Inductive and mesomeric effects on the π orbitals of halobenzenes

Ionization potentials for the π bands of a wide range of fluoro-, chloro-, and bromobenzenes have been measured by the technique of photoelectron spectroscopy. An empirical model incorporating inductive and mesomeric terms is discussed and its validity tested with the observed energies. Satisfactory agreement is obtained with dipole moment data. The theory accurately predicts ionization energies only for molecules having no ortho halogen atoms. Deviations from linearity are adequately explained in terms of an ortho effect which is proportional to the number of halogens situated in ortho positions. An analogous effect has previously been observed in the electronic spectra of substituted benzenes. Application of the model to 1a _2u band shifts in the fluorobenzenes leads to the conclusion that the energy of this band in benzene lies in the region of 11·5 eV.     

An experimental and computational study of the valence photoelectron spectra of halogenated pyrimidines

The electronic structures of pyrimidine and a selection of its halogen-substituted derivatives have been investigated using ultraviolet photoelectron spectroscopy and ab initio quantum chemical methods. Assignments are proposed for all of the features in the PES spectra by comparison with the vertical ionization energies of the molecular orbitals calculated using the partial third-order quasiparticle approximation as applied to electron propagator theory and a corrected density functional method based on the B3LYP functional. The shifts of the outermost five molecular orbitals of the pyrimidine ring structure in the halogen-substituted derivatives with respect to the binding energies of the equivalent orbitals in the parent pyrimidine molecule are discussed as a function of the identity and ring position of the halogen atom.     

Photoelectron spectra of bis-cyclopentadienyl metal dihalides

He(I) and HE(II) photoelectron spectra are reported for (η-C₅H₅)₂MX₂ (M = Ti; X = F, Cl, Br, I: M = Zr, Hf; X = Cl, Br: M = Ta; X = Cl, Br) and (η-MeC₅H₄)₂MX₂ (M = Nb, X = Cl: M = Mo; X = Cl, Br, I). A substantial variation is found in the ordering of the halogen and cyclopentadienyl ionizations, the order being dependent on the metal as well as on the halogen. The compounds may be divided into three classes, namely, those in which the electrons in cyclopentadienyl e₁ orbitals ionize at a lower energy than those occupying halogen pπ orbitals, those in which halogen pπ electrons have lower ionization energy than cyclopentadienyl e₁ electrons and those in which the corresponding electrons arise from extensively delocalized molecular orbitals with significant contributions from both these categories of fragment orbital.     

A pes study on 1,3-thiazole and its monosubstituted halogen derivatives

The photoelectron (HeI) spectra of 1,3-thiazole and its 2F, 2Cl, 2Br, 4Br and 5Br-derivatives are reported. The assignment of the first few bands of the various spectra to the corresponding molecular orbital is based, for thiazole, on the results of an ab initio SCF—MO calculation, while for the various halogen derivatives, on reasonings based on perturbation theory.In particular, the first five outermost molecular orbitals of thiazole probably correspond to π₃ (9.50 eV), π₂ (10.24), σ_N (10.48; orbital mainly localized on the nitrogen atom), σ_S (12.78; orbital mainly localized on the sulphur atom) and π₁ (13.5).     

Gas phase and solid state X-ray photoelectron spectra of the substituted acetylenes (SF5)n C2 (CF3)2−n (n = 0, 1, 2): A comparison of the pentafluorosulfur and trifluoromethyl groups

The core level X-ray photoelectron spectra (XPS) of CF₃CCCF₃, CF₃CCSF₅ and SF₅CCSF₅ have been measured in the solid state. Gas phase spectra of CF₃CCCF₃ and CF₃CCSF₅ have also been obtained. The XPS data, interpreted with the point charge potential model and semiempirical MNDO (minimum neglect of differential overlap) molecular orbital calculations, indicate that the electron withdrawing effect of the ?CF₃ group is greater than that of the ?SF₅ group. Results further suggest that sulfur 3d orbitals do not play a detectable role in the bonding or charge distribution in these molecules. Carbon 1s linewidths of ?CF₃ carbon atoms are found to be much narrower than those arising from the acetylenic carbon atoms. The narrower lines correlate with the much higher binding energy of the ?CF₃ carbon atoms. Large shifts (nearly 1 eV) in heteroatom core level binding energy differences (for example, F 1s — C 1s) between the gas phase and solid state data are observed. These shifts are attributed to solid state effects (Madelung potential, intermolecular bonding interactions, and/or extramolecular relaxation contributions). From these comparisons it is clear that solid state effects are not uniform in their influence on the photoionized sites in these molecules.     

