Structure and properties of non-classical polymers II. Band structure and spin densities

Theoretical investigation of the band structure of three types of nonclassical polymers, namely alternant (one- and two-dimensional), nonalternant and heteroatomic, are carried out. Although polyradicals, these polymers have a considerable delocalization energy which may determine their relative stability.The spin-density distribution of the alternant type of non-classical polymers corresponds to a ferrimagnetic ground state at 0 K.The non-classical polymers represent a new class of organic systems as their band structure and magnetic properties essentially differ from those of common polymers.     

Crystal and Molecular Structure of Monoammonium Salt of 5‐nitroaminotetrazole

Xray structural investigation of the monoammonium salt of 5nitroaminotetrazole was performed. The crystals are orthorhombic: a = 10.077(1), b = 17.009(1), c = 6.6472(6), V = 1139.33(17)³, space group Pbca, Z =8, _calc = 1.715 g/cm³. Monodeprotonation of 5nitroaminotetrazole during formation of the salt occurs at the N(4) nitrogen atom of the heterocycle. The anion has an almost flat structure; the bond lengths suggest delocalization of electron density in the molecule. The negative charge is distributed among three nitrogen atoms and two oxygen atoms of the anion. Changes in the geometrical parameters of 5nitroaminotetrazole on monodeprotonation are considered.     

Reaction characteristics of cellulose in the LiCl/1,3‐dimethyl‐2‐imidazolidinone solvent system

In order to elucidate the nature of the LiCl/1,3dimethy2imidazolidinone (DMI) solvent system as one of the homogeneous reaction media of cellulose, cellulose acetate (CA) and Omethylcellulose (MC) were prepared using this solvent system, and the distribution of substituents within anhydroglucose units was examined by ¹³CNMR. It was found that (i) homogeneous cellulose solutions can be easily prepared by heating 2, 5–12 and 100 parts of weight of cellulose, LiCl, and DMI, respectively, and (ii) the relative reactivity of hydroxyl groups is in the order C6 > C2 > C3 for both CA and MC. A remarkable feature of this solvent system is that the reaction efficiency in etherification is very high compared with other homogeneous solvent systems.     

High temperature corrosion behaviour of M(Co,Ni)CrAIY-coatings on high temperature materials-microanalytical investigations

In our former investigations, the phenomenon high temperature corrosion (HTC) was described exemplarily on selected materialcoating combinations for blades of stationary gas turbines, mainly in the temperature region of up to 750° C. To answer the question, in which manner higher gas inlet and/or higher material surface temperature would influence the HTC behaviour, a special analytical technique—the integral layer profile analysis—was introduced to ameliorate concentration profile methods. By examining IN 738 LC specimens with CoCrAlY plasma spray coating, stressed by hot gas at 900° C, the limits of this system are shown by explaining the corrosion mechanism; the method is also used for other systems, e.g. U 520 with NiCrAlY coating at 750° C.     

Crystal and Molecular Structure of 2‐nitro‐1‐ureidoguanidine

An Xray structural investigation of 2nitro1ureidoguanidine has been carried out. The crystals are monoclinic: a = 4.4690(2), b = 15.566(1), c = 9.4131(7), = 94.896(5)°, V = 652.4(3)> ³, space group P2₁/n, Z = 4, _calc = 1.650 g/cm³. The molecule consists of two planar fragments: carbamide and nitroguanidine. The geometrical characteristics of the molecule are analyzed. The system of intra and intermolecular hydrogen bonds in the crystal is considered.     

Effect of Oxygen Redistribution on the Normal and Superconducting Properties of Yttrium High‐Temperature Superconductors

An anomaly is found in the vicinity of T_max 560$ K on electric resistance versus temperature curves for yttrium hightemperature superconductors of the 123 phase. Xray phase analysis suggests a structural phase transition in the vicinity of T_max due to oxygen redistribution in the mobile sublattice of this compound. Investigation of samples with Y partially replaced by Ca has shown that the structural phase transition does not depend on carrier concentration in the conduction band.     

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.     

Band structure of even and ions of odd polyenes

The analysis of experimental data for singlet transitions (E _n) of even polyenes (I), cations (II) and anions (III) of odd polyenes show that for infinite chains E (I)/E (II)=E (I)/E (III) = 2:1. It is shown that the energy gap is equal for the three systems. In cases (II) and (III) there is a level (NBMO) in the gap which is vacant in (II) and occupied in (III). That is why the first optical transition in (II) and (III) depends on the semiwidth of the gap.     

