Philippe Millié

Theoretical and experimental studies of the triatomic doubly charged ions CO2+2, OCS2+, and CS2+2

The term schemes of the doubly charged ions CO2+2, OCS2+, and CS2+2 have been calculated by the CIPSI method using a new and economical basis set of polarized atomic orbitals and difference orbitals. The calculated energies are compared with existing and new experimental data obtained from Auger spectra, double charge transfer, and photoionization including the PIPICO technique. A complete assignment of the manifolds of singlet states is achieved with good agreement between theory and experiment. Only the lowest doubly charged ion states are well described by simple two‐hole configurations, while three‐hole one‐particle configurations are important at higher energy. It is confirmed that the triplet ground states of these ions are well populated by photoionization, while the excited singlet states are revealed most clearly by the double charge transfer technique.

Dipolar coupling between electronic transitions of the DNA bases and its relevance to exciton states in double helices

The present communication addresses the question of the magnitude of dipolar coupling between the lowest electronic transition moments of the DNA nucleosides and its relevance to Frenkel exciton states in double helices. The transition energies and moments of the nucleosides are determined from absorption spectra recorded for dilute water solutions. Dipolar interactions are computed for some typical nucleoside dimers according to the atomic transition charge distribution model. The properties of the exciton states of two particular double helices, (dA)20·(dT)20 and (dAdT)10·(dAdT)10, are calculated considering three closely lying molecular electronic transitions (S1 and S2 for adenosine, S1 for thymidine). It is shown that (i) the oscillator strength is distributed over a small number of eigenstates, (ii) important mixing of the three monomer electronic transitions may occur, (iii) all eigenstates are spatially delocalised over the whole length of the double helix and (iv) the extent of exciton states over the two strands depends on the base sequence.

Single photon double ionization studies of CS2 with synchrotron radiation

Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF‐LCAO‐MO calculation leads to a tentative assignment of the observed states.

Ab initio Hartree-Fock instabilities in closed-shell molecular systems

The Hartree-Fock instability of twelve polyatomic systems is studied at theab initio level. It is found that all the systems with at least one double bond, exhibit a non-singlet instability. On the other hand instabilities of singlet type as well as instabilities of non-real type appear only in a small number of cases. The existence of these instabilities is discussed with respect to the location of low-lying excited states and to the weight of ionic structure.

Ab initio study of the electronic structure and hyperfine coupling properties in simple hydrocarbon radicals. II. Short‐range and long‐range interactions in alkyl free radicals

Nonempirical calculations of the ground−state energy, proton and carbon−13 coupling constants of methyl, ethyl, n−propyl, and cyclopropyl radicals have been performed in the frame of spin−restricted LCAO−SCF open−shell and first−order double−perturbation theories. The rotation barrier is negligible for the ethyl radical. The two barriers of the n−propyl radical are, respectively, 0.3 kcal/mole (0.4 exptl) and 4.7 kcal/mole for the rotations about the Ċ−Cα and Cα−Cβ bonds. The cyclopropyl radical is found to be nonplanar with an out−of−plane angle of 41° and an inversion barrier of 3.80 kcal/mole. For the equilibrium conformations, the computed carbon−13 splittings of the radical carbon of methyl (1), ethyl (2), n−propyl (3), and cyclopropyl (4) are +31.04, +37.90, +37.72, and +138.83 G, respectively; the theoretical α −carbon−13 splittings are −18.14 (2), −17.15 (3), and −8.35 G (4); the β −carbon−13 splitting is +11.8 G for the stable conformation of the n−propyl radical. The calculated coupling constant...

Electronic States and Decay Mechanisms of the N2o2+ Dication

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.

Spectroscopy, dynamics and structures of jet formed anthracene clusters

Abstract The spectroscopy and dynamics of the lowest excited states of the jet formed anthracene clusters (n

Energetics of a model NH–π interaction: the gas phase benzene–NH3 complex

The dissociation energy of the benzene–ammonia complex formed in a supersonic expansion has been determined (D0=1.84±0.12 kcal mol−1) from the features of its photoionisation curve as obtained by mass-resolved two-color resonant two-photon ionisation. The complex structure, stabilised by a π-type hydrogen bond between the benzene ring and the ammonia molecule located above, has also been obtained by a semi-empirical model. The neutral structure found is in good agreement with experiment and the best ab initio calculations in the literature. The ionic structures calculated enable us to interpret the slowly increasing photoionisation curve as a consequence of a large equilibrium geometry change between neutral and ion. The present study also shows that the benzene–ammonia complex is less bound than its homologue with water by ca. 0.6 kcal mol−1. However, the value found indicates that the NH–π interaction can be taken into account when modelling the structure of biological systems.

Investigating bonding in small silicon–carbon clusters: Exploration of the potential energy surfaces of Si3C4, Si4C3, and Si4C4 using ab initio molecular dynamics

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.

Electronic excitations in organized molecular systems. A model for columnar aggregates of ionic compounds

Abstract A model for the description of electronic excitations in organized molecular systems of finite size in which intermolecular distances are comparable to molecular dimensions is presented. A methodology based on the excitonic theory coupled to quantum chemistry is developed and applied to columnar aggregates of triarylpyrylium tetrafluoroborates. Excitation energy and interactions among transition moments (diagonal and off-diagonal terms of the matrix Hamiltonian) are calculated taking into account the precise geometry of the aggregate. Its energetic topography shows that diagonal energy is sensitive to edge and orientational effects. Diagonalization of the electronic Hamiltonian in the strong exciton interaction limit, provides the eigenstates from which localization indexes, radiative lifetimes, absorption and fluorescence spectra are calculated. It is found that aggregate growth induces a blue-shift in absorption and an increase in radiative lifetime. Orientational disorder causes a red-shift in fluorescence. Previously published experimental data are discussed in the frame of the present model.