Band structure of nonclassical polymers

It is shown that fully conjugated alternant nonclassical polymers are comparatively stable systems as a result of the considerable delocalisation energy.The energy characteristics of the polymers are strongly determined by the electron spin distribution of the degenerate nonbonding molecular orbitals; the full spin configuration,S>0, is favoured. The spin densities depend on the electron correlation and alternate; this corresponds to a ferrimagnetic state of the polymer at 0°K.     

Novel techniques for plasma diagnostics: Electron paramagnetic resonance and laser-induced fluorescence spectroscopy

Diagnostic techniques for low-pressure, cold plasmas have mostly been limited to emission and mass spectrometry. Herein, the techniques of gas-phase electron paramagnetic resonance and laser-induced fluorescence spectroscopy are briefly reviewed. Particular attention is paid to their attributes which make them good candidates for plasma diagnostic tools. It is found that gas-phase electron paramagnetic resonance can be used to determine and monitor the absolute concentration of a number of important plasma species, e.g., free radicals and atoms. Laser-induced fluorescence can also monitor, with even more sensitivity, but perhaps not so well absolutely, the concentrations of many plasma species, e.g., free radicals, metastable excited states, and molecular ions.     

Copper(I) 3-methylsalicylate,an efficient catalyst for N-arylation of heterocycles under moderate reaction conditions

Copper(I) 3-methylsalicylate mediates N-arylation reactions of both aliphatic and aromatic heterocycles under moderate reaction conditions. Both electron rich and electron poor aryl bromides and a diverse set of N-heterocycles were allowed to react and gave N-arylation products in high yields which demonstrated the efficiency and utility of this catalyst.     

On bridging the gap between extended Hückel and NDO Type LCAO-MO theories

The possibility of incorporating overlap effects as well as electron interaction terms in simple LCAO-MO theories is discussed. A recent suggestion by de Bruijn is criticized and a revised scheme is developed.     

Comments on the electronic properties of cyclopropane,cyclobutane, and cyclohexane

Spectral quantities of cyclopropane, cyclobutane, cyclohexane, and of several derivatives, have been calculated by a semiempirical all-valence electron SCF-CI MO method. In cyclopropane, HOMO is practically localized in the carbon-frame, and LVMO is purely so. In cyclobutane, these two MO's are based on C-H bonds, while cyclohexane holds an intermediate position. Despite the overall similarity-experimental and computed-of the spectra of these molecules, assignments are non-parallel. Like cyclopropane, cyclobutane can extend conjugation, but to a diminished degree; cyclohexane behaves in this respect like an acyclic alkane. An interpretation of this gradation, in terms of the nature of high-lying MO's, is proposed.     

Nature of weak inter- and intramolecular interactions in crystals. 1. The F...O and F...H contacts in the crystal of 2-trifluoroacetyl-5-trifluoromethylpyrrole

The topological analysis of the electron density distribution in the crystal of 2-trifluoroacetyl-5-trifluoromethylpyrrole revealed that the F...H and F...O intermolecular contacts correspond to attractive interactions. The energies of these interactions were estimated from the experimental data and it was shown that these contacts are similar to the C—H...O contacts. Analysis of the deformation electron density revealed that the F...O contacts correspond to transfer of the lone electron pair of a fluorine atom to the antibonding -orbital of the C=O bond.     

Conformation analysis of formic acid. Extended basis set SCF and CEPA calculations

SCF computations using extended DZP and TZP basis sets have been performed to determine the structure of syn and anti formic acid and the transition state for rotation of the OH group. Effects of electron correlation were accounted for by means of CEPA calculations which predict the anti conformer to be 5 kcal/mol and the transition state 14.7 kcal/mol higher in energy than the syn conformer, with probable error estimates of 0.7 kcal/mol and 2 kcal/mol respectively.     

