Christine Dézarnaud-Dandine

Topological study, using a coupled ELF and catastrophe theory technique, of electron transfer in the Li+Cl2 system

A topological study of electron transfer in the {Li+Cl2} system, based on catastrophe theory and using ELF as a starting point, has been achieved. It enables the determination of the method-independent invariants that might be used for characterizing the well-documented harpooning process in all types of geometry. This study has shown that the process is characterized by an electron jump from the covalent {Li+Cl2} state, stable at large separation distances, to the ionic charge transfer state {Li++Cl2-}. The topological invariant associated with this behaviour is a dual cusp catastrophe, whose existence has been found at any level of calculation, that is, MP2, DFT and CASSCF. The latter catastrophe is mostly associated with the actual cleavage of the Cl2- species. Analysis of the topological parameters allows the actual regions of PES crossings, in any geometry, to be localized. Our results suggest that the dual cusp catastrophe characterizes the diabatic surface crossings that are subjacent in the classical adiabatic analysis of the overall reaction path.

Empty levels in germanium compounds studied by XAS, ISEELS, ETS, DEAS and ab initio calculations: GeH4, GeCl4 and Ge(CH3)3Cl

Abstract The lower-lying empty levels of gaseous GeH4, GeCl4 and Ge(CH3)3Cl have been studied using X-ray absorption spectroscopy (XAS), inner shell electron energy loss spectrocopy (ISEELS), electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DEAS) and ab initio calculations. The inner shell excitation spectra include Ge 2p in GeH4, GeCl4 and Ge(CH3)3Cl; Ge 3s, Ge 3p and Cl 1s and Cl 2p in GeCl4. The Ge 2p and Cl 1s excitation spectra were obtained in gas phase at room temperature with a direct ion-yield technique at the LURE SUPER ACO storage ring in Orsay. The excitation spectra generated from p core orbitals are very sensitive to the change of ligand and geometry. Ab initio calculations employing the 1h-1p CI scheme performed for the Ge 1s, Ge 2s, Ge 2p absorptions for GeH4 and GeCl4 allow to assign and reproduce the main experimental below edge features whereas the composition of empty orbitals are deduced from the calculated oscillator strengths. The continuum above edge features reveal the presence of double excitations and shape resonances. Moreover complementary experimental results are reported for GeCl4. The Ge 3s, ge 3p and Cl 2p inner shell electrn energy loss spectra recorded under electric dipole scattering conditions give a complete picture of empty levels of different symmetries. Furthermore the ET spectrum (0–7 eV) of GeCl4 shows two resonance states located at 1.7 and 5.6 eV whereas the DEA spectrum is dominated by formation of the GeCl3− anion at zero impact energy and formation of Cl− and GeCl2− anions around 5.6 eV.

X-ray absorption study of potential barrier effect in group 14 tetrachlorides (GeCl4, SnCl4) and group 15 trichlorides (PCl3, AsCl3)

Abstract Inner-shell excitation spectra of GeCl 4 , SnCl 4 , PCl 3 and AsCl 3 were obtained in gas phase at room temperature with a total ion-yield technique at the LURE Super ACO and DCI storage rings in Orsay. The inner-shell excitation spectra include Ge 2p, Sn 2p, P 1s, As 2p, and Cl 1s in GeCl 4 , SnCl 4 , PCl 3 and AsCl 3 , respectively, and are interpreted in the framework of the potential barrier concept, according to electric dipole selection rules. The well-resolved below-edge features in the metal 2p excitation spectra of GeCl 4 and SnCl 4 show a similar shape. Moreover, complementary experimental results of ETS with the support of MS-X α calculations are reported and correlated with the C 1s, Si 2p, Ge 2p and Sn 2p term values of the lowest vacant orbitals for CCl 4 , SiCl 4 , GeCl 4 and SnCl 4 , respectively, in order to study the trend of the stabilization energy of the various empty orbitals in the neutral and in the core ionized molecules. The P 1s spectrum of PCl 3 has been assigned and the As 2p spectrum of AsCl 3 compared with the published P 2p spectrum of PCl 3 . The 1s excitation spectra of the chlorine ligand in the tetrachloride (GeCl 4 , SnCl 4 and trichloride (PCl 3 , AsCl 3 ) series are very sensitive to the change of symmetry and bonding. A linear correlation between (MCl) bond lenght (with M  Ge, Sn, P, As) and the energy of the Cl 1 s → σ ∗ (MCl) shape resonance observed above the Cl is ionization threshold has been obtained.

A theoretical study of the bonding in NO, (NO)2, (NO)2− and (NO)22− using a topological analysis of the electron localization function

The topological analysis of the ELF function calculated at the DFT level for (NO)2, (NO)2− and (NO)22− reveals a regular variation in the NN bonding. In the neutral species, a very weak attraction results form the fluctuation of a small electron population associated with the N lone pairs. A similar but even weaker attraction is found between the O atoms, thus favouring the cis geometry. The same trends are found in the cis anion, although in the more stable trans isomer a NN bonding basin appears. The latter bonding basin is more pronounced in the dianions. The molecular orbital localization function (MOLF) method which is briefly described, is proposed for gaining information on the σ/π separation.

