Spin conversion of positronium in paramagnetic solutions studied by age-momentum-correlation method

Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based ⁺ AMOC method installed at the Stuttgart relativistic positron beam facility (E _kin ^e+ 4MeV) offers substantial advantages over the conventional E coincidence technique resulting in a fast data taking due to the unity detection efficiency of the ⁺ scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positronium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e. g., oxidation or complex formation.     

A possibility to study the properties of substances using positronium as the simplest "labeled" atom

A bound state of positron (e⁺) and electron (e^-) (positronium Ps) can be considered as a kind of hydrogen-like atom and the simplest free radical, which has a positron instead of a heavy nucleus. Annihilation g-radiation is in fact a label of this short-lived atom. Therefore, positronium chemistry is radiochemistry of the lightest short-lived labeled atom. Application of positronium chemistry to various chemical problems is essentially dependent on our knowledge of analogies and differences in the formation and chemical conversion of Ps, a atomic hydrogen, and other free radicals. In this review, experimental results of the authors and some other publications are briefly analyzed from this point of view.     

An identity in chemical thermodynamics

In this work we consider the equilibrium state of a multicomponent system in which chemical reactions occur, and its sensitivity to the change of any of the thermodynamic parameters influencing the equilibrium. A general expression is obtained for the sensitivity coefficientn _i/Y, wheren _i is the equilibrium amount of theith chemical species in the system, andY is a thermodynamic parameter such as temperature, pressure, initial amount of a species, chemical potential of a species, etc. We demonstrate that the sensitivity coefficients are linear combinations of contributions originating from certain special reactions, called response reactions, which are defined in the paper.     

Chemical reactions of positronium studied by age-momentum correlation (AMOC) using a relativistic positron beam

Correlated measurements of the individual positron lifetimes (so-called positron ages) and of the Doppler shift of the 2-annihilation photon energy (Age-Momentum Correlation, AMOC) allow the evolution of the positron states to be observed in the time domain. The AMOC technique is thus especially useful when chemical reactions of positronium induce transitions between positron states. The full information contained in the AMOC data may be extracted from the two-dimensional AMOC histogramme, the so-called AMOC relief, by fitting a suitable two-dimensional model function. In this way quantitative information on the spin conversion of positronium by a paramagnetic radical in the systems HTEMPO/methanol and HTEMPO/benzene and on the formation of positron bound states in aqueous solutions of various sodium halides has been deduced.     

From Chemical Processes to Electrochemical Processes: The Key to Minimal Entropy Production

Exothermic chemical reactions exploited in the chemical industry today represent opportunities for the development of fuel cell like devices that can perform these reactions electrochemically. Thereby, the excessive generation of waste heat is avoided and electric power is produced. Moreover, the reactions that exhibit a positive entropy change (S > 0) when performed electrochemically may enable the direct conversion of heat into power to compensate for S. The rapid development of fuel cell technology appears to be an ideal precursor for such new developments. On the other hand, interest form the chemical industry for the development of fuel cell technology can boost the market penetration of conventional fuel cells. In some cases, conventional fuel cell types like the MCFC and the SOFC may be used without change, in other cases maybe only the anode or cathode catalyst materials have to be modified or replaced. New, dedicated electrochemical reactors may be developed. Because of the different reactions, reactants, and products, a new research area for the design of new electrocatalyst lies ahead.     

Metastable ion studies. IX—Thermochemistry and ion structures among fragmentating ions,an electron impact and field ionization investigation

Metastable peak shapes, dimensions and relative abundances have been measured for the fragmentations ions were generated from but 1-ene, cis-and trans- but-2-ene, 2-methylpropene, methylcyclopropane and cyclobutane. Forward and reverse activation energies for the above reactions have been estimated and daughter ion structures proposed. The pathway for the consecutive reaction which generates the metastable peak (m/e 56 → m/e 53) is proposed to be Observations on metastable peaks for fourteen assorted daughter ion fragmentations are also presented and the thermochemistry and daughter ion structures for some of the reactions are discussed. Metastable peaks for the fragmentation of ions generated by field ionization are also described.     

