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RESEARCH TOPICS

1. Structural signatures in photoemission spectra. Advanced quantum mechanical studies of model oligomer chains have built evidences for useful relationships between the valence electronic structure and the chemical bonding characteristics (configuration, conformation). Emphasis has been led on a combination of such calculations with photoelectron spectroscopy as direct tools for investigating the molecular architecture within the top layers of polymer materials. In order to cope reliably (i.e. size-consistently and semi-quantitatively) with electronic correlation and relaxation effects, photoionization spectra of saturated hydrocarbons have been first (1990-1995) simulated using various one-particle Green's function schemes in the quasi-particle approximation. These calculations have enlightened the role played by cooperative methylenic hyperconjugation effects on the one-electron levels at the top of the inner-valence region of saturated hydrocarbons and polyethers. In unconjugated polymers, this part of the spectrum (18 to 22 eV) is extremely sensitive to the mixing of C2s and C2p+ H1s states, and thus provides very specific signatures for alterations of the conformation. Examples of such alterations are the fold-stacking sequences at the surface of microcrystalline lamellae of polyethylene (a controversial topic since the early 50's), the folding of n- alkane chains due to thermal agitation in the gas phase or within self-assembled layers of large alkanethiol compounds, and the helicoidal distorsion of branched or 'polar' polymers such as isotactic polypropylene or polyoxymethylene. (Figure 1)

 

2. The issues of size-consistency, charge-consistency and size-extensivity, and the implications of translation symmetry in many-body quantum mechanics. Size-dependence aspects of many-body methods such as the one-particle Green's Function Theory or Møller-Plesset Perturbation Theory, have been extensively analysed using Feynman diagrams and the crystal orbital formalism for stereoregular polymers. The conclusions drawn from these analyses have been systematically and numerically tested on model chains at different orders of approximation. Contrary to a consensus in the scientific literature, it has been shown, and this for the very first time in 1995, that the linked-cluster properties of many-body expansions based on the Dyson evolution operator U(t,t0) are not always sufficient to ensure that physical observables (e.g. ionization energies) computed by means of such approaches have the correct scaling with particle number. A noticeable exception arises with the static self-energy S(¥), which corresponds in 1p-GF theory to a one-electron scattering potential due to many-electron corrections to the HF ground state one-electron density. Because of the long-range character of the Coulomb interaction, this contribution to the self-energy may diverge logarithmically with increasing system size, as a result of slight but intrinsic violations of the exact particle number due to couplings of states with different particle number. Note that in an order-by-order (e.g. MBPT, Many-Body Perturbation Theory) expansion, the wavefunction is normalized and these logarithmic divergences identically cancel in pairs of antigraphs. Such cancellations are only partial with renormalized Dyson theories, which in contrast are thus prone to size-divergences with increasing system size. This finding has led us to differentiate major theoretical concepts such as size-consistency and size-extensivity as the correct scaling in the dissociation and thermodynamic limits, respectively. It also motivated further investigations of the convergence properties of lattice summations in post-SCF methods such as : CPHF (Coupled Perturbed Hartree-Fock), RPA (Random Phase Approximation), MBPT (see e.g. J.Q. Sun and R.J. Bartlett, J. Chem. Phys., 106, 5554, 1997), Coupled-Cluster (see e.g. M. Nooijen and R.J. Bartlett, Int. J. Quant. Chem. 63, 601, 1997), ... Clearly, a violation of the charge-consistency has also other implications such as a loss of gauge invariance of linear response properties (e.g. electric polarizability) obtained using approximate Dyson theories (see section 4). The size-dependence properties of shake-up bands in ionization spectra have been recently analyzed on similar grounds by fully adapting the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme, one of the most accurate and efficient 1p-GF approaches to calculate valence one-electron and shake-up ionization spectra, to the formalism of crystalline orbitals for stereoregular polymers.(Figure 2)

 

3. Calculations of photoionization intensities. High-level theoretical schemes, combining an orthogonalized plane wave decription of the emitted photoelectron to a one-particle Green's function calculation of Dyson orbitals for the ionization process, have been developed and implemented to calculate highly accurate photoionization cross sections under a X-ray photon beam. Excellent results have been obtained for randomly-oriented (gas phase) samples of small molecules (H2O, N2, CO, CH4, C2H2).(Figure 3)

 

