Conjugated circuits and resonance energies of benzenoid hydrocarbons

A concept of conjugated circuits contained in Kekulé forms of benzenoid hydrocarbons is considered. Circuits of size (4n + 2) in which CC single and double bonds formally alternate are enumerated and serve as the basis for the characterization of a given system. Resonance energies of benzenoid systems are given by contributions of conjugated circuits of different size. The scheme permits the expression of resonance energies of different molecules in terms of selected reference compounds, such as linear acenes. The approach is illustrated on a dozen benzenoid hydrocarbons and the calculated resonance energies are on average within 0.05 eV of the values obtained by SCF MO calculations.     

Resonance energies of fullerenes with 4-membered rings

We report the resonance energies (REs ) of several fullerenes with 4-membered rings and their isomers with only 5- and 6-membered rings computed using the conjugated-circuit model [RE (CC )] and the topological resonance energy (TRE ) model. Both aromaticity indices were normalized by dividing by the size of the considered fullerene [RE (CC )/e and TRE /e]. The results parallel the predictions by Gao and Herndon using the much more advanced SCF –UHF π-electron approach. A good linear correlation is found between the topologically defined indices [RE (CC )/e and TRE /e] and normalized SCF –UHF π-electron energy. © 1995 John Wiley & Sons, Inc.     

计算共振能的一个新的经验方法

本文提出了一个很简单的计算共振能(RE)的方法。对苯系芳烃,可用下式计算: RE(cV)=0.215(0.89N_0+0.75N_1+0.36N_2)式中,N_0是顶点度为3的碳原子数,N_1是顶点度为2但被两个顶点度为2的原子所连结的碳原子数,N_2是顶点度为2但被一个顶点度为2和另一个顶点度为3的原子所连结的碳原子数。此或略加修正也可用于计算含四元环共轭烃、半苯型烃和奥系化合物等共轭烃的共振能。     

The conjugated-circuit model: The optimum parameters for benzenoid hydrocarbons

A search for the optimum set of parameters for the conjugated-circuit computations on benzenoid hydrocarbons in reported. The SCFπ-MO resonance energies (REs) of Dewar and de Llano were used as standards for the determination ofR _n (n= l,2,3) parameters, which correspond to 4n + 2 conjugated circuits. The following set of parameters:R ₁ = 0.827 eV.R ₂ = 0.317 andR ₃ = 0.111 eV produced the best agreement between the REs calculated by the conjugated-circuit model and the REs calculated using the SCF π-MO model.     

Elektronenstruktur und Stabilität kondensierter Kohlenwasserstoffe

The theory of significant electron structures is applied to benzenoid, nonbenzenoid and semibenzenoid condensed hydrocarbons. Stabilization energies are obtained in agreement with SCF MO resonance energies. The weights of the structures enabled to calculate benzene characters and other character indices.Clar's postulate of localized benzene-like regions is justified. The structural and energetic properties of semibenzenoid hydrocarbons are derived in a systematic manner.     

A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700–1100 K) are calculated by using B3LYP/6‐31+G(d,p) geometries and BMK/6‐311+G(3df,2p) single‐point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity–enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.     

Thermochemical parameters for benzenoid hydrocarbons

The heats of atomization of aromatic hydrocarbons are correlated with an incremental 5-term scheme that includes CH and CC bond energy terms, resonance energies, and two steric parameters. Regression analysis of the experimental data with respect to the proposed parameters gives reasonable values for each term. A simple method for calculating resonance energies is illustrated that agrees with the results of SCFLCAOMO calculations.     

Benzenoid rings resonance energies and local aromaticity of benzenoid hydrocarbons

In this article, we consider partitioning of the analytical expression for resonance energy (RE) in smaller benzenoid hydrocarbons, to individual benzenoid rings of polycyclic molecules. The analytical expression for molecular RE, available since 1976, is given by the count of all linearly independent conjugated circuit in all Kekulé structures in a molecule. Analytical expression for local ring RE (RRE) is given by counting all linearly independent conjugated circuits involving single benzenoid ring in all Kekulé structures, which when added, gives the molecular RE. If for benzene ring the RRE is taken to be 1.000, rings in polycyclic benzenoid hydrocarbons have their ring RRE, which give the degree of their local aromaticity, smaller than 1.000. The difference to 1.000 is a measure of the similarity of a ring to benzene in this one-dimensional (1-D) representation of local aromaticities of benzenoid hydrocarbons. The plot of RRE against the distance of the same ring from benzene in the Local Aromaticity Map, in which benzenoid rings are characterized ring bond orders and average variations of adjacent CC bonds, shows linear correlation (with r = 0.91), reducing the local aromaticity in benzenoid hydrocarbons to 1-D molecular property. © 2018 Wiley Periodicals, Inc.     