General therodynamic identities for sensitivity coefficients

Expressions are obtained for the sensitivity coefficientsn _i /Y of the equilibrium state of a homogeneous multicomponent system in which chemical reactions occur, and in which ideal behavior of the components isnot assumed;n _i is the equilibrium amount of theith chemical species, andY is a parameter influencing the equilibrium. These therodynamic identities represent generalizations of the results of the same kind, recently reported for systems with ideal behavior [I. Gutman, I. Fishtik and I. Nagypál, J. Math. Chem. 16 (1994) 229]. Also in the most general case,n _i /Y is found to be equal to the sum of contributions originating from certain special equilibria, previously named as Hessian response reactions.     

Mathematical foundation of the Hessian response equilibrium concept

In our previous studies of systems with multiple chemical equilibria we examined sensitivity coefficientsn _k Y Y, wheren _k is the equilibrium amount of thekth chemical species, andY is a parameter influencing the equilibrium. The chemical transformations in the system are described by means of stoichiometrically independent reactions (SIRs). It was shown that the sensitivity coefficients can be expressed in terms of contributions associated with so-called Hessian response equilibria (HEQs). We now prove some fundamental properties of HEQs, among which are the following: 1. The HEQs are independent of the choice of SIRS. 2. The number of linearly independent HEQs is equal to the number of SIRS.On leave from: Faculty of Science, University of Kragujevac, Kragujevac, Yugoslavia.     

Kinetics of recombination,dismutation, and disproportionation reactions involving neutral ketyl radicals and radical anions

The kinetics of fast elementary recombination of neutral ketyl radicals of benzophenone and its four derivatives (BPH?), the dismutation of benzophenone radical anions, the disproportionation between BPH? and stable nitroxyl radicals, ( ), and the electron transfer have been investigated in both individual solvents and binary mixtures of different viscosities. Reaction (1) for unsubstituted BPH in water, water glycerol, and n-hexane is controlled by diffusion with 2k₁ ? k_diff. In aliphatic alcohols and toluene, which form solvation complexes with BPH?, reaction (1) is diffusion-enhanced and activation-controlled, respectively, with 2k₁ < k_diff. In a viscous solvent such as 1-propanol–glycerol mixture (100 ? η ? 450 cP) reaction (1) is diffusion-controlled. Reaction (2) in alkaline 1-propanol and alkaline 1-propanol–glycerol mixture is activation controlled. The rates of reactions (3) and (4) for benzophenone radicals and nitroxyl radicals of the imidazoline series decrease as the viscosity of the water–glycerol and 1-propanol–glycerol mixtures is increased. The reactions are molecular mobility limited; nevertheless, the numerical values of k₃ (k₄) are 2–6 times as small as the corresponding k_diff values due to the low steric factor of the reactions (therefore called pseudodiffusion-controlled reactions). The theoretical estimates of k₃ (k₄) are in good agreement with the experimental results. The elimination of spin forbiddance in the process of radical recombination in viscous solvents is discussed.     

Effect of isotopic composition on free-radical reactions in polyolefins

On substitution of hydrogen by deuterium the radiation yield of free radicals in polyolefins decreases ca. 2-fold. Under ultraviolet illumination (γ < 300 nm) in deuterated polyethylene (DPE), allyl radicals are converted into alkyl radicals (hfs constants a = 3.2 Oe, a = 4.4 Oe); in deuterated polypropylene (DPP), allyl and peroxide radicals are converted into alkyl radicals (a = 3.7 Oe). At 77°K under ultraviolet light in γ-irradiated polyethylene (PE) and DPE, triene, tetraene, dienyl, trienyl, and tetraenyl radicals are formed. Deuterium substitution has no effect on absorption spectra of polyenyl radicals and polyenes. In polyolefins the quantum yields of photochemical reactions of allyl and peroxide radicals are 0.1–1.0. Photochemical reactions of peroxide radicals result in a radical concentration increase of ca. 3-fold. In reactions of peroxide radicals in the dark, a kinetic isotope effect has been observed. Activation energies of substitution reactions of peroxide radicals are in PE, 9.3 ± 0.3 kcal/mole; in DPE, 10.2 ± 0.3 kcal/mole; in PP, 12.7 ± 0.4 kcal/mole; in DPP, 14 ± 0.5 kcal/mole. Possible mechanisms of the effect of polyolefin isotope composition on radical formation by high energy irradiation as well as on photochemical and dark reactions of free radicals are discussed. The effect of the energy released from phototransformations and radiationless deactivation of macroradicals on the migration of free valence is also considered.     

Time Positron Diagnostics of Metal β-Diketonates

The effects of the nature of the metal atom and -substituents in aluminum(III), chromium(III), cobalt(III), and iron(III) acetylacetonates on the lifetime and intensity of a long-lived component in positron and positronium annihilation were studied by the method of positron diagnostics. An addition of a positron to the -system of a chelate ring is supposed, which makes it possible to qualitatively estimate the basicities of the complexes. Annihilation of positrons and halogen-containing positronium -diketonates is accompanied by the formation of a protonated complex; for -organylthioacetylacetonates, annihilation follows the inhibition mechanism.     

Reactions of Cl2P3/2: Absolute rate constants for reaction with H2, CH4, C2H6, CH2Cl2, C2Cl4, and c?C6H12

The absolute rate constants have been measured for several gas-phase chlorine atom-molecule reactions at 25°C by resonance fluorescence. These reactions and their corresponding rate constants in units of cm³ mole^?1 sec^?1 are: The effects of varying the substrate pressure, total pressure, light intensity and chlorine-atom source on the value of the bimolecular rate constants have been investigated for all these reactions. Conditions under which no competing side reaction occurs were established and the reported rate constants were measured under these conditions. For reactions (2), (5), (6), (7), and 8, there is a discrepancy of a factor of two between the rate constants measured in this work and values in the literature; it is suggested that this is due to an error in the previously measured value of k/k upon which the relative measurements in the literature ultimately depend.     

A low cost time-energy correlation apparatus for positronium annihilation

A two parameter correlation system to measure the energy of a thermalizing ortho-positronium as function of its age was constructed. The Doppler broadening of the 2 annihilation line due the quenching ofo-Ps is measured in coincidence with the age of the positronium. We use only one high resolution nuclear ADC for the energy spectrum, together with a self build low resolution ADC for the timing information and an integrated logical unit. In this way, AMOC (Age Momentum Correlation) becomes possible with small extra expenses.     

Reactions of dodecacarbonyltriruthenium with oxadienes

The results of the studies of thermal reactions of Ru₃(CO)₁₂ with oxadienes (4-methylchalcone and benzylideneacetone) have been surveyed. The reactions yield a number of hitherto unknown bi- and polynuclear complexes, whose structures were determined by X-ray diffraction analysis, NMR, and IR spectroscopy. The complexes obtained contain ³-coordinated structural moieties,viz., five-membered chelate oxaruthena rings and/or dihydropyranyl rings. The fundamental differences in the chemical behavior of Ru and Fe carbonyls in the reactions with oxadienes and the differences between the reactions of Ru carbonyls with oxa- and azadienes have been discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 159–166, January, 1995.The authors are grateful to A. S. Batsanov who determined the structure of4a.This work was carried out with the financial support of the Russian Foundation for Basic Research (Projects No. 93-03-4028 and 94-03-08338).     