4. Linear and quadratic responses. New theoretical methods have been derived using an algebraic diagrammatic construction scheme for the Coupled-Perturbed one-Electron Propagator (CPEP) to calculate linear and quadratic responses (e.g. electric polarisabilty and first hyperpolarizality) through second and third-order in electronic correlation. In analogy with the Coupled Perturbed Hartree-Fock (CPHF) method, the CPEP approach consists in a self-consistent procedure based on calculations of correlated one-electron densities of molecules embedded in an external field. Such densities are obtained from integration along the Coulson contour of the related 1st-order pertubed one-electron Green's Function. Feynman-Goldstone diagrams have then been used to rationalize the size-dependence and gauge invariance of CPEP response properties. Among the 96 terms contributing e.g. to the first-order pertubed one-electron density at second-order in correlation, r(1)[2], 30 terms provide non vanishing contributions to the trace of r(1)[2], which strictly cancel each others in pairs or triplets of diagrams. Like for the standard electron propagator, normalization of the wavefunction guarantees the charge consistency, and thus gauge invariance and size-extensivity of CPEP schemes derived from an order-by-order expansion. Calculations of electric polarizabilities at second-order in correlation have later illustrated the viability of such an approach. (Figure 4)

 

5. Shake-up and correlation bands in the ionization spectra of extended systems. High-level (ADC3) Green's function calculations on model molecular chains (hydrogen chains, polyenes, carbon chains and rings, benzene, biphenyl and polycyclic aromatic hydrocarbons) have shown that most bands recorded in the valence photoionization spectra of low band gap extended systems relate to excited (shake-up) states of the cations, an aspect of band structure theory which still remains completely overlooked. For pentacene, as an extreme example, it has been found that ionization bands pertaining to p-orbitals are subject to a severe shake-up contamination at very low binding energies (down to 8.0 eV), whereas the orbital picture of ionization holds up to binding energies of 14.6 eV within the s-band system. In general, the inner valence structures in the ionization spectrum of large conjugated systems exclusively relate to correlation bands. A severe breakdown of the orbital picture of ionization has even been found for the innermost valence levels of alkane compounds converging to polyethylene, a typical insulator. For a series of oligomers CnH2n+2 converging to polyethylene, the number of shake-up states grows proportionally to n2 while their intensities scales like n-2. This balance is consistent with the delocalization properties of canonical orbitals and constraints induced by translational symmetry in extended periodic systems. By introducing the band-Lanczos technique, it has been possible to fully investigate the correlation bands of large saturated hydrocarbons. These lead to surprisingly long tails (extending beyond 60 eV), which borrow a substantial proportion (10 %) of the total intensity. (Figure 5a-b) (Figure 5c-d)

 

6. Comparison with the results of easier OVGF (Outer Valence Green's Function) calculations has led to a simple rule to predict such situations where a severe breakdown of the orbital picture of ionization prevails at the ADC(3) level. Futher comparison with TDDFT calculations of excitation energies in PAH radical cations have shed light on serious limitations of the currently existing functionals in treatments of s-p charge-tranfer processes. In contrast, comparison with results obtained at the confines of non-relativistic quantum mechanics through focal point analyzes of the ionization threshold of polyacenes confirm the robustness of the OVGF and ADC(3) approaches in studies of strongly correlated systems. Suited extrapolations of the ADC(3) results for the one-electron ionization energies characterizing the p-band system of PAH molecules to Dunning’s correlation basis set of triple zeta quality (cc-pVTZ) enable theoretical insights into He I measurements which approach chemical accuracy (1 kcal/mol, or 43.4 meV).

 

7. Dynamics of large clusters and supramolecular systems. The intra-ring and inter-ring circumrotational dynamics of benzylic amide [2] catenanes has been investigated by means of molecular mechanics (MM) calculations of energy barriers along with evaluations of kinetic rate constants using Transition State Theory (TST). The calculated activation energies and circumrotational frequencies compare extremely well with values inferred from temperature-dependent NMR measurements. Catenanes have been shown to resort in solution to very complex molecular rearrangements to minimize the barriers on the circumrotational path, via the formation or snapping of hydrogen bonds, p-stacks, phenyl-phenyl T-shaped interactions and amide rotamer interconversions. Analysis of the structural characteristics of transition states on the circumrotational pathway of three different benzylic amide catenanes (bearing phenyl, pyridyl and thiophenyl-1,3-dicarbonyl groups) furnishes a simple mechanistic interpretation of the large variation of circumrotational frequencies observed in NMR experiments, over more than 6 orders of magnitude (from 10-3 to 10+3 s-1 at 298K). Although sterically allowed, further MM and TST studies of the same catenanes deposited on highly oriented pyrolitic graphite (HOPG) have shown that a full circumrotational process can be strongly hindered both by physisorption forces, intramolecular interactions and entropy effects. For the thiophenyl-based catenane, rates of the order of one macrocyclic ring rotation per second are nonetheless reached at moderate temperatures (~ 400K). This theoretical prediction, together with the large freedom for translational and rotational motions of catenanes on HOPG, is very promissing for the making of practical nanoscale devices with interlocked molecules. (Figure 6) (Figure 7)

