Rate constant of unimolecular decomposition of the radical (CH3)2juvyCCH(CH3)2

The energy of activation of CH ₃ ^. radical rupture from the radical (CH₃)₂juvyCCH(CH₃)₂ is 142.2 kJ mol^–1; the selfcombination rate constant is k_c {(CH₃)₂juvyCCH(CH₃)₂}=10^7.3 dm³ mol^–1 s^–1.

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CH ₃ ^. (CH₃)₂juvyCCH(CH₃)₂ 142,2 /, k_c {(CH₃)₂juvyCCH(CH₃)₂}=10^{7,3 3–1 –1}.

     

Kinetics and mechanism of the oxidation of benzyl alcohol by potassium chlorochromate

The kinetics of the oxidation of benzyl alcohol by potassium chlorochromate, KCrClO₃, has been studied in dimethyl sulfoxide-dichloromethane medium. The reaction is catalyzed by acid. The effects of temperature and solvent composition were studied and activation parameters evaluated. Probable mechanisms are discussed.

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, KCrClO₃, . . . .

     

Square, hexagonal, and row phases of PTCDA and PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag-Si(111)square root(3) x square root(3)R30 degrees surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.     

Surfactant polymer interactions between strongly interacting cationic surfactants and anionic polyelectrolytes from conductivity and turbidity measurements

The interactions between oppositely charged surfactant/polymer mixtures have been studied using conductivity and turbidity measurements. The dependence of aggregation phenomenon on the chain length and head group modifications of conventional cationic surfactants, i.e., hexadecyl- (HTAB), tetradecyl- (TTAB), and dodecyltrimethylammonium bromides (DTAB) and dimeric cationic surfactants, i.e., decyl- (DeDGB) and dodecyldimethylgemini bromides (DDGB), is investigated. It was observed that cationic surfactants induce cooperative binding with anionic polyelectrolytes at critical aggregation concentration (cac). The cac values are considerably lower than the critical micelle concentration (cmc) values for the same surfactant. After the complete complexation, free micelles are formed at the apparent critical micelle concentration (acmc), which is slightly higher in aqueous polyelectrolyte than in pure water. Among the conventional and dimeric cationic surfactants, DTAB and DeDGB, respectively, have been found to have least interactions with oppositely charged polyelectrolytes.     

Density functional calculations of large systems containing heavy elements by means of the regionalization algorithm

The compounds containing transition metals orother heavy elements play important roles in chemistry.They possess diverse properties which have been util-ized in numerous functional materials. Some of theseproperties are also involved in biological processes.However, it is very difficult to study these compoundsdeeply by means of quantum chemical methods, be-cause in these systems usually there are a large amountof electrons, the distribution of the frontier orbitals isdense, the correlation be…     

Preparation of trout liver microsomes for iron speciation in P-450 enzymes by AE–FPLC with ICP–(ORS)MS detection

Cytochromes P-450 are members of a superfamily of hemoproteins involved in the oxidative metabolism of various physiological and xenobiotic compounds in eukaryotes and prokaryotes. The multiplicity of this group of enzymes has been widely studied by chromatographic techniques, mainly high-performance liquid chromatography (HPLC). Because these enzymes are membrane-bound proteins, sample preparation for chromatographic separation of P-450 enzymes requires a solubilization step. The sample-preparation procedures are critical, because detergents affect not only the efficiency of protein solubilization but also their further chromatographic resolution. Trout liver microsomes have been taken here as a model sample to investigate iron speciation in cytochrome P-450. Trouts were treated intraperitoneally with -naphthoflavone, a potent inducer of some P-450 enzymes, and a microsomal suspension containing 7.4±0.1 nmol mL^–1 P-450 enzymes was obtained by ultracentrifugation. Lubrol PX was selected as detergent for solubilization, resulting in about 90% solubilization recovery. The solubilized cytochromes P-450 were further separated by AE–FPLC, with UV detection, or coupled to ICP–MS with an octapole reaction system, ICP–(ORS)MS (monitoring Fe signals at masses 54, 56, and 57). A sampling procedure and chromatographic conditions are developed and were successfully applied to iron speciation in trout liver P-450 enzymes. ICP–(ORS)MS detection of P-450 enzymes is Fe-specific and so will give accurate information on the prosthetic group of the protein, which can constitute an advantageous alternative to classical methods for detection of these hemoproteins.     

