The effect of carboxylate anions on the formation of clathrate hydrates of tetrabutylammcnium carboxylates

The solid-liquid phase diagrams of binary mixtures of water with tetrabutylammonium carboxylate having an unsaturated alkyl group in the carboxylate anion ((n-C₄H₉)₄NOOCR; R=C₂H₃–C₉H₁₇) were examined in order to confirm the formation of clathrate-like hydrates. The results are summarized as follows: (1) the formation of a clathrate-like hydrate is newly confirmed for all the 13 carboxylates examined; (2) these hydrates are classified into three groups I, II, and III on the basis of the hydration numbers; (3) the group I hydrates, which are formed by the carboxylates with R=C₂ and R=C₃, have hydration numbers around 30 and are the most stable hydrates among those examined in this study; (4) the group II hydrates, with hydration numbers around 39, are formed by all the carboxylates with R=C₄ and C₅ including sorbate and are less stable than the group I hydrates; (5) the group III hydrates, with hydration numbers around 30 like the group I hydrates, are formed by carboxylates with long alkyl chains such as 2-octenoate and 2-decenoate and are generally unstable.     

Synthesis and study of the properties of some azomethine compounds

8 azomethine compounds hitherto undescribed in the literature, which are derivatives of salicylaldehyde and amines from the group sulfanilamide, acridine, tolidine, and some other compounds, are synthesized. The acid dissociation constants of the azomethines are determined spectrophotometrically (using the isobestic points method), and by the solubility method. These constants are of the order 10^–7--10^–11.     

Thermochemistry of hydride and phenyl groups on the surface of amorphous silicon dioxide

The thermodynamic properties of hydride and phenyl groups on the surface of amorphous silicon dioxide are investigated in the presented work. The characteristics of the surface silane centers (SiH) are determined from the data obtained by infrared spectroscopy and caloric measurements. The conversions of hydrogen and benzene with the surface are described by thermodynamic calculations at reactions take place in the gaseous phase.To model the reaction between hydrogen and the surface the thermodynamic data for (OH)_4−nSi_n (n=0-4) in the gaseous phase are used. The surface groups and the model molecules are comparable because the thermodynamic characteristics depend only from the local environment.The thermodynamic properties of (OH)₃SiC₆H₅ in the gaseous phase are determined to describe the reaction between benzene and the surface. The predications of these calculations are confirmed by the spectroscopic results. The properties of the surface phenyl groups (SiC₆H₅) are concluded from these data.     

Total syntheses of lichen xanthones : Revision of structures

The preparations of several chlorinated derivatives of norlichexanthone 6a by unambiguous methods are described. The ¹H NMR spectra of these compounds are discussed and several structures previously assigned for lichen xanthones are questioned. The suggested revisions are summarized in Table 3.     

Triterpene glycosides of plants of the family Caryophyllaceae

In this review, tables of 25 glycosides are given and the previously unknown structures of 11 of them are established. Characteristic features of the glycosides of the family Caryophyllaceae are considered. Methods are shown for isolating the glycosides and, using acanthophylloside B — a compound having carbohydrate chains of complex structure — as example, methods of determining their structures are demonstrated.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 417–433, July–August, 1981. Original article submitted May 6, 1981.     

Effects of adjacent groups of benzimidazole on antioxidation of polybenzimidazoles

The chemical oxidative stabilities of poly(2,2-(m-phenylene)-5,5-bibenzimidazole) (PBI-ph), poly(2,5-benzimidazole) (ABPBI), poly(2,2′-hexyl-5,5′-bibenzimidazole) (PBI-hex), and poly(2,2′-imidazole-5,5′-bibenzimidazole) (PBI-imi) are studied. By means of FTIR and ¹HNMR analysis, more information about the degradation process of PBI-ph is found as: CH₂ groups are left in the residual polymers; after the N-H bond and the trisubstituted benzene ring are oxidized by oxidative free radicals, the meta-phenylene is relatively stable. Through Fenton tests, the chemical oxidative stabilities of these PBIs are compared and results show that PBI-ph is the stablest material while PBI-imi is the unstablest one. Through FTIR analysis, the structure changes to those degraded PBIs are compared. The conjugated structure formed between meta-phenyl and benzimidazole can protect the main chain of PBI-ph from the attack of oxidative free radicals. Additionally, effects of acid on PBI-ph degradation rate are evaluated and the results show that phosphoric acid can slow down the chemical oxidative degradation.     

