Dimitris Skouteris

ABC: a quantum reactive scattering program

Abstract This article describes a quantum mechanical reactive scattering program for atom–diatom chemical reactions that we have written during the past several years. The program uses a coupled-channel hyperspherical coordinate method to solve the Schrodinger equation for the motion of the three nuclei on a single Born–Oppenheimer potential energy surface. It has been tested for all possible deuterium-substituted isotopomers of the H + H 2 , F + H 2 , and Cl + H 2 reactions, and tried and tested potential energy surfaces for these reactions are included within the program as Fortran subroutines.

Observation of a transition state resonance in the integral cross section of the F+HD reaction

We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2–5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately ...

Experimental and theoretical differential cross sections for the reactions Cl+H2/D2

Experimental and theoretical differential cross sections for the reactions between Cl atoms and two isotopic variants of molecular hydrogen (H2 and D2) are presented. The experimental results have been obtained by using the crossed molecular beam method with mass spectrometric detection. The theoretical results have been computed using both the quasiclassical trajectory and quantum mechanical (QM) methods. The potential energy surface employed for the calculations is the ab initio BW2 surface by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The theoretical results have been directly compared to the experiments in the laboratory frame at a collision energy (Ec) of 4.25 and 5.85 kcal/mol for the Cl+H2 reaction and of 4.9 and 6.3 kcal/mol for the Cl+D2 reaction. The agreement between QM results and experiment is quite satisfactory for the Cl+D2 reaction, especially for the low collision energy, while for Cl+H2 is less good, especially when considering data at the lower Ec.

Combined Crossed Molecular Beam and Theoretical Studies of the N(2D) + CH4 Reaction and Implications for Atmospheric Models of Titan†

The dynamics of the H-displacement channel in the reaction N((2)D) + CH(4) has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH(4)N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N((2)D) + CH(4) reaction, with the production of the CH(3)N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere of Titan are discussed.

A quantum mechanical and quasi-classical trajectory study of the Cl+H2 reaction and its isotopic variants: Dependence of the integral cross section on the collision energy and reagent rotation

Quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations have been performed for the Cl+H2, Cl+D2, Cl+HD→ HCl(DCl)+D(H) reactions in order to determine integral cross sections as a function of collision energy and for different reagent rotational quantum numbers using the recent ab initio BW2 potential energy surface (PES) by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The results are compared with experimental data obtained by using the Doppler-selected time-of-flight technique. It has been found theoretically by both the QM and QCT methods that reagent rotation enhances reactivity in agreement with experiment. The QM results are found to be in quantitative agreement with the experimental excitation functions for the Cl+p-H2 and Cl+n-H2 reactions, whereas those obtained quasi-classically fail to reproduce the experimental data. These results are in strong contrast with those reported on the previous G3 PES, in which QM and QCT calculations predicted that reactivity decreases with r...

Crossed-beam dynamics, low-temperature kinetics, and theoretical studies of the reaction S(1D) + C2H4.

The reaction between sulfur atoms in the first electronically excited state, S((1)D), and ethene (C(2)H(4)) has been investigated in a complementary fashion in (a) crossed-beam dynamic experiments with mass spectrometric detection and time-of-flight (TOF) analysis at two collision energies (37.0 and 45.0 kJ mol(-1)), (b) low temperature kinetics experiments ranging from 298 K down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the C(2)H(4)S singlet and triplet potential energy surfaces. The rate coefficients for total loss of S((1)D) are found to be very large (ca. 4 x 10(-10) cm(3) molecule(-1) s(-1)) down to very low temperatures indicating that the overall reaction is barrierless. From laboratory angular and TOF distributions at different product masses, three competing reaction channels leading to H + CH(2)CHS (thiovinoxy), H(2) + CH(2)CS (thioketene), and CH(3) + HCS (thioformyl) have been unambiguously identified and their dynamics characterized. Product branching ratios have also been estimated. Interpretation of the experimental results on the reaction kinetics and dynamics is assisted by high-level theoretical calculations on the C(2)H(4)S singlet potential energy surface. RRKM (Rice-Ramsperger-Kassel-Marcus) estimates of the product branching ratios using the newly developed singlet potential energy surface have also been performed and compared with the experimental determinations.

Formation of nitriles and imines in the atmosphere of Titan: combined crossed-beam and theoretical studies on the reaction dynamics of excited nitrogen atoms N(2D) with ethane

The dynamics of the H-displacement channels in the reaction N(2D) + C2H6 have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (18.0 and 31.4 kJ mol(-1)). From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms and the product energy partitioning have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the C2H6N ground state doublet potential energy surface. C-C bond breaking and NH production channels have been theoretically characterized and the statistical branching ratio derived at the temperatures relevant for the atmosphere of Titan. Methanimine plus CH3 and ethanimine plus H are the main reaction channels. Implications for the atmospheric chemistry of Titan are discussed.

Time-dependent wavepacket calculations for the system on a LEPS surface: inelastic and reactive probabilities

A time-dependent wavepacket program has been used to determine rotationally and vibrationally inelastic and reactive probabilities for collisions between a N atom and a N2 molecule in their ground electronic states (4S and respectively). The calculation is performed on a single LEPS surface and therefore any spin effects (expected to be negligibly small) are neglected. Complete rovibrational product state resolution is achieved. The collision is found to be to a large extent vibrationally adiabatic, while there is significant mixing of rotational states. An interesting interference effect is observed.A time-dependent wavepacket program has been used to determine rotationally and vibrationally inelastic and reactive probabilities for collisions between a N atom and a N2 molecule in their ground electronic states (4S and respectively). The calculation is performed on a single LEPS surface and therefore any spin effects (expected to be negligibly small) are neglected. Complete rovibrational product state resolution is achieved. The collision is found to be to a large extent vibrationally adiabatic, while there is significant mixing of rotational states. An interesting interference effect is observed.

A comparison of the quantum state-specific efficiency of N + N2 reaction computed on different potential energy surfaces.

State-to-state exact quantum probabilities of the N + N2 exchange reaction have been calculated on the recently proposed L4 potential energy surface fitted to high level ab initio points using full-dimensional time-independent quantum techniques. Thermal rate coefficient values calculated on L4 were found not to differ from those calculated on a previously proposed potential energy surface. On the contrary, state-specific reaction probabilities calculated on the two surfaces are shown to differ significantly. Arguments for attributing the difference to specific features of the considered potential energy surfaces are provided.

Theoretical study of reactions relevant for atmospheric models of titan: interaction of excited nitrogen atoms with small hydrocarbons

The potential energy surface of the systems N(2D) + CH4 , C2 H4 , and C2 H6 have been investigated at B3LYP/aug-cc-pVTZ//CCSD(T)/aug-cc-pVTZ level in order to assist the interpretation of available experimental information very relevant for its implication for the chemical models of the atmosphere of Titan, and possibly of objects where both N2 and small hydrocarbons like methane are present, such as Triton and Pluto.