Density and phase diagram of the magnesium–lead system in the region of Mg2Pb intermetallic compound

The phase diagram of magnesium–lead system has been investigated by a new method for phase analysis on the basis of a strong penetrating radiation. The measurements have shown that the standard phase diagram of this system contains inaccuracy in the region of the Mg₂Pb intermetallic compound. New data on the temperature dependences of the solid and the melt densities have been obtained. The density change during the phase transitions has been directly measured.     

Intramolecular interactions and nature of the lowest electronically excited states in compounds modeling the structural unit of lignin. I. Molecular form

Quantum-chemical calculations of the electronic structure of molecules of model compounds of lignin in the ground and electronically excited states have been made by the CNDO/S method. The paper gives results on the energies and strengths of the oscillators of the electronic transitions and on the type of excited singlet and triplet states, shows the main configurations of the HOMOs and LUMOs participating in the transitions and their energies and statistical weights, and gives the distribution of charges and their redistribution on the passage of the molecules from the ground into the excited states. Donor-acceptor interactions in the molecules under investigation are discussed on the basis of the results obtained.Siberian Scientific-Research Institute of Pulp and Board, Bratsk. A. A. Zhdanov Irkutsk State University. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 265–274, March–April, 1988.     

Structure of regecoline

A new alkaloid, regecoline, with mp 312–314°C, has been isolated from the fraction of phenolic-basic alkaloids ofColchicum kesselringii Rgl. growing in the Syrdar'-inskaya region. The structure of this alkaloid has been established on the basis of the results of a study of chemical transformations, spectral characteristics, and mutual transitions with kesselringine.V. I. Lenin Tashkent State University and M. I. Kalinin Turkmen Agricultural Institute, Ashkhabad. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 248–253, March–April, 1985.     

Dependence of the characteristics of the UV spectra of chalcones of the chroman and 1,4-benzodioxane series on their chemical and electronic structures

A study of the UV spectra of chalcones of the chroman and 1,4-benzodioxane series at 293 and 183 K has been carried out, and the bands have been assigned on the basis of quantum-mechanical calculations. It has been shown that the localization of the transitions in most of the molecules on one of their cross-conjugated fragments is stipulated by the localization of several of their highest occupied MO's, which is attributed to specific features of the electronic structure of the CO group. In cases in which the energies of the MO's of fragments which are conjugated by means of a CO group are degenerate delocalization of the transitions takes place. As a result, a discontinuous dependence of the degree of localization of the transitions on the number and positioning of the substituents is observed. This results in partial violation of the rule of the selective displacement of bands upon substitution.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 271–276, May–June, 1989.We thank D.V. Shatkovskaya for discussing the results of this work.     

Mechanism of the reaction of CO with NO on palladium catalysts

A mechanism has been proposed for the reaction of CO + NO + (O₂) on palladium catalysts and the elementary stages of the reaction have been subjected to thermodynamic analysis. A kinetic model of the reaction has been constructed. The model of the reaction CO + NO has been analyzed in comparison with the reaction CO + O₂; the existence of a multiplicity of stationary states and critical transitions in the system CO + NO has been demonstrated. Numerical calculation has explained the accelerating influence of oxygen and the inhibiting influence of CO on the rate of the reaction CO + NO, observed experimentally.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 484–489, July–August 1987.     

High-pressure phase behaviour of the binary system {CO2 + cis-decalin} from (292.75 to 373.75) K

The phase behaviour of the {CO₂ (1) + cis-decalin (2)} binary system has been experimentally studied at temperatures ranging from (292.75 to 373.75) K. Saturation pressures, ranging from (15.9 to 490.5) bar, were obtained using a variable volume high-pressure cell by visual observation of phase transitions at constant overall composition. For this system, no literature data are available and the results obtained in this study reveal the occurrence of vapor–liquid, liquid–liquid, and vapor–liquid–liquid phase transitions in the investigated temperature range. A total of 133 experimental points are reported including bubble points, dew points, liquid–liquid phase equilibria, and coordinates of the three-phase line. The experimental data can be reasonably predicted by the PPR78 model in which the temperature-dependent binary interaction parameter is calculated by a group contribution method.     