Structure of molecular nanoparticles of petroleum asphaltenes

Ab initio RHF/3-21G** and MM+ molecular mechanics method are employed to calculate the structural chemical parameters of molecular nanoparticles of petroleum asphaltenes. It is found that naphtheno-aromatic rings have a non-planar cup-like character. The values of dihedral angles α ₁ between the planes of aromatic and naphthene rings, which are calculated by the RHF/3-21G** method, are within 156–163°. The values of dihedral angles α₂, which were calculated by the molecular mechanics method, are within 156–164°.     

Changes of vibrational lifetimes with minor structural modification of small polyatomic molecules

Substantial changes of population lifetimes of CH-stretching modes are observed when two atoms are exchanged in CH₂CCl₂ to form trans CHClCHCl and when three deuterons are substituted in C₆H₆ to form 1,3,5.-C₆H₃D₃. The measured lifetimes are in good agreement with estimates based on Fermi resonance-mixing which is inferred from infrared and Raman spectra.     

Are kinetic parameters of non-isothermal thermogravimetric degradation of polymers unequivocal?

Due to the complex character of the thermal degradation of polymers as a solid-gas chain reaction, an unequivocal kinetic characteirzation is possible only for stationary states of both radical concentration and reaction mechanism. These conditions are hardly realizable in non-isothermal thermogravimetry. Additional the weight losses are depedent on the volatility of the reaction products. That is not always certain in polymer degradation. As a consequence the deduced ‘kinetic parameters’ are not unequivocal. They are conversion and heating rate dependent and may be influenced by sample shape and size. Thus the ‘kinetic parameters’ are in fact from the point of view of mathematics the fitting parameters of a ‘rate equation’ like relation, specific for the used reaction conditions only. From the point of view of chemical kinetics they are neither attributable to a determined reaction mechanism nor can they be used for predictions. Dedicated to the 70^th Anniversary of Dr. Jo Flynn     

NMR studies of the electronic structure of coumarins

¹H and¹³C NMR spectra of a series of coumarin dyes used as active laser media are studied. Electron densities and ring currents are calculated. Formulas for calculating¹³C NMR chemical shifts from quantum mechanical data for aminocoumarins are suggested. NMR data and electronic structure calculations are used to explain the generation properties of coumarins. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 495–502, May–June, 1997.     

The effects of errors in measurements of absorbance and time on the calculated value of a pseudo-first-order rate constant

When data on the variation with time of the absorbance of a reactant or product are used to evaluate the rate constant of a first- or pseudo-first-order reaction, the precision of the result depends on the precisions with which both the time and the absorbance are measured. The natures of the dependences, and the ways in which they are affected by both constant and linearly varying background absorbances, are examined. If the standard error σ_t of a measurement of time is below about 0.005 t_{$\text{1}\text{2}$}, the standard error of the rate constant is virtually identical for an experiment in which the concentration of the reactant is followed as for one in which the concentration of the product is followed, but for larger values of σ_t it is better to follow the concentration of the reactant. In any event, errors in the measurements of time are much more likely to be significant than they are usually assumed to be. Other sources of error in such experiments, including constant and time-dependent background absorbances, are examined more briefly, with emphasis on the requirements that should be satisfied in work of the highest quality.     

The polarography of amidines

Summary The polarographic properties of some aliphatic, arylaliphatic, phenoxyaliphatic and aromatic amidines have been investigated. Aliphatic and arylaliphatic amidines are not reducible in the range 0 to –1.8 volts versus the saturated calomel electrode. Phenoxyacetamidines are reduced to the phenol and acetamidine. Aromatic amidines are reduced to the benzylamine and ammonia. The half-wave potentials of both reducible classes of compound are independent of Ph over the range 7–9 Ph.     

Experimental study of electron impact excitation of multiply charged ions

We report on measurements of angular differential cross sections for the excitation of multiply charged ions by electron impact. An ion beam is crossed by an electron beam; electrons which are inelastically scattered at different angles are identified by their energy loss due to the excitation process. Absolute excitation cross sections are obtained by comparing the signals of the elastic and the inelastic electron-ion scattering. Results obtained for the 3s→3p excitation of Ar⁷⁺ are discussed.     

