Geometries and electronic properties of carbon-nitrogen clusters C8N4 and C8N4+/-

The geometry optimization and frequency analysis for the low-lying electronic states of C(8)N(4) and its ions are performed at the DFT/6-31G(d) level. Their energies are calibrated at the CCSD(T)/6-31G(d) level of theory. Ionization energy, electron affinity, binding energy of C(8)N(4), and anion photoelectron spectra of C(8)N(4)(-) are provided at the CCSD(T)/6-31G(d) level. Mulliken populations, leading configurations, bond orders, and compositions of molecular orbitals are used to examine the bonding characteristics in the low-lying electronic states of C(8)N(4) and its ions. It is surprising to find that the ground state of C(8)N(4) is the open shell (5)A(1) state. Interestingly for the low-lying electronic states of C(8)N(4) and its ions, their structures significantly corrugated, which may be caused by their larger [N]/([N]+[C]) ratios. In addition, the similarities and differences between C(8)N(4) and C(10)N(2)(II) are analyzed and discussed.     

Assignment of the photoelectron spectra of FeS3(-) by density functional theory, CASPT2, and RCCSD(T) calculations

The geometric structures of FeS(3) and FeS(3)(-) with spin multiplicities ranging from singlet to octet were optimized at the B3LYP level, allowing two low-lying conformations for these clusters to be identified. The planar D(3h) conformation contains three S(2-) atomic ligands (S(3)Fe(0/-)), whereas the C(2v) structure contains, in addition to an atomic S(2-) ligand, also a S(2)(2-) ligand that is side-on-bound to the iron cation: an η(2)-S(2)FeS conformation. Subsequently, energy differences between the various states of these conformations were estimated by carrying out geometry optimizations at the multireference CASPT2 level. Several competing structures for the ground state of the anionic cluster were recognized at this level. Relative stabilities were also estimated by performing single-point RCSSD(T) calculations on the B3LYP geometries. The ground state of the neutral complex was unambiguously found to be (5)B(2). The ground state of the anion is considerably less certain. The 1(4)B(2), 2(4)B(2), (4)B(1), and (6)A(1) states were all found as low-lying η(2)-S(2)FeS(-) states. Also, (4)B(2) of S(3)Fe(-) has a comparable CASPT2 energy. In contrast, B3LYP and RCCSD(T) mutually agree that the S(3)Fe(-) state is at a much higher energy. Energetically, the bands of the photoelectron spectra of FeS(3)(-) are reproduced at the CASPT2 level as ionizations from either the (4)B(2) or (6)A(1) state of η(2)-S(2)FeS. However, the Franck-Condon factors obtained from a harmonic vibrational analysis at the B3LYP level show that only the (4)B(2)-to-(5)B(2) ionization, which preserves the η(2)-S(2)Fe-S conformation, provides the best vibrational progression match with the X band of the experimental photoelectron spectra.     

Spectroscopic characterization of the ground and low-lying electronic states of Ga2N via anion photoelectron spectroscopy

Anion photoelectron spectra of Ga(2)N(-) were measured at photodetachment wavelengths of 416 nm(2.978 eV), 355 nm(3.493 eV), and 266 nm(4.661 eV). Both field-free time-of-flight and velocity-map imaging methods were used to collect the data. The field-free time-of-flight data provided better resolution of the features, while the velocity-map-imaging data provided more accurate anisotropy parameters for the peaks. Transitions from the ground electronic state of the anion to two electronic states of the neutral were observed and analyzed with the aid of electronic structure calculations and Franck-Condon simulations. The ground-state band was assigned to a transition between linear ground states of Ga(2)N(-)(X (1)Sigma(g) (+)) and Ga(2)N(X (2)Sigma(u) (+)), yielding the electron affinity of Ga(2)N, 2.506+/-0.008 eV. Vibrationally resolved features in the ground-state band were assigned to symmetric and antisymmetric stretch modes of Ga(2)N, with the latter allowed by vibronic coupling to an excited electronic state. The energy of the observed excited neutral state agrees with that calculated for the A (2)Pi(u) state, but the congested nature of this band in the photoelectron spectrum is more consistent with a transition to a bent neutral state.     