The conformational preferences of γ-lactam and its role in constraining peptide structure

Summary The conformational constraints imposed by -lactams in peptides have been studied using valence force field energy calculations and flexible geometry maps. It has been found that while cyclisation restrains the of the lactam, non-bonded interactions contribute to the constraints on of the lactam. The -lactam also affects the (,) of the residue after it in a peptide sequence. For an l-lactam, the ring geometry restricts to about-120°, and has two minima, the lowest energy around-140° and a higher minimum (5 kcal/mol higher) at 60°, making an l--lactam more favourably accommodated in a near extended conformation than in position 2 of a type II -turn. The energy of the +60° minimum can be lowered substantially until it is more favoured than the-140° minimum by progressive substitution of bulkier groups on the amide N of the l--lactam. The (,) maps of the residue succeeding a -lactam show subtle differences from those of standard N-methylated residues. The dependence of the constraints on the chirality of -lactams and N-substituted -lactams, in terms of the formation of secondary structures like -turns is discussed and the comparison of the theoretical conformations with experimental results is highlighted.     

Theoretical study of the electronic structure of diazomethane

The least-energy dissociation path of the ground state of CH₂N₂ was determined fromab initio calculations using in a complementary way basis sets of minimal size (STO-3G) and double-zeta (DZ) quality. The results indicate that the least-energy point of attack of the N₂ molecule on CH₂ (¹ A ₁) is roughly perpendicular to the molecular plane (93 °), the C and N atoms being almost co-linear (angle C-N-N203 ° with outermost N atom pointing away from CH₂). The potential barrier of 1.2 eV found previously on theC _2v dissociation path, disappears completely along the least-energy dissociation path (point groupC _s (out-of-plane)). These findings corroborate the Woodward-Hoffman rules for this process since the outermost orbitals of the two intersecting states found in point groupC _2v (...2b ₁ and ...8a ₁) both correlate to the same irreducible representation (10á) in point groupC _s (out-of-plane).Larger basis set calculations (DZ + polarization functions on all centers, 3d _c and 3d _N developed here), were also carried out on CH₂N₂ (¹ A ₁,³ A ₂ and¹ A ₂) at the¹ A ₁ equilibrium geometry and on CH₂ (³ B ₁) and N₂ (¹ _g ⁺ ) at their respective equilibrium geometries. These calculations, together with consideration of correlation energy differences, yieldD ₀ ⁰ (CH₂N₂,¹ A ₁) = 19 kcal/mole and vertical excitation energies of 67 and 73 kcal/mole for the³ A ₂ and¹ A ₂ states respectively. The latter value is in good agreement with the measured experimental value: 72.4 kcal/mole corresponding to the maximum of intensity in the¹ A ₂¹ A ₁ absorption band.     

Recent progress in tartaric acid‐modified Raney nickel system for enantio‐differentiating hydrogenation

Tartaric acidmodified Raney nickel (TAMRNi) is an enantiodifferentiating catalyst for hydrogenation of ketones. The stereochemical models explaining the enantiodifferentiation of ketoester and 2alkanone were unified as an extended stereochemical model by the experimental supports. Based on this new model, a working hypothesis to improve the enantiomeric excess (ee) of the ketoester/TAMRNi system was developed, and the ee was improved to 96%. By further fine tuning of this system, almost perfect enantiodifferentiation resulting in over 98% ee was achieved.     

Photochemical addition of diethyl ether to β,β,β',β'-tetrakis(trifluoromethyl)divinyl ether

The 1:1 adducts of diethyl and ,,','-tetrakis(trifluoromethyl)divinyl ether (1),i.e., 3,5-(ee)-bis[2,2,2-trifluoro-1-(trifluoromethyl)ethyl]-2,6-dimethyl-1,4-dioxane (2) (3 isomers) and 4-ethoxy-1,1,1-trifluoro-2-trifluoromethyl-3-[3,3,3-trifluoro-2(trifluoromethyl)propenyloxy]pentane (3), have been obtained by UV-irradiation of a solution of divinyl ether1 in diethyl ether. The X-ray structural investigation of the all-(e)-isomer of dioxane (2) has been carried out.Deceased.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 85–88, January, 1994.     

