Electron propagator calculations show that alkyl substituents alter porphyrin ionization energies

The effect of alkyl substituents on the four lowest vertical ionization energies of porphyrins is determined with ab initio electron propagator calculations on porphine and octamethylporphyrin. With the use of the partial third-order approximation, predicted ionization energies are in close agreement with recent photoelectron spectra. These data and the associated Dyson orbitals, which describe changes in electronic structure that accompany photoionization, enable assignment of photoelectron spectra and determination of alkyl-induced shifts. Hyperconjugation is most evident in the Dyson orbitals associated with the third and fourth ionization energies of octamethylporphyrin and is least prominent in the Dyson orbital of the second ionization energy. There is a positive correlation between the shift in an ionization energy produced by alkyl substitution and the degree of hyperconjugation in the associated Dyson orbital. Alkyl substitutions, therefore, may be employed to adjust the ionization energies of porphyrins and, consequently, their reactivity patterns that depend on charge-transfer capabilities and disposition to electrophilic attack.     

The Si-Si effect on ionization of beta-disilanyl sulfides and selenides

The ionization energies of conformationally constrained, newly synthesized beta-disilanyl sulfides and selenides were determined by photoelectron spectroscopy. These ionization energies reflect substantial (0.53-0.75 eV) orbital destabilizations. The basis for these destabilizations was investigated by theoretical calculations, which reveal geometry-dependent interaction between sulfur or selenium lone pair orbitals and sigma-orbitals, especially Si-Si sigma-orbitals. These results presage facile redox chemistry for these compounds and significantly extend the concept of sigma-stabilization of electron-deficient centers.     

UV photoelectron spectroscopic study of substituent effects in quinoline derivatives

The molecular and electronic structure of eight substituted quinolines has been investigated by HeI/HeII photoelectron spectroscopy, Green's function calculations, and comparison with the spectra of related compounds. The correlation between nitrogen lone pair ionization energies and basicity in 18 substituted quinolines is discussed. The influence of different substituents has been quantified via the scheme that is based on experimental energy shifts. The relationships between nitrogen ionization energies, pK(a) values, and medicinal activity are also discussed.     

Polarization energies of tetrathiafulvalene derivatives

Ultraviolet photoelectron spectroscopic studies were carried out on dimethyl-, tetramethyl- and dibenzo-derivatives of tetrathiafulvalene in the gaseous and solid state. As the difference between the threshold ionization potentials in both states, we obtained the polarization energies of those compounds in the solid state. Dibenzotetrathiafulvalene (DBTTF) showed the differential relaxation shifts due to the different electronic bands. Polarization energies of the two methyl-derivatives are very small, 0.9 and 1.1 eV. While that of DBTTF is the largest, 2.3 eV, among the donor-like compounds measured, due to the large molecular polarizability. Comparing these values with the polarization energies of related compounds, it is suggested that the DBTTF form very stable charge-transfer complexes with some appropriate acceptor molecules     

Tryptophol cation conformations studied with ZEKE spectroscopy

The relative energies of several conformations of the tryptophol cation are determined by zero kinetic energy (ZEKE) photoelectron spectroscopy and photoionization efficiency measurements. Recently published high-resolution electronic spectroscopy on the neutral species determined the absolute configuration of the different conformers in the S1 spectrum. These assignments are utilized in the photoelectron experiments by pumping through conformer specific S1 resonances yielding ZEKE spectra of the specific, assigned conformations. The adiabatic ionization of one specific conformation is definitively determined, and two others are estimated. The photoelectron spectra, coupled with calculations, reveal that structural changes upon ionization are dominated by interactions of the hydroxyl group with the changes of electronic structure in the aromatic system.     

High ionization potential conjugated polymers

We report the synthesis of a series of poly(p-phenylene ethynylene)s (PPEs) with high ionization potentials and associated high excited-state electron affinities. Their photophysical properties were investigated using steady-state and time-resolved fluorescence techniques. The ionization potentials of the polymer thin films were determined using ultraviolet photoelectron spectroscopy (UPS), and those with the highest ionization potentials displayed high sensitivity for the detection of electron-donating aromatic compounds. The effects of sterics, chemical structure, and electronic properties on the polymers' sensory responses were investigated by fluorescence quenching experiments in both solution and solid thin films. In addition, we report that in some cases the excited-state charge-transfer complexes (exciplexes) of the PPEs with analytes were observed. These latter effects provide promising opportunities for the formation of sensitive and selective chemical sensors.     

He I photoelectron spectroscopy and theoretical investigation on diaceto disulfide, CH3C(O)OSSOC(O)CH3

A novel species, diaceto disulfide (CH3C(O)OSSOC(O)CH3), has been generated through the heterogeneous reaction between sulfur monochloride (S2Cl2) and silver acetate (AgOC(O)CH3). Photoelectron spectroscopy (PES) and theoretical calculations are performed to investigate its electronic and geometric structures. This molecule exhibits gauche conformation with both C=O groups syn to the S-O bond. The dihedral angle around the S-S bond is calculated to be -93.1 degrees at the B3LYP/6-311++G(3df,3pd) level. After structural optimizations of the most stable conformer, a theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) was performed. The ionization energies of different bands in the photoelectron spectrum are in good agreement with the calculated values from the OVGF method. The first vertical ionization energy of CH3C(O)OSSOC(O)CH3 is determined to be 9.83 eV by photoelectron spectroscopy, which corresponds to the ionization of an electron mainly localized on the sulfur 3p lone pair molecular orbital.     