Penning ionization of 1-bromoadamantane and bromocyclohexane by collision with He*(23S) metastable atoms: spin–orbit coupling effect and anisotropic interaction around bromine atom

The He I ultraviolet photoelectron spectra and Penning ionization electron spectra for 1-bromoadamantane and bromocyclohexane are measured and assigned on the basis of the outer valence Green’s function calculations. Spin–orbit splitting is observed clearly for the lone pair orbitals localized on the Br atoms. Slope parameters of collision energy dependence of partial ionization cross sections for two split branches in the bromocyclohexane spectra are nearly equal. Two possible explanations are discussed by a strong spin–orbit coupling effect and the coexistence of two isomers. The former breaks down the spatial electron distribution picture of molecular orbital in electron energy spectra; the latter is supported by the calculated results of anisotropic interaction around the Br atom using the unrestricted second-order Møller–Plesset perturbation theory with the 6-31+G(d,p) basis set. By comparison of the anisotropic interactions around the Br and Cl atoms, a shielding effect by the hydrocarbon group which leads to decreasing attractive interactions around the substituted Br atom is found to be relatively weak even for the large bromohydrocarbons.     

The photoelectron spectra of the halomethanes

The photoelectron bands associated with ionization from the formally non-bonding p orbitals of the halogen atoms in the halomethanes have been interpreted in terms of a pseudo one-electron hamiltonian. Account has been taken of interactions between the halogen atoms, of interactions between the halogen p orbitals and the σ-bonding orbitals, and of spin-orbit coupling. This model leads to a systematic assignment of all the bands of the chloro and bromomethanes, and gives a satisfactory account of the spin-orbit splittings in the bromomethanes.     

The temperature dependence of penning ionization electron energy spectra: He(23S)—ar,N2, NO,O2, N2O,CO2

Using two intense thermal energy He(2³S) sources of different temperatures (≈400 and ≈ 1000 K, resp.) and a transmission-calibrated electron energy analyzer with about 30meV resolution, the dependence of He(2³S) Penning ionization electron spectra on collision energy for the target species Ar, N₂, NO, O₂, N₂O and CO₂ have been studied. The energy shifts of the Penning electron energy distributions and the branching ratios for the population of different electronic states in the molecular ions are determined quantitatively and compared for the two different collision temperatures. These results and the shapes of the observed Penning electron energy distributions are discussed in the light of current models for the Penning procen; the observed temperature dependences are correlated with the nature of the ionized orbitals in cases of only one entrance channel (closed shell targets) and, in addition, to the existence of qualitatively different entrance channels (open shell targets).     

SF6电子动量谱学研究中干涉效应的观测

利用非共面对称的高效率(e,2e)电子动量谱仪测量了SF₆分子外价分子轨道的二维电子能量- 动量密度谱. 通过理论计算与实验结果的比较,发现B3LYP密度泛函理论计算结果可以较好地解释实验测量的轨道电子动量分布. 此外,对于最外层的4个来自F2p孤对电子贡献的非键分子轨道,实验上观测到非常明显的多中心干涉图样.     

Thin layers of gallium-arsenide: A molecular orbital study

The quasirelativistic INDO/1 method has been used to generate molecular orbitals for some {GaAs}_n clusters up to 160 atoms. On the basis of these one-electron energy levels the density of states (DOS) and density of hole functions have been calculated. Various projections of DOS functions are discussed. The calculations are compared with those generated by periodic crystal orbitals of the EHT quality and with experimental ESCA spectra on thin layers of GaAs.     