Non-empirical calculations and basis set effects on the inversion geometry of the aniline molecule

The geometry of the amine group and the barrier to internal conversion in aniline have been studied by single-determinantab initio SCF calculations using several basis sets from minimal to double-zeta quality. The results obtained from different types and sizes of basis sets are discussed. Calculations performed with the two most flexible basis sets yield inversion barriers of 0.9–1.1 kcal/mole and angles of pyramidalization at the nitrogen atom of 38–39 ° which are in good agreement with the experimental results (1.3 kcal/ mole and 38 °). Orbital and overlap population analyses are performed and compared with the expected mesomeric and inductive effect. The calculated dipole moment 1.48–1.49 D also agrees with the experimental values (1.48–1.50 D).Dedicated to Professor O. E. Polansky, Mülheim/Ruhr, on the occasion of his 60th birthday.     

Photochromic dihetarylethenes. 7. Synthesis of bis(thienylazoles), photochromic analogs of diarylethenes

Procedures for the synthesis of 4,5-bis[2,5-dimethyl(3-thienyl)]-1,3-azoles based on 1,2-bis[2,5-dimethyl(3-thienyl)]-2-hydroxyethan-1-one, 2-chloro-1,2-bis[2,5-dimethyl(3-thienyl)]ethan-1-one, and 1,2-bis[2,5-dimethyl(3-thienyl)]ethane-1,2-dione were developed. Dithienylethenic compounds in which the thienyl rings are linked through the azole rings exhibit photochromic properties.     

Studies of solvent effects

The effect of water on the conformational preferences of acetylcholine has been studied within the discrete, the continuum and the combined discrete-continuum models described in parts I and II of this series. All the models lead to the conclusion that the trans-gauche form which is, following refined quantum-mechanical computations, the intrinsically preferred one and the one observed in the crystal of acetylcholine and of a number of analogues should remain also the preferred conformation in water. This result agrees with NMR studies. The results of the empirical discrete model used here compare favorably to those obtained by an ab initio super-molecule treatment. The continuum model utilized here represents a net improvement above such models utilized in other works.     

Disordered Crystal Structure of 4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacozane perchlorate

Crystals of the ionic complex (salt) of 4,7,13,16,21,24hexaoxa 1,10diazabicyclo[8.8.8]hexacozane perchlorate, [H₂(Crypt2.2.2)]²⁺ · 2ClO₄ ^-, were synthesized and studied by Xray structural analysis: space group C2/c, a = 20.198(3), b = 10.119(2), c = 12.938(2), = 90.97(1)°, Z = 4, 3030 measured independent reflections, R = 0.067. In these crystals, all atoms of the 2.2.2 dication are disordered over two positions with occupancies of 0.518(4) and 0.482(4). Two conformations of the disordered 2.2.2 dication are such that two H atoms at two nodal N atoms point to its cavity.     

Absolute Calibration of TALIF of Atomic Nitrogen by NO Titration—Experimental and Theoretical Analysis

The method of absolute calibration of the TALIF signal of atomic nitrogen by NO titration in the afterglow of a flow tube reactor has been analyzed using TALIF and emission spectroscopy. An increase in production of atomic nitrogen at titration beginning and an asymmetric parabolic curve for emission of NObands have been observed. Of three possible explanations of these effects the model of wall recombination was favored: it led to fitted wall recombination coefficients referring to wall loss factors, which where in the same dimension as literature values and was able to simulate a hysteresis effect, which has been observed experimentally. According to the observations the titration end point cannot be used to determine the atomic nitrogen density in the afterglow before any NO has been added into the system as the titer produces part of this density itself. But for absolute calibration purposes referring to the wall recombination model the linear decrease of the TALIF signal with increasing titer concentration can still be used as straight line of calibration. Only a correction of the calibration factor by a factor of 0.59 for 5 mbar titrations has to be taken into account as the titer shows wall interaction not only in a catalytic way, but is partly consumed.     

Bestimmung der Aktivitätskoeffizienten im chromatographischen Modell „binärer Lösungen“

An explanation is given for the two possible ways of determination of the activity coefficients of solutes, when applying a new thermodynamic model of adsorption and partition chromatography, i. e. the binary solutions model.

     

A theoretical study of electrophilic reactions

The electronic structures of protonated formyl and acetylium cations and their deprotonation paths leading to HCO⁺, COH⁺ and CH₃CO⁺have been studied by means of ab initio calculations. The results support Olah's theory that dipositive species can be the de facto reagents in electrophilic reactions.