Revealing the electronic properties of the B-B bond: the bis-catecholato diboron molecule.

The electronic properties of a diboron molecule, namely bis(catecholato)diboron (2-(1,3,2-benzodioxaborol-2-yl)-1,3,2-benzodioxaborole) (B2Cat2), have been studied by comparing the results of photoemission (XPS) and near edge X-ray absorption spectroscopy (NEXAFS) experiments with the outcome of DFT calculations. The B 1s, C 1s and O 1s K-edges have been investigated for both the isolated gas phase molecule and the adsorbed one on the Au(111) surface. The main features of the polarized NEXAFS spectra at each of the three edges considered are not significantly affected by the presence of the substrate, with respect to the isolated molecule, indicating that the molecule-gold interaction is weak. Moreover, the comparison between the observed dichroism in the NEXAFS spectra of the adsorbed B2Cat2 and that in the NEXAFS spectra of the isolated molecule has confirmed the orbital symmetry assigned in the gas phase absorption spectra. The transitions to π(B-B) bonding and π*(B-B) anti-bonding final states represent the most relevant probe of the chemistry of the B2Cat2 molecule. We show that their theoretical description requires that the treatment of the relaxation changes among different excited state configurations, which we successfully implemented by using ΔSCF-DFT (ΔSCF) calculations.

Electronic Structure Characterization of a Thiophene Benzo-Annulated Series of Common Building Blocks for Donor and Acceptor Compounds Studied by Gas Phase Photoelectron and Photoabsorption Synchrotron Spectroscopies.

The near-edge x-ray-absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) spectra of benzo[ b]thiophene (BBT) and dibenzothiophene (DBT) in the gas phase have been measured at the carbon K-edge and sulfur LII,III-edge regions. The assignment of the spectral features has been provided by theoretical calculations based on density functional theory (DFT) and its time-dependent generalization (TDDFT) in the linear response regime. Observed trends in computed C 1s and S 2p ionization potentials (IPs) have been rationalized in terms of both the inductive effects due to the presence of S and the increased π-electrons delocalization arising from the benzo-annulation process. The analysis of the NEXAFS carbon K-edge and sulfur LII,III-edge regions provided information on both low-lying delocalized virtual π orbitals, and higher-lying localized σ*(C-S) states. The evolution of the NEXAFS carbon K-edge spectral features along the series thiophene (T) and derivatives, BBT and DBT, is informative of a stabilizing effect due to increased aromaticity. This effect is however more pronounced in going from T to BBT compared to the introduction of a second annulated phenyl ring in DBT. The nature of the most intense sulfur LII,III-edge NEXAFS spectral features is instead conserved along the series reflecting thus the localized nature of the virtual states involved in the S 2p core-excitation process.

Study of the electronic structure of short chain oligothiophenes.

The electronic structure of short-chain thiophenes (thiophene, 2,2'-bithiophene, and 2,2':5',2″-terthiophene) in the gas phase has been investigated by combining the outcomes of Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) at the C K-edge with those of density functional theory (DFT) calculations. The calculated NEXAFS spectra provide a comprehensive description of the main experimental features and allow their attribution. The evolution of the C1s NEXAFS spectral features is analyzed as a function of the number of thiophene rings; a tendency to stabilization for increasing chain length is found. The computation of the binding energy allows to assign the experimental XPS peaks to the different carbon sites on the basis of both the inductive effects generated by the presence of the S atom as well as of the differential aromaticity effects.

S2p core level spectroscopy of short chain oligothiophenes.

The Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) of short-chain oligothiophenes (thiophene, 2,2'-bithiophene, and 2,2':5',2″-terthiophene) in the gas phase have been measured in the sulfur L2,3-edge region. The assignment of the spectral features is based on the relativistic two-component zeroth-order regular approximation time dependent density functional theory approach. The calculations allow us to estimate both the contribution of the spin-orbit splitting and of the molecular-field splitting to the sulfur binding energies and give results in good agreement with the experimental measurements. The deconvolution of the calculated S2p NEXAFS spectra into the two manifolds of excited states converging to the LIII and LII edges facilitates the attribution of the spectral structures. The main S2p NEXAFS features are preserved along the series both as concerns the energy positions and the nature of the transitions. This behaviour suggests that the electronic and geometrical environment of the sulfur atom in the three oligomers is relatively unaffected by the increasing chain length. This trend is also observed in the XPS spectra. The relatively simple structure of S2p NEXAFS spectra along the series reflects the localized nature of the virtual states involved in the core excitation process.