A THEORETICAL STUDY OF THE FORMATION OF STABLE ANION INTERMEDIATES VIA ELECTRON CAPTURE BY HETEROSUBSTITUTED THREE MEMBERED RING XCH2CY2 COMPOUNDS (X = O, S, NH, PH; Y = H, F)

Abstract A comparison has been made between the electron acceptor properties of various three membered ring molecules, XCH2CY2 (X = O, NH, S, PH; Y = H, F). In a first step, the emphasis has been put on the ability of these substrates to accommodate an extra electron in their lowest-lying antibonding (valence and diffuse) MOs (vertical anions). A strong dichotomy was found between compounds bearing a heteroatom of the second row and those bearing a heteroatom of the third row. For the second row heteroatoms, the electron was essentially located in the diffuse space attached to the H atoms, via the XH (σ∗) and CH2 (π∗) antibonding MOs; while for first row ones, it remained mainly located on the heteroatom, in a Rydberg + valence p-type MO. The open anions resulting from ring opening have then been studied. Two series of anions resulted: when F-substituents were present on the terminal carbon atom, a 1–3 dipolar structure was obtained, whereas the possibility of resonant forms was present when F-substituents were borne by the central C atom. The possibility of obtaining stable anionic species has finally been qualitatively examined.

Experimental and ab initio study of the molybdenum K and L edge excitation in Mo(CO)6 and MoF6

Abstract The L inner-shell absorption spectra of molybdenum in gas phase Mo(CO)6 and MoF6 have been obtained at room temperature with a direct ion-yield technique at the LURE Super ACO storage ring in Orsay. Transitions from the 2 p 3 2 and 2 p 1 2 initial states, respectively, have pronounced “white line” features assigned to the allowed 2p → 4d transitions and as expected the Mo 2p spectra are found to be very sensitive to the nature of the ligands and their bonding. Most of the intensity in the Mo 2s excitation spectra goes above the ionization threshold. Ab initio calculations employing the 1h-1p CI scheme, performed for the K and L excitation of molubdenum, permit to assign spectral features below the ionization thresholds and to deduce from the oscillator strength the metal character of the vacant orbital.

Theoretical investigation on the acceptor role of σ∗ MOs in three and four-membered ring thio compounds and their open chain analogues. Comparison of ground state, lowest energy triplet state and anion properties

Abstract Ab initio SCF-CI (6-31G∗∗/MP2 and 6-31G∗∗/MP4) computation of the singlet ground state, lowest energy triplet and anion states have been carried out for a series of divalent sulfur compounds. The aim of this study was to provide an insight into the acceptor capacity of σ ∗ MOs involved in the temporary states created in X-ray photoabsorption and electron transmission spectroscopies. A roughly parallel behavior is observed between triplet and anion species, either for vertical (GS geometry) or relaxed moieties. Qualitative correlations are presented between the calculated energies and molecular properties such as the CS bond length or the energy of the first virtual σ ∗ MO.

A theoretical investigation on the electron-acceptor properties of polar CS bonds of CH3−nFnSH (n = 0–3) thiols and CH3−nFnSCH3−nFn (n = 0–3) sulfides

Abstract The structures and stabilities of fluoro-substituted thiols and sulfides of general formula CH 3− n F n SH and CH 3− n F n SCH 3− n F n ( n = 0–3), have been calculated at the MP2/6-311G ∗∗ and MP2/6-311+ +G ∗∗ levels. The role of F-substitution in molecules and fragments resulting from CS and SH bond cleavage has been studied. In molecules, F-substitution does not bring about important changes of the CS bond lengths, and, accordingly, no important change is observed in the energy of the vertical anion species which remain much less stable than the neutral ground state. The same type of conclusion holds for the homolytic dissociation of neutral molecules (CS and SH bonds), yielding radical moieties, which exhibit only little variation upon F-substitution. When dealing with anion fragments, the situation is more striking. It is noteworthy that the well-documented effect of methyl anions stabilization by halogen substitution is appreciably increased in the new class of compounds of formula CH 3− n F n S. For the latter, the EA rises from 1.47 eV in CH 3 S to 3.08 eV in CF 3 S. On the ground of energy balance between {A 0 + B − ∞ } and {A − + B 0 ∞ } it is proposed that CF 3 SH and CF 3 SCH 3 are good candidates for the adiabatic formation of stable 3-electron bonds.

XANES and EXAFS in gaseous Mo (CO)6

Abstract X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) have been observed in the molybdenum L 2,3 edge absorption spectrum of Mo(CO) 6 in the gas phase at room temperature, with a direct ion-yield technique at the LURE Super ACO storage ring in Orsay. Molecular shape resonances are identified and weak oscillations in both the 2p 3 2 and 2p 1 2 continua are interpreted as EXAFS oscillations due to the CO ligands.

Theoretical exploratory study of the strong conformational and diastereotopic effects found in the vertical anion of α-halogen-substituted open-chain and three-membered ring compounds bearing a heteroatom (N, O, P)

Abstract A theoretical exploratory comparison of steric effects in the ground state and unstable temporary anion species formed by electron attachment has been achieved for open-chain and three-membered ring compounds bearing O, N, and P atoms. The quantum calculations have been carried out at the MP2/6–311G∗∗ level. When dealing with open-chain compounds, it is found that a very important enhancement of rotation barriers is found in the anion states. This effect mostly arises from through-space interaction of the odd electron which is predominantly located in σ ∗ CH and σ ∗ XH MOs, concentrated around the H atoms, and the O and N-lone pair electrons. The role of F and Cl substituents is analyzed with respect to unsubstituted compounds. The same type of effect is observed in cyclic compounds. No such strong interactions are present in P-substituted compounds. It is thus emphasized that very strong diastereotopic effects would exist in temporary anionic moieties having high negative EAs. in O and N-substituted series.