A new nuclear spin selective reaction of molecular oxygen

During high-temperature (623–673 K) oxidation of polyarylenes (polypyromellitimide and polyphenylquinoxaline), the molecular oxygen is enriched in the¹⁸O nonmagnetic isotope and impoverished in the¹⁷O magnetic isotope. The isotope selection increases with the increase in the degree of conversion of oxygen. The spin-selective reaction responsible for the selection of the¹⁷O isotope is the addition of molecular oxygen to triplet exited aromatic fragments of macromolecules to give endoperoxides. This reaction, which is selective in terms of the electron spin, is also nuclear-spin selective resulting in a magnetic isotope effect. The selection of nonmagnetic isotopes,¹⁶O and¹⁸O, is caused by competition between the reversible and irreversible decomposition of endoperoxide and by the classical isotope effect in these reactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1402–1405, August, 1994.The authors are grateful to E. M. Galimov and I. V. Nikulina for high-quality isotope analyses and to the Russian Foundation for Basic Research for financial support (grant 93-03-5227).     

Spin catalysis: dynamics of three-spin systems

The kinetic theory of a new phenomenon, spin catalysis, has been developed. This phenomenon takes place in three-spin systems, when the third radical (or a paramagnetic ion) induces spin conversion in the pair, removes spin forbiddance, and thus changes the reactivity of the first two radicals. Kinetic equations for yields of spin catalyzed reactions have been derived. Conditions for spin catalysis, values of the catalytic effect, and the kinetic behavior are discussed. A new kind of quantum beats in chemical reactions is predicted.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1646–1652, September, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-05227).     

Effect of repulsive interaction on the structural and dynamic features of liquid water. The role of molecular polarizability

The effect of variation of the potential function on the structural and dynamic properties of model water when the surface of pair interactions is invariant is studied by the molecular dynamics method. Variations for the rigid and polarizable models are compared. Translated fromZhurnal Strukturnoi Khimii, Vol.40, No. 2, pp. 296–303, March–April, 1999.     

Radical steps in the reactions of organometallic compounds with 2-methyl-2-nitrosopropane: experimental evaluation of the validity of the nitroxyl method for investigations of homolytic reactions of organometallic compounds

Radical adducts formed in the reactions of organometallic compounds of group II-V elements and of some organochromium and organoiron compounds with 2-methyl-2-nitrosopropane and phenyl(tert-butyl)nitrone were studied by ESR. Nitroso compounds and nitrones can be used as selective spin traps for investigations of homolytic reactions involving organometallic compounds.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 916–920, May, 1994.     

Magnetic isotope effect

The electron spin selectivity of radical reactions is accompanied by nuclear spin selectivity, i.e., sorting of the isotopic nuclei relative to their magnetic moment. This property of spin-selective reactions produces separation of magnetic and nonmagnetic isotopes known as the magnetic isotope effect. The chemical physics of this phenomenon is examined along with the conditions and magnitude of its manifestation as well as the prospects for its use in theoretical and experimental chemistry.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 3, pp. 144–152, May–June, 1995.     

Study of the isotope effect in the isotope exchange reaction between PuIV and PuIII by extraction redox-chromatography

The equilibrium isotope effect in the isotope exchange reaction between Pu^IV extracted with tributyl phosphate and Pu^III in an aqueous nitric acid solution has been studied by extraction chromatography. For the 238/242Pu isotope pair the isotope separation coefficient () at 25 °C has been found to be 1.0022±0.0005.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1557–1558, September, 1994.The authors are grateful to Prof. N. N. Krot for discussion of the results and valuable remarks and to V. P. Perminov for help in designing the computer programs.This work was financially supported by the International Science Foundation (Grant MDB 000).     

Sterospecific Syntheses and Spectroscopic Properties of Isomeric 2,4,6,8-Undecatetraenes. New Hydrocarbons from the Marine Brown Alga Giffordia mitchellae. Part IV.

Giffordene (=(2Z,4Z,6E,8Z)-2,4,6,8-undecatetraene; 9f ) and five steroisomers are new C₁₁H₁₆ hydrocarbons from the marine brown alga Giffordia mitchellae. Their synthesis is based on non-stereoselective Wittig reactions of (E)-2-alkenals with appropriate acetylenic phosphoranes and subsequent chromatographic separation of the resulting (E/Z)-pairs. The uniform enynes (>98% purity) are then stereospecifically reduced to (Z)-alkenes with Zn(Cu/Ag) in aq. MeOH at r.t. ¹³C- and ¹ H-NMR data of the new tetraenes are presented. Biosynthetically, giffordene ( 9f ) originates from dodeca-3,6,9-trienoic acid via an unstable (3Z,5Z,8Z)-1,3,5,8,-undecatetraene followed by a thermally allowed antarafacial 1,7-sigmatropic hydrogen shift to the (2Z,4Z,6E,8Z)-isomer 9f .     