Analysis of organic acids in wines by Fourier-transform infrared spectroscopy

Fourier-transform infrared (FTIR) spectroscopy has been a major point of development in many wine laboratories in recent years. It enables almost instant analysis of several properties of wine, usually with very good precision and accurate results. Nevertheless, validation procedures should not be forgotten and should be fully performed. Recovery experiments were performed by spiking wine samples with different amounts of organic acids (tartaric, malic, lactic, acetic and citric—the most prominent in wines). After FTIR analysis of the total acidity and of each organic acid concentration, recoveries were calculated. For total acidity recovery results were, in general, good and very close to 100% (64–111%). On the other hand, for individual organic acid concentrations, the recovery results were lower than 100% (11–73%) for all spiking additions. These results could be explained by spectroscopic interferences between the organic acids. Because they have similar infrared spectra, it is not easy to distinguish between them and, therefore, to achieve accurate calibration. When total acidity, with a different infrared spectrum from the other abundant compounds in the wine, was taken as a single property the recovery results were acceptable.     

Dissociation of Fumaric Acid: Spectrophotometric Investigation in Aqueous Solutions from 10 to 90 <Superscript>∘</Superscript>C and Theoretical Considerations

The dissociation constants of fumaric acid were extracted from UV-vis spectra in the 10–90 ^∘C range. These values were used to extract thermodynamic parameters that showed the temperature effects on the dissociations reactions to be dominantly driven by the solvent. The molar absorption coefficients for the fumaric acid, the bifumarate and fumarate species, can be accurately reproduced with the two-term Gauss–Lorentz equation. Deconvolution of these bands showed strong π–π^∗ transitions for all species and weaker charge-transfer-to-solvent transitions for the charged species. TD-DFT calculations were used to identify the most important molecular orbitals involved in the vertical excitations of the fumaric acid species. The electron population and their states of delocalization were also estimated with topological analyses of the electron density and of the Becke–Edgecombe Electron Localization Function.     

Photochemical cleavage and release of carboxylic acids from alpha-keto amides

In aqueous media, alpha-keto amides LGCH(2)COCON(R)CH(R')CH(3) (1a, R = Et, R' = H; 1b, R = (i)()Pr, R' = Me; 1c, R = Ph, R' = H) with various carboxylate leaving groups (LG) at the C-3 position undergo photocleavage and release of carboxylic acids with formation of diastereomeric 5-hydroxyoxazolidin-4-ones 2a,c in the cases of 1a,c or 5-methyleneoxazolidin-4-ones 3b in the case of 1b. For 1a,b, Phi(photocleavage) = 0.24-0.38, whereas Phi(photocleavage) = ca. 0.05 for 1c. The proposed mechanism involves transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates 4a-c that eliminate carboxylate anions. The resultant imminium ions, H(2)C=C(OH)CON(+)(R)=C(R')CH(3) 5a-c, cyclize intramolecularly to 3b or undergo intermolecular addition of water followed by tautomerization and cyclization to give 2a,c. These inter- or intramolecular trapping reactions of 5 release protons that decrease the pH and cause bleaching of the 620 nm band of the pH indicator, bromocresol green. Determination of the bleaching kinetics by laser flash photolysis methods in the case of 1a gives time constants of 18-137 mus, depending on the leaving group ability of the carboxylate anion, whereas amides 1b show only a small leaving group effect. For 1a, the large leaving group effect is consistent with rate-limiting carboxylate elimination from 4a, whereas the proton release step would be largely rate determining for 1b. Photolyses of 1a (LG = CH(3)CO(2)(-), PhCH(2)CO(2)(-)) in neat CH(3)CN results in carboxylate elimination to form imminium ion 5a, followed by internal return to give aminals.     

Speciation of cyclo(Pro-Gly)<Subscript>3</Subscript> and its divalent metal-ion complexes by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly)3 (CPG3), a model ion carrier peptide. Metal salts (CatXn) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH4Ac). Species detected include [M+H]+, [M+Cat-H]+, [M2+Cat]2+, [M+Cat+Ac]+, and [M+Cat+X]+. The relative stabilities of complexes formed with different cations (Mg2+, Ca2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) were determined from the abundance of 1:1 and 2:1 M/Cat species relative to that of the unbound peptide. The largest metal ions (Ca2+, Sr2+, and Mn2+) formed the most stable complexes. Reducing the buffer concentration increased the overall extent of metal binding. Results show that the binding specificity of CPG3 depends upon the size of the metal ion and its propensity for electrostatic interaction with oxygen atoms. Product ion tandem mass spectrometry of [M+H]+ and [M+Cu-H]+ confirmed the cyclic structure of the peptide, although the initial site(s) of metal attachment could not be determined.