Energy structures and electronic-transition wavelengths for trinuclear polymethine dyes

A study has been made on the energy structures and long-wave electronic transitions for linear trinuclear polymethine dyes by the method of additive generating functions. The necessary and sufficient conditions have been established for the occurrence of local levels related to the central nucleus. It is found that the bisvinylene shift is dependent on the structure of the central nucleus in this class of dyes. The relationship fits the available measurements, and it is predicted that there is a positive bridge effect, namely an increase in the bisvinylene shift in trinuclear dyes by comparison with binuclear ones.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 10–21, January–February, 1987.     

Changes in Coordination Number of Co Ion and Fluorescence Spectrum of 1-Naphthol during the Sol-Gel Reaction of TEOS

Changes in coordination number of Co²⁺ and fluorescence spectra of 1-naphthol during the sol-gel transitions of TEOS have been investigated as a function of time. The change in the coordination number of Co²⁺ has been observed as follows. Six-coordinated Co²⁺ decreased quickly within several hours in the first stage of the reaction corresponding to water consumption by initial hydrolysis reaction. Then six-coordinated Co²⁺ increased around gelation due to polycondensation. After the gelation four-coordinated Co²⁺ increased, where isomorphous-replacement of Co²⁺ into the –O–Si–O– networks occurred. In the Co²⁺/1-naphthol mixed system, separate ion pair of 1-naphthol is preferentially coordinated on the six-coordinated Co²⁺ where water molecule(s) plays an important role to geometrical relaxation of excited-state 1-naphthol.     

Homo- and hetero-nuclear chromium(III) complexes with natural ligands. Part 3. Chromium(III)/vanadium(IV) species isolated from the redox systems

Four new heteronuclear Cr^III/V^IV complexes have been isolated from the redox [Cr^III–V^v–L₁–L₂] systems (L₁ = glycine, glutaminic and nicotinic acids, L₂ = cysteine and glutathione). The complexes have been analysed by spectroscopic methods (diffuse reflectance u.v./vis., i.r.) and by FAB mass spectra. A significant bathochromic shift of the d–d Cr^III and V^IV transitions in heteronuclear complexes (d¹–d³) in comparison to the Cr^III homonuclear species (d³/d³) has been related to the interaction of two metal centres. Spectral analyses by the digital filter and band deconvolution methods are presented.     

Investigation of the circular dichromism spectra of bromine-substituted nucleic acid fragments II. Circular dichroism spectra of 5-bromine-substituted pyrimidine nucleotides

Summary The CD spectra of 5-bromine-substituted derivatives of nucelotides of uracil and cytosine, namely the ribo- and 2-deoxyribonucleoside 5-monophosphates, have been obtained in neutral, acid, alkaline, and alcoholic solutions. Three CEs have been recorded in the 220–300 nm region, corresponding to -* transitions in the B_2u, B_1u, and E_1ua bands. It has been concluded that an intramolecular hydrogen bond exists between 0–2 and the 2-hydroxyl in the 5-bromine-substituted ribonucleotides of uridine and cytidine. A transition of the conformational type localized at a pH equal to the pK value of the heterocyclic base has been detected for 5-bromo-UMP and UMP. On comparing the main characteristic of the CD spectra of the compounds investigated with the spectra of the unbrominated analogs given in the literature, it has been concluded that the halogenation of a pyrimidine base in position 5 does not fundamentally change the ratio of the most probable rotational stereomers, and the compounds investigated have the anti conformation in solution.Institute of Molecular Genetics, Academy of Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 204–208, March–April, 1979.     