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach.

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.

Theoretical study of near-edge X-ray absorption fine structure spectra of metal phthalocyanines at C and N K-edges.

The inner shell excitation of CuPc, NiPc, and H(2)Pc phthalocyanines at both C and N K-edges has been investigated theoretically by density functional theory calculations. The selected molecules allow one to study the effect on the spectra of the presence and the nature of the atom in the central cavity of the macrocycle. The individual characteristics of the spectra can be rationalized in terms of the position of the unequivalent C and N atomic sites, showing that sensible changes are present in the spectral features deriving from the N atoms directly bound to the atom at the center of the Pc macrocycle. The minor variations present in the spectral C 1s profiles of the phthalocyanines reflect the little perturbation experienced by the peripheral atomic sites.

The valence electronic structure and conformational flexibility of epichlorohydrin.

The electronic structure of epichlorohydrin is investigated in the whole valence region by a combined experimental and theoretical study. The issue of controversial assignments of the molecular electronic structure is here addressed. Photoelectron spectra (PES) and Threshold Photoelectron spectra (TPES) of room temperature molecules in the gas phase are recorded. Geometries and energies of the stable conformers due to internal rotation of the C-C-C-Cl dihedral angle, gauche-II (g-II), gauche-I (g-I), and cis, are calculated, and the effect of the conformational flexibility on the photoionization energetics is studied by DFT and 2h-1p Configuration Interaction (CI) methods. Strong breakdown of the Koopmans Theorem (KT) is obtained for the four outermost ionizations, which are further investigated by higher level ab initio calculations. The full assignment of the spectrum is put on a firm basis by the combination of experimental and theoretical results. The orbital composition from correlated calculations is found closer to the DFT orbitals, which are then used to analyze the electronic structure of the molecule. The Highest Occupied Molecular Orbital (HOMO) and HOMO--2 are n(O)/n(Cl) mixed orbitals. The nature of each valence MO is generally preserved in all the conformers, although the magnitude of the n(O)/n(Cl) mixing in HOMO and HOMO--2 varies to some extent with the C-C-C-Cl dihedral angle. The low energy part of the HOMO PE band is predicted to be substantially affected by the conformational flexibility, as experimentally observed in the spectra. The rest of the spectrum is described in terms of the dominant conformer g-II, and a good agreement between experiment and theory is found. The inner-valence PE spectrum is characterized by satellite structures, due to electron correlation effects, which are interpreted by means of 2h-1p CI calculations.

Photoabsorption and S 2p photoionization of the SF6 molecule: resonances in the excitation energy range of 200-280 eV.

Photoabsorption and S 2p photoionization of the SF(6) molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t(2g) and 4e(g) shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s(-1)6t(1u) excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.

Theoretical study of sulfur L-edge XANES of thiol protected gold nanoparticles.

The Scalar Relativistic-Zero Order Regular Approximation-Time Dependent Density Functional Theory has been employed to study the sulfur L-edge XANES spectrum of the [Au(25)(SCH(3))(18)](+) model cluster, with the aim to reproduce and rationalize previous experimental data. The salient experimental features are properly described by the present calculation. The model cluster contains two different types of bidentate "staple" ligand thiol fragments, and it has been possible to assign the spectral features according to the different location of the initial core orbital on one of the two different fragments. This finding suggests that in the real nanoparticle two different non-equivalent type of sulfur bidentate ligands are present, arranged with the typical staple geometry.

S K-edge NEXAFS spectra of model systems for SO2 on TiO2 (110): a TDDFT simulation.

