An alternative convention describing the (n, γ)-reaction rate suited for use in the k0-method of NAA

An alternative convention for use in the k₀-method describing the (n, )-reaction rate upon reactor neutron irradiation has been derived by dividing the cross-section in a (v)=₀v₀/v part and a pure resonance integral, instead of splitting up the neutron spectrum. It describes the (n, )-reactions with the Westcott factor g(T)1 but without resonances below 0.35 eV, and should yield better results for those with resonances below this limit. The resulting formulas are simpler than the ones currently used. An important practical aspect of this new convention is that no irradiations under Cd-cover are needed to determine the parameters to be used in the k₀-method. The parameters determined previously for (n, )-reactions with g(T)=1 can still be used.     

Synthesis ofσ-aryl andσ-carboranyl-π-cyclopentadienyliron dicarbonyl via the aryl and carboranyl compounds of copper

Conclusions A simple method was proposed for the synthesis of-aryl- and-carboranyl--cyclopentadienyliron dicarbonyl via the aryl and carboranyl compounds of copper and BrFe(CO)₂C₅H₅-.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1673–1675, July, 1977.     

51V NMR studies of a system vanadium pentoxide-potassium pyrosulfate

Two states of pentavalent vanadium in the system K₂S₂O₇–V₂O₅ have been found by the⁵¹V NMR method. For state 1 a polynuclear structure with greater distorted local environment compared with V₂O₅ is retained. For state 2 the nearest environment of vanadium differs significantly from V₂O₅, and is probably characterized by the higher ionic nature of the vanadium-oxygen bond.

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⁵¹V K₂S₂O₇–V₂O₅ . V₂O₅. , V₂O₅ , , -.

     

Orbital interactions in anti- and syn- tricyclooctadienes and their homoderivatives

Photoelectron spectroscopy and molecular orbital calculations of the Extended Hückel, MINDO/3 and STO-3G Hartree-Fock type have been applied to anti- and syn-tricyclo[4.2.0.0^2,5]octadiene (1 and 2) and their homo and bishomo derivatives. The resulting ordering of the one-electron levels for 1 and 2 are 7a _g ( ₊), [6b _u (), 5b _u (_–)], 4a _u (), 3a _u () and 7a ₁(₊), 5b ₂(), 6b ₂(_–), 3a ₂(), 4b ₁(), respectively. The present results differ substantially from those previously published.     

Ionization potentials of halides. Substituent effects in Cl-, Br-, and I-centered radical cations

The first vertical ionization potentials (I) of halides HalX (Hal = Cl, Br, I; X is an inorganic or organic substituent) are linearly related to the inductive (_I), resonance (_R ⁺), and polarizability () constants of the substituents X (I = a + b_I + c_R ⁺ + d). As the atomic number of the Hal element in the Hal^·+X radical cations increases, the inductive interaction is strengthened while the polarizability interaction is weakened. Conjugation remains virtually independent of the Hal atom. The resonance _R ⁺-constants of the MX₃ (M = Si, Ge, Sn, Pb) substituents bound to the Hal^·+ radical cation centers were first calculated.     

Decomposition of N2O over CuxMg1?xAl2O4 catalysts

The results show that there is not difference between the catalytic activities of two series of Cu_xMg_1–xAl₂O₄ catalysts, despite a small but significant difference in cation distribution. The activity is mainly affected by the presence of CuO.

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, , , , - Cu_XMg_1–XAl₂O₄. CuO.

     

Thermal analysis of manganese dioxide in controlled atmospheres

TG and DTA curves of-MnO₂ have been obtained in nitrogen, air and oxygen. The reactions MnO₂Mn₂O₃ and Mn₂O₃ Mn₃O₄ were observed in the ranges 450° to 600° and 750° to 1100° respectively, and the decomposition temperatures are affected by the partial pressure of oxygen. The endotherm at 1200° is unaffected by the atmosphere and is not accompanied by weight loss. It is, therefore, due to a polymorphic transformation of Mn₃O₄ rather than formation of MnO. Reaction to form MnO was observed by TG in nitrogen above 1400°, but did not occur on heating in oxygen to 1500°.

