Theoretical and experimental studies of the dissociation dynamics of methaniminium cation, CH2NH2+→CHNH++H2: Reaction path bifurcation

From Anodic Oxidation of Aliphatic α-Amino Acids to Polypeptides by Quantum Electrochemistry Approach: Beyond Miller-Urey Experiments

For years, polypeptide formation has fascinated the scientific world because its understanding could lead to one of the possible explanations for the origin of life. Anodic oxidation of aliphatic α-amino acids in aqueous electrolytes can result either in their decomposition or in their polymerization into polypeptide. This behavior depends experimentally on both amino acid concentration and pH. The elucidation of the involved mechanisms remains a challenge because of the multitude of products which can be obtained. In this context, the electrochemical behavior of glycine and alanine on a biased platinum surface was examined at the nanoscale by quantum electrochemistry via the effective screening medium method. Several electrochemical systems with different concentrations and pH values have been explored. Simulations of the anodic oxidation of the amino acids have not only confirmed their electropolymerization and decomposition at high and low concentrations, respectively, but also have revealed unsuspected mechanisms at the origin of polypeptide formation. This sheds new light on electrochemistry of α-amino acids, on occurrence of polypeptides, and more generally on organic electrochemistry.

REDUCED GRADIENT METHODS AND THEIR RELATION TO REACTION PATHS

The reaction path is an important concept in theoretical chemistry. We discuss different definitions, their merits as well as their drawbacks: IRC (steepest descent from saddle), reduced gradient following (RGF), gradient extremals, and some others. Many properties and problems are explained by two-dimensional figures. This paper is both a review and a pointer to future research. The branching points of RGF curves are valley-ridge inflection (VRI) points of the potential energy surface. These points may serve as indicators for bifurcations of the reaction path. The VRI points are calculated with the help of Branin's method. All the important features of the potential energy surface are independent of the coordinate system. Besides the theoretical definitions, we also discuss the numerical use of the methods.

Ground and Excited State Dissociation Dynamics of Ionized 1,1-Difluoroethene

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.