Conformational Analysis by Photoelectron Spectroscopy

The conformation of organic molecules may be determined by means of photoelectron (PE) spectroscopy if there are orbital interactions present which depend on a dihedral angle. Straightforward application of the method requires that all ionization bands of interest can be assigned unambiguously and that inductive effects and secondary orbital interactions do not occur or can be accounted for appropriately. In many cases, this method complements conventional methods of conformational analysis. PE conformational analysis is particularly suited to compounds containing vicinal lone pairs or π systems.     

OCS的多光子电离高分辨光电子能谱

硫氧化碳OCS是线性三原子分子，这类小分子的激发态、离子态能级结构、能级之间的相互作用及电离过程，是研究中所关心的问题．Tanaka等[1]和Kopp[2]测量了OCS的VUV吸收光谱，Frey和Schlag等[3]以同步辐射光源，用光电离共振（PIR）谱方法、Kovac[4]和Wang，Shirley等[5]以Hel为电离光源，分别采用传统的光电子能谱和高分辨光电子能谱技术研究了CO2、CS2和OCS分子从电子振动基态吸收单个光子而电离的过程．Yang和Anderson等问为了作选态的离子一分子反应利用可调谐激光rt光子吸收将OCS选择激发到某一中间态，OCS再吸收光子后…     

Photoelectron Spectra of Nonmetal Compounds and Their Interpretation by MO Models

The results of (low energy) photoelectron spectroscopy render possible a better appreciation of the “Nature of the Chemical Bond”. The application of this new experimental method is demonstrated utilizing representative compounds of the nonmetal elements, and a close symbiosis delineated with molecular orbital models. In particular, general consequences are discussed concerning electron deficiency, σ- and π-interactions, electron pair delocalization, and substituent effects or geometric perturbations. Photoelectron spectroscopic ionization energies permit evaluation of parameters for specified molecular groups, allow correlation with numerous other experimental data, and are didactically valuable in the teaching of general chemistry.     

The Kinetic and Mechanistic Evaluation of NMR Spectra. New analytical methods (18)

In addition to the static parameters of the chemical shifts and coupling constants, which serve as a source of knowledge for molecular structure and stereochemistry, an NMR spectrum can frequently furnish dynamic quantities characterizing relaxation and exchange phenomena. The information about nuclear switching processes has proved to be particularly useful in practice for the detection of internal molecular motions and for the estimation or determination of the corresponding energy barriers. A plethora of studies of this nature has in the past been performed on simple proton spectra. Methodological developments of recent years have led to a significant reduction of the effort required for the quantitative dynamic evaluation of NMR spectra arising from complex spin systems or involving other nuclei. In many cases it has, moreover, become possible to extract detailed mechanistic information inaccessible by other means. The practical execution of such analyses will be explained and illustrated by a selected number of applications.     

Pheromones in Nanogram Quantities: Structure Determination by Combined Microchemical and Gas Chromatographic Methods [New Analytical Methods (35)]

What can a chemist do with a few nanograms of valuable organic substance, pure or impure? The answer may be, a great deal. Its components may be quantified, separated, isolated, degraded, derivatized, extracted by solvent partition, as well as the more obvious recording of their mass spectra. Provided a substance is not contaminated by large quantities of solvent, manipulations are possible without much loss or dilution. Necessity is the mother of invention. The examples given in this review show what can be accomplished today in nanochemistry and should act as a stimulus to direct the scientist to discover other applications appropriate to his needs and his circumstances. The examples are largely taken from the field of insect chemistry, but their general applicability is emphasized. This review illustrates how gas chromatography and reaction gas chromatography, alone or combined with mass spectrometry are potent tools in the hands of the chemist to deduce the unique structure of volatile organic substances available only in nanogram quantities.     

Photoelectron Spectroscopy,Photoionization Mass Spectroscopy,and Theoretical Study on CCl3SSCN

Trichloromethanesulfenyl thiocyanate, CCl3SSCN, was generated and studied by photoelectron spectroscopy (PES), photoionization mass spectroscopy (PIMS), and theoretical calculations. This molecule exhibits a gauche conformation, and the torsional angle around S-S bond is 91.4 o due to the sulfur-sulfur lone pair interactions. After ionization, the ground-state cationic-radical form of CCl3SSCN*+ adopts a trans planar main-atom structure with Cs symmetry. The highest occupied molecular orbital (HOMO) of CCl3SSCN corresponds to the electrons mainly localized on the sulfur 3p lone pair MO. The first ionization energy is determined to be 10.40 eV.     

Photoelectron spectroscopy of mono and binuclear iron and chromium cyclooctatetraene complexes

The ultraviolet photoelectron spectra of mono and binuclear cyclooctatetraene (COT) complexes (CO)₃FeCOT (I) [(CO)₃Fe]₂COT (II), CpCrCOT (Cp: 1,3 cyclopentadienyl) (III) and (CpCr)₂COT (IV) are reported. The interpretation of the low energy part of the spectra is followed by a discussion concerning the metal–ligand (COT) and metal–metal interactions. The calculated gas phase structure of CpCrCOT is presented and its main features are discussed.     