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Zusammenfassung Die TG und DTA Kurven von-MnO₂ wurden in Stickstoff, Luft und Sauerstoff ermittelt. Die Umwandlungen MnO₂ Mn₂O₃ und Mn₂O₃ Mn₃O₄ wurden im Temperaturbereich von 450° bis 600° bzw. 750° bis 1100° beobachtet. Die Zersetzungstemperaturen werden durch den partiellen Sauerstoffdruck beeinflußt. Der endendothermische Vorgang von 1200° bleibt von der Atmosphäre unbeeinflußt, ist mit keinem Gewichtsverlust verbunden. Er ist eher einer polymorphen Umwandlung von Mn₃O₄ als der Bildung von MnO zuzuschreiben. Die Bildung von MnO wurde thermogravimetrisch in Stickstoff oberhalb 1400° beobachtet, nicht jedoch in Sauerstoff bis zu 1500°.

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Résumé Etude de-MnO₂ par TG et par ATD, dans l'azote, dans l'air et dans l'oxygène. Entre 450 et 600° on observe la réaction MnO₂Mn₂O₃ et entre 750 et 1100° Mn₂O₃ Mn₃O₄. La pression partielle de l'oxygène influence la température de la décomposition. A 1200°, il apparaît un phénomène endothermique undépendant de la nature de l'atmosphère et qui ne s'accompagne d'aucune variation de poids. C'est pourquoi il est attribué à une transformation polymorphique de Mn₃O₄ plutôt qu'à la formation de MnO. Celle-ci s'observe en TG au-dessus de 1400°, dans l'azote, mais ne se produit pas par chauffage dans l'oxygène jusqu'à 1500°.

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, -MnO₂. , MnO₂ Mn₂O₃ Mn₂O₃ Mn₃O₄ 450°–600° 750°–1100°, , . 1200° . , n₃₄, MnO. MnO 1400°, , 1500° .

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The authors wish to thank Dr. D. Dollimore and Dr. R. C. Mackenzie for suggesting the problem, and the S.R.C. for a research assistantship to D.M.T.     

The activity patterns of the rare earth oxides in heterogeneous catalysis

The catalytic activity of rare earth oxides for simple catalytic reactions show patterns which broadly resemble each other and also reactions in which the number of 4f-electrons changes. If simple concepts of electron transfer are augmented, it emerges that any rate controlling step which involves the adsorption of a species by electron transfer to it, leads to an activity pattern with maxima for f⁰ and f⁷ and f¹⁴ structures.

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, , 4f-. , , , , f¹ f⁸ f⁰, f⁷ f¹⁴ .

     

Diatomic interactions in momentum space. The 2pπ u and 3dπ g states of H 2 + system

The recently proposed method of momentum electron density for interatomic interactions is applied to the two states of the H ₂ ⁺ system. The processes of the attractive 2P _u and repulsive 3d _g interactions are analysed based on the behaviour of the momentum density and Compton profile. The results are compared with the previous ones for the 1 _{S g} and 2p _u states. The guiding principle of contraction and expansion for the energy-density relation in momentum space is shown to be common to both the and states.     

Massenspektrometrische Untersuchung des Substituenteneffektes bei einfach substituierten Benzophenonen

Ionization potentials of benzophenone and six meta- and parasubstituted benzophenones have been measured and correlated withBrown ⁺-constants. The -value of this correlation gives evidence that the ionization process of benzophenones is similar to the ionization process of compounds with one benzene ring: removal of an electron from the -electron system.Appearance potentials of the benzoyl ions and the corresponding six substituted benzoyl ions have been determined and correlated withHammett -constants. The deviations of ions with the substituents m-NH₂ and p-NH₂ from the correlation line are mainly due to the kinetic shift of these ions.For the decomposition of benzophenone ions to benzoyl ions the correlation of metastable-ion intensities withHammett -constants shows a similar -value as the correlation of the intensities of the decomposition in the ion source.

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Mit 3 Abbildungen     

Predissociation lifetime of the 1σ g 2 2σ g diabatic state of He 2 + : a one-channel approach

Summary This work is concerned with the application of a one-channel model to obtaining predissociation lifetimes and transition rates in a system of crossing diabatic states. The calculation focuses on the first shape resonance of the 1 _g ² 2 _g diabatic state of He ₂ ⁺ , which is relatively stable with respect to tunneling. This resonance predissociates as a result of the 1 _g ² 2 _g state being crossed by the 1 _g 1 _u ² dissociative diabatic state near the resonance level. We have estimated its predissociation lifetime to be of the order of 10^–11 s.     