 

8. The spinning of fullerene cages in clusters and in the solid phase has been similarly investigated on MM/TST grounds. The lowest lying intra-cluster rotations were shown to spread over several C60 cages towards the surface and edges of the clusters. Coaxial rotations also appeared in the regime. The increase in the size of the cluster makes the calculated activation barriers and rate constants converge to the values reported in the solid phase. The satisfactory result of this elementary form of kinetic theory implies that the energy of the internal degrees of freedom is randomized sufficiently fast with respect to the spining coordinates. These studies on catenanes and fullerenes have initiated other investigations of the structure and rotational dynamics of inclusion complexes of C60 based e.g. on calixarenes. (Figure 8)

 

9. Structural and electronic properties of carbon clusters. Despite the importance of correlation bands, ADC(3) simulations of the ionization spectra of linear (n=3,5,7,9) and cyclic (n=3-10) Cn species have led to the identification of striking structural signatures for the carbon rings versus the carbon chains. One can in particular easily trace from our convolutions the energy degeneracies, and in the outer-valence region, the S-P near-energy degeneracies which characterize the electronic structure of these doubly conjugated (cumulenic or polyynic) rings. In relation to its doubly anti-aromatic nature C8 is subject to stronger many-body effects than the other rings. The strongly correlated character of carbon clusters has also been evidenced in high-level ph-GF/ADC(2) studies of the electronic excitation spectra of linear C3, C5, C7 clusters which contain numerous low-lying single and double valence excitations. The structures, rotational moments, vibrational normal modes and infra-red spectra of ionized carbon clusters Cn+ (n=4-19) and Cn- (n=3-13) have also been investigated in detail using Density Functional Theory (DFT). In this research, we focused on the identification of useful spectroscopic (IR, or rotational) fingerprints for tracing a number of changes induced in the molecular structure of carbon chains and rings by adiabatic ionization and electron attachment processes. The C4n+2 and to a lesser extent the C4n+1 cyclic systems have been found to evolve from an essentially regular (i.e. cumulenic) pattern to a more alternating (i.e. polyynic) structure in their ionized forms, whereas the opposite trend is observed for the C4n and C4n+3 rings. Similarly, linear carbon clusters, which can be regarded as mostly cumulenic in their neutral form, tend to become more polyynic after ionization. Evidences have been given, for the very first time, that upon electron attachment, most carbon chains become bent because of Renner-Teller effects. From the very unusual topology of their frontier orbitals, it appears that closed anionic clusters such as C5-, C9- and C13- are non-planar, even-twisted, cumulenic rings. (Figure 9)

 

10. Structural and electronic properties of Boron-Nitrogen Clusters. In spite of their structural similarity with the isoelectronic C6, C8 and C10 rings, the BnNn clusters (n=3,4,5) do not exhibit a significant breakdown of the orbital picture of ionization, with the exception of the N2s bands (eb > 25 eV). The main (one-hole) ionization bands provide specific signatures for ring topologies based on n equivalent vertices, whereas a reduction of the curvature of N-B-N bridges with increasing system size can be followed from the N2s bands. The B3N3- anion is slightly stable against electron loss, whereas negative vertical electron affinities are obtained for B4N4 and B5N5. The BnNn+ species with n=4,6,8,10 are found to possess as lowest energy form a fully regular structure of Dnh symmetry, as the neutral species. In both cases, their IR vibrational spectrum invariably contains four lines, relating to one out-of-plane, non-degenerate, and three in-plane, doubly degenerate, normal modes. On the other hand, the BnNn clusters with n=3,5,7,9 are found to evolve from a fully regular Dnh structure to a more alternating one upon an adiabatic ionization process. In this case, ionization strongly enhances the infrared activity. Rotational moments, IR spectra and adiabatic ionization potentials as well provide specific markers of these contrasted behaviors.

