Quantum-chemical study of degradation of molecular analogs of butadiene-styrene copolymer fragments during stretching by end atoms

A comparative study of changes in the electronic structure during stretching of molecular fragments by end atoms is performed by using semiempirical quantum chemical method PM3 with geometry optimization for cis- and trans-hexa-3-ene, 3-methyl-1-pentene and sec-butylbenzene, which model butadiene-styrene copolymer units. In contrast to molecules modeling butadiene-1,4 units, the degradation of molecules modeling butadiene-1,2 and styrene units is found to proceed in two stages. At the first stage, when a maximum in the corresponding stress-strain curves is attained, the molecules with side vinyl and phenyl groups do not break down but experience intramolecular structural transition that is accompanied by the isomerization and displacement of double bonds. At the second stage, the molecules degrade.     

Quantum-chemical study of allylic compounds with two bonded heavy atoms

It has been shown by the CNDO method that the bathochromic shift of the long-wave absorption band in the transition from allylstannane to compounds of the type C=C-C-Sn-X and C=C-Sn-X (where X is a heavy atom) is connected with the formation of a low-energy vacant *_S-X orbital, localized mainly in the region of the Sn-X chemical bond, and of an occupied _Sn-X orbital, the energy of which is somewhat higher than of the _C-Sn orbital. The dependence of the position of the long-wave absorbance region on conformation is related to the fact that, in planar and nonplanar conformers, the long-wave transitions are of a different type ( * and *, respectively); the bathochromic shift is determined to a large degree by the difference in the energies of the highest occupied MO ( - ) in the s-trans form. In the nonplanar conformers the heavy atom orbitals interact with the -orbital of the ethylene moiety through the bridge group; this leads to a significant delocalization of the HOMO and to a considerable change in its energy. On the other hand, their interaction with the *-orbital in compounds of the C=C-C-Sn-X type is very low and does not favor the delocalization of lower vacant MO. In vinyldistannane the *-orbital is noticeably delocalized, due to the interaction with the *_Sn-Sn orbital in planar and with the *_Sn-Sn orbital in nonplanar conformers.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 636–641, March, 1992.     

Quantum-chemical modeling of the detachment of hydrogen atoms by the sulfate radical anion

Nonempirical quantum-chemical calculations of the transition states of reactions between the sulfate radical anion and organic compounds of various classes were performed, and the activation energies of the corresponding reactions were calculated. A correlation dependence between the calculated activation energies and the strength of substrate C-H bonds was found. The polarized continuum model COSMO was used to study the influence of solvents on the kinetics of the reactions.     

Quantum-chemical study of aza derivatives of styrylnaphthalenes and their photocyclization products

Quantum-chemical calculation of 1- and 2-styrylnaphthalenes and their aza derivatives 2- and 4-styrylquinolines were carried out by the DFT B3LYP/6-31G* method. The relative stability of the products of their photoisomerization, (E)- and (Z)-isomers and of the photocyclization products, the corresponding derivatives of dihydrophenanthrene (DHP), was calculated. It has been shown that, by the DHP stability, the diarylethylenes in question are divided into two groups in qualitative agreement with experimental data on their photochemical activity in the photocyclization reaction.     

Quantum-chemical study of the structure and reactivity of 4,5-dihydropyrazol-5-ones and their thio and seleno analogs: VIII. Solvation effects and tautomerism of 4,5-dihydropyrazol-5-ones and their thio and seleno analogs

The effect of hydration on the stability of tautomeric forms of 1-methyl-4,5-dihydro-1H-pyrazol-5-one, 1-methyl-4,5-dihydro-1H-pyrazole-5-thione, and 1-methyl-4,5-dihydro-1H-pyrazole-5-selone was analyzed by nonempirical quantum-chemical methods at different theory levels. The results of calculations by all these methods, including density functional theory, with two types of models (continuum and discrete) showed stronger stabilization of the NH tautomers of all the examined heteropyrazolones in water, as compared to their CH and XH tautomers. The strongest stabilization effect is reached in the calculation of discrete complexes in terms of the self-consistent reaction field model. The degree of differentiation of the stability of tautomeric forms considerably decreases when electronic correlation is taken into account, whereas the use of polarization functions on hydrogen atoms in addition to polarization functions on heavy atoms almost does not affect the position of the tautomeric equilibrium.     

Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on various centers in pyrrolo[2,1-b]thiazole (1) has been evaluated. The results of semiempirical (MNDO, AM1, and PM3) andab initio (6-31G^*) calculations were compared. The MNDO and 6-31G^* methods give “chemically proper” and qualitatively coincident results. Analysis of the intramolecular (geometric and electronic) reorganization of molecule1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic reaction, have been recognized. The energy parameters of intramolecular prototropic rearrangements in cation1 and the “blocking” factor value of methyl groups reducting the corresponding complex stability have been evaluated. It has been established that the relative stability of the protonated forms does not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but the range of variations is considerably narrowed in the series S>Se>Te. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1707–1711, October, 2000.     

Quantum-chemical study of the electronic structures and spectral properties of oxazole analogs of 1,4-distyrylbenzene and the isomeric distyrylnaphthalenes

The electronic structures and spectral properties of a number of oxazole analogs of 1,4-distyrylbenzene and the isomeric 2,6-, 1,5-, and 2,7-distyrylnaphthalenes were studied within the framework of the Pariser-Parr-Pople method. The long-wave band of the electronic spectra of these compounds is formed by the S_o S₁ lower electron transition, which has general molecular character and is accompanied by intramolecular charge transfer. Intense electron transitions from fluorescent level S₁ to higher electron-excitation levels are absent over the range of the absorption and fluorescence bands.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 609–615 May, 1980     

Mass-spectrometric study of phenothiasilines and of their derivatives and analogs

Conclusions A study was made of some phenothiasiline derivatives and analogs and the main paths of their fragmentation were established.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1619–1622, July, 1981.     

Quantum-chemical study of the sorption of halide ions by active carbons

Series of the affinity of anions to a carbon matrix similar to the well-known Eisenman series have been obtained in the framework of the SCF-MO-LCAO method in a modified Boyd-Whitehead approximation for the purpose of studying the features of the ionic selectivity of carbon-containing materials. The influence of hydration on the values of the ion-exchange energies has been considered. The theoretical and experimental selectivity series have been compared. The mechanisms of specific and nonspecific anionic sorption have been determined on the basis of an analysis of various energy characteristics and the features of the distribution of the electron density.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 356–361, May–June, 1985.     

Fragmentation study and analysis of benzoylurea insecticides and their analogs by liquid chromatography-electrospray ionization-mass spectrometry

Two insecticides, diflubenzuron and hexaflumuron, and their analogs have been separated by liquid chromatography (LC) and their fragmentation mechanisms were studied by electrospray ionization-ion trap mass spectrometry (ESI-MSⁿ) in both positive- and negative-ion modes. Sequential product ion fragmentation experiments were performed in order to explain the degradation pathways and identify their predominant fragment ions. It was indicated that the characteristic fragmentations are the loss of neutral molecules such as HF, HNO₂, and HCl to form stable ring structure or the cleavage of the acyl amine to form conjugated structure. Furthermore, the separation and determination of two benzoylurea (BU) insecticides and their analogs in the water samples from Weiming Lake have been described by LC-ESI-MS in negative mode. By the use of deprotonated molecule for quantitative analysis at low capillary exit voltage, low detection limits, good linearity and reproducibility for standard solutions were presented.     

Generation of the atrane cations by nuclear-chemical method: Quantum-chemical study

Comparative analysis of equilibrium structures of cations [RM(OCH₂CH₂)₂NCH₃]⁺ (M = Si, Ge; R = F, Me, Ph) and the corresponding neutral compounds HRM(OCH₂CH₂)₂NCH₃ was performed based on quantum-chemical calculations. The possibility was analyzed for expanding nuclear-chemical method to generation of cations having the atrane structure. Quantum-chemical method was applied to estimate the heights of barriers to hydrolysis by the М–О bond in ocanes acting as the sources of the corresponding cations with the atrane structure.     