Neuartige massenspektrometrische Zerfallsreaktionen bei α,ω-disubstituierten Alkanen. 15. Mitteilung über das massenspektrometrische verhalten von stickstoffverbindungen [1]

The mass spectral behaviour of α,ω-disubstituted alkanes and, especially, that of different N-substituted α,ω-diaminoalkanes has been investigated. It was found that the two amino groups which are separated by CH₂-groups can fragment only to a small extent indepently from each other. Yet those fragmentation reactions are predominant in which both functional groups participate. The main reactions of this type are:

• 1 Loss of the N-substituent (R) from the molecular ion, leading to the [M⁺—R]-ions.
• 2 Loss of NH₃, primary or secondary amines from the [M⁺—R]-ion in the case of monodi-, tri- and tetra-substituted diamino compounds respectively.
• 3 α-Cleavage to the non charged nitrogen atom by forming the ions
• 4 S_Ni-type fragmentation.

The mechanisms of these fragmentation patterns were deduced by using D-labelled derivatives, from metastabile peaks and high resolution mass spectrometry. These reactions seem to be typical for disubstituted alkanes.     

HNO,an Intermediate in (Light-induced) Rearrangement Reactions of Nitrosooxy Compounds and Nitrosamines

IR-spectroscopic investigations of light-induced rearrangement reactions of nitrosooxymethane (CH³ONO³), nitrosooxyethane (CH₃CH₂ONO) and N,N-dimethylnitrosamine ((CH₃)₂NNO) in low-temperature rare-gas matrices have established that these molecules are transformed in two photolysis steps to the previously unknown C-nitroso compounds nitrosomethanol (CH₂(OH)(NO)), 1-nitrosoethanol (CH₃CH(OH)(NO)), and methyl(nitrosomethyl)amine CH₂(NO)(NH)(CH₃). Evidence for a similar rearrangement reaction has been advanced for N-Nitrosopyrrolidine which is converted to C-nitrosopyrrolidine . The matrix-isolation technique in combination with wavelength-selective irradiation allowed to trap and characterize an intermediate of rearrangement which revealed to be nitroxyl (HNO) complex (CH₂…HNO, CH₃CHO…HNO, CH₃N = CH₂…HNO, and ). Since these findings have a close resemblance with rearrangement reactions of more complex nitrosooxy compounds, nitrosamines, or nitrosohydrazines used in organic synthesis, it is suggested that also in these reactions nitroxyl is present as an intermediate species.     

Intersequence cyclization in copolymers of N-vinyl carbamates

Monomer reactivity ratios determined for the copolymerization of tert-butyl N-vinylcarbamate (r_A = 0.55 ± 0.05) with phenyl N-vinylcarbamate (r_B = 2.08 ± 0.15) indicate that the monomer units are distributed randomly along the polymer chains. The following sequence of reactions was used to cause intersequence cyclization between tert-butyl N-vinylcarbamate and phenyl N-vinylcarbamate units in the copolymers. The extent of cyclization obtained was in accord with that expected for random copolymers.      

Thermal decomposition reactions of n‐alkylated 2‐aminobiphenyls to carbazole and phenanthridine

Thermal cyclization reactions were examined by passing vapors of N‐alkylated 2‐aminobiphenyls 1a‐c and 2 over calcium oxide at 450‐600°C under nitrogen carrier gas. The reactions yielded 9‐methylcarbazole 3 , carbazole 4 , phenanthridine 5 and phenanthrene 6. The major product for the reactions of 1a, 1b and 2 was phenanthridine 5 while that of 1c was carbazole 4.     

Synthesis of 5,6,7,8‐tetrahydroindolizines via a domino‐type transformation based on the rhodium catalyzed hydroformylation of N‐(β‐methallyl)pyrroles

Variously substituted 5,6,7,8‐tetrahydroindolizines can be easily synthesized via a domino reactions sequence under rhodium catalyzed hydroformylation of N‐(β‐methallyl)pyrroles. The later are readily prepared from properly functionalized pyrroles via phase‐transfer N‐allylation in the presence of 18‐crown‐6 and potassium tert‐butoxide.