 

11. Structural and conformational fingerprints in valence electron momentum spectra. Valence electron momentum spectra (EMS) [also referred to as (e,2e) binary spectra] recorded on n-butane have been analyzed at the ADC(3) level, in combination with Kohn-Sham (B3LYP) density functional calculations of momentum-space orbital densities. The ADC(3) calculations give evidence for a complete breakdown of the innermost C2s level, which in the spectrum appears experimentally as a broad set of shake-up lines centered at 25 eV, accompanied by a correlation tail extending up to at least 60 eV, in agreement with earlier predictions [see section 5]. The observed angular dependence of the EMS intensities confirms the exclusive relationships of the spectral features recorded at electron binding energies of ~24 eV and beyond with the first C2s level. Refinement of the analysis in 2003 on the basis of a Boltzmann-weighted average of the ionization spectra and momentum orbital densities of the all-staggered and gauche forms of n-butane demonstrate, for the very first time, that conformational rearrangements can be finely traced with Electron Momentum Spectroscopy. Both the one-electron binding energies and momentum distributions consistently image the distortions and topological changes that molecular orbitals undergo due to torsion of the carbon backbone, and thereby exhibit conformational fingerprints which can be traced experimentally. An exhaustive study of the EMS records of n-butane very superbly confirms the intimate relationships that prevail between the configuration (r) and momentum (p) spaces which lie at the roots of Quantum Mechanics. It also clearly advocates a proper description of the effect of thermal motions in spectroscopic measurements of all kind (see section 12). (Figure 10) Over the years, many more works on structurally versatile molecules like n-pentane, n-hexane, dimethoxymethane, or ethanol have confirmed the influence of the molecular conformation on momentum distributions inferred from an angular analysis of (e,2e) ionization intensities in experiments employing Electron Momentum Spectroscopy. Latest works on this topic indicate that these momentum distributions may also fingerprint changes that individual orbitals undergo due to nuclear motions and bond breakings in the final ionized state, within a time scale of the order of a few femtoseconds, as is for instance the case with a charge transfer induced by an ultra-fast chemical dissociation of the ethanol radical cation in its lowest electronic state into a methyl radical and a protonated form of formaldehyde.

 

11. Structural, electronic, vibrational and spectroscopic properties of boranes and carboranes. 1,2-, 1,6-, 1,10- C2B8H10 as well as 1,2-, 1,7- and 1,12- C2B10H10 closo-carboranes were studied at the HF and DFT levels (B3LYP/6-31G* and B3LYP/6-31++G**). Energies, optimized geometries, Mulliken charges, harmonic frequencies, electric dipole and quadrupole moments were computed and compared with earlier calculations and available experimental data.

 

12. Internal conversion of sulfoxide, sulfonyl and xanthate precursor chains of conjugated polymers. The gas-phase internal elimination of sulfoxide, sulfone and xanthate precursors of model oligomers of PPV (poly-para-phenylene-vinylene), PITN (polyisiothianaphthene), PITNV (polyisothianaphthene vinylene), and PEDOTV (poly-ethylene dioxythiophene vinylene) have been investigated in detail using a newly developed functional, MPW1K (Modified Perdew Wang -1 parameter model- for Kinetic). From a calibration against benchmark quantum mechanical (CCSD[T]) calculations of the conversion path of model precursors of ethylene, accuracies of about 2.5 kcal mol-1 are expected for the computed activation energies. Depending on the substituents attached on the Ca and Cb atoms, asynchroneous internal conversions proceed either through an E1-like or carbanion-like mechanism. It has been found that entropy contributions to the activation energies strongly favor direct radical dissociations of sulfone precursors of PPV. Further radical side reactions following an Ei conversion through an alkyl substituent of these precursors also significantly contribute to the formation of sp3 defects and cross-linked structures - an advantageous feature for the making of electroluminescent polymers.(Figure 11)

 

