probe of the potential parameter transferability.

The intermolecular potentials for hexafluorobenzene (HFBz) and 1,3,5-trifluorobenzene (TFBz) interacting with alkali (M(+); M = Li, Na, K, Rb, Cs) and halogen (X(-); X = F, Cl, Br, I) ions are provided as a combination of electrostatic and nonelectrostatic terms. The ion-HFBz and ion-TFBz electrostatic components are formulated as a sum of Coulombic potentials associated with the interactions between the ion charge and point charges on the molecular frame, whose distributions are consistent with the permanent quadrupole moment of HFBz and TFBz, respectively. The corresponding nonelectrostatic components are represented as a sum of effective potential functions, each one having a specific physical meaning, related to ion-molecular bond pair interactions. In the present paper, we test the transferability of the ion-bond potential parameters. Moreover, the powerfulness of the model is analyzed by comparing predicted binding energies and equilibrium geometries for the family of M(+)-HFBz, X(-)-HFBz, M(+)-TFBz, and X(-)-TFBz systems with available ab initio results.

Reactivity enhanced by under-barrier tunneling and resonances: the F+H2→HF+H reaction

Accurate quantum mechanical rate constants (all contributing partial waves, fine energy grid Boltzmann averaging) for the title reaction are obtained by the hyperquantization algorithm, covering the range from above room temperature down to the cold regime (few K). The good agreement with available experiments down to � 200 K, obtained by blending ab initio description of the transition state and molecular beam scattering experimental characterization of the entrance channel, establishes the reliability of the approach to describe deviations from Arrhenius behavior at those low temperatures where quantum mechanics can induce specific selectivity in chemical reactivity 2003 Elsevier Science B.V. All rights reserved.

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.

Exact state-to-state quantum dynamics of the F + HD --> HF(v' = 2) + D reaction on model potential energy surfaces.

In this paper, we present the results of a theoretical investigation on the dynamics of the title reaction at collision energies below 1.2 kcal/mol using rigorous quantum reactive scattering calculations. Vibrationally resolved integral and differential cross sections, as well as product rotational distributions, have been calculated using two electronically adiabatic potential energy surfaces, developed by us on the basis of semiempirical modifications of the entrance channel. In particular, we focus our attention on the role of the exothermicity and of the exit channel region of the interaction on the experimental observables. From the comparison between the theoretical results, insight about the main mechanisms governing the reaction is extracted, especially regarding the bimodal structure of the HF(v = 2) nascent rotational state distributions. A good overall agreement with molecular beam scattering experiments has been obtained.

A study to improve the van der Waals component of the interaction in water clusters

A portable model potential, representing the intermolecular interaction of water as a combination of a few effective components given in terms of the polarizability and dipole moment values of the molecular partners, is here proposed as a building block of the force field of water clusters in molecular dynamics simulations. In this spirit, here, we discuss the key properties of the model potential and its application to water dimers, trimers and tetramers with the purpose of extrapolating the results to very large clusters mimicking the liquid phase. The suitability of the model potential for dynamics investigations is checked by comparing on one hand the value of the second virial coefficient calculated for the gaseous dimer with experimental data measured over a wide range of temperature (273–3000 K) and, on the other hand, the calculated radial distribution functions and density with those obtained from experiments performed using liquid water.

Exact quantum 3D cross sections for the Ne+H2+→NeH++H reaction by the hyperspherical method. Comparison with approximate quantum mechanical and classical results

Exact, fully converged three-dimensional quantum mechanical cross sections for the title reaction have been computed on the analytical potential energy surface of Pendergast, Heck, Hayes and Jaquet. The close-coupling hyperspherical method of Launay and LeDourneuf has been used for the calculations. Results explicitly shown here correspond to reaction probabilities as a function of total energy and J, integral cross sections and product rotational distributions, for the first three reactant vibrational levels and the ground j=0 reactant rotational level. Integral cross sections confirm the main experimental findings: (a) vibrational excitation greatly enhances reactivity and (b) the reactivity threshold is near the opening of the v=2 reactant channel. Product rotational distributions show an unimodal shape, with its maximum lying at intermediate values of the open product rotational quantum numbers. Results have been compared with previously available centrifugal sudden (CS) and reactive infinite order sudden (R-IOS) results, as well as with quasiclassical trajectory (QCT) calculations. As a general trend, CS and R-IOS integral cross sections show the same qualitative shape as the exact ones, the CS ones being very close to exact but those of R-IOS are between four and five times lower. The QCT results are three times lower and fail to reproduce the threshold behaviour. CS rotational distributions are slightly hotter than exact ones, while QCT results are closer to the exact ones except for the fact that they populate rotational levels not allowed when considering both the zero-point energy and the total energy conservation.

Benzene-hydrogen bond (C6H6-HX) interactions: the influence of the X nature on their strength and anisotropy.

The intermolecular potential energy of the C6H6-SH2 and C6H6-NH3 dimers is formulated as combination of independent electrostatic and nonelectrostatic contributions. The relevant parameters of the nonelectrostatic terms, derived from molecular polarizability components, have been proved to be useful to describe in a consistent way both size repulsion and dispersion attraction forces. The representation adopted for the electrostatic contribution asymptotically reproduces the dipole quadrupole interaction. To test the validity of the proposed potential formulation, the features of the most stable configurations of the systems predicted have been compared with the available ab initio and experimental data. Moreover, the strength of the C6H6-HX interaction has been analyzed comparing the obtained results with the corresponding ones for the C6H6-H2O and C6H6-CH4 systems, investigated previously with the same methodology. Information on the relative orientation dependence of the partners, arising from the anisotropy of the intermolecular interaction, evaluated at different intermolecular distances, has been also obtained. Such information is crucial to evaluate sterodynamics effects in bimolecular collisions.