An EELS and EXELFS study of amorphous hydrogenated silicon carbide

A series of thin films of amorphous hydrogenated silicon carbide (a-SiCH) produced by RF plasma decomposition of propane and silane has been studied by electron energy-loss spectroscopy (EELS) and extended energy-loss fine structure (EXELFS) studies. The composition of the films has been determined by EELS and the nearest neighbour spacings have been determined by EXELFS. These results, along with the energy of the plasmon loss peaks, have been compared with the deposition conditions for each film. The results show that for a large gas ratio (C₃H₈/(C₃H₈+SiH₄)) the films have a high proportion of carbon and are similar to a-CH in structure, whereas those films prepared with Y = 0.4 or 0.5 have nearest neighbour spacings consistent with those for tetrahedrally bonded carbon. The films prepared with lowest Y have nearest neighbour spacings similar to those for amorphous silicon carbide. The results for a-SiCH have been compared with the results of EELS and EXELFS of CVD diamond films, amorphous carbon and amorphous silicon.     

The combinatorics of graphical unitary group approach to many electron correlation

The combinatorics of Gel'fand states which are useful in the graphical unitary group approach to many electron correlation problem and spin free quantum chemistry is considered. Using operator theoretic methods it is shown that the generators of Gel'fand states are S-functions.     

Application of the independent electron pair approach to the calculation of excitation energies,ionization potentials,and electron affinities of first row atoms

Excitation energies, first ionization potentials and electron affinities of first row atoms are calculated with a spin-adapted independent electron pair approximation (IEPA) combined with the direct determination of pair natural orbitals (PNOs). To enable comparison with molecular calculations Gaussian basis sets are used which are small enough to be also applicable to molecules. IEPA results for the above mentioned properties are accurate to 0.1–0.3 eV which is almost one order of magnitude better than the corresponding SCF-results. The same accuracy can be expected for molecules in which a localization of the doubly and singly occupied orbitals is possible, for instance for small hydrides. This is supported by the results of calculations on carbon hydrides.     

Molecular orbital calculations based on linear combinations of fragment orbitals

A semiempirical MO method based on localized fragment orbitals has been developed, which is particularly suited for the construction of orbital correlation diagrams for the discussion of the electronic structure of complex molecules in terms of fragments and their interactions. The method allows for the inclusion of experimental ionization potentials and electron affinities of the fragments within the calculation of the Fock matrix elements and may thus form the basis of an interpretation of photoelectron spectra, comparable to the interpretation of UV spectra by means of the MIM method of Longuet-Higgins and Murrell. Several levels of approximation are discussed using the acrolein molecule as an example.     

Calculation of vertical ionization potentials of ketene by perturbation corrections to Koopmans' theorem

The vertical ionization potentials of ketene are calculated by perturbation corrections to Koopmans' theorem. The present results are compared with those from the pseudonatural orbital coupled electron pair approach and the experimental values.     

Electronic structure of unstable intermediates

The geometry of the linear molecule HBO, has been investigated within the restricted Hartree-Fock LCAO-MO-SCF approximation. The calculated bond lengths for the near Hartree-Fock calculation were R(H-B)=2.1913 bohr, R(B-O)=2.2284 bohr. Several one electron properties have been calculated for the minimum energy configuration.     

Staged Development of Modified Silicon Dioxide Films

The hydrolytic generation of SiO₂ films from chlorosilanes or alkoxysilanes is interrupted by incorporating labile organic groups which stop SiO₂ formation at a processable prepolymer stage. The monomers for the prepolymer have electron withdrawing substituents in the -position. The organic groups are removed from the prepolymer at low temperature, extruding ethylene. The formation of SiO₂ proceeds by intramolecular condensation of the electronegative substituents which are now in a hydrolytically unstable bond with silicon and hydroxyl groups or ambient moisture. Films of the prepolymer spun onto silicon wafers are converted into uniform SiO₂-rich films at temperatures between 150–400°C.     