Photoelectron spectra of fluorotribromomethane and fluorotrichloromethane

High resolution He(I) photoelectron spectra are reported for fluorotribromomethane CFBr₃ and fluorotrichloromethane CFCl₃. The assignments are based on CNDO/2 calculations, symmetry arguments, the fine structures of the bands, and comparison with the photoelectron spectra of related compounds. Linear relationships have been found between the Pauling electronegativity values for the halogen atoms and the observed vertical ionization energies corresponding to a₂ of a₂′ orbitals of CFX₃ and related compounds CHX₃, OPX₃, BX₃ and PX₃ (X = F, Cl, Br, I).     

A multispectroscopic investigation of the electronic structure of luminescent,tetrahedrally coordinated vanadium-oxygen compounds: Case of YVO4

Photoelectron (XPS) and core ionization loss (CILS) spectra of YVO₄ have been measured and compared to our earlier appearance potential (APS) data on the same compound, literature data on X-ray emission of YVO₄, X-ray emission and absorption of Na₃VO₄, and the energy level diagram of molecular orbitals of the VO₄^3- cluster. Strong many-body effects, with participation of a 2p core hole on vanadium atoms, are found to lead to a drastic local perturbation of the outer molecular orbitals of the anionic cluster VO₄^3-, at the site of the excited vanadium atoms. Identification of the spectroscopic features in XPS on one hand and the edge spectroscopies CILS and APS on the other hand, enables determination of an empirical energy level diagram of excited configurations, which is different in different spectroscopies used. As a byproduct of our experiment, data on the excitation of oxygen (within the VO₄^3- cluster), yttrium (outside the VO₄^3- cluster), and carbon impurity are presented and discussed.     

Spectral Manifestations of Anion-Cation Interactions of Water-Soluble Porphyrins

The influence of anion-cation interactions on the structure and electronic absorption spectra of cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin and anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin has been investigated by absorption spectroscopy and molecular modeling. It is shown that the shifts of bands in the electronic absorption spectra appearing when passing from aqueous solutions to solutions in organic solvents are due to the interaction of ionized peripheral substituents of the porphyrin macrocycle with counterions. The bathochromic shift of the Q _x transition is due to the influence of counterions on the energy characteristics of the molecule itself, primarily, as a result of Coulomb interactions changing the relative position of the orbitals and the distribution of electron density on them. The hypsochromic shift of the Q _y transition is a consequence of the increase in the dihedral angle between the plane of the porphyrin macrocycle and the aromatic rings with ionized groups due to the decrease in the -electron interaction between them under the action of counterions.     

Photoelectron spectra of F3SiC2H4Si(CH3)3 molecule using monochromatized synchrotron radiation

A variety of photoelectron spectra for gas phase F₃SiC₂H₄Si(CH₃)₃ molecule have been measured using monochromatized undulator radiation and a hemispherical electrostatic analyzer. Valence photoelectron spectrum shows many peaks for ionization from shallow and deep molecular orbitals in the binding energy region of 9–40 eV. A calculation of ionization energies using the outer valence Green's function method indicates energies in agreement with experimental results below 17.5 eV. Spectra for Si L-shell electron emission show chemical shifts of Si atoms induced from different chemical environments around two Si atoms and also exhibit spin–orbit splitting for 2p photoelectrons. Further photoelectron spectra for C K-shell and F K-shell are discussed in comparison with those of related molecules.     

Valency electron molecular orbital calculations

The self-consistent perturbation method used previously with the CNDO/2 approximations is generalized to obtain diamagnetic susceptibilities and nuclear shielding constants, as well as polarizabilities, for N₂, BF, CO and F₂ both with a minimal basis set and with a slightly extended set containing an extra 2p orbital. The importance of origin-dependent terms is pointed out. For quantities that depend on the first-order perturbed molecular orbitals, inclusion of the extra 2p orbitals produces results in better agreement with experiment.     