Magnetic isotope effect in the photolysis of organotin compounds

Photolysis of organotin molecules RSnMe3 is shown to be a spin selective radical reaction accompanied by fractionation of magnetic, (117,119)Sn, and nonmagnetic, (118,120)Sn, isotopes between starting reagents and products. A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope effect.     

Construction of optimal bases to calculate molecular characteristics of excited states

Ideas are set forth on construction of molecular bases adjusted to both the excited state (ES) involved and the characteristic being determined. Within a single approach this method takes into account the state orthogonality condition and the spin state purity. The potential of the method is demonstrated with respect to the calculation of the energy of doublet ES and polarizabilities of ES of diatomic molecules.Dnepropetrovsk Chemical Engineering Institute. Dnepropetrovsk State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 10–13, July–August, 1993.Translated by L. Smolina     

Pyrroles as C-nucleophiles in reactions with acylacetylenes

The reactions of substituted pyroles with terminal acylacetylenes occur selectively to form 2-(Z/E-2-acylvinyl)pyrroles. When the reactions are performed on the surface of silica gel,C-vinylation is noticeably accelerated to form predominantlyE-isomers. ESR spectroscopy with the use of a spin trap demonstrated the ion-radical character of the process. The structures of the adducts synthesized, which exist asanti-s-cis- andsyn-s-cis-rotomers, were studied by IR, UV, and NMR spectroscopy. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1562–1567, August, 1999.     

Kinetic isotope effects in the reactions of bromine atoms with CH4 and CD4

The competitive reactions of Br atoms with CH₄ and CD₄ were studied over the temperature range of 562° to 637°K. Over this temperature interval, the kinetic isotope effect, k_H/k_D, varied from 3.05 to 2.47 for the reactions The rate constant ratio k_H/k_D, expressed in Arrhenius form, was found to equal (1.10 ± 0.05) exp (1030 ± 60/RT). A comparison is presented between the experimental result and the result obtained theoretically from absolute rate theory using the London-Eyring-Polanyi-Sato (LEPS) method of constructing the potential energy surface of the reaction. The agreement between theory and experiment is very poor, and this is believed to arise from the highly unsymmetrical nature of the potential energy surface involved in these reactions. A comparison is also presented between the k_H/k_D values obtained in the Br + CH₄–CD₄ experiments and the available data on the corresponding Cl + CH₄–CD₄ reactions.     

Structure and spin-purity conditions for reduced density matrices of arbitrary order

For arbitrary k, the separation of spin variables is performed in the reduced density matrix of the kth order (RDM -k) on the basis of the Fock coordinate function method. The independent spatial components of RDM -k are analyzed. For RDM -k of the total spin eigenstate, their number is proved never to exceed its spin multiplicity 2s + 1. Integral and other nontrivial interrelations between spatial components are established which turn out to be the necessary and sufficient conditions of spin purity of a wavefunction corresponding to a given RDM -k. It is shown that the r-rank k-particle spin distribution matrix F, defined as a spatial coefficient at the spin-tensorial operator of rank r in the RDM -k expansion, can be obtained by reduction of the (k + r)-particle charge density matrix F. In particular, all spatial components of RDM -2 are explicitly expressed in terms of the four-electron charge density matrix only. This allows us to purpose some approximative formulas for the McWeeny-Mizuno spin–orbit and spin–spin coupling functions in the case of the weak spin contamination.     

Determination of molecular weight and its distribution of rigid‐rod polymers determined by phase‐modulated flow birefringence technique

The phase‐modulated flow birefringence (PMFB) method is widely accepted as one of the most sensitive and accurate techniques suitable for experimental tests on the molecular theory of polymer solutions. The objective of this study is to develop a systematic method to determine molecular weight and distribution of rigid‐rod polymers by the PMFB technique. Using molecular theory for rigid polymers, birefringence Δn and orientation angle χ have been expressed as a function of molecular weight and distribution. Δn has been shown to be proportional to Σc_iM, and cot 2χ turned out to have a linear relationship with Σc_iM/Σc_iM. From the experimental results for PBLG solutions, birefringence and orientation angle data were in some degree matched with the theory presented. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 509–515, 2000     

Light-scattering Characterization of the Molecular Weight Distributions of Some Intractable Polymers

Recent developments of using laser light scattering (LLS) to characterize the molecular weight distribution f(M) of special polymers such as Kevlar, Tefzel, Teflon, branched epoxy clusters, gelatin, dextran, segment copolymers and polymer mixtures, are reviewed. The basic principle of combining static (classic) and dynamic LLS results is outlined. In dynamic LLS, the line-width (or the translational diffusion coefficient) distribution G(Γ) can be obtained from the precisely measured intensity–intensity time correlation function. The key problem is transforming G(Γ) to a corresponding molecular weight distribution f(M) is to establish a calibration between D (the translational diffusion coefficient) and M. Typical examples were used to illustrate different calibration methods, including the methods of using a series of narrowly distributed polymer standards with different molecular weights, using two or more broadly distributed polymer samples, and one broadly distributed polymer samples plus an additional experimental method (e.g. viscometry or size exclusion chromatography). The advantages and disadvantages of the LLS method are discussed by comparison with size exclusion chromatography. © 1997 John Wiley & Sons, Ltd.     