Geometries, electronic states, and spectroscopic properties of nitrogen-doped fullerene fragment C10N2(II) and its ions

The DFT(B3LYP)/6-31G(d)//CCSD(T)/6-31G(d) method is used to investigate the low-lying electronic states of C(10)N(2)(II) and its ions. Mulliken populations, leading configurations, bond orders, and compositions of molecular orbitals are employed to explore the nature of bonding in the electronic states of C(10)N(2)(II) and its ions. Electron affinity, ionization energy, binding energy of C(10)N(2)(II), and anion photoelectron spectra of C(10)N(2)(II)(-) are also estimated at the CCSD(T)/6-31G(d) level. On the other hand, the similarities and differences between C(10)N(2)(I) and C(10)N(2)(II) are compared and discussed.     

Electronic Structure of Bis(2,4-pentanedionato-O,O')oxovanadium(IV). A Photoelectron Spectroscopy, Electronic Spectroscopy, and ab Initio Molecular Orbital Study

The electronic structure of the title VO(acac)(2) complex has been investigated using effective core potential configuration interaction ab initio calculations, UV-photoelectron spectroscopy, and electronic spectroscopy. The metal-ligand bonding with the equatorial acac(-) ligands is dominated by sigma interactions involving the filled ligand orbitals and the empty orbitals of the d(1) vanadium(IV) ion. The oxovanadium interactions involve a larger metal-d participation thus resulting in a strong V-O bonding having partial triple-bond character. Additional three-orbital-four-electron stabilizing interactions involving the filled acac(-) MOs and the oxovanadium orbitals further reinforce both the axial and equatorial bonds. The unpaired metal-d electron is completely localized in the nonbonding d(x)()()2(-)(y)()()2 orbital. The low ionization energy of the photoelectron spectrum has been fully assigned on the basis of combined DeltaSCF and configuration interaction calculations. The same theoretical approach has, in addition, provided a good fitting of frequencies associated with "d-d" and charge transfer electronic transitions.     

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.     

A study of reversible gamma-induced structural transformations in vitreous Ge23.5Sb11.8S64.7 by high-resolution X-ray photoelectron spectroscopy

The structural origin of reversible gamma-induced effects in vitreous Ge(23.5)Sb(11.8)S(64.7) has been investigated by high-resolution X-ray photoelectron spectroscopy (XPS). The changes in valence band spectrum from gamma-irradiation suggest a decrease of sulfur lone pair electron concentration accompanied by changes in bonding states of S and Ge. The appearance of additional doublets in the core-level XPS spectra of Ge, Sb, and S atoms for gamma-irradiated sample is described by the formation of over- and under-coordinated charged defect pairs (Ge(3)(-)-S(3)(+)) as a result of radiation treatment. The results verify the switching of Ge-S covalent bonds into S-S bonds as the main microstructural mechanism for gamma-induced optical effects in this glass.     

Soft x-ray spectroscopy of Ba24Ge100: electronic phase transition and Ba-atom rattling

The electronic states of Ba24Ge100 are studied by soft x-ray photoelectron spectroscopy (XPS) at a high-energy photon factory. A large reduction in the density of states (DOS) at the Fermi level is clearly shown before and after the electronic phase transition at 200 K. The changes in the spectrum widths and the fine structures of the core-level Ba 4d spectra give a very reasonable indication of the Ba-atom rattlings in the clathrate polyhedra. On-resonance experiments using the excitation from Ba 3d to 4f levels display that the wave functions of Ba 5d and 6s orbitals give only a small contribution to make a Fermi surface through the hybridization with the Ge20 cluster orbitals. Importantly, reliable values of the DOS at the Fermi level NEF are successfully deduced, using two data sets of DOS obtained from high-resolution XPS and the total magnetic susceptibilities by a superconducting quantum interference device, to be 0.149 and 0.0427 states eV(-1) (Ge atom)(-1) for a high-temperature and for a low-temperature phase.     

Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions

We report photoelectron images and spectra of deprotonated thiophene, C(4)H(3)S(-), obtained at 266, 355, and 390 nm. Photodetachment of the α isomer of the anion is observed, and the photoelectron bands are assigned to the ground X(2)A(') (σ) and excited A(2)A(") and B(2)A(") (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (σ) and out-of-plane (π(?)) orbitals. The adiabatic electron affinity of α-(●)C(4)H(3)S is determined to be 2.05 ± 0.08 eV, while the B(2)A(") term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinity/acidity thermodynamic cycle, the C-H(α) bond dissociation energy of thiophene is calculated as DH(298)(H(α)-C(4)H(3)S) = 115 ± 3 kcal/mol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C(4)H(3)O(-), reveal a previously unobserved vibrationally resolved band, assigned to the B(2)A(") excited state of the furanyl radical, (●)C(4)H(3)O. The observed band origin corresponds to a 2.53 ± 0.01 eV B(2)A(") term energy, while the resolved vibrational progression (853 ± 42 cm(-1)) is assigned to an in-plane ring mode of α-(●)C(4)H(3)O (B(2)A(")).     

Description of the geometric and electronic structures responsible for the photoelectron spectrum of FeO4(-)

The relative stabilities of all low-lying conformations of FeO(4)(0/-) stoichiometry were investigated at the quantum mechanical BPW91, CASPT2, and RCCSD(T) levels of theory. For both the anionic and neutral clusters, the determination of the most stable structure appears to be a demanding task. The density functional theory and wave function second-order perturbation theory computational techniques place the doublet state of the tetrahedron-like O(4)Fe(-) conformation substantially lower, up to 0.81 eV, than the doublet state of η(2)-(O(2))FeO(2)(-). The coupled-cluster method reduces the energy difference to less than 0.01 eV. This equal stability of the ground states of O(4)Fe(-) and η(2)-(O(2))FeO(2)(-) leads to the assignment of the experimental photoelectron spectrum of FeO(4)(-). The lowest binding energy band (X band) is ascribed to the (2)A(1) → (1)A(1) ionization of η(2)-(O(2))FeO(2)(-), while the higher energy band (A band) mainly corresponds to the (2)E → (1)A(1) transition between the O(4)Fe(0/-) conformations. For a specific conformation, CASPT2 calculates the best electron detachment energies. The highest energy peak in this band with the weakest intensity could be ascribed to the (2)A(2) → (3)A(2) transition between the η(2)-(O(2))FeO(2) conformations. The two progressions are the result of ionizations from the anti-bonding orbitals of predominant iron 3d. For a specific conformation, CASPT2 calculates the best electron detachment energies. A BPW91 Franck-Condon simulation of the observed vibrational progressions further confirms the proposed assignments.     

The band 12 issue in the electron momentum spectra of norbornane: a comparison with additional Green's Function calculations and ultraviolet photoemission measurements

In continuation of a recent study of the electronic structure of norbornane [J. Chem. Phys., 2004, 121, 10525] by means of electron momentum spectroscopy (EMS), we present Green's Function calculations of the ionization spectrum of this compound at the ADC(3) level using basis sets of varying quality, along with accurate evaluations at the CCSD(T) level of the vertical (26.5 eV) and adiabatic (22.1 eV) double ionization thresholds under C(2v) symmetry. The obtained results are compared with newly recorded ultraviolet photoemission spectra (UPS), up to binding energies of 40 eV. The theoretical predictions are entirely consistent with experiment and indicate that, in a vertical depiction of ionization, shake-up states at binding energies larger than approximately 26.5 eV tend to decay via emission of a second electron in the continuum. A band of s-type symmetry that has been previously seen at approximately 25 eV in the electron impact ionization spectra of norbornane is entirely missing in the UPS measurements and theoretical ADC(3) spectra. With regard to these results and to the time scales characterizing electron-electron interactions in EMS (10(-17) s) as compared with that (10(-13) s) of photon-electron interactions in UPS, and considering the p-type symmetry of the electron momentum distributions for the nearest 1b(1) and 1b(2) orbitals, this additional band can certainly not be due to adiabatic double ionization processes starting from the ground electronic state of norbornane, or to exceptionally strong vibronic coupling interactions between cationic states derived from ionization of the latter orbitals. It is therefore tentatively ascribed to autoionization processes via electronically excited and possibly dissociating states.     