On the synthesis and isolation of chlorocarbazoles obtained by chlorination of N-substituted carbazoles

Chloro derivatives of N-methylcarbazole ( 1 ), N-phenylcarbazole ( 2 ), N-acetylcarbazole ( 3 ), N-benzoylcarbazole ( 4 ) and 2-methoxy-N-methylcarbazole are synthesized. They are compounds 1a, 1b, 1c, 1d, 1e, 2a, 2b, 3a, 3b, 3c, 3d, 4a, 4b, 5a, 5b, 5c, 5d and 5e . Some of them are described for the first time. By using semiempirical PM3 method theoretical substituent effects on the chlorinating reaction are calculated. A chlorination mechanism of carbazoles and N-substituted carbazoles are compared.     

溴化钾团簇相变的分子动力学研究

众所周知，团簇的尺寸介于原子或分子与大块物质之间。由于它有较大的表面／体积比而有独特的物理化学性质。团簇在成核，晶体生长，材料科学和纳米器件加工等领域起了至关重要的作用。近年来，计算机模拟已成为物理，化学，生物，天文，地质等领域的重要研究工具。计算机模拟不仅可以     

Potential energy curves of several excited states of the Ne2* excimer: Assignment of the transient absorption spectra of the excimer

Most of the excited states of Ne₂, which are correlated with the Rydberg state transitions 2p → 3s, 3p, and 4s of Ne, are studied by ab initio CI calculations. Two transient absorption spectra from the lowest excimer state Σ_u⁺ recently observed by Arai et al., are discussed on the basis of calculated potential energy curves. Possible assignments are presented. The calculated transition energies are in good agreement with the observed ones.     

Reactivity of fluoroalkanes in reactions of coordinated molecular decomposition

Experimental results on the coordinated molecular decomposition of RF fluoroalkanes to olefin and HF are analyzed using the model of intersecting parabolas (IPM). The kinetic parameters are calculated to allow estimates of the activation energy (E) and rate constant (k) of these reactions, based on enthalpy and IPM algorithms. Parameters E and k are found for the first time for eight RF decomposition reactions. The factors that affect activation energy E of RF decomposition (the enthalpy of the reaction, the electronegativity of the atoms of reaction centers, and the dipole–dipole interaction of polar groups) are determined. The values of E and k for reverse reactions of addition are estimated.     

Chemisorption of microimpurities of nitrogen(II) and (IV) oxides in the presence of an excess amount of oxygen

Chemisorbents and chemical absorbents synthesized by impregnation of a γ-Al₂O₃ substrate with active components in the form of transition metal oxides and potassium salts of metallic acids are recommended for application. The sorbents are intended for neutralization of microconcentrated nitrogen mono- and dioxides from an oxygen-enriched medium. The synthesis parameters and physicochemical properties of the sorbents are presented and the conditions of the most effective operation of each type of sorbents are specified.     

Qualitative and quantitative measurements of radioiodines

Some methods for measurements of radioiodines are discussed. Three isotopes of iodine are presented in detail;¹²⁵I,¹²⁹I and¹³¹I. Limited discussions of¹²³I and¹²⁶I are also given. Measurements of these isotopes are examined for both the NaI(TI) and liquid scintillation systems. The properties of the decay schemes are utilized to explain the mode of measurements of the isotopes in these two types of detectors. Methods of standardization of the radioiodines are discussed. Special emphasis is given to the direct standardization of¹²⁵I as compared to so called “Mock¹²⁵I” standards.     

Alternate multi-layered adsorption of macro-cations and -anions on the colloidal spheres. Influence of the deionization of the complexation mixtures with coexistence of the ion-exchange resins

The alternate multiple adsorption layers of macrocations and macroanions on the surfaces of colloidal spheres, in which the complexation mixtures are deionized with ion-exchange resins are studied with help of the electrophoretic light-scattering, dynamic light-scattering and transmitted electron-microscopy techniques. The results are compared with those without resins. Colloidal silica spheres (110 nm in diameter) and monodispersed polystyrene spheres (220 nm) are used as colloidal spheres. The macrocations used are poly (4-vinyl-N-n-butyl pyridinium bromide) and poly (allylamine hydrochloride). Sodium poly (styrene sulfonate) and sodium polyacrylate are used as macroanions. The macroion-colloid complexations are formed firmly when the complexation suspensions are deionized with the resins.