Radiolysis of aqueous solutions of poly(vinyl alcohol) at 77 K

Paramagnetic products of low-temperature X-ray radiolysis of aqueous poly(vinyl alcohol) solutions (2.5 and 5% by weight) were studied by ESR spectroscopy. Experimental spectra were ascribed to a superposition of signals from hydroxyl radicals and –CH₂?^?C(OH)–CH₂? macroradicals (C_α-macroradicals), respectively. No ESR signals corresponding to trapped electrons were observed that was attributed to the peculiarities of microheterogenous structure of the frozen aqueous polymer solutions. Annealing at 115 K resulted in partial conversion of OH radicals to C_α-macroradicals. It was suggested that main part of hydroxyl radicals was stabilized in phase of polycrystalline ice while macroradicals were formed in “mixed” water–polymer phase. The radiation–chemical yields of paramagnetic species stabilized in the systems under study were determined.     

Temperature dependence of scavenger effect on trapped electron photobleaching in 8 M NaOH and 5 M K2CO3 glasses

Upon photobleaching, trapped electrons seem to react with the scavenger molecules without the vertical transition through the conduction band.     

Formation of excited singlet toluene in the radiolysis of alkane containing toluene in the solid phase at 77 K

When alkane containing a small amount of toluene is irradiated by γ-ray or X-ray in the solid state at 77 K, luminescence from excited toluene is observed. The formation of the excited toluene is not due to direct excitation of toluene by γ-ray or Čerenkov light, but to the rapid energy transfer from the irradiated alkane to toluene. The intensity of luminescence from the excited toluene in the cyclopentane matrix does not correspond at all to the amounts of toluene ions, which are formed by pre-irradiation. The energy transfer is not caused by a free hole and an electron, but by an exciton. Furthermore, the effect of additives on the fluorescence suggests that the Wannier exciton or mobile ion-pair is responsible for the energy transfer in cyclopentane containing toluene in the solid phase at 77 K.     

The reactivity of the electron formed in the radiolysis of aerated alkaline aqueous solutions containing tetracycline hydrochloride,at 77 K

The present study deals with the radiolysis of tetracycline hydrochloride dissolved in aerated alkaline aqueous solutions containing 0.1, 0.5 and 1M NaOH, at 77 K, as followed by ESR. The rate constants for the reactions between the electron and physical and chemical traps which are present in these solutions are calculated. These values are k_ph= =9.6·10¹⁵ l·mol^–1·s^–1 and k_ch=1.3·10¹⁰ l·mol^–1·s^–1. The reactivity of electrons that are formed in the radiolysis of water decreases in the following proportions: physical traps: chemical traps: molecules of water: 4.8·10¹⁴: 6.5·10⁸: 1.0. The electrons react preferentially with the solute instead of the solvent.From a thesis submitted by S.M.L.Guedes to the University of São Paulo in partial fulfillment of the requirements for a Doctor of Science Degree in Nuclear Technology.     

Deuterium kinetic isotope effect in the oxidation of sodium (2-D2)valerate with manganate in 3M aqueous NaOH solutions

Sodium (2-²H₂)valerate has been obtained by exchange technique and the kinetic parameters characterizing the oxidation of both sodium valerate of normal isotopic composition and sodium ,-dideuteriovalerate with manganate in 3M aqueous NaOH solutions have been determined. A discussion of the kinetic deuterium isotope effect observed in the oxidation of CH₃CH₂CH₂CD₂COONa is given.     

Photoisomerization of cis-1-(2-naphthyl)-2-phenylethene in methylcyclohexane at 77 K: no hula-twist

The cis-trans photoisomerization of cis-1-(2-naphthyl)-2-phenylethene (c-NPEA and c-NPEB) was studied in methylcyclohexane glass at 77 K. The progress of the reaction was followed by fluorescence spectroscopy. Formation of the extended trans conformer (t-NPEB) is revealed by the growth of sharp vibronic bands on the broad structureless fluorescence spectrum of c-NPE. Principal component analysis with self-modeling of a matrix consisting of spectra generated by irradiation of c-NPE at different excitation wavelengths and spectra of t-NPE measured under the same conditions as a function of excitation wavelength reveals that the diabatic photoisomerization is a completely conformer-specific one-bond twist process: c-NPEB --> t-NPEB. These observations are consistent with observations in solution at ambient temperature. They demonstrate unequivocally that free volume restrictions imposed by the amorphous glassy environment do not open a hula-twist process.     

Photoisomerization of cis-1-(3-methyl-2-naphthyl)-2-phenylethene in glassy methylcyclohexane at 77 K

The cis-trans photoisomerizations of cis-1-(3-methyl-2-naphthyl)-2-phenylethene (c-3-MPE) was studied in methylcyclohexane (MCH) glass at 77 K. The fluorescence spectra of c- and t-3-MPE are excitation wavelength (λ(exc)) independent because the steric requirement of the methyl group restricts the conformational space of each isomer to a single conformer. Photocyclization, the dominant reaction pathway of c-3-MPE in solution, is entirely suppressed in MCH glass at 77 K. The only reaction on 313 nm irradiation of c-3-MPE in MCH glass is cis-trans isomerization. As the reaction progresses, the structureless fluorescence of c-3-MPE is replaced by the vibronically resolved fluorescence of the stable conformer of the trans isomer. The results are consistent with photoisomerization by the conventional one bond twist (OBT) pathway. Previously reported results on the photoisomerization of cis-1-(2-naphthyl)-2-(o-tolyl)ethene (c-NTE) are reinterpreted. Calculated geometries and energy differences for c- and t-3-MPE and c- and t-NTE [DFT using B3LYP/6-311+G(d,p)] are consistent with the interpretation of the experimental results.     