Photoionization and Pyrolysis of a 1,4‐Azaborinine: Retro‐Hydroboration in the Cation and Identification of Novel Organoboron Ring Systems

The photoionization and dissociative photoionization of 1,4‐di‐tert‐butyl‐1,4‐azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low‐lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE_0K were obtained. The loss of isobutene in a retro‐hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C₄H₆BN isomers 1,2‐ and 1,4‐dihydro‐1,4‐azaborinine and the C₃H₆BN isomer 1,2‐dihydro‐1,3‐azaborole were determined from threshold photoelectron spectra.     

Electrochemistry and photoelectron spectroscopy of oxomolybdenum(V) complexes with phenoxide ligands: effect of para substituents on redox potentials,heterogeneous electron transfer rates,and ionization energies

Complexes of the form (Tp*)MoOCl(p-OC(6)H(4)X) and (Tp*)MoO(p-OC(6)H(4)X)(2) (Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X = OEt, OMe, Et, Me, H, F, Cl, Br, I, and CN) were examined by electrochemical techniques and gas-phase photoelectron spectroscopy to probe the effect of the remote substituent (X) on electron-transfer reactions at the oxomolybdenum core. Cyclic voltammetry revealed that all of these neutral Mo(V) compounds undergo a quasireversible one-electron oxidation (Mo(VI)/Mo(V)) and a quasireversible one-electron reduction (Mo(V)/Mo(IV)) at potentials that linearly depend on the electronic influence (Hammett sigma(p) parameter) of X. The first ionization energies for (Tp*)MoO(p-OC(6)H(4)X)(2) (X = OEt, OMe, H, F, and CN) were determined by photoelectron spectroscopy. A nearly linear correlation was found for the Mo(VI)/Mo(V) oxidation potentials in solution and the gas-phase ionization energies. Calculated heterogeneous electron-transfer rate constants show a slight systematic dependence on the substituent.     

Spectroscopic,Theoretical, and Electrochemical Studies of 1,2-Dithiins

The lowest oxidation potentials for 1,2-dithiins are in the range of 0.67-0.96 V in acetonitrile and 0.81-1.04 V in dichloromethane. These oxidation potentials are less anodic than expected based on the ionization potentials of 1,2-dithiin determined by photoelectron spectroscopy. Theoretical calculations suggest that the reason for this difference is a change in optimized geometry between 1,2-dithiin and its oxidized species.     

Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride.

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.     

Photoelectron spectra and electronic structures of some chlorosulfonyl pseudohalides

The electronic structures of chlorosulfonyl pseudohalide ClSO2X (X = Cl, NCO, N3) are studied by photoelectron spectroscopy (PES) combined with the OVGF calculation at 6-311++G(3df) basis sets. The first ionization potentials for ClSO2NCO and ClSO2N3 are determined to be 12.02 and 11.43 eV, respectively, for the first time, and their features in the PE spectra were assigned based on comparison with related compounds and with high level quantum calculations. Photoelectron spectra of ClSO2NCO and ClSO2N3 suggest that the interactions between chlorine "lone-pair" electrons and two pseudohalogen groups have significant effect on the HOMOs of two compounds, besides the influence of their electronegativity on the first ionization energies.     

Electronic structure of aryl- and carbonyl-containing diazomethane derivatives,as disclosed by photoelectron spectroscopy

Vertical ionization energies (IE) of aryl and carbonyl derivatives of diazomethane have been determined by photoelectron spectroscopy. The types of the highest occupied molecular orbitals (HOMO) in these molecules are identified.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 325–328, February, 1993.     

The electronic structure and the He (I) photoelectron spectrum of bis(π-pentadienyl)dinickel

The electronic structure of bis(π-pentadienyl)dinickel (1) has been investigated by means of semi-empirical MO calculations of the INDO type and by means of He(I) photoelectron (PE) spectroscopy. The vertical ionization potentials obtained by a Green's function approach are in good agreement with the measured ionization energies. It is demonstrated that the Ni 3d ionization events occur at lower energies than the lowest ligand band, a sequence that differs from the case of bis(π-allyl)nickel (2) where ligand π orbitals are ionized at lower energies than Ni 3d orbitals. This difference between the two π complexes can be traced back to a less efficient metal to ligand charge transfer in the binuclear complex 1 leading to a destabilization of the MOs with large Ni 3d amplitudes. According to the semi-empirical INDO-hamiltonian the direct interaction between the two 3d manifolds in the closed shell ground state of 1 is small.     