Proton-magnetic-resonance spectra of some organoethoxysilanes

The PMR spectra of 33 organoethoxysilanes have been investigated. Comparison of the chemical shifts of C₂H₅O and CH₃ groups attached to the silicon atom with the values of the remaining substituents at the central silicon atom gives reason to suppose that the contribution of each atom in p– interaction decreases with increased number of ethoxy groups at the silicon atom, while the over-all effect of increased electron density at the silicon heteroatom arising from this interaction increases. The Taft constants for several substituents are calculated.     

The nature of the inductive effect of azinyl groups

The induction constants of azinyl groups are factored into two components, a -induction ( _X ) and a field (_F) component. It is shown that the _X constants of azinyl groups can be thought of as sums of contributions from the phenyl group and the endocyclic nitrogen atoms. A satisfactory correlation is found between the _F constants and the size and direction of the dipole moment of the heterocyclic substituents in the framework of the Kirkwood-Westheimer equation.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 672–676, May, 1987.     

Sigma-polarization in 5-membered heterocyclic ring systems

Extended Hückel calculations on the 5-membered heterocyclic molecules furan, pyrrole, imidazole, pyrazole, 1,2,3-triazole, and the four possible oxadiazoles suggest a significant polarization of the -framework. This -polarization appears to follow simple electronegativity considerations. The calculated -polarizations are independent of, and may be opposed to, the corresponding -polarizations. For furan and pyrrole the above results reproduce the different direction of the experimental dipole moments of these two molecules. A good correlation is observed between the total (+) calculated electron densities and the experimental proton and carbon-13 chemical shifts in all of the systems studied.

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Zusammenfassung Rechnungen mit der erweiterten Hückel-Methode über die Fünferheterozyklen Furan, Pyrrol, Imidazol, Pyrazol, 1,2,3-Triazol und die vier möglichen Oxadiazole lassen eine beträchtliche Polarisation des -Gerüsts vermuten. Die -Polarisation scheint einfachen Elektronegativitätsbetrachtungen zu folgen. Die -Polarisation ist unabhängig von der entsprechenden -Polarisation und kann entgegengesetzt gerichtet sein. Für Furan und Pyrrol reproduzieren die oben genannten Ergebnisse die verschiedenen Richtungen der experimentellen Dipolmomente beider Moleküle. Eine gute Korrelation wird zwischen der gesamten ( +)-Elektronendichte und der experimentellen chemischen Verschiebung für Protonen und C¹³ in allen Systemen beobachtet.

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Résumé Des calculs par la méthode de Hückel étendue portant sur les molécules à cycle pentagonaux: furane, pyrrole, imidazole, 1,2,3 triazole et les quatre oxadiazoles, suggèrent l'existence d'une polarisation significative du squelette . Cette polarisation apparaît en accord avec de simples considérations d'électronégativité. Les polarisations calculées sont indépendantes des polarisations correspondantes et peuvent leur être contraires. Pour le furane et le pyrrole les résultats ci-dessus reproduisent les directions différentes des moments dipolaires expérie mentaux des deux molécules. On observe une bonne corrélation entre les densités totales (+) calculées et les déplacements chimiques expérimentaux du proton et du carbone 13 dans tous les systèmes étudiés.

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This work was supported by a grant from the University of Puerto Rico. It was presented at the Fourth Caribbean Chemical Symposium, January 3–7, 1967, Kingston, Jamaica.The Puerto Rico Nuclear Center is operated by the University of Puerto Rico through a contract with the U. S. Atomic Energy Commission.     

Effect of substituents on ionization potentials of phosphorus compounds. Conjugation in radical cations

The first vertical ionization potentials (I) of phosphorus compounds P(X_i)₃, OP(X_i)₃, SP(X_i)₃, (4-XC₆H₄)₃P, and PCX are related to the inductive, resonance, and polarizability parameters of inorganic, organic, and organometallic substituents X by dependences of the type I = I _H + a_I + b_R ⁺ + c, where I _H is the I value for X = H. The I values are also affected by hyperconjugation. The ratio of the contributions of the resonance (b_R ⁺) and polarizability (c) effects to the I value is determined by the degree of delocalization of the unpaired electron and the positive charge in the radical cations formed upon ionization of neutral molecules. The _R ⁺ resonance parameters of organosilicon, organogermanium, and organotin substituents bound to the P ^·+ radical cation center were calculated for the first time.     