The effect of the antioxidant spermine on the photophysical reactions of tryptophan in aqueous solution at 77 K

Fluorescence, phosphorescence and electron paramagnetic resonance techniques were used to investigate the effect of the antioxidant spermine on the initial photophysical reactions of tryptophan (Trp) in aqueous salt solutions at 77 K. At low concentrations of Trp (3.5 X 10(-5) M) a ground state complex was formed between one Trp and two spermine molecules (a 1:2 complex). Complexed Trp was photodegraded at a rate 65% lower than the free molecule due to a change in the charge-transfer character of the excited 1La state. At high concentrations of Trp (3.5 X 10(-3) M) the phosphorescence was almost completely quenched due to hydrogen-bond formation between two neighbouring Trp molecules. A strong complex was formed between this Trp dimer and one spermine molecule on addition of spermine (a 2:1 complex). Spermine enhanced intersystem crossing in one of the two Trp molecules in the 2:1 complex and phosphorescence was observed. From this triplet state the tryptophyl radical was formed with high efficiency by hydrogen-atom transfer. The yield of radical formation from the triplet state in the 2:1 complex was much larger than from the excited singlet state in the 1:2 complex.     

Reactivity of H. formed in the radiolysis of benzyl alcohol containing tetracycline hydrochloride at 77 K

The radiolysis of tetracycline hydrochloride dissolved in benzyl alcohol has been studied at 77 K by ESR. The H^. and e^– which are formed in the radiolysis of benzyl alcohol at 77 K migrate over a distance corresponding to about 95 and 995 molecules of solvent, respectively, before they are captured by the tetracycline hydrochloride solute. This distance corresponding to H^. is smaller than the distance that it migrates in a neopentane matrix. The migration of H^. in neopentane matrix is more favoured than in benzyl alcohol matrix. When the mole ratio between solute and solvent is 110000, the reactivity of H^. observed by ESR is the following: a) 20% of H^. reacts preferentially with solute because E_H(sin )² < (E_mp)_solvent; b) 80% of H^. reacts exclusively with the solvent in the firsst collision because E_H(sin )² > (E_mp)_solvent. The crystal structure of benzyl alcohol presents inherent factors which do not favour the migration of H^. at 77 K.From a thesis submitted by S. M. L. Guedes to the University of São Paulo in a partial fulfillment of the requirements for a Doctor of Science Degree in Nuclear Technology.     

Selective extraction of Hg(II) from aqueous mineral acid solution containing iodide ions using 2-benzylpyridine in benzene

A method for the selective extraction of mercury has been developed. The extraction of Hg(II) by 2-benzylpyridine (BPy) in benzene from dilute mineral acid solution containing iodide ions has been investigated, and variables such as concentration of acids, iodide and the extractant have been optimized. The optimum conditions for the extraction of Hg(II) by 0.1M BPy/benzene are: 0.01M (HCl, HNO₃, H₂SO₄)+0.01M KI. The distribution coefficients and separation factors of 19 elements relative to Hg(II), have been reported. Effect of anions such as ascorbate, acetate, citrate, oxalate and thiosulfate has also been studied. The method developed could find useful applications in selective extraction of small amounts of mercury from environmental samples.     

Ionophoric binding of alkaline earth metal ions (M2+) by tris-(arylazo oximato)osmium(II) anions, [OsA3]−

Summary High-yield synthetic routes to trinuclear complexes of the type [M(OsA₃)₂] (M = Mg, Ca, Sr or Ba; A = anion of arylazo oxime) by reaction of Na[OsA₃] and M(ClO₄)₂, or of [HOsA₃] with MCO₃, are described. The new complexes have been characterized on the basis of spectroscopic and physico-chemical results. The alkaline earth metal ions are held in an O₆ matrix of two facial [OsA₃]^– units, each behaving as a tridentate (O, O, O) ligand. Quantitative transport of one equivalent of M²⁺ from the aqueous to the organic (CH₂Cl₂) phase can be achieved with two equivalents of [OsA₃]^–. When the aqueous phase is acidified with two mol of H⁺, [M(OsA₃)₂] decomposes into M²⁺ and [HOsA₃], and M²⁺ returns to this phase in its free state.     

Heat capacity, Raman, and Brillouin scattering studies of M2O-MgO-WO3-P2O5 glasses (M=K,Rb)

The authors report the results of temperature-dependent Brillouin scattering from both transverse and longitudinal acoustic waves, heat capacity studies as well as room temperature Raman scattering studies on M2O-MgO-WO3-P2O5 glasses (M=K,Rb). These results were used to obtain information about structure and various properties of the studied glasses such as fragility, elastic moduli, ratio of photoelastic constants, and elastic anharmonicity. They have found that both glasses have similar properties but replacement of K+ ions by Rb+ ions in the glass network leads to decrease of elastic parameters and P44 photoelastic constant due to increase of fragility. Based on Brillouin spectroscopy they show that a linear correlation between longitudinal and shear elastic moduli holds over a large temperature range. This result supports the literature data that the Cauchy-type relation represents a general rule for amorphous solids. An analysis of the Boson peak revealed that the form of the frequency distribution of the excess density of states is in agreement with the Euclidean random matrix theory. The reason of the observed shift of the maximum frequency of the Boson peak when K+ ions are substituted for Rb+ ions is also briefly discussed.