Thermal Analysis Studies on Human Skin and Skin Barrier Modulation by Fatty Acids and Propylene Glycol

The thermal behaviour of human stratum corneum (SC) with various hydration levels was studied using differential thermal analysis DSC within the temperature range of –130 to 120°C. SC containing 20% water, resembling the intact condition, shows thermal transitions at around –20°C (representing water in skin), –10, 40, 70°C (representing skin lipids), 85°C (representing protein-associated lipids) and 100°C (representing skin protein). Dehydration of SC causes the transitions at –20 and 100°C to be invisible. Lipid extraction followed by dehydration eliminates all transitions. Further hydration produces a transition of water at around 0°C with a huge change in enthalpy. The perturbation effects of penetration enhancers fatty acids (FA) and propylene glycol (PG) were studied using DTA on SC after pretreatment with PG alone and FA/PG. The application of PG alone shifted the transitions at 70 and 85°C to lower temperatures. Additionally, the application to dehydrated stratum corneum removes the transitions at –10°C. Saturated fatty acids, e.g. nonanoic and decanoic acids, exert barely noticeable effects on the thermal behaviour of SC suggesting that they easily mix with the skin lipids. Thermal analysis also revealed that the cis-9- and 13-isomers of octadecenoic acid (monounsaturated fatty acids) form a separate domain containing mostly the pure fatty acids within the SC lipids and suppress the lipid transitions at 70/80°C. Polyunsaturated fatty acids linoleic and -linolenic acids — form separate domains but do not completely suppress the SC lipid transitions at 70/80°C as monounsaturated acids do. This study suggests different ways of perturbation by various fatty acids.This revised version was published online in November 2005 with corrections to the Cover Date.     

Quantum-chemical calculations of the spectroscopic parameters of heteroaromatic compounds containing sulfur and selenium atoms

Conclusions A new single set of spectroscopic parameters of the CNDO method for compounds containing sulfur and selenium atoms, which permits the simultaneous calculation of the ionization potentials and energies of the electronic transitions on a semiquantitative level, has been proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2251–2253, October, 1986.     

Identification of some endogenous metabolites in the leaves of dwarf and tall lines of the cotton plant

The components of cottonplant leaves have been studied. A GC-MS analysis has been made of the secondary metabolites (SMs) in the total extractives of the leaves of cotton plants with different phenotypic characteristics. It has been found that a change in the genotype leads to a change in the composition of the SMs.Deceased.Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 484–489, May–June, 1995. Original article submitted October 31, 1994.     

The pure rotational spectrum of NaCH3 ( )

The pure rotational spectrum of NaCH₃ and NaCD₃ in their states has been recorded using millimeter/sub-mm direct absorption techniques in the 300–510 GHz range. This work is the first gas-phase detection of sodium monomethyl, which was created by the reaction of sodium vapor with tetramethyl tin. Ten rotational transitions were measured for NaCH₃ for the K=0 through K=5 components and, in select cases, up to K=10, and four transitions (K=0–7) for NaCD₃. Rotational constants have been accurately determined for both isotopomers, suggesting a sodium–carbon bond length of 2.30 Å and an H–C–H bond angle of 107.3°.     

Quenching of luminescence of terbium(III) by platinum(IV) complexes

The rate constants (k_q) of the bimolecular energy-transfer reactions in systems consisting of the terbium(III) aquo complex and platinum(IV) complexes have been determined. The influence of the electronic and nuclear factors on the values of the rate constant of the reactions has been considered. It has been shown that the platinum(IV) complexes make up an adiabatic family of quenchers, while the energy-transfer reactions between terbium(III) aquo complexes are characterized by a high degree of nonadiabaticity x -10^–12). Estimates of the energies of the pure electronic transitions to electronically excited states of platinum(IV) complexes which are not observed spectroscopically have been obtained on the basis of the dependence of k_q on the change in the free energy of the energy-transfer process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 618–623, September–October, 1989.     