Hydrogenation of CO on thoria supported group VIII (Pd, Pt, Ir) metal catalysts

Comparative studies of the transformation of CO/H₂ mixtures on Pd supported (MgO, Al₂O₃, ThO₂) catalysts reveal the promoting effect of thoria, particularly towards hydrocarbon products.

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CO/H₂ Pd MgO, Al₂O₃ ThO₂ ThO₂, .

     

Exchange kernel of density functional response theory from the common energy denominator approximation (CEDA) for the Kohn–Sham Green's function

A complete and explicit expression for the exchange kernel f _x of density functional response theory (DFRT) is derived in terms of the occupied Kohn-Sham (KS) orbitals _i. It is based on the common energy denominator approximation (CEDA) for the KS Green's function (O. V. Gritsenko and E. J. Baerends, Phys. Rev. A 64, 042506 (2001)). The kernel f _x ^CEDA is naturally subdivided into the Slater f _S ^CEDA and the 'response' f _resp ^CEDA parts, which are the derivatives of the Slater _S and response _resp potentials, respectively. While f _S ^CEDA is obtained with a straightforward differentiation of _S , some terms of f _resp ^CEDA are obtained from the solution of linear equations for the corresponding derivatives. All components of f _x ^CEDA are explicitly expressed in terms of the products _i ^* _j of the occupied KS orbitals taken at the positions r ₁ and r ₂, as well as the potentials of these products at r ₃. The coefficients in these expressions are obtained by inversion of the matrix, associated with the overlap matrix of the products _i ^* _j and _k ^* _l . Terms are indicated, which generate in an external electric field an ultra-nonlocal potential _x , counteracting an external field, and possible approximations to f _x ^CEDA are considered.     

Absorption spectra and bond type in Co(II) aquoamino complexes

Absorption spectra are reported for solutions containing Co(II) and ammonia. Spectra are calculated for five Co(II) aquoamino complexes. The spectra are used to determine Dq and B, parameters of the bonds in these compounds. It is found that Dq and B change nonlinearly and nonadditively. The parameters _c, Dt, Ds, and have also been established for [Co(H₂O)₆]²⁺ and [Co(NH₃)₆]²⁺. The parameter variation can be explained if the H₂O molecules form weaker bonds to the central atom than do the NH₃ molecules, but are capable of d-p interaction.     

Substituent parameters for radical reactions Cerium(IV) — substituted toluenes — acrylonitrile system

Summary In the present study, a new approach for the determination of ', that is, radical substituent parameter, has been described. The technique is based on two equations, one reported byYamamoto andOtsu and the other byStreitwieser andPerrin. The system used for the determination of the radical substituent parameter (') was Ce(IV) — substituted toluenes — acrylonitrile system. To explain inhibition, it has been suggested that the reaction scheme involves initiation of polymerization by Ce(IV) and termination by primary radical of the substrate. The kinetic expression based on the reaction scheme formulated could explain the unusual observation of a negative intercept found in the plot of [M]/R _p versus 1/[M]. Whereas a plot of log /₀ against ⁺ did not give a straight line, a good linear plot with zero intercept was obtained when log /₀ was plotted against (⁺ + '). This proves the validity of ' values which have been determinded.     

Comparative study of measured and critically evaluated resonance integral to thermal cross-section ratios. Part II

The present paper deals with the experimental determination and the critical evaluation of Q_o, the ratio of the resonance integral (I_o) to the 2200 m.s^–1 cross-section (_o) for 57 analytically interesting (n, ) reactions, including a revision and updating of formerly published results for 13 isotopes. Full account is given for deviations from the ideal epithermal neutron flux distribution. The Budapest-Gent cooperative determinations, yielding parallel but independent results, lead to recommended Q_o-values with an average uncertainty as low as 3%. Finally, with the aid of independently evaluated or measured _o-data, a list of recommended I_o-values is given.