Gas Chromatographic Separation of Enantiomers on Cyclodextrin Derivatives

In investigations concerned with the phenomenon of molecular chirality, the use of gas chromatography for the enantiomeric analysis of stable, volatile compounds is a technique of steadily growing importance. ^[1] In the last three years an important breakthrough in gas-chro-matographic separation of enantiomers has been achieved by using alkylated cyclodextrins (α, β, and γ) as chiral stationary phases in high-resolution capillary columns. In academic and commercial practice two different and complementary strategies have been adopted up to now. In the first, alkylated cyclodextrins are diluted with polysiloxanes and coated on glass or fused silica capillary columns. In the second, lipophilic per-n-pentylcyclodextrins and hydrophilic di-n-pentyl- and hydroxyalkylpermethylcyclodextrins are coated directly in the form of liquid phases onto suitably pretreated glass or fused silica surfaces. These techniques permit enantiomer separations not only for polar diols and alcohols, derivatized hydroxycarboxylic acids, amino acids, sugars, and alkyl halides, but also for nonpolar alkenes, cyclic saturated hydrocarbons, and metal π complexes. An important aspect for practical applications is that in many cases the enantiomers can be separated without previous derivatization. Whereas the resolution of racemates of unfunctionalized hydrocarbons is attributed to an enantioselective host–guest inclusion complex, some observations indicate that for polar guest molecules additional enantioselective interactions are also involved. The new chiral stationary phases can be used over a wide range of temperatures (25 to 250°C). The technique described is likely to become widely adopted as a simple, accurate and highly sensitive method for the enantiomeric analysis of chiral compounds that can be vaporized without decomposition. It will also stimulate future research aimed at finding universal cyclodextrin phases and elucidating the mechanisms of enantioselectivity.     

Gas Chromatographic Separation of Enantiomers on Optically Active Metal-Complex-Free Stationary Phases. New Analytical Methods (24)

If a stationary phase A employed in gas chromatography possesses a chemical affinity for substance B, which is to be separated, then the retention behavior is not only determined by the normal physical equilibrium between the gas and liquid phases but also by the chemical equilibrium A + B ? AB. If A and B are chiral and A is present in optically active form while B is a racemic mixture, then it is possible to achieve a gas chromatographic enantiomer resolution without the isolation of diastereomers: the energetically different diastereomeric associates A_R B_R and A_R B_S are formed rapidly and reversibly. This enantiospecific resolution principle was first demonstrated in 1966 by the quantitative resolution of racemic amino acid derivatives on optically active peptide phases in analogy to the well-known stereospecificity of enzymes. The anchoring of the chiral resolving agent to thermally stable polysiloxanes together with the employment of high resolution capillary columns and the use of appropriate derivatization strategies has led to the development of enantiomer resolution into a routine modern method for many classes of substances. The demonstration of enantiospecificity in the gas chromatographic separation process is of fundamental interest, and its systematic study can result in a significant contribution to the understanding of the molecular mechanism of “chiral recognition”. The gas chromatographic separation of enantiomers has also proven to be an accurate and sensitive method for the determination of the enantiomeric composition of natural products and products of enantioselective transformations (asymmetric syntheses, “chiral pool” transformations, kinetic resolutions, biomimetic reactions) and for the quantification of racemization, e.g. in the synthesis and hydrolysis of peptides. In any research program devoted to the phenomenon of chirality, the gas chromatographic separation of the enantiomers of volatile compounds constitutes an indispensable modern instrumental technique.     

Photoelectron microscopy and applications in surface and materials science

We review the recent achievements of photoelectron microscopy (PEM), which is a rapidly developing technique that is significantly advancing the frontiers of surface and materials science. The operation principles of scanning photoelectron microscopes (SPEM), using different photon optic systems to obtain a micro-probe of sub-micrometer dimensions, and of the full-field imaging microscope, using electrostatic lenses for magnification of the irradiated sample area, are presented. The contrast mechanisms, based on photon absorption and photon-induced electron emission, are described and the expected development in the photon and electron optics and detection systems are discussed. Particular attention is paid to the present state-of-art performance of the microscopes collecting photoelectrons (PEs), which carry specific information about the lateral variations in the chemical, magnetic and electronic properties of the material under investigation. Selected results, obtained recently with instruments installed at synchrotron light facilities, are used to illustrate the potential of PEM in characterising micro-phases and dynamic processes with a lateral resolution better than 100 nm.     

Two-Dimensional NMR Spectroscopy: Background and Overview of the Experiments [New Analytical Methods (36)]

An exact knowledge of the structure, dynamics, and reactions of molecules provides the key to understand their functions and properties. NMR spectroscopy has developed, through the introduction of two-dimensional methods, into the most important method for the investigation of these questions in solution. A great variety of different techniques is available. However, for their successful application not only the appropriate equipment is required, but also the right choice of experiments and optimum measurement parameters, as well as a careful evaluation of the spectra. This contribution describes the necessary background for modern NMR spectroscopy. With the aid of the so-called product operator formalism it is possible to understand pulsed Fourier transform NMR spectroscopy both qualitatively and quantitatively. Very few, readily understandable assumptions are sufficient for confident application of these methods. This article attempts to introduce in a simple manner this formalism as well as phase cycles necessary for the understanding of pulse sequences, and to train the reader through the discussion of several 2D NMR techniques. An overview of the most important techniques is given in the second part of this article.     

