The analysis of combustion products : Some improvements in the methods of analysis of peroxides

The conditions for the estimation of peroxides by three established, methods have been re-examined in detail. 1. It is shown that the reduction of organic hydroperoxides by stannous chloride (barnard and hargrave method) can be carried out quantitatively and rapidly in alkaline solution. The advantages of this modification are discussed. 2. The exact conditions are described for accurately measuring the concentrations of hydrogen peroxide and of the lower organic peroxides by the thiocyanate method. Amyl alcohol is recommended as a decolourizer for the reagent since the red colour produced by the action of the peroxides is stabilized, permitting accurate spectrophotometric measurements, Using this reagent, 0.03 micrograms of peroxide per ml can be estimated. 3. The limits of the titanous sulphate method of estimating hydrogen peroxide have been determined and a method suitable for the determination of 0.3 micrograms of hydrogen peroxide per ml is described.     

Stable [Pb(ROH)(N)](2+) complexes in the gas phase: softening the base to match the Lewis acid

Experiments have been performed in the gas phase to investigate the stability of complexes of the general form [Pb(ROH)(N)](2+). With water as a solvent, there is no evidence of [Pb(H(2)O)(N)](2+); instead [PbOH(H(2)O)(N-1)](+) is observed, where lead is considered to be held formally in a +2 oxidation state by the formation of a hydroxide core. As the polarizability of the solvating ligands is increased through the use of straight chain alcohols, ROH, solvation of Pb(2+) is observed without proton transfer when R >or= CH(3)CH(2)CH(2)-. The relative stabilities of [Pb(ROH)(4)](2+) complexes with respect to proton transfer are further investigated through the application of density functional theory to examples where R = H, methyl, ethyl, and 1-propyl. Of three trial structures examined for [Pb(ROH)(4)](2+) complexes, in all cases those with the lowest energy comprised of three solvent molecules were directly bound to the central cation, with the fourth molecule held in a secondary shell by hydrogen bonds. The implications of this arrangement as a favorable starting structure for proton transfer are discussed. Conditions for the stability of particular Pb(II)/ligand combinations are also discussed in terms of the hard-soft acid-base principle. Charge population densities calculated for the central lead cation and oxygen donor atoms across the ROH range are used to support the proposal that proton transfer occurs when a ligand is hard. Stability of the [Pb(ROH)(4)](2+) unit is commensurate with a decrease in the ionic character of the bond between Pb(2+) and a ligand; this in turn reflects a softening of the ligand as the alkyl chain increases in length. From the calculations, the most favorