The Monte Carlo growth method (MCGM), a recently developed method of Potential Energy Surface (PES) exploration, is adapted to the case of molecular clusters. The choice of the relevant parameters of this method for the study of these systems is discussed. The MCGM is applied to two different molecular systems: the acetonitrile and the molecular nitrogen clusters. The MCGM is compared to two classical PES exploration methods: the Monte Carlo simulated annealing technique (MCSA) and the dynamic quenching method (DQM) used in Molecular Dynamics. The MCGM is much more effective than the MCSA and yields similar results to those of the DQM, without needing any initial cluster guess. Obtained isomers for clusters up to (CH3CN)6 and (N2)14 are described. In both systems, remarkably stable patterns are observed: the anti-parallel pair in the case of the acetonitrile, the pentagonal bipyramid and the icosahedron for nitrogen clusters. Lowest-energy isomers of (N2)n clusters are isomorphic to those of Arn.

An all-electron MO calculation in Gaussian functions is presented for the methyl radical and its ions and the vinyl radical. Roothaan's open-shell technique, using two effective Hamiltonians, is applied for solving the LCAO-SCF equations.

A non-empirical LCAO–MO–SCF calculation using a basis of Gaussian orbitals had been performed on 1,2-benzyne. The nature of the 1,2-bond is analyzed by means of a localization procedure. A second-order perturbation calculation leads us to estimate at 157 kcal/mole the abstraction energy of two hydrogens from benzene.

A localization procedure for free radicals using two unitary transformations is presented. Starting with Gaussian molecular orbitals obtained by the restricted SCF formalism of Roothaan, two sets of quasi-localized spinorbitals corresponding to the usual chemical formula have been found for vinyl and formaldiminoxyl radicals. This method has been also applied to the Hückel orbitals of benzyl.

The reaction between acetylene ion and ethylene is revisited to investigate possible dynamical effects in one of the reactions, which are considered as benchmark systems for statistical models. Reactant ions are produced by photoionisation with synchrotron radiation and are selected in internal energy by a coincidence method between threshold photoelectrons and photoions. Measured absolute reaction cross-sections decrease with collision energy (0.1–1eV CM), but increase with acetylene ion vibrational energy for the three exothermic channels giving C4H5+, C3H3+ and C2H4+ ionic products. Even though RRKM calculations are shown to fit rather well experimental results for both the bimolecular (C2H2++C2H4) and unimolecular (C4H6+) systems, some experimental results clearly indicate dynamical effects in this reaction for the C2H4+ production channel, in particular the strongly backward peaked C2H4+ angular distribution. We propose an alternative model combining reaction control both by dynamics and by statistics: the first step in the reactant approach is the capture between reactants, followed by a charge transfer process controlled by dynamics. In a second step, either the two reactants separate leading to C2H4+ ionic products, or there is a rearrangement into 1,3-butadiene (C4H6+) ion which dissociates statistically into C4H5++H and C3H3++CH3 products. Charge transfer is shown to be possible at intermediate intermolecular distances, slightly shorter than the capture distance, provided that one takes into account the charge–quadrupole interaction in addition to the ion-induced dipole interaction. This work clearly shows that a good fit of experimental data by RRKM calculations does not prove that the reaction elementary mechanism is controlled by statistics.

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si-C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.

AbstractFor Abstract see ChemInform Abstract in Full Text.

We show by X-ray induced photoelectron spectroscopy (XPS) that the amphiphilic tetrapyridinoporphyrazinium CuS18 undergoes different redox modifications when in the amorphous phase or in LB films. In the case of the LB films, CuS18 appears in a two-electron reduced state, but the XPS spectrum is surprisingly dominated by a copper(II) signal. On the contrary, amorPhous CuS18 is spontaneously reduced on the copper atom when exposed to various X-ray irradiations. Semi-empirical calculations show that the extra electrons can be added either on the macrocycle (LB films) or on the metallic ion (amorphous phase), probably depending on a slight geometric distortion around the cupric ion within the LB films.

A b initio calculations of the potential energy surfaces of the first low-lying states of N2O++ (3Σ−, 1Δ, 1Σ+, and 3Π) have been performed. Photodissociation mechanisms have been investigated. The dissociation into NO++N+ appears to be a direct process for the first three lowest states. On the other hand, for the 1Δ and 1Σ+ states, high and wide potential barriers are found for the dissociation into N+2+O+ . The 3Σ− state seems to be the most likely state to dissociate along this direction. The predissociation by the 3Π state appears to be an unlikely process. From these theoretical results, the main experimental features are explained. In addition, a simple valence-bond model predicting the bond formation between two positively charged ions is proposed.

Ab initio calculations on the HgN2 system have been performed using classical methods of quantum chemistry. They show the existence of a weakly bound complex in the excited state correlating to Hg(3P) + N2(1Σg+) in the linear configuration Hg-N-N. This complex plays an important role in different relaxation paths of Hg(3P) observed in matrices. The (HgN2)* exciplex spectrum is explained using these theoretical potential curves. The E-V transfer probability is calculated and the experimental distribution of N2 vibrational levels populated by transfer is well reproduced. These results show that the interaction between one atom of mercury and one molecule of nitrogen is sufficient to explain the principal features observed in matrix experiments.

Double-core-vacancy excited states are observed in the photoabsorption spectra of gas-phase silicon molecules, Si${\mathit{X}}_{4}$(X=H,${\mathrm{CH}}_{3}$,F,Cl,Br). We interpret the structures as originating from 1s2p and 1s2s electron excitations into unoccupied valence orbitals. The energies of the observed structures are found at much higher energy than the sum of individual core excitations primarily because of the Coulomb repulsion of the core holes within the silicon atom. Using Hartree-Fock calculations, we interpret the energy separation of the first doublet structure as the singlet-triplet splitting because of the 1s-2p (1s-2s) exchange term. Molecular effects due to the ligands are found in the absolute energies and in the second peak shape. We estimate the energy of the double-core ionization continua, 1${s}^{\mathrm{\ensuremath{-}}1}$2${p}^{\mathrm{\ensuremath{-}}1}$ and 1${s}^{\mathrm{\ensuremath{-}}1}$2${s}^{\mathrm{\ensuremath{-}}1}$, to be around 25 eV above the double excited states.

Energies of the electronic states of the triatomic dication N2O2+ in the Franck–Condon zone of neutral N2O have been determined by a combination of (1) double charge transfer spectroscopy to locate singlet states, (2) photoionization measurements to locate the lowest triplet state, and (3) configuration-interaction calculations to identify the states and to predict the energies of other triplets. It seems likely that two distinct charge separation reactions compete in the relatively slow decay of the N2O2+ ground state.

Potential energy surfaces for HgH2 have been calculated using a nonempirical relativistic effective core )potential incorporating configuration interaction by means of the CIPSI algorithm (configuration interaction by perturbation with multiconfigurational zeroth-order wave function selected by iterative process). Core-valence polarization and correlation energy are included via a perturbative treatment. Spin–orbit coupling is introduced through an effective Hamiltonian. These theoretical results are used to discuss the experimental ones for the Hg(3P1)+H2→HgH(X 2Σ+)+H reaction. The different behaviors of the two van der Waals complexes are explained. Dynamical studies and classical trajectories calculations confirm that the dominant pathway is a direct dissociation on the first excited surface and is responsible for a highly peaked rotational distribution of the HgH product, in agreement with experiment.