The time dependent density functional theory approach has been employed to simulate the S K edge absorption spectra of model systems for the adsorption of SO(2) on TiO(2) (110) regular surface, employing cluster models to mimic the rutile surface. The spectra calculated for the adsorbate models are compared with the spectrum of the free SO(2) in order to discuss the nature of the adsorbate-substrate interaction in terms of the differences in the core excitation spectra. The comparison with the experimental NEXAFS spectra, measured at different temperatures, is satisfactory at low temperature while it reveals the difficulty of reproducing the complex experimental situations induced by the temperature increase with an adsorption model based on a perfect TiO(2) surface.

X-ray absorption spectroscopy of VOCl3, CrO2Cl2, and MnO3Cl: an experimental and theoretical study.

X-ray absorption spectra of gas-phase VOCl(3) and CrO(2)Cl(2) have been measured in the metal L(2,3)-edge and O K-edge regions. The assignment of the spectral features is based on the relativistic two-component ZORA TDDFT approach. The calculations provide results in excellent agreement with the experimental spectra and prove the importance of including both configuration mixing and spin-orbit coupling in the theoretical description to obtain a reliable simulation of the transition metal L(2,3)-edge. The calculations are extended also to the MnO(3)Cl molecule to discuss the spectral variations along the series of the oxychlorides both in the metal L(2,3) and ligand O K spectra.

Spin-orbit effects in the photoabsorption of WAu12 and MoAu12: a relativistic time dependent density functional study.

The electronic structure of both WAu12 and MoAu12 has been calculated at the density functional theory (DFT) level, employing the zero order regular approximation at the scalar relativistic level and including a spin-orbit coupling. The effect of the inclusion of the spin-orbit coupling is discussed, and the differences assigned to the nature of the encaged atom (W or Mo) are identified. Then, the excitation spectra of both clusters are calculated at the time-dependent DFT level, also in this case at both scalar relativistic and spin-orbit levels. The inclusion of spin-orbit coupling is mandatory for an accurate description in the low energy region. At higher energy, where the density of states is higher, the convoluted intensity can be properly described already at the scalar relativistic level. The consequences of the spin-orbit coupling on the excitation spectrum of the clusters indicate that while in WAu12 the lowest excitations are essentially shifted in energy with respect to the scalar relativistic results, in MoAu12, a dramatic splitting in many lines is actually predicted, revealing a quite different behavior of the two clusters.

Spin-orbit relativistic calculations of the core excitation spectra of SO2.

The time dependent density functional theory approach within the two-component zero-order relativistic approximation has been applied to the calculation of the core excitation spectra of SO2 molecule. The results obtained reproduce correctly the high resolution experimental spectra and allow the assignment of the spectral features both of the valence and Rydberg regions in the S 1s and O 1s spectra. For the S 2p threshold a correct description of the spin-orbit coupling as well as of the molecular field splitting appears mandatory for a reliable description of the spectrum and a detailed attribution of the complex Rydberg manifold of core excited states.

Time dependent density functional investigation of the near-edge absorption spectra of V2O5.

We have performed Time Dependent Density Functional Theory (TDDFT) calculations employing a cluster model of the core excitation spectra of vanadium pentoxide, V(2)O(5). The excitation energies and dipole transition moments are determined for all the core edges, vanadium and oxygen K- and vanadium L-edges, treating them at the same level of accuracy. The agreement between the TDDFT theoretical spectra and the experimental data is rather good, particularly at the V and O K-edges. A quantitative reproduction of the fine pre-edge structures appears more difficult for the V L-edge. The comparison between the TDDFT results and the results obtained at the simpler one electron Kohn-Sham (KS) level indicates that the V and O K edges can be correctly described within a single particle approximation (KS), while the strong modification of the V L-edge structures from the KS to the TDDFT description emphasizes the importance of configuration mixing to treat the metal 2p excitations. The origin of the calculated pre-edge features is analyzed in detail with the help of the atom-projected density-of-states of the unoccupied levels. This analysis emphasizes the V 3d dominant character of the final states in the conduction band, probed by the V L-absorption. The strong octahedral distortion of the V(2)O(5) structure allows the mixing of the 3d state with the V 4p components, which are mapped by the oscillator strength in the V K-edge spectrum. The high intensity of the O 1s transitions reflects the presence of a significant O 2p component in the conduction band.