Magneto-Optical Investigation of the Exchange-Coupled Dimer Cs(3)Mo(2)Br(9)

The electronic spectrum of the confacial bioctahedral complex Cs(3)Mo(2)Br(9) has been investigated by single-crystal absorption, Zeeman, and MCD spectroscopies. A total of seven distinct band regions were resolved and assigned to transitions essentially within the t(2)(2)t(2)(2) configuration that arises when the pair of t(2)(z)() orbitals in the t(2)(3)t(2)(3) configuration is decoupled by strong Mo-Mo sigma bonding. The excited state separations, and in particular the orbital g values, are sensitive to Mo-Mo pi bonding and show clearly that the pi bonding is weaker in the bromide than in the chloride complex.     

EPR spectroscopy of [Fe2O2(5-Et3-TPA)2]3+: electronic origin of the unique spin-Hamiltonian parameters of the Fe2(III,IV)O2 diamond core

The electronic origins of the magnetic signatures of [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3), where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon (17)O substitution in the Fe(2)O(2) core, and an unusually large, positive zero-field splitting parameter, D = 38 +/- 3 cm(-1). Density functional calculations identify the anisotropic (17)O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe(2)O(2) core in the three singly occupied molecular orbitals of the S = (3)/(2) ground state. The near-axial g-matrix arises from DeltaS = 0 spin-orbit mixing between the singly and doubly occupied d(pi) orbitals of the iron d-manifold. The large D is due to DeltaS = +/-1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe(2)O(2) core, and define the low-lying valence orbitals responsible for reactivity.     

Structural and electronic properties of reduced transition metal oxide clusters, M4O10 and M4O10- (M = Cr, W), from photoelectron spectroscopy and quantum chemical calculations

Anion photoelectron spectroscopy and quantum chemical calculations at the density functional theory (DFT), coupled cluster theory (CCSD(T)), and complete active space self-consistent field (CASSCF) theory levels are employed to study the reduced transition metal oxide clusters M(4)O(10)(-) (M = Cr, W) and their neutrals. Photoelectron spectra are obtained at 193 and 157 nm photon energies, revealing very different electronic structures for the Cr versus W oxide clusters. The electron affinity and HOMO-LUMO gap are measured to be 3.68 ± 0.05 and 0.7 eV, respectively, for the Cr(4)O(10) neutral cluster, as compared to 4.41 ± 0.04 and 1.3 eV for W(4)O(10). A comprehensive search is performed to determine the ground-state structures for M(4)O(10) and M(4)O(10)(-), in terms of geometry and electronic states by carefully examining the calculated relative energies at the DFT, CCSD(T), and CASSCF levels. The ground states of Cr(4)O(10) and Cr(4)O(10)(-) have tetrahedral structures similar to that of P(4)O(10) with the anion having a lower symmetry due to a Jahn-Teller distortion. The ground states of W(4)O(10) and W(4)O(10)(-) have butterfly shape structures, featuring two fused five-member rings with a metal-metal multiple bond between the central metal atoms. The much stronger WW bonding than the CrCr bonding is found to be the primary cause for the different ground state structures of the reduced Cr(4)O(10)(0/-) versus W(4)O(10)(0/-) oxide clusters. The photoelectron spectra are assigned by comparing the experimental and theoretical adiabatic and vertical electron detachment energies, further confirming the determination of the ground electronic states of M(4)O(10) and M(4)O(10)(-). The time-dependent DFT method is used to calculate the excitation energies of M(4)O(10). The TD-DFT results in combination with the self-consistently calculated vertical detachment energies for some of the excited states at the DFT and CCSD(T) levels are used to assign the higher energy bands. Accurate clustering energies and heats of formation of M(4)O(10) are calculated and used to calculate accurate reaction energies for the reduction of M(4)O(12) to M(4)O(10) by CH(3)OH, as well as for the oxidation of M(4)O(10) to M(4)O(12) by O(2). The performance of the DFT method with the B3LYP and BP86 functionals in the calculations of the relative energies, electron detachment energies, and excitation energies are evaluated, and the BP86 functional is found to give superior results for most of these energetic properties.     