Thermochemistry and kinetics of hydrogen abstraction by methyl radical from polycyclic aromatic hydrocarbons

Thermodynamic and kinetic properties relating to hydrogen abstraction by methyl radical from various sites in polycyclic aromatic hydrocarbons (PAHs) have been investigated. The reaction enthalpies (298 K), barriers (0 K), and activation energies and pre-exponential factors (700-1100 K), have been calculated by means of density functional theory, specifically with B3-LYP/6-311G(d,p) geometries, followed by BMK/6-311+G(3df,2p) single-point energy calculations. For uncongested sites in the PAHs, a reasonable correlation is obtained between reactivities (as characterized by the reaction barriers) and reaction enthalpies. This is reflected in a Bell-Evans-Polanyi (BEP) relationship. However, for congested sites, abstraction is accompanied both by lower reaction enthalpies (due to relief of steric strain) and also by reduced reactivities (due to significantly increased steric hindrance effects in the transition structures), so that the BEP relationship does not hold. In addition, the reaction enthalpies and kinetic parameters for the series of linear acenes indicate that abstraction is more difficult from the central rings.     

分子连接性指数~mX~z与不饱和链烃沸点的定量关系研究

提出了一个计算分子中成键原子点价δ_i~z的新方法，以δ_i~z为基础建构的 新的分子连接性指数为~mX~z = ∑(δ_i~z·δ_j~z·δ_k~z……)~(-0.5)，其中 ~0X~z，~1X~z的定义为：~0X~z = ∑(δ_i~z)~(-0.5), ~1X~z = ∑(δ_i~z·δ _j~z)~(-0.5)，并研究了~0X~z,~1X~z与不饱和链烃沸点的相关性。结果表明，该 拓扑指数具有良好的结构-性质相关性。以~0X~z,~1X~z和碳原子数N为结构参数分 别与80个单烃烃、39个单炔烃、169个不饱和链烃（包括烷烃、炔烃及烯炔）的沸 点进行关联所得到的三元回归方程为：单烯烃，log(779.13-bp) = 2.822433-0. 0133346 ~0X~z - 0.0638379 ~1X~2 + 0.0111229N (R = 0.99895, F = 202783. 65, s = 3.36)；单炔烃，log(797.47-bp) = 2.809912-0.0108374~0X~z - 0. 0864540 ~1X~z + 0.0233028N (R = 0.99935, F = 98657.36, s = 3.65);不饱和 链烃，log(741.26-bp) = 2.779526 + 0.0194388~0X~z - 0.0519158~1X~z - 0. 0211047N (R = 0.99467, F = 82387.26, s = 7.74)。应用这些经验公式可以预测 不饱和链烃的沸点。     

Localised vs delocalised molecular orbitals in aromatic hydrocarbons

In many series of polycyclic aromatic hydrocarbons, e.g. the acenes, phenes, starphenes, pyrenes, perylenes and coronenes, the first two IP's follow simple rules and can be predicted with sufficient accuracy from the IP's of the basic chromophores. For example, the PE spectra of all angularly annellated anthracenes show a persistent (“autonomous”) band at 7.4 eV; in the tetracenes such a band is found at 7.0 eV, in the pentacenes at 6.6 eV. The PE spectrum of hexaphene is essentially a superposition of the spectra of tetracene and anthracene, and the spectrum of heptaphene resembles that of tetracene, with all bands showing twice the normal integrated intensity. The bands in the spectra of the linearly annellated pyrenes can be classified into pyrene and acene-type bands of which only the latter respond to annellation (“moving” bands). Similar observations can be made in angularly annellated perylenes and the linearly annellated coronenes. Isotopic hydrocarbons, i.e. compounds with identical UV band positions, show surprising similarities in their PE spectra, despite the widely differing topologies. The origin of these regularities is discussed in terms of the sextet formalism and Hückel theory. Applications to structure elucidation are pointed out.     

Doubly charged ion mass spectra. 2—aromatic hydrocarbons

Doubly charged ion mass spectra for 15 aromatic hydrocarbons have been obtained using a Nier-Johnson geometry, Hitachi RMU-7L mass spectrometer operating at 1.6 kV accelerating voltage. The doubly charged ion spectra have features that are characteristic of the individual compounds. Unsaturated aromatic molecules show intense molecular ions in contrast to saturated, substituted or heteroatom compounds which undergo extensive fragmentation. Ionization energies for forming doubly charged molecular ions and appearance energies for the prominent doubly charged fragment ions have been measured. Calculations of the SCF energies and structures of various doubly charged ions have been carried out. Measured and calculated ionization/appearance energies are in reasonable accord and lend support to the suggested ion structures.     

