L'analyse des distributions d'énergie cinétique par la méthode du maximum d'entropie.

Energy is not always fully randomized in an activated molecule because of the existence of dynamical constraints. An analysis of kinetic energy release distributions (KERDs) of dissociation fragments by the maximum entropy method (MEM) provides information on the efficiency of the energy flow between the reaction coordinate and the remaining degrees of freedom during the fragmentation. For example, for barrierless cleavages, large translational energy releases are disfavoured while energy channeling into the rotational and vibrational degrees of freedom of the pair of fragments is increased with respect to a purely statistical partitioning. Hydrogen atom loss reactions provide an exception to this propensity rule. An ergodicity index, F, can be derived. It represents an upper bound to the ratio between two volumes of phase space: that effectively explored during the reaction and that in principle available at the internal energy E. The function F(E) has been found to initially decrease and to level off at high internal energies. For an atom loss reaction, the orbiting transition state version of phase space theory (OTST) is especially valid for low internal energies, low total angular momentum, large reduced mass of the pair of fragments, large rotational constant of the fragment ion, and large polarizability of the released atom. For barrierless dissociations, the major constraint that results from conservation of angular momentum is a propensity to confine the translational motion to a two-dimensional space. For high rotational quantum numbers, the influence of conservation of angular momentum cannot be separated from effects resulting from the curvature of the reaction path. The nonlinear relationship between the average translational energy and the internal energy E is determined by the density of vibrational-rotational states of the pair of fragments and also by non-statistical effects related to the incompleteness of phase space exploration. The MEM analysis of experimental KERDs suggests that many simple reactions can be described by the reaction path Hamiltonian (RPH) model and provides a criterion for the validity of this method. Chemically oriented problems can also be solved by this approach. A few examples are discussed: determination of branching ratios between competitive channels, reactions involving a reverse activation barrier, nonadiabatic mechanisms, and isolated state decay. (c) 2005 Elsevier B.V. All rights reserved

Unimolecular Dissociation of Halogenobenzene Cations by Phase space Theory.

peer reviewedThe Orbiting Transition State version of the Phase Space Theory (PST) is used to calculate the KER distributions in the dissociation channel of X (X=I,Cl,Br)-loss from C6H5X+. The results are compared to the experimental distribution and to that obtained by PST

How ergodic is the Fragmentation of the Pyridine Cation ? A Maximum Entropy Ananlysis

peer reviewedThe experimental KER and the statistical distributions are compared by the Maximum Entropy Method. An Ergodicity Index F(E) is defined to measure the phase space sampling efficiency. This is applied to the KERD of C4H4+ cation produced by the C5H5N+ -> HCN+C4H4+ fragmentation path. In this particular case the F(E) is found to decrease steadily with increasing internal energy

Perte d'un atome d'hydrogène par le cation du benzène. Pourquoi l'énergie cinétique libérée est-elle si importante?

The kinetic energy release distributions (KERDs) associated with the hydrogen loss from the benzene cation and the deuterium loss from the perdeuteriobenzene cation have been remeasured on the metastable time scale and analyzed by the maximum entropy method. The experimental kinetic energy releases are larger than expected statistically, in contradistinction to what has been observed for the C-X fragmentations of the halogenobenzene cations. H(D) loss from C6H6+ (C6D6+) occurs via a conical intersection connecting the (2)A(2) and (2)A(1) electronic states. Two models are proposed to account for the experimental data: (i) a modified orbiting transition state theory (OTST) approach incorporating electronic nonadiabaticity; (ii) an electronically nonadiabatic version of the statistical adiabatic channel model ( SACM) of Quack and Troe. The latter approach is found to be preferable. It leads to the conclusion that the larger the energy stored in the transitional modes, which partly convert to the relative interfragment motion, the shorter the value of the reaction coordinate at which the adiabatic channels cross, and the larger the probability of undergoing the (2)A(2) -> (2)A(1) transition required for hydrogen loss

Le role des forces à longues distances dans la determination de la libération d'énergie cinétique translationnelle. La formation de cations C4H4+ à partir du Benzène et de la Pyridine.

Kinetic energy release distributions (KERDs) for the benzene ion fragmenting into C4H4+ and C2H2 have been recorded by double-focussing mass spectrometry in the metastable energy window and by a retarding field experiment up to an energy of 5 eV above the fragmentation threshold. They are compared with those resulting from the HCN loss reaction from the pyridine ion. Both reactions display a similar variation of the kinetic energy release as a function of the internal energy: the average release is smaller than statistically expected, with a further restriction of the phase-space sampling for the C5H5N+ dissociation. Ab initio calculations of the potential-energy profile have been carried out. They reveal a complicated reaction mechanism, the last step of which consists in the dissociation of a weakly bound ion-quadrupole or ion-dipole complex. The KERDs have been analyzed by the maximum entropy method. The fraction of phase-space effectively sampled by the pair of fragments has been determined and is similar for both dissociations. Both reactions are constrained by the square root of the released kinetic energy, epsilon1/2. This indicates that in the latter stage of the dissociation process, the reaction coordinate is adiabatically decoupled from the bath of the bound degrees of freedom. For the C6H6+ fragmentation, the analysis of the experimental results strongly suggests that, just as for the symmetric interaction potential, the translational motion is confined to a two-dimensional subspace. This dimensionality reduction of the translational phase space is due to the fact that the Hamiltonian of both weakly bound complexes contains a cyclic coordinate