外场作用下蒽分子的激发特性研究

蒽(anthracene)具有良好的热稳定性以及较高的荧光量子产率的优点, 是最早用于研究有机发光器件(organic light-emitting device, OLED)的材料之一. 在本文中, 主要利用量子化学方法研究了不同外电场对蒽分子激发特性的影响规律. 首先采用密度泛函理论(density functional theory, DFT)在6-311G(d, p)基组水平上对蒽分子基态结构进行优化, 基于稳定基态结构, 利用含时密度泛函(time-dependent density functional theory, TDDFT)以及同一基组水平, 计算出蒽分子的前十个激发态的激发能、跃迁偶极矩、振子强度和紫外吸收光谱等数据. 然后以密度泛函B3P86方法优化出的不同外电场下蒽分子基态结构为基础, 使用TDDFT方法研究了不同外电场对蒽分子前线轨道能级和激发特性的影响规律. 结果显示, 无场时蒽分子在紫外区域234.50 nm处有一个较强的吸收峰, 对应基态电子跃迁至第5激发态吸收光子波长; 在外电场作用下, 蒽分子电子由基态跃迁到激发态的各项光谱参数均有显著变化, 加场后蒽分子的吸收光谱发生了红移, 由紫外波段移向了紫外–可见光波段, 与实验值相符合. 分子前线轨道的计算结果也表明蒽分子的最高占据轨道(highest occupied molecular orbital, HOMO)和最低未占据轨道(lowest unoccupied molecular orbital, LUMO)能量差值在不同电场下存在差异. 关键词： 蒽 外电场 激发特性     

Cooperativity effects between σ-hole interactions: a theoretical evidence for mutual influence between chalcogen bond and halogen bond interactions in F2S···NCX···NCY complexes (X = F,Cl, Br,I; Y = H,F, OH)

Quantum chemical calculations are performed to study the cooperativity effects between chalcogen bond and halogen bond interactions in F₂S···NCX···NCY complexes, where X = F, Cl, Br, I and Y = H, F, OH. These effects are investigated in terms of geometric and energetic features of the complexes, which are computed by second-order Møller–Plesset perturbation theory (MP2). For each F₂S···NCX···NCY complex studied, the effect of cooperativity on the chalcogen bond is dependent on the strength of halogen bond. The results indicate that the interaction energies of chalcogen and halogen bonds in the triads are more negative relative to the respective dyads. The interaction energy of chalcogen bond is increased by 31%–49%, whereas that of halogen bond by 28%–62%. The energy decomposition analysis reveals that electrostatic force plays a main role in the cooperativity effects between the chalcogen bond and halogen bond interactions. The topological analysis, based on the quantum theory of atoms in molecules, is used to characterise the interactions and analyse their enhancement with varying electron density at bond critical points.     

Optical reflection spectra of cadmium halides

The reflection spectra have been measured at liquid helium temperature in the region 4–11.5 eV for CdCl₂ and CdBr₂ crystals. Exciton bands observed around the optical gap are associated with the halogen ion excitation under the influence of the strong crystal field. Characteristic sharp lines are also observed in the deep interband energy regions.     

二乙酰分子内价轨道4ag+4bu的电子动量谱学研究

在高分辨率(ΔE=115eV, Δp≈01a.u.)电子动量谱仪上获得了二乙酰分子(d iacetyl)最内价轨道4ag_g+4bu_u的电离能谱和动量谱的实验结果， 并用Hartree-Fock和密度泛函理论方法做了理论计算.实验结果与理论计算符合较好. 关键词： 二乙酰 内价轨道 电离能 电子动量谱     

Inner-shell excitation of formaldehyde,acetaldehyde and acetone studied by electron impact

Excitation and ionisation of the carbon and oxygen 1s electrons of formaldehyde, acetaldehyde and acetone have been examined by small-angle inelastic scattering of 2.5-keV electrons, with energy resolutions in the range 0.20–0.50 eV FWHM. Features in the electron energy loss spectra below the inner-shell ionisation thresholds have been assigned to transitions to unoccupied valence (π*) and Rydberg orbitals. Broad maxima in the inner-shell continua of all three molecules have been interpreted as resonances associated with transitions to σ*(CO) orbitals. The assignments of the acetaldehyde and acetone spectra have been supported by comparison with those for CH₄ and H₂ CO. The adiabatic first ionisation potential of H₂ NO and H₂ CF have been estimated from the inner-shell spectra of formaldehyde using the equivalent-core analogy.