Photoionization cross section and angular distribution calculations of carbon tetrafluoride.

Correlation in the photoionization dynamics of carbon tetrafluoride is studied in the framework of the time-dependent density-functional theory (TDDFT) approach by employing a multicentric basis set expansion of the scattering wave function linear combination of atomic orbitals (LCAO) TDDFT. Results obtained with the statistical average of orbital potentials and LB94 exchange-correlation (xc) potentials are compared with photoabsorption, photoionization, and electron-scattering experiments as well as with past theoretical calculations. Inadequacies in both the V(xc) parametrizations employed have been suggested from the analysis of the intensity plots for the D2A1 ionization. The formation of resonant scattering states in selected continuum channels has been studied through the analysis of the dipole-prepared scattering wave function; our findings are then compared with results of electron-scattering calculations. Overall, the LCAO-TDDFT results highlight the effectiveness of the approach for the calculation of the unbound spectrum of fairly large molecules.

X-ray absorption spectroscopy of titanium oxide by time dependent density functional calculations.

The potentiality of the time dependent density functional theory (TDDFT) for the description of core excitation spectra (XAS) in transition metal oxides is analyzed, considering the rutile form of TiO(2) as a test case. Cluster models are adopted to mimic the bulk, embedded within an array of point charges to simulate the Madelung potential. All of the edges, titanium and oxygen K and titanium L edges, are considered, and the TDDFT results are compared with the experimental data in order to assess the performance of the theoretical approach in dealing with this complex class of compounds. Satisfactory results have been obtained for the Ti and O K edges, while in the case of the Ti L edge some discrepancies with the experiment are still present. The configuration mixing explicitly included in the TDDFT model strongly influences the distribution of the 2p metal oscillator strength. The origin of the spectral features is investigated with the help of the partial density of the virtual states (PDOS) calculated for each core hole considered, which can be qualitatively compared with the theoretical spectra calculated in the Kohn-Sham one-electron approach.

Time dependent density functional study of the photoionization dynamics of SF6.

The B-spline linear combination of atomic orbitals method has been employed to study the valence and core photoionization dynamics of SF6. The cross section and asymmetry parameter profiles calculated at the time dependent density functional theory level have been found to be in fairly nice agreement with the experimental data, with the quality of the exchange-correlation statistical average of orbital potential results superior to the Van Leeuwen-Baerends 94 (LB94) ones [Phys. Rev. A 49, 2421 (1994)]. The role of response effects has been identified by a comparison of the time dependent density functional theory results with the Kohn-Sham ones interchannel coupling effects and autoionization resonances play an important role at low kinetic energies. Prominent shape resonances features have been analyzed in terms of "dipole prepared" continuum orbitals and interpreted as due to a large angular momentum centrifugal barrier as well as anisotropic (nonspherical) molecular effective potential. Finally, the method has been proven numerically stable, robust, and efficient, thanks to a noniterative implementation of the time dependent density functional theory equations and suitability of the multicentric B-spline basis set to describe continuum states from outer valence to deep core states.

Theoretical study on the circular dichroism in core and valence photoelectron angular distributions of camphor enantiomers.

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xalpha methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.

Valence photoionization dynamics in circular dichroism of chiral free molecules: the methyl-oxirane.

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Time-dependent density-functional theory for molecular photoionization with noniterative algorithm and multicenter B-spline basis set: CS2 and C6H6 case studies.

In this work a new direct (noniterative) algorithm to solve the time-dependent density-functional theory equations for molecular photoionization has been proposed and implemented, using a multicentric basis set expansion of B-spline functions and complete exploiting of the molecular point-group symmetry. The method has been applied to study the photoionization dynamics of CS2 and C6H6: the results confirmed the expectation of large screening effects in CS2. For C6H6 the screening effects have been found to play a minor role than in CS2, however, also in this case the quality of the final results is definitely improved. The method has proven suitable to study with confidence molecules of medium size, and there is still room for further improvement working on more elaborate treatment of the exchange-correlation functional.