S. Yu. Grebenshchikov

Isotope dependence of the O+O2 exchange reaction: Experiment and theory

The isotope dependence of the O+O2 exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic 18O+16O2→18O16O+16O and the endothermic 16O+18O2→16O18O+18O reaction between 233 and 353 K: k8=(3.4±0.6)×10−12 (300 K/T)1.1±0.5 cm3 s−1 and k6=(2.7±0.4)×10−12 (300 K/T)0.9±0.5 cm3 s−1. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔEZPE≈±22 cm−1, are phenomenologically incorporated. Only c...

Saddle-node bifurcations in the spectrum of HOCl

A detailed analysis of the bound-state spectrum of HOCl (hypoclorous acid) in the ground electronic state is presented. Exact quantum mechanical calculations (filter diagonalization) are performed employing an ab initio potential energy surface, which has been constructed using the multireference configuration-interaction method and a quintuple-zeta one-particle basis set. The wave functions of all bound states up to the HO+Cl dissociation threshold are visually inspected in order to assign the spectrum in a rigorous way and to elucidate how the spectrum develops with energy. The dominant features are (1) a 2:1 anharmonic resonance between the bending mode and the OCl stretching mode, which is gradually tuned in as the energy increases, and (2) a saddle-node bifurcation, i.e., the sudden birth of a new family of states. The bifurcation is further investigated in terms of the structure of the classical phase space (periodic orbits, continuation/bifurcation diagram). It is also discussed how the spectrum of...

Highly excited vibrational states of HCP and their analysis in terms of periodic orbits: The genesis of saddle-node states and their spectroscopic signature

We present quantum mechanical bound-state calculations for HCP(X) using an ab initio potential energy surface. The wave functions of the first 700 states, corresponding to energies roughly 23 000 cm−1 above the ground vibrational state, are visually inspected and it is found that the majority can be uniquely assigned by three quantum numbers. The energy spectrum is governed, from the lowest excited states up to very high states, by a pronounced Fermi resonance between the CP stretching and the HCP bending mode leading to a clear polyad structure. At an energy of about 15 000 cm−1 above the origin, the states at the lower end of the polyads rather suddenly change their bending character. While all states below this critical energy avoid the isomerization pathway, the states with the new behaviour develop nodes along the minimum energy path and show large-amplitude motion with H swinging from the C- to the P-end of the diatomic entity. How this structural change can be understood in terms of periodic class...

Unimolecular dissociation of HOCl: unexpectedly broad distribution of rate constants

Abstract The dissociation of HOCl in its electronic ground state is investigated by means of quantum dynamics calculations (filter diagonalization and harmonic inversion of the autocorrelation function) and an ab initio potential energy surface. At threshold, the state-resolved rate constants are scattered over seven orders of magnitude, which is significantly broader than the distribution predicted by random matrix theory. This remarkable state specificity is the fingerprint of the regular dynamics of HOCl even at high energies.

UNIMOLECULAR DISSOCIATION OF NO2. I. CLASSICAL TRAJECTORY AND STATISTICAL CALCULATIONS ON A GLOBAL POTENTIAL ENERGY SURFACE

The title reaction is examined with classical and statistical (RRKM, SACM) mechanics on an ab initio three-dimensional global potential energy surface of the lowest adiabatic electronic state of NO2. The energy-resolved rate constants obtained within different theoretical frameworks are compared with each other and with the available experimental data. The question of the transition state definition in this barrierless reaction is analyzed and the applicability of the statistical hypothesis is discussed. Although the rate of internal vibrational energy redistribution is estimated to be larger than 5 ps−1 at the quantum dissociation threshold, which is much larger than the dissociation rate, a substantial fraction of classical trajectories remains regular. This causes statistical transition-state-like theories to overestimate the actual dissociation rate defined from the full classical dynamics calculations.

The triplet channel in the photodissociation of ozone in the Hartley band: Classical trajectory surface hopping analysis

The triplet channel in the photodissociation of ozone in the Hartley band, O3 + hv-->O(3P) + O2(3sigma), is investigated by means of a classical trajectory surface hopping method using ab initio diabatic potential energy surfaces for the B and the R states. Because of the strong recoil in the R state along the breaking O-O bond, O2(3sigma) is produced with a high rotational energy. The nonadiabatic transition probability depends markedly on the coordinate along the crossing seam. As a consequence a unique correlation is found between the internuclear geometry at the crossing and the final vibrational state of O2(3sigma). The calculated distribution of the translational energy is in good accord with the measured distribution.

Absorption spectrum and assignment of the Chappuis band of ozone

New global diabatic potential energy surfaces of the electronic states 1B1 and 1A2 of ozone and the non-adiabatic coupling surface between them are constructed from electronic structure calculations. These surfaces are used to study the visible photodissociation in the Chappuis band by means of quantum mechanical calculations. The calculated absorption spectrum and its absolute intensity are in good agreement with the experimental results. A vibrational assignment of the diffuse structures in the Chappuis band system is proposed on the basis of the nodal structures of the underlying resonance states.

Ab initio vibration–rotation‐tunneling spectra and dynamics of H2⋅F− and its isotopomers

Ab initio computations of the potential energy surface (PES) of the ground electronic state of H2⋅F− have been performed as a function of the stretching F–H2 (R) and H2 rotation (θ) coordinates. Minima on the PES correspond to linear H–H–F− structures, while the transition state is T‐shaped. The F− to H2 distance increases in the transition state from 2.07 to 3.10 A, demonstrating strong coupling between the θ and R degrees of freedom. The vibration–rotation‐tunneling spectra are calculated by diagonalizing the five dimensional Hamiltonian matrix that describes free rotation of the triatomic (three coordinates) plus the internal θ and R motions. For total angular momentum J=0, the spacing between levels in the tunneling doublets increases from 0.029 to 6.74 cm−1 as the stretching quantum number n corresponding to R motion varies from 0 to 5. The splittings increase even more strongly with the bending quantum number. For J=1, each level in the tunneling doublets is further split by Coriolis forces. K‐doubl...

Ka-mixing in the unimolecular dissociation of NO2 studied by classical dynamics calculations

Abstract Coriolis and centrifugal vibration–rotation coupling in the unimolecular dissociation of ground electronic state NO 2 has been examined by using classical trajectories. The time evolution of the projection K a of the rotational angular momentum N in a body-fixed frame is analyzed. The main result is a relation between the decomposition lifetime and the degree of K a -mixing. For example, less than 30% of the available K a space becomes populated for an average lifetime of 5 ps. This is consistent with the conclusions, based on time-resolved experiments, that rotation–vibration transfer is slower than reaction except just above the threshold.

Revivals in triatomic molecules: influence of mode mixing

Abstract We investigate the occurrence of revivals and fractional revivals of three-dimensional molecular wavepackets in DNO and HNO. In both cases the initial wavepackets, describing essentially stretching of the NO bond, are generated as superposition of the stationary wavefunctions of the three-dimensional Hamiltonian. Fractional revivals up to the order 1 8 are observed in both molecules. While DNO behaves like a pseudo diatomic molecule, mode mixing is rather pronounced in HNO, leading to a much more complicated appearance of the autocorrelation function, less pronounced fractional revivals, and particularly to a lengthening of the revival time with respect to DNO.