Incidence of the relativistic corrections in electronic properties of 5d compounds: MS–Xα Calculations on hexachloroiridates (III and IV) and tungsten trioxide

Multiple-scattering–X_α (MS –X_α) calculations have been performed on IrCl, IrCl, and WO clusters. Relativistic calculations have been performed by the inclusion of the approximation of Wood and Boring in the MS –X_α method, i.e., mass–velocity and Darwin terms are taken into account self-consistently, while the effects of the spin–orbit operator are estimated by first-order perturbation theory. The strong contraction of s and p orbitals can be seen on selected displays of radial wave functions and significant changes in the energy diagrams between the nonrelativistic and the relativistic calculations can be obtained. The comparison with photoemission spectra clearly shows the necessity of including relativistic corrections in the calculations. The calculated spin–orbit parameters are in agreement with experiment for core levels, while a strong dependence of the spin–orbit parameters on the one electron energies is pointed out; this dependance is unconnected from the atomic contributions to the molecular orbitals.     

Co‐Conformational Distribution of Nanosized [2]Catenanes Determined by Pulse EPR Measurements

The co‐conformational ensembles of three differently sized [2]catenanes were studied by measuring pair correlation functions corresponding to the separation of nitroxide spin labels—one attached to each of the two macrocycles—with the double electron–electron resonance (DEER) experiment. A geometric model for the [2]catenanes was derived that approximates the macrocycles by circles and takes into account the topological constraint. Comparison of the experimental to the theoretically predicted pair correlation functions gives insight into the co‐conformational distribution and the size of the macrocycles. It was found that the macrocycles of the medium‐ and large‐sized catenanes in chloroform are close to fully expanded, while they are partially collapsed in glassy o‐terphenyl. For the small‐sized catenane, moderate interaction between the unsaturated sections of the macrocycles in chloroform is indicated by a slight overrepresentation of short label‐to‐label separations in the pair correlation function.     

Synthesis and properties of the first representatives of terminal acetylenes in 3-imidazoline-1-oxyl 3-oxide series

Method for the synthesis of m-and p-isomers of 4-[2-(ethynylphenyl)vinyl]-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl 3-oxides by the cross-coupling of 4-[2-(3-iodophenyl)vinyl]-and 4-[2-(4-iodophenyl)vinyl]-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl 3-oxides with (trimethylsilyl)acetylene followed by desilylation was elaborated. The reactions at the CH-fragment of the ethynyl group were performed. The Mannich reaction proceeds with the loss of a spin label, whereas the oxidative homocoupling, with its retention. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2051–2054, October, 2007.     

Stereocomplex Formation from Enantiomeric Polyamides Derived from Tartaric Acid

Summary: The formation of stereocomplexes from the pair of enantiomorphs of the chiral polyamide poly(hexamethylene di‐O‐methyl tartaramide) was investigated for a variety of experimental conditions. DSC and X‐ray diffraction data evidenced that efficiency in enantiomeric association is highly sensitive to the procedure used for preparing the complex. A comparative isothermal crystallization study revealed that the stereocomplex crystallized from the melt at a rate lower than the enantiomerically pure components. The radial growth of individual spherulites was also delayed in the crystallization of the complex. No evidence of stereocoupling was detected for other poly(alkylene di‐O‐methyl tartaramide)s with the alkylene unit length different from six. It was concluded that molecular interlocking of hydrogen bonds in the enantiomeric pair is highly selective in this family of polymers.

Spherulitic growth at 200 °C from the melt.     

ESR molecular dynamics study at the oxidation sites of polypropylene with nitroxyl spin labels and spin probes

Novel methods suitable for the selective insertion of nitroxyl spin labels and spin probes at the oxidation sites of isotactic polypropylene have been developed based on reactions with the oxidation products, on spin trapping with nitroso and nitrone compounds and taking advantage of the spontaneous generation of nitroxyl radicals as intermediates in the stabilization mechanism by the HALS. The analysis of the esr spectra of the doxyl spin label and of the spin probes from the HALS, performed according to the stochastic Liouville theory, has allowed information to be obtained on the molecular mobility at the oxidation sites as a function of the temperature.