13. Temperature effects on the structure and optical properties of model oligomers of poly-para-phenylene-vinylene (PPV). The influence of thermal motions on the UV-visible absorption spectra of trans-stilbene and large oligo-phenylene-vinylenes in the gas and in crystalline phases have been investigated by means of molecular dynamics simulations based on Allinger's MM3 force field, in conjunction with semi-empirical ZINDO-S/CIS calculations of vertical electronic excitation energies and transition moments. It has been found that, at room temperature, the thermal broadening of UV-Vis spectral bands is comparable to that due to vibronic couplings. In a related study of the pedalling motions of trans-stilbene, the accuracy of the MM3 force field has been checked against the results of a Focal Point Analysis of rotational barriers and conformational energy differences calculated using various ab initio methods, up to the benchmark CCSD(T) level in an asymptotically complete basis set (cc-pV¥Z). Discrepancies between the MM3 barriers and CCSD(T) values extrapolated to the cc-pV¥Z basis set do not exceed 0.3 kcal mol-1. The reliability of the ZINDO-S/CIS scheme has also been checked by comparison with the results of TD-DFT(B3LYP) and high-level CASPT2 calculations. (Figure 12)

 

14. Conformational analysis. A very accurate determination of the relative energies and abundances of structurally flexible molecules is a compulsory step for a reliable study of thermodynamic state functions, ionization spectra, electron momentum spectra, infra-red and raman spectra, ... because of the impact of the conformation on orbital energies, electron density distributions, or the molecular rotations and vibrations. In this framework, we calculated the energy differences between stationary points on the potential energy surface of n-pentane, stilbene, dimethoxymethane and n-hexane from results obtained using many-body quantum-mechanical methods and basis sets of improving quality, along with suited procedures for extrapolating these results to the CCSD(T) level at the limit of an asymptotically complete basis set. The molar fractions of the conformers were calculated at various temperatures, using these benchmark energy differences and statistical thermodynamical partition functions accounting for hindered rotations.

 

15. Nucleation of organic semi-conductors on inert surfaces. The microscopic theory of nucleation for the "epitaxial" growth of inorganic materials has been adapted to the nucleation of organic small molecules on an inert substrate like the gate dielectric of an Organic Thin Film Transistor. The parameters required to explore the model were calculated with the standard MM3 force field and also include experimentally determined vapor pressure data, as well as film growth data. The growth of pentacene, tetracene and perylene on inert substrates has been studied in terms of this theory, especially focussing on the two-dimensional (2D) to three-dimensional (3D) nucleation transition. It is demonstrated that 3D nucleation leads to ill-connected grains, while 2D nucleated grains form continuous films suitable for charge transport. The analysis of this transition allows for the experimental determination of the molecule-substrate interactions for a given molecule on a given surface. It was found that the deposition conditions for 2D growth shift to less favorable substrate temperatures and deposition rates as the difference between interlayer interactions and molecule-substrate interactions increase, and the intralayer interactions decrease. Moreover, those interactions affect the nucleation rate and therefore the ultimate 2D grain size that can be obtained. (Figure 13)

 

16. Autoionization bands. Ionization of bromomethanes (CH3Br, CH2Br2, and CHBr3) upon collision with metastable He*(23S) atoms has been studied by means of collision-energy-resolved Penning ionization electron spectroscopy (PIES). Lone-pair (nBr) orbitals of Br4p characters have larger ionization cross sections than sC-Br orbitals. Relative PIES intensities directly measure the spread of orbitals outside the collision boundaries. The collision energy dependence of the partial ionization cross sections (CEDPICS) enables detailed insights into the anisotropies of the interaction potentials for the entrance ionization channels, and into stereo-nucleophilicity therefore of the related orbitals. Bands observed at electron energies of ~2 eV in the He*(23S) PIES spectra of CH2Br2 and CHBr3 have no counterpart in ultra-violet (He I) photoionization spectra (UPS) and theoretical (ADC(3)) one electron and shake-up ionization spectra. Energy analysis of the processes involved demonstrates that these bands relate to autoionization of dissociating (He+-Br-) pairs.