In Situ Crystallization of Bacterial Cellulose III. Influences of Different Polymeric Additives on the Formation of Microfibrils as Revealed by Transmission Electron Microscopy

Effects of polymer additives on the formation of microfibrils of bacterial cellulose have been examined by transmission electron microscopy. Among additives with different degrees of polymerization (DP) or substitution (DS), carboxymethyl cellulose sodium salt (CMC) with DP = 80 and DS = 0.57 is the most effective in producing separate, smaller-size microfibrils. By increasing the concentration of this CMC from 0.1 to 1.5%, the percentage of microfibrils measuring 3–7 nm wide is increased and levels off at around 1.0%. Other polymer additives such as xyloglucan are less effective than CMC in producing microfibrils with smaller sizes and the resulting microfibrils still tend to aggregate. The number of charged substituents and the molecular weight seem to be important factors in the production of highly separate smaller-size microfibrils. The reduction in average microfibril size is well correlated to the decrease in mass fraction of cellulose I in bacterial cellulose crystals. On the basis of these results, the mechanism of the crystallization of celluloses I and I is discussed. The effect of colony types, smooth and rough, on the formation of microfibrils in the presence of CMC is also described.     

Stereochemical characteristics of the ammonia reaction with perfluoro-2-methyl-2-pentenethiocyanate-3: The defining role of the steric factor determined by geometric modeling

Geometric modeling has been used to analyze the stereochemical characteristics of the ammonia reaction with perfluoro-2-methyl-2-pentenethiocyanate-3. This reaction is stereospecific because it results in only one E stereoisomer of 2-aminoperfluoro-4,4-dimethyl-4,5-dihydro-5-ethylidene-1,3-thiazole (3). It is shown that such stereospecificity is determined by a lower steric hindrance in the anion from which the E isomer is formed as compared to that in the anion from which a Z isomer would be formed. Strong steric hindrances associated with the amino group attached to a double bond exclude the heat-dependent cyclization of E-2,4-diaminoperfluoro-4-methyl-2-pentenethiocyanate-3 (4), which would result in the formation of a thiazole ring. The substitution of the fluorine atom at the double exocyclic bond in thiazoline E-3 by an amino group does not occur because of the electron destabilization of the transition state resulting from the repulsion of the unshared electron pairs of the carbanion and the sulphur atom .A. N. Nesmeyanov Institute of Heteroorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 343–349, February, 1992.     

Amino- und Hydroxylaminosubstituierte α-Azidozimtsäureester und α-Aminozimtsäureester durch kathodische Reduktion ungesättigter Nitroazidoester

Cathodic reduction of nitro-substituted azidocinnamic esters on mercury under aprotic and protic conditions was studied. Nitro and azido groups behave as independent entities so that selectively amino- resp. hydroxylamino--azido compounds are obtained, which could not be synthesised by conventional methods. Under aprotic conditions, with added acetic anhydride, medium yields of acylated 4-hydroxylamino- and 4-amino--cinnamic acid derivatives are isolated. Evidence of homogeneous electron transfer from nitroanion radicals to azido groups is given.

     

Solid-State Evidence for Alkali Metal to Arene Pi-Complexation

We report a variety of alkali metal cation- interactions, documented by X-ray crystallography. These include interactions with the neutral arenes benzene, phenol, and indole. We also include structural results for lithium, sodium, potassium, rubidium, and cesium cation- complexes in which the arene has enhanced electron richness owing to an adjacent or integral charge.     

Radiation effect of a high energy electron beam on thin films of saturated and unsaturated fatty acids

Studies have been carried out on electron beam induced polymerization of LB films of saturated and unsaturated long chain aliphatic acids, in an attempt to obtain information about the dose dependence of conversion in relation to molecular arrangement. The results indicate that polymerization of unsaturated aliphatic acid LB films occurred when irradiation was carried out in a nitrogen atmosphere and that the LB films of a disordered state are more sensitive to radiation than tightly packed LB films.