Parameterized valence bond calculations for benzenoid hydrocarbons using clar structure

SCF (Dewar-de Llano) calculations are used to parameterize a valence bond theory with a basis of Clar structures. A precise correlation of calculated resonance energies is found (corr. coeff. 0.9998). Graphtheoretic algorithms for carrying out the calculations are illustrated. It is suggested that this theoretical approach would be useful in considering structure and reactivity problems in very large polycyclic benzenoid aromatic hydrocarbons.     

Speciation of activators in electroluminescent thin films: an EXAFS study of cerium and terbium doped strontium sulfide

Extended X-ray absorption fine structure (EXAFS) was used to determine the local structure of the luminescent centers in Tb³⁺ and Ce³⁺ -doped strontium sulfide thin films deposited by Atomic Layer Epitaxy (ALE). The rare earths were observed to enter mainly the substitutional sites but at the same time a part of the atoms form RES clusters (RE = rare earth). The presence of both substitutional sites and RES clusters has been observed for the first time in this study for rare earths in II–VI compounds.     

Energetics and dynamics of fragmentation of protonated leucine enkephalin from time- and energy-resolved surface-induced dissociation studies

Dissociation of singly protonated leucine enkephalin (YGGFL) was studied using surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for studying ion activation by collisions with surfaces. The energetics and dynamics of seven primary dissociation channels were deduced from modeling the time- and energy-resolved fragmentation efficiency curves for different fragment ions using an RRKM-based approach developed in our laboratory. The following threshold energies and activation entropies were determined in this study: E(0) = 1.20 eV and DeltaS++ = -20 eu(1) (MH(+)-->b(5)); E(0) = 1.14 eV and DeltaS++ = -14.7 eu (MH(+)-->b(4)); E(0) = 1.42 eV and DeltaS++ = -2.5 eu (MH(+)-->b(3)); E(0) = 1.30 eV and DeltaS++ = -4.1 eu (MH(+)-->a(4)); E(0) = 1.37 eV and DeltaS++ = -5.2 eu (MH(+)-->y ions); E(0) = 1.50 eV and DeltaS++ = 1.6 eu (MH(+)-->internal fragments); E(0) = 1.62 eV and DeltaS++ = 5.2 eu (MH(+)-->F). Comparison with Arrhenius activation energies reported in the literature demonstrated for the first time the reversal of the order of activation energies as compared to threshold energies for dissociation.     

(艹力仒)酸及其类似物紫外光电子能谱的量子化学研究

用一种新的半经验SCF MO方法HAM/3计算菸酸,菸酸甲酯,吡啶,甲酸甲酯和甲酸的分子轨道电离能。提出菸酸与菸酸甲酯紫外光电子能谱的轨道指认,并且研究了这些分子基态的电子结构。在取代效应基础上用经验方法分析了HAM/3指认的可靠性。文中还给出和讨论CNDO/2的计算结果。     

Ag@SiO2@Y2O3:RE3+（RE=Eu,Tb）核壳结构发光材料的制备与性能

采用均相沉淀法制备了Ag@SiO2@(Y,RE)(OH)CO3.H2O(RE=Eu,Tb)核壳结构微球,经过700℃焙烧后成功制备出Ag@SiO2@Y2O3:RE3+(RE=Eu,Tb)核壳结构发光材料。XRD谱图表明Ag核具有结晶良好的面心立方结构;SiO2层为无定型;Y2O3层为立方晶系。FTIR谱图表明核壳之间以化学键相结合。TEM照片表明合成了核壳结构的表面光滑的复合微球,分散良好,大小均匀,Ag核的粒径分布为50±20 nm;SiO2层的厚度为20～30 nm;Y2O3:RE3+(RE=Eu,Tb)层厚度约为125 nm。电子衍射图像表明Ag@SiO2@Y2O3:RE3+(RE=Eu,Tb)为多晶结构。UV-Vis光谱表明表面包覆使Ag离子的等离子体共振吸收峰发生了红移。荧光光谱表明Ag@SiO2@Y2O3:Eu3+具有Eu3+的特征红光发射,Ag@SiO2@Y2O3:Tb3+具有Tb3+的特征绿光发射,但是发光强度均比纯的Y2O3:RE3+有所减弱,说明贵金属的引入对稀土Y2O3:RE3+(RE=Eu,Tb)的发光起到了荧光猝灭的作用。     

SCF-SW-Xα Calculations on molybdenum—halogen cluster compounds

The electronic structures of the cluster cations Mo₆ X⁴⁺₈ (X = F, Cl, Br, I) have been calculated using the SCF—SW—Xα method. The eigenvalue spectra obtained are discussed and compared with results of other quantum-chemical studies of such systems. The results of population analysis are used to discuss the charge distribution in the clusters studied. A qualitative discussion of the Mo-Mo and Mo-X bond strengths is also presented. Finally, calculated bond energies are compared with adsorption energies.