Indole derivatives 137. Quantum-chemical study of the conformation of some phenylhydrazones and their enehydrazine tautomers

It was shown by the data from quantum-chemical calculations that the phenylhydrazones of certain unsymmetrical carbonyl compounds (methyl ethyl ketone, acetoacetic and levulinic acids) have the transoid conformation in relation to the N-N bond. The relation between the regioselectivity of the Fischer reaction and the structure of the calculated compounds is discussed.For Communication 136, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1038–1040, August, 1991.     

Experimental and theoretical study of the basicity of azoles and their analogs

The thermodynamic ionization constants of 30 imidazole, pyrazole, triazole, and thiazole derivatives and their structural analogs were determined by potentiometric titration in acetonitrile. The effect of individual structural factors on the basicity of the azole molecules is discussed. It is demonstrated that the best theoretical basicity index is the magnitude of the electrostatic energy of interaction () of the unshared pair of the nitrogen atom being protonated with the system of charges. The values were calculated by means of the MO self-consistent-field (MO SCF) method with the Pariser-Parr-Pople approximation and by the simple Hückel method. The pK_a- correlation equations obtained make it possible to predict the pK_a values of azoles with an average accuracy of 0.5 pK_a units for acetonitrile solutions and 0.3 units for aqueous solutions.Communication XI of the series Basicity and Structure of Azomethines and Their Structural Analogs. See [1] for communication X.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 552–557, April, 1972.     

Quantum-chemical study of the structure and reactivity of pyrazol-5-ones and their thio and seleno analogs: X. 1-methylpyrazol-5-one and its thio and seleno analogs in H-complex formation reactions in the gas phase and in solutions

The effects of specific solvation and self-association of chalcogenpyrazol-5-ones are assessed using nonempirical quantum-chemical, density functional theory (DFT), and MP2 second-order perturbation theory methods. The formation of H-complexes with water, methanol, and DMSO stabilizes all tautomeric forms, the NH tautomers of all hetero analogs being the most affected. The NH tautomers form with water 1:2 complexes which reveal cooperativity. The complexes of chalcogenpyrazolones with DMSO are more stable than the respective complexes with water, and, therewith, the extra stabilization in continuum is less pronounced than in the case of hydration. Quantitatively, the effects of tautomer self-association compare with the effects of interaction of chalcogenpyrazolones with proton-donor solvents.     

Regression rates of polymers during burning of their volatile degradation products

Regression rates as functions of air flow rates and oxygen content of the oxidant gas as well as the distance of the polymer surface from the top of the reaction vessel have been investigated for poly(methyl methacrylate), poly(α-methyl-styrene), a polymide, and isotactic polypropylene. The results are discussed from the standpoint of reaction kinetics, considering regression rates and energies of activation for diffusion and degradation.     

Theoretical study of the mechanism of the enantiomerization of sulfuranes and their analogs

Journal of Structural Chemistry -     

Electron microscopic study of wheat straw degradation during composting

We have studied the microbial attack on wheat straw that occurs during the process of composting for growth substrate production for the edible mushroomAgaricus bisporus. Both transmission (TEM) and scanning (SEM) electron microscopy have been used. Samples were taken at all stages of the composting process. They show an ecological succession of microbial attack initially consisting of a predominantly bacterial flora with only few fungi present. Very few microorganisms are present on the surface of dry straw as the starting material. If stable litter (horse manure/wet straw) is used as a starting material, dense bacterial populations are already present. Bacterial numbers increase very rapidly in the initial stages of composting, and then decline, but with an increase in the proportion of spore forms. The initial microbial attack takes place on the straw cuticle and in phloem tissue and then spreads to encompass all the tissue types. Actinomycetes become visible in the preparations as hyphae and/or spores after the end of the first stage of the composting process (day 14). Microbial attack on the straw structure was non-uniform. Even at late stages of composting, areas of noncolonized plant cells are present. Various patterns of degradation of the plant wall were found, including tunnelling, the formation of concave depressions, and wholesale erosion causing fraying of the secondary thickening. Microbial colonies were often seen to be anchored to the plant walls by a polysaccharide layer (glycocalyx). This adhesion may allow greater efficiency of degradation in a localized area, but may also inhibit the rapid spread of the degrading microbial flora. The final material of the composting is heavily degraded, with many of the plant fibers being separated, but it retains a surprising degree of structural integrity.