 

17. Photon- and electron-impact ionization spectra of cage compounds. The valence one-electron and shake-up ionization spectra of norbornane, norbornene, stella-2,6-diene (STDE), stella-2,6-dione (STDO), bicyclo[2.2.2]octane-2.5-dione (BCOD) and bicyclo[2.2.1]hepta-2,5-dione (BCHD) have been studied at the OVGF and ADC(3) levels, with as main focus the identification of spectral fingerprints for cyclic strains and through-bond p-conjugation. Results obtained for norbornene indicate that EMS is now at a stage to finely probe the effective topology of molecular orbitals at varying distances from the molecular center, and the way the individual atomic components interact with each other. A band of s-type symmetry which is observed at ~25 eV in the (e, 2e) electron impact ionization spectrum of norbornane is entirely missing in the UV photoelectron and theoretical ADC(3) ionization spectra measured or calculated for this compound. Analysis of the interaction time scales characterizing these spectroscopies seems to indicate therefore that intramolecular electronic decays of shake-up states into doubly ionized states in norbornane leads ultimately to ultra-fast nuclear dynamical and Coulomb fragmentation processes with a time scale around the femtosecond regime. Our work on cage compounds (stella-2,6-dione, stella-2,6-diene, bicyclo-[2.2.2]-octane-2,5-dione and bicyclo-[2.2.2]-heptane-2,5-dione) overall demonstrates that it is impossible to reliably assign complex (e,2e) ionization spectra using only Hartree-Fock or Kohn-Sham orbital theories.

 

18. Probing the shape and stereochemistry of orbitals in conformationally flexible compounds. The interplay between the valence electronic structure, the topology and reactivity of orbitals, and the molecular structure of biphenyl has been studied by means of Penning ionization electron spectroscopy in the gas phase upon collision with metastable He*(23S) atoms. Analysis of the collision energy dependence of partial ionization cross sections confirm the presence of p-2 p*+1 shake-up states at ~13.2 eV, in agreement with theoretical ADC(3) results. The cross sections of p-ionization bands exhibit a strongly negative collision energy dependence and indicate that the interaction potential that prevails between the molecule and the He*(23S) atom is strongly attractive in the p-orbital region. On the other hand, the partial ionization cross sections pertaining to s-ionization channels are characterized by more limited collision-energy dependencies, as a consequence of repulsive interactions within the s orbital region. A comparison of ADC(3) simulations with Penning ionization electron spectra and ultraviolet photoelectron spectra measured on thin films of biphenyl deposited at 170 K and 109 K on copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar to those prevailing in the gas phase. (Figure 14)

 

19. The fate of dicationic states in molecular clusters of benzene and related compounds. Calculations employing density functional theory indicate that, rather than undergoing fragmentation, dicationic clusters of benzene, hexafluorobenzene, and naphthalene produced by sequential one-electron or sudden double ionization experiments on the neutrals can relax via the formation of inter-ring covalent C-C bonds, along with a series of proton transfers that enable a substantial reduction of inter- and intra-molecular Coulomb repulsions. The theoretically predicted chemically bound structures correspond to deep local energy minima on the potential energy surface pertaining to the lowest electronic state of the dications, and can therefore be regarded as metastable (kinetically long-lived) species. This discovery invalidates on theoretical grounds the liquid-droplet model of multiply charged clusters, and sheds very unexpected light on possible consequences in chemistry of the Intermolecular Coulombic Decay (ICD) mechanism [Cederbaum, L.S. et al, Phys. Rev. Lett. 1997, 79, 4778] for deep inner-valence ionized states. Propagation of charge rearrangement reactions and proton transfers to several monomers may eventually lead to the formation of rather extended dicationic assemblies. (Figure 15)

 

20. Probing Dyson orbitals with Green's Function Theory and Electron Momentum Spectroscopy. The ADC(3) scheme has been utilized, for the first time, for computing accurate spherically averaged electron momentum distributions derived from Dyson orbitals and analyzing the results of an experimental study of the valence electronic structure of difluoromethane employing high-resolution Electron Momentum Spectroscopy with various impact energies. The eigen-energies associated to Dyson orbitals also very accurately reproduce the (e,2e) ionization spectrum. Shortcomings of empirical analyses of (e,2e) experiments based on Kohn-Sham orbitals and eigen-energies are comparatively discussed. A failure of the target HF approxim ation is noted for the momentum distribution pertaining to the 1b1+3b2+5a1 levels. Benchmark ADC(3) Dyson orbitals have been further employed in a refutation of a work by Saha et al in J. Chem. Phys. 123 (2005), 124315 regarding fingerprints of the gauche conformational isomer of 1,3-butadiene in electron momentum distributions inferred from gas phase (e,2e) measurements. The ionization spectra and Dyson orbital momentum distributions calculated for the trans-conformer alone are amply sufficient for quantitatively unravelling the shape of all available experimental momentum distributions, provided the contribution of an intense p-2 p*+1 shake-up line at ~13.1 eV is taken into account (Figure16). The ADC(3) ionization spectrum of the gauche conformer is completely incompatible with high-resolution photoelectron measurements employing a synchrotron radiation beam. Further shake-up lines have been similarly identified from an analysis of experiments employing EMS upon thiophene and cyclopentene.