Mixed metal complexes in solution. Thermodynamic and spectrophotometric study of copper(II)-citrate heterobinuclear complexes with nickel(II), zinc(II) or cadmium(II) in aqueous solution

Summary A thermodynamic study of Cu^II–M^II-citrate (M^II=Ni^II, Zn^II or Cd^II) ternary systems has been performed by means of potentiometric measurements of hydrogen ion concentration at different temperatures (10, 25, 35 and 45°C) and at I=0.1 mol dm^–3 (KNO₃).The different binary and ternary systems involved have been further characterized by visible spectra and by calculating the spectra ( versus ) of all the Cu^II complexes.The thermodynamic data suggest strong entropic stabilization for the species under discussion. As regards the visible spectral characteristics of Cu^II(d-d transitions), the substitution of one Cu^II ion in the dimer [Cu₂(cit)₂H_–2]^4– by Ni^II or Zn^II to form heterobinuclear [CuM(cit)₂H_–2]^4– complexes, gives rise to a change in the visible spectrum.     

Compensation effect in the theory of nonadiabatic reactions

It has been shown in the framework of the theory of nonadiabatic transitions that consideration of the exponential dependence of the electronic matrix element of a transition on the distance between the reactants leads to a compensation effect. An expression for the isokinetic temperature in terms of the activation energy and the characteristic vibrational frequency has been obtained for reactions taking place in the normal region in nonpolar media. The conditions for the observation of the compensation effect and the conditions determining the sign of the isokinetic potential have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 223–226, March–April, 1986.     

Internal rotation and phase transitions in lanthanum tetrahydroborates

Conclusion The investigation carried out shows that in the low-symmetry compouds La(BH₄)₃ and La(BH₄)₃·3THF, as in the high-symmetry MBH₄, phase transitions occur. An unexpected result is the fact that the symmetry of the environment of the boron ions for these phase transitions practically does not change and remains close to cubic. The activation energy for reorientation of the BH _– ⁴ ion (1.2 kcal/mole) is somewhat overstimated, if we start from the difference in the electronegativity of the boron and hydrogen ions (2.0 and 2.15). This may mean that the nature of the barrier to reorientation of the BH _– ⁴ anion in lanthanum tetrahydroborates is connected with the van der Waals interaction of the tetrahedral BH _– ⁴ groups with the environment. From the¹³⁹La NMR data it follows that the coordination sphere of the triply-charged lanthanum is characterized by low-symmetry of the arrangement of the BH_4– and THF (OC₄H₈) ligands, and the La³⁺-(BH₄)^– bond has predominantly ionic character. Consequently, the potential energy of the BH _– ⁴ anions in the lattice is the sum of the strong isotropic ionic component and the substantially weaker anisotropic van der Waals component. In the low-temperature phases, for both compounds we observe structurally nonequivalent positions for the BH _– ⁴ anions, retained in the high-temperature phase. The phase transitions in the studied compounds are connected with a change in the nature of the hydrogen atom distribution. Due to the low height of the barriers, the reorientational mobility sets in even at low temperatures, and in the phase transition region the value of _c reaches 10¹⁰–10¹¹ Hz. For such a high reorientational frequency, there may be substantial effects from correlation rotation of the anions, leading to a change in the hydrogen atom distribution in the structure and an order-disorder type phase transition.Institute of Inorganic Chemistry, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 1, pp. 69–72, January–February, 1988.     

Phase transitions in Langmuir monolayer of long chain attached fluorescein studied by second harmonic generation technique

Optical second harmonic generation (SHG) measurements coupled with Π-A isotherms have been shown to be helpful, for the following and comprehension of orientational orders and phase transitions in Langmuir monolayers. Using the SHG-(Π-A) measurements, monolayers of 5-hexadecanoylaminofluorescein on the water surface were examined by monolayer compression. The phase transitions were noticeably revealed. Dependence of the square root of the intensities polarizations quotient in the molecules surface density, allowed establishing tilting orientation alignment phases. In addition, change in the monolayer symmetry C_∞v→C_2v as it goes through the LE–LC phase transition, was clearly recognized. It was concluded that a possible change in β is taking place due to aggregate formation.