CFCl3的理论和紫外光电子能谱研究

采用紫外光电子能谱(PES)和量子化学方法,研究了以CFCl3化合物为代表的系列化合物(CFCl3、CF2Cl2、CF3Cl、CCl4)不同离子态的电子结构和性质。结果表明,四种化合物CF3Cl、CF2Cl2、CFCl3、CCl4的第一电离能依次下降。结合从头算自洽场分子轨道(abinitioSCFMO)和外壳层格林函数法(OVGF)计算对化合物的PES进行了分析和指认,表明化合物的外层轨道中Cl的孤对电子成分对电离能存在明显的影响;外层格林函数法计算得到电离能与实验吻合很好;同时发现在外壳层格林函数法计算结果中由于考虑相关能,得到的分子轨道存在能级顺序的交错。     

Vacuum ultraviolet photoionization and photoelectron studies in the new millennium: recent developments and applications

Vacuum ultraviolet (VUV) photoionization mass spectrometry and photoelectron spectroscopy have played a central role in providing energetic and spectroscopic information for neutrals and cations. The most important data obtainable in a VUV photoionization and photoelectron experiment are ionization energies and 0 K ion dissociation thresholds or appearance energy (AE), from which 0 K bond dissociation energies for neutrals and cations can be deduced. The recent developments in VUV lasers and third-generation synchrotron sources, together with the introduction of the pulsed-field ionization (PFI), photoelectron (PFI-PE), and PFI-photoion (PFI-PI) methods, have revolutionized the field of photoelectron and ion spectroscopy by significantly improving the energy resolution to the range of 0.025–1.0 meV (full width at half maximum, FWHM). These resolutions, which make possible the measurement of photoelectron spectra for many simple molecules at the rotational-resolved level, are ≈100-fold better than those observed in traditional photoelectron studies, making the PFI-PE technique a true spectroscopic method. The recent introduction of the synchrotron-based PFI-PEPICO scheme has shown that AE values for a range of molecules can be determined with an unprecedented precision limited only by the PFI-PE measurement. The synchrotron-based PFI-PEPICO and PFI-PI schemes show great promises for future studies of state- or energy-selected ion-dissociation dynamics and energy-selected ion-molecule reaction dynamics. Further improvement in energy resolution for PFI-PE and PFI-PI measurements has been demonstrated using the two-color photo-induced Rydberg ionization (PIRI) spectroscopic scheme, which involves the photo-induced ionization of intermediate long-lived high-n ( n≥100) Rydberg states. The incorporation of this method by VUV photoexcitation to prepare intermediate high-n ( n≥100) Rydberg states is also expected to greatly increase the energy range of PFI studies. The availability of this array of laser- and synchrotron-based PFI methods, including PFI-PE, PFI-PEPICO, PFI-PI, PFI-ion-pair, and PIRI schemes, ensures an exciting and bright future for VUV photoionization and photoelectron studies in the new millennium.     

PFI-ZEKE photoelectron spectra of the methane cation and the dynamic Jahn-Teller effect

High resolution pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectroscopy has been used to record the photoelectron spectra of CH4, CDH3, CD2H2 and CD4. The observed extensive progression of rotationally resolved transitions between 100,800 cm-1 and 104,100 cm-1 reveals for the first time the complex energy level structure of the methane cation. The high resolution enabled the determination of accurate values for the adiabatic ionization potentials of the different isotopomers. Based on a simple one-dimensional model for the pseudorotation in the different isotopomers, progress has been made towards the understanding of the Jahn-Teller effect at low energies. The static Jahn-Teller distortion in the ion could be determined directly from the vibrationless photoelectron transition in CD2H2. The analysis of the rotational structure in this spectrum with a rigid rotor model leads to an approximate experimental C2v structure. The dynamics of the other methane isotopomers near the adiabatic ionization potentials is dominated by large amplitude vibrational motions between equivalent structures. The corresponding ground state tunneling motions takes place on a picosecond time scale.     

负离子光电子能谱技术发展及应用

负离子光电子能谱已经成为探索光谱和化学动力学基础问题的最为重要的技术之一。本文简要介绍了负离子光电子能谱的发展历史,回顾了负离子光电子能谱发展过程中的几种主要技术,包括为了提高分辨率而开发出的零动能能谱和慢电子速度成像技术以及为在气相条件下研究多电荷负离子及其对应中性分子电子特性而开发的电喷雾源与负离子光电子能谱结合的技术。随后介绍其在锕系元素及化合物、含铍化合物的电子结构及成键特征研究中的应用及进展。     

Gas-phase structure, conformation, and sulfur 2p photoelectron spectroscopy of pentafluorosulfur fluorosulfonate, SF5OSO2F

The structure of SF(5)OSO(2)F has been investigated using gas-phase electron diffraction and quantum-chemical calculations. It is found to exist primarily in the gauche form (SF(5) group gauche relative to the S-F bond of the SO(2)F group with phi(S-O-S-F = 71(7) degrees ). A small contribution of the trans conformer cannot be excluded. Photoelectron spectroscopy gives ionization energies for the sulfur 2p electrons that reflect the relative electronegativities of fluorine and oxygen. The widths of the peaks in the photoelectron spectra indicate that there is considerable vibrational excitation associated with the core ionization of the sulfur atoms.     

Cn films (n=50, 52, 54, 56, and 58) on graphite: cage size dependent electronic properties

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.