Exploration of ground and excited electronic states of aromatic and quinoid S,S-dioxide terthiophenes. Complementary systems for enhanced electronic organic materials

We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both pi systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.     

Probing Rapidly‐Ionizing Super‐Atom Molecular Orbitals in C60: A Computational and Femtosecond Photoelectron Spectroscopy Study

Super‐atom molecular orbitals (SAMOs) are diffuse hydrogen‐like orbitals defined by the shallow potential at the centre of hollow molecules such as fullerenes. The SAMO excited states differ from the Rydberg states by the significant electronic density present inside the carbon cage. We provide a detailed computational study of SAMO and Rydberg states and an experimental characterization of SAMO excited electronic states for gas‐phase C₆₀ molecules by photoelectron spectroscopy. A large band of 500 excited states was computed using time‐dependent density functional theory. We show that due to their diffuse character, the photoionization widths of the SAMO and Rydberg states are orders of magnitude larger than those of the isoenergetic non‐SAMO excited states. Moreover, in the range of kinetic energies experimentally measured, only the SAMO states photoionize significantly on the timescale of the femtosecond laser experiments. Single photon ionization of the SAMO states dominates the photoelectron spectrum for relatively low laser intensities. The computed photoelectron spectra and photoelectron angular distributions are in good agreement with the experimental results.     

Geometries and electronic structures of nitrogen-doped fullerene fragment C10N2(I) and its ions

The geometries and relative energies of the low-lying electronic states of C(10)N(2)(I), cation, and anion are investigated by the DFT/CCSD(T) method. Vibrational frequency calculation is performed to analyze the stability of optimized geometries of these states. The binding energy, ionization energy, electron affinity of C(10)N(2)(I) and the anion photoelectron spectra are estimated at the CCSD(T)/6-31G(d) level. The ground states of neutral C(10)N(2)(I), cation, and anion are the (1)A(1), (4)B(2), and (2)A(2) states, respectively. The structure of C(10)N(2)(I) can be described as resulting from the fusion of 2 five-numbered rings and 1 six-numbered ring. Results demonstrate that the 2 five-numbered rings are more active than the six-numbered ring in C(10)N(2)(I) during electron excitation and the C(1) atom site within each N(11)-C(1)-C(5)-C(10) unit exhibits more inert than other atom sites during electron ionization and electron attachment.     

Increasing the band gap of iron pyrite by alloying with oxygen

Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS(2)) can be increased from ～1.0 to 1.2-1.3 eV by replacing ～10% of the sulfur atoms with oxygen atoms (i.e., ～10% O(S) impurities). O(S) formation is exothermic, and the oxygen atoms tend to avoid O-O dimerization, which favors the structural stability of homogeneous FeS(2-x)O(x) alloys and frustrates phase separation into FeS(2) and iron oxides. With an ideal band gap, absence of O(S)-induced gap states, high optical absorptivity, and low electron effective mass, FeS(2-x)O(x) alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies.     

Ab initio Studies on the Fe-S Protein Model Complexes

In this paper, ab initio calculations of the iron-sulfur protein model complexes have been completed on Fe_2S_2(SH)_2~(n-) and Fe_4S_4(SH)_4~(n-)(n=2, 3). The results indicate that the occupied terminal sulfur characteristic orbitals are found in the front orbital site and the energy levels of the occupied Fe 3d-like orbitals appear internally below the S-H bonding orbitals in the valence band. Although the energy sequence is different from what was reported in literature, our results are in agreement with the relevant experimental facts. We have discussed the reason that variations of the active sites are produced by various oxidation levels. The action mechanism of the Fe-S proteins as electron carriers in the biological processes is also explored preliminarily.