纳米通道内混合气体流动的分子动力学模拟

采用非平衡分子动力学方法, 模拟了混合气体在纳米通道中的Poiseuille流动. 结果显示气体混合物的化学成分与物理结构不再均匀一致, 随着亲水性气体粒子的减少, 亲水性粒子逐渐被吸附于壁面, 而疏水性粒子主要分布于通道中间. 当亲水性粒子为10%时, 混合气体在壁面处形成有序的“类固体”. 在本文的模拟条件下, 流体速度分布显示混合气体流动速度随着疏水性粒子比例的增加而升高; 同时, 混合气体滑移速度也从负滑移速度逐渐转变为正滑移速度.     

Proximity Effects in Organic Chemistry—The Photoelectron Spectroscopic Investigation of Non-Bonding and Transannular Interactions

Classical structural formulas often convey the impression only those relationships between the atoms are of importance which hold a molecule together as symbolized by the chemical bonds. However, many interactions between atoms or groups of atoms are not adequately denoted in this manner. Nevertheless, their existence can have important consequences for ground state energies (cis-difluoroethylene is more stable than trans-difluoroethylene), conformations (the syn form of methyl vinyl ether is more stable than the anti form), reactivities (an endo cyclopropane ring in 7-anti-norbornyl derivatives accelerates solvolysis by a factor of 10¹⁴), UV spectra (“superposition” of the π-systems of acridine and purine-bonded through a four-membered chain-results in hypochromism), CD spectra (the inherently symmetrical but dissymmetrically perturbed chromophore of 2-deuterionorbornadiene allows the observation of three transitions in the near UV), and ESR spectra (the long-range coupling with H-4 in the bridgehead bicyclo[2.1.1]hex-1-yl radical equals 22.5 G). Since orbital interactions are involved in most intramolecular effects of this kind, photoelectron spectroscopy has proven an informative and valuable extension to other methods.     

Fundamentals of Silicon Chemistry: Molecular States of Silicon-Containing Compounds

Silicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the “renaissance” of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic “molecular fingerprints” as well as the resulting benefit for the preparative chemist. The comparison of “equivalent” states of “chemically related” molecules is emphasized, based on perturbation arguments and supporting quantum-chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon “building blocks”, the energy-dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of “silicon substituents” on molecular properties. Future-directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion-pair formation of silicon-substituted π systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.     

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。     

Photoelectron Spectra and Electronic Structures of Some Acceptor‐Substituted Cyclopropanes: Linear Correlation of Substituent Effects on MO Energies with Molecular Structures

The relationship between electronic and geometrical structures in acceptor‐substituted cyclopropanes has been investigated by B3LYP DFT calculations and photoelectron (PE) spectroscopy. The spectra of cyclopropanecarbaldehyde ( 2 ), cyclopropanecarboxylic acid ( 3 ), cyclopropanecarboxylic acid methyl ester ( 4 ), nitrocyclopropane ( 5 ), isothiocyanatocyclopropane ( 6 ), cyanocyclopropane ( 7 ), and 1,1‐dicyanocyclopropane ( 8 ) have been analyzed. The first ionization potential (IP₁) of compounds 2 – 5 was found to be 0.1–0.4 eV higher than that of the analogous isopropyl derivatives indicating—contrary to expectation—that in these compounds the cyclopropyl group acts as a weaker electron donor than an isopropyl group. In the other compounds, IP₁ values are 0.4–1.1 eV lower than in the open‐chain congeners. The Walsh orbitals ω_S and ω_A of the three‐membered ring are substantially stabilized to different extents by interactions with substituent orbitals, and this is reflected in shortened distal and elongated vicinal C? C bonds. Although the nitro group in compound 5 causes large stabilizations of both ω_S and ω_A, their energy difference Δω remains rather small; this is in agreement with a relatively small difference Δr of the C? C bond lengths. For the investigated monosubstituted cyclopropanes 2 – 7 , the largest effects with respect to Δω and Δr are caused by the formyl group in carboxaldehyde 2 . Comparison of the results for nitriles 7 and 8 indicates that the effects of the cyano groups are additive. A linear relationship between Δω and Δr was established by B3LYP DFT calculations on geometrically distorted cyclopropane ( 1 ) and from the PE data of 2 – 8 .     

Problems and Possibilities of Modern Trace Analysis

As a result of the development of measuring techniques, analytical chemists can now determine very small quantities as well as trace concentrations of the order of 10^?8% with good accuracy. In exceptional cases it is even possible to detect 10⁵ atoms of an element. In this review, definitions of important analytical terms are outlined and a critical appraisal of the principal methods of trace analysis is given.