 

21. Aromaticity of Giant Polycyclic Aromatic Hydrocarbons with Hollow Sites: Super Ring Currents in Super-Rings. Our work focus particularly on magnetic criteria of aromaticity, namely 1H NMR and nucleus independent chemical shifts (NICS), and on their relationships with further electronic properties (Figure 17). The computed shifts and NICS indices indicate that an external magnetic field induces exceptionally strong ring currents in even-layered PAH donuts with hollow sites, in particular in the layer directly adjacent to the central cavity of the double-layered compounds. Exceptionally strong ring currents also correlate with particularly low band gaps and electronic excitation energies, and to abnormally high polarisabilities, indicating in turn that these compounds have a more pronounced metallic character. Comparison is made with further depictions of aromaticity in these systems employing topological, structural and energetic criteria.

 

22. Calculations of transition energies in large conjugated systems within chemical accuracy (1 kcal/mol). Benchmark theoretical studies of the electronic ground state and of the vertical and adiabatic ionization energies, electron affinities, and singlet-triplet (S0-T1) excitation energies of benzene (n=1) and n-acenes (C4n+2H2n+4) ranging from naphthalene (n=2) to hexacene (n=6) or heptacene (n=7) have been published, on the ground of single- and multi-reference calculations based on restricted or unrestricted zero-order wave functions. High-level and large scale treatments of electronic correlation in the ground state are found to be necessary for compensating giant but unphysical symmetry-breaking effects in unrestricted single-reference treatments. The composition of multi-configurational wave functions, the topologies of natural orbitals in symmetry-unrestricted CASSCF calculations, the T1 diagnostics of Coupled Cluster theory and further energy-based criteria demonstrate that all investigated systems exhibit a 1Ag singlet closed-shell electronic ground state. The above electronic transition energies can be therefore determined within chemical accuracy (1 kcal/mol, i.e. ~0.04 eV) by applying the principles of a Focal Point Analysis onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ or aug-cc-pVXZ basis sets (X = D, T, Q, 5) and single-reference methods [HF, MP2, MP3, MP4SDQ, CCSD, CCSD(T)] of improving quality, in order to extrapolate CCSD(T) results to asymptotically complete basis sets (cc-pV∞Z, aug-cc-pV∞Z). … etc. Highly quantitative insights into experiments employing electron transmission spectroscopy on systems characterised by negative electron affinities, corresponding to so-called metastable anions, are in particular amenable with such an approach, provided diffuse atomic functions are deliberately removed from the basis set, in order to enforce confinement of the incoming electron within the electro-attractive part of the molecular electrostatic potential (Figure 18, Figure 19) and enable a determination of pseudo-adiabatic electron affinities (with respect to the timescale of nuclear motions). Comparison is made with calculations of electron affinities employing Density Functional Theory and especially designed models that exploit the integer discontinuity in the potential or incorporate a potential wall in the unrestricted Kohn-Sham orbital equation for the anion. In line with the absence of Peierls distortions, extrapolations of results indicate a vanishingly small S0-T1 energy gap of 0 to ~4 kcal/mol (~0.17 eV) in the limit of an infinitely large polyacene.

23. Theoretical Study of the Oxidation Mechanisms of Naphthalene Initiated by Hydroxyl Radicals. The oxidation mechanisms of naphthalene by OH radicals under inert (He) conditions have been studied using Density Functional Theory along with various exchange-correlation functionals. The studied reactions so far comprise addition of OH radicals under inert (He) conditions, H abstraction processes by OH radicals, subsequent O2 addition reaction pathways, and isomerization of the resulting peroxy radicals through cyclisation processes as well as intramolecular hydrogen transfers. Comparison has been made with benchmark CBS-QB3 theoretical results for reaction and activation energies. Kinetic rate constants for elementary unimolecular and bimolecular reaction steps were correspondingly estimated by means of Transition State Theory and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Effective rate constants were then calculated according to steady state analyses upon two-step model reaction mechanisms, enabling detailed studies of the influence of the temperature and pressure on the regioselectivity of the studied chemical reactions, as well as a quantitative understanding of the available experimental kinetic data.