The trapping of excitons by impurities and its dependence on the impurity concentration

When excitons migrate through molecular crystals containing impurities the relative probabilities of their decay at a site of the host lattice or at an impurity can often be deduced experimentally from the material's luminescence spectrum. Theoretically, the mean probability $\text{K}$ that an exciton will be captured by an impurity can be calculated by solving a diffusion equation with a term that takes account of the exciton-impurity interaction. The results of such calculations are reported, with particular emphasis on the dependence of $\text{K}$ on the impurity concentration c for small values of c. It is found that if the impurities behave as absorbing spheres $\text{K}$ is proportional to c, but if the interaction potential is of the multipole-multipole form then d(log$\text{K}$)/d(logc) depends on the exciton's mean free path and on c, and can be much less than unity. The physical reasons for these results are discussed.     

Hydrolysis of very reactive methylation agents - a rapid injection N. M. R. investigation.

By use of the recently described Rapid Injection N. M. R. technique it has been possible to measure the hydrolysis rates of $\text{1}$, $\text{2}$, and $\text{3}$ at various temperatures; the activation parameters for these rapid reactions have been determined and discussed.     

Protonation of acenaphthoquinone. A novel peripheral 10 π-electron aromatic system

Acenaphthoquinone ($\text{1}$) undergoes ppotonation to afford the respective double protonation product $\text{2}$. The proton and carbon NMR parameters of $\text{2}$ are discussed.     

Relationship between the general laws of vapor-liquid equilibria and the analytical curve of emission spectrometry

The relationship between the vapor-liquid equilibrium and the analytical curve of emission spectrometry is discussed on the basis of Hirata's equation, i.e., $\text{y}{}_1\text{y}{}_2$ = α($\text{x}{}_1\text{x}{}_2$)^β, where y is the mole fraction in the vapor phase, x in the liquid phase, α and β are constants for binary systems in a certain concentration range, and the indices $\text{1}$ and $\text{2}$ refer to the two components. Evaporation produced by different atomization-excitation systems can be characterized by apparent evaporation constants, α' and β', the values of which fall between the true values, α and β, of the normal distillation, and unity. Two powder techniques were selected representing the limiting cases: injection spark excitation (α' ≈ α, β' ≈ β) and controlled arc excitation (α' ≈ 1, β' ≈ 1). Using these excitation methods, the analytical curves of the impurities in industrial alumina and electrocorundum were studied using cup-electrode techniques with arc excitation. As a result, one of Hirata's rules could be selected for a given pair of elements. An unambiguous correlation between the properties of the analytical curves and Hirata's rules could be established.     

NMR determination of absolute site-specific natural isotope ratios of hydrogen in organic molecules. Analytical and mechanistic applications

It has been shown that the “internal” isotope distribution within a given molecular species at the natural abundance level is accessible by a new method, SNIF-NMR, which is based on deuterium NMR. Relative internal factors, R_i/j,have been defined which enable the isotope content of a given site, i, to be compared to that of another molecular site, j, taken as the reference. Several referencing methods intended to provide direct access to relative $\text{externals}$, T_i , and $\text{absolute}$, (D/H)_i , site-specific parameters, are now discussed from both the theoretical and the experimental points of view. In the $\text{intramolecular referencing method}$, which involves a time-consuming chemical transformation of the sample, the risk exists of more or leas systematic errors resulting from discriminating fractionation effects. However this technique offers, conversely, an interesting way of investigating kinetic isotope effects without the need for specific labelling. In spite of its lower spectral precision the $\text{external referencing method}$ has the advantage of being fast and less sensitive to systematic errois and may be used for direct rough routine determinations of the site-specific isotope contents. More precise results can be obtained, at the price of contaminating the sample, when an $\text{intermolecular reference}$ is added and signal heights are used, remembering however that the intensity parameters then have no strict physical meaning in terms of absolute isotope contents. The site-specific parameters, T_i and (D/H)_i thus accessible, provide new information on the mechanisms of the fractionation effects occurring in natural conditions and examples are considered.     

Second excited singlet state emission spectroscopy of thiocarbonyls. I. Thiophosgene

Well resolved emission spectra have been recorded after dye laser excitation of low-lying vibronic levels in the $\text{B}$(¹A₁) state of Cl₂CS. The effects of substrate pressure, added SF₆ and excitation wavelength on the spectra are reported. A Franck-Condon analysis suggests that the excited state CS bond length is 0.5 Å longer than in the ground state.     

Angle-resolved x-ray photoelectron spectroscopy

In this review, various aspects of angle-resolved x-ray photoelectron spectroscopy (ARXPS) as applied to solid state- and surface chemical- studies are discussed. Special requirements for $\text{instrumentation}$ are first considered. The use of $\text{grazing-emission}$ angles to enhance surface sensitivity and study surface concentration profiles of various types is then discussed. Various effects that may limit the accuracy of such measurements such as surface roughness, electron refraction, and elastic scattering are considered. Several examples of surface-specific electronic structure changes as studied by grazing-emission ARXPS (e.g., valence-band narrowing and core-level shifts) are also reviewed. The use of $\text{grazing-incidence}$ geometries for surface enhancement is also briefly considered. Single-crystal studies providing additional types of information via ARXPS are next discussed. For core-level emission from single-crystal substrates or adsorbed overlayers, $\text{x-ray photoelectron diffraction (XPD)}$ is found to produce considerable fine structure in polar- or azimuthal- scans of intensity. Such XPD effects can be very directly related to the atomic geometry near a surface, for example, through simple intramolecular or intermolecular scattering processes. A straightforward single scattering or kinematical theory also appears to describe such effects rather well, thus far permitting several structures to be solved by analyses of azimuthal intensity scans. Likely future developments and possible limitations of such XPD structure studies are also discussed. Finally, $\text{valence-band ARXPS}$ is considered, and it is shown that pronounced direct-transition effects can be observed provided that the specimen Debye-Waller factor is not too small. A simple free-electron final-state model is found to predict these direct-transition effects very well, and future studies at low temperatures and with higher angular resolution seem promising.     

Models that demonstrate peptide bond formation by prior thiol capture I. Capture by disulfide formation

Unsymmetrical disulfides formed from $\text{L}$-cysteine esters and $\text{o}$-Cbz-$\text{L}$-alaninyloxy-benzenethiol $\text{9}$, 2-Cbz-$\text{L}$-alaninyloxy-5-chlorophenylmethylenethiol $\text{8}$,4-acetoxyxanthenylmethylene thiol $\text{10}$, and 1,5-diacetoxy-2-methyl-3-methoxy-4-thioxanthone $\text{11}$ are observed to undergo intramolecular O,N-acyl transfer in yields up to 60%, with accompanying disulfide interchange. The significance of these results for a general amide forming strategy of prior cysteine capture are discussed.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.     

Sterically crowded cyclohexanes-2 synthesis,structure and dynamics of hexaspiro[2.0.3.0.2.0.3.0.2.0.3.0]heneicosane

The synthesis, structure and dynamics of hexaspiro[2.0.3.0.2.0.3.0.- 2.0.3.0]heneicosane $\text{3}$ are described. $\text{3}$ adopts a chair conformation in the crystal state but prefers a chair-to-twistboat equilibrium in solution. The activation parameters of the chair-to-chair interconversion of $\text{3}$ and the closely related hexaspiro[2.0.2.0.2.0.2.0.2.0.2.0]- octadecane ([6]rotane) $\text{4}$ have been determined by DNMR. Some consequences appertaining to the conformation and dynamics of other fully (cyclo-)alkylated cyclohexanes are discussed.     

The synthesis and x-ray crystal structure of a tetracyclo[4.2.1.1.2,503,7]decane derivative. A rigid,twist boat-containing ring system.

Attempted intramolecular pinacolization of cage diketone $\text{4}$ unexpectedly gave ketol $\text{5}$ instead. Compound $\text{5}$ was shown to possess the rare tetracyclo[4.2.1.1.^2,50^3,7]decane ring system by means of an X-ray crystal structure of hydroxy-mesylate $\text{10}$ derived from $\text{5}$ by sodium borohydride reduction followed by mesylation. A possible mechanism for the transformation of $\text{4}$ into $\text{5}$ is presented, and certain noteworthy features of the crystal structure of $\text{10}$ are discussed. The base-catalysed fragmentation of $\text{10}$ to give enone $\text{11}$ is also described.     

Integral cross sections for electronic excitation in KHg collisions

The energy dependence of the integral cross section for the electronic excitation in collisions of K and Hg is investigated for energies between 50 eV and 1500 eV. By the measurement of the spectra of the emitted light the 4²P_{$\text{3}\text{2}$} and the 4²P_{$\text{1}\text{2}$} states of potassium are found to be dominant. For these the energy dependence of the cross sections is studied in detail. By the measurement of the polarization the contributions to the 4²P_{$\text{3}\text{2}$} state are differentiated with respect to |m_j|.     

o-quinone methides 2. Stereoselectivity in cycloaddition reactions of o-quinone methides with vinyl ethers

Cycloaddition reactions between vinyl ethers 3 and $\text{o}$-quinone methides 2, thermally generated from 2-hydroxybenzyl alcohols 1, have been studied. The structure and conformational preferences of the 4-substituted 2-ethoxy-(2,3)-dihydro-2$\text{H}$-benzopyrans 4–9 obtained, which show new interesting features, are discussed together with competitive kinetic data. The cycloaddition process is concerted and involves $\text{o}$-quinone methides in the $\text{E}$-configuration. The OEt-$\text{endo}$ transition state seems to be preferred with ethyl vinyl ether and $\text{Z}$-1-propenyl ethyl ether, whereas with $\text{E}$-1-propenyl ethyl ether the stereoselectivity of the cycloaddition process depends on substituents on the methylene group of the starting alcohol 1. These results are discussed in terms of $\text{endo}$ and $\text{exo}$ preference of the propenyl ether methyl group.     

Nucleophilic reactions of fluoroolefins. I. Reactions of 1-(2-tetrahydrofuryl)-pentafluoropropene with alkoxide ions

Unlike unsubstituted perfluoropropene [1,2] 1-(2-tetrahydrofuryl)-pentafluoropropene $\text{1}$ reacts with sodium alkoxides in parent alcohol solutions to give 1-alkoxy-1-(2-tetrahydrofuryl)-tetrafluoropropenes $\text{2}$ as major products and 1-alkoxy-2H-1-(2-tetrahydrofuryl)-pentafluoropropanes $\text{3}$ as very minor products. The yield of adducts $\text{3}$ is, to some extent, increased by the reduced basicity of the alkoxide ion. Results are discussed in terms of the supposed properties of the carbanionic intermediates involved.N.m.r., m.s., and i.r. data for, and some chemical properties of compounds $\text{2}$, viz., further reactions with sodium alkoxides and hydrolysis are described.     

Synthesis and conformational analysis of 68,11-dihalo[5]metacyclophanes

The synthesis of the 8,ll-dihalo[5]metacyclophanes $\text{1b-1d}$ (halogen = chlorine or bromine) from 1,2-dimethylenecycloheptane ($\text{2}$) is described. Spin saturation transfer and line shape analysis of the temperature dependent ¹H NMR spectra showed $\text{1b-1d}$ to occur in two conformations A (85-89%) and B (15-11%, respectively). The conformation of the pentamethylene bridge was fully analyzed from vicinal proton coupling constants which obey the Karplus relation inspite of severe distortions of the carbon valence angles. The thermodynamic parameters of the ground and transition state were determined for the equilibrium between the conformers A and B and are discussed in terms of steric interactions in these highly strained molecules.     

Measurement of dilute metallic impurity concentration in lead by differential scanning calorimetry

The traditional method for purity determination by analysis of the peak shape of the melting transition has included fitting to a linear relation between sample temperature (T) and the reciprocal fraction molten ($\text{1}\text{7}$). This technique, however, necessitates the use of a series of calibrants to determine the proper choice of limits for $\text{1}\text{7}$ as a function of purity. In this paper, a non-linear relation between T and $\text{1}\text{7}$ is developed and applied to the determination of metallic impurities in Pb in the range of a few to 1000 parts per million. The results are found to be independent of the range of $\text{1}\text{7}$ used. The use of differential scanning calorimety in this determination is discussed, and rate effects are also mentioned.     

Nucleosides, 45.-Assignment of glycosylation sites in o-hexopyranosyl-ribonucleosides by 13C-NMR

Evidence is presented that glycosylation of a ribose-OH group in nucleosides results in a significant downfield shift for the appended ¹³C nucleus and smaller upfield displacements for the adjacent carbons, providing an efficient tool for differentiating between 2′-$\text{o}$-, 3′-$\text{o}$- and 5′-$\text{o}$-glycosyl-ribonucleosides. Therewith, the products formed on enzymatic galactosylation of uridine, inosine, and adenosine are unequivocally assigned β(1→3)-glycosidic linkages ($\text{3a}$ – $\text{3c}$).     

Electroanalytical simulations. Orthogonal collocation simulation of fast second-order chemical reactions coupled to an electron transfer with a heterogeneous equivalent formulation

The combination of orthogonal collocation and the heterogeneous equivalent technique is extended to simulate cyclic voltammograms of fast second-order follow-up reactions coupled to an electron transfer at an electrode surface. The $\text{ED}$ (reversible electron transfer with irreversible follow-up dimerization) and $\text{EC}$₂ (reversible electron transfer with irreversible second-order follow-up reaction) models are considered. The non-linear boundary equations are solved numerically. No linear approximation of the concentration profiles is required. The use of non-linear space coordinate transformations is described. Peak potential and peak current function results are compared with literature values and agreement is found. The transition between the second-order $\text{EC}$₂ and the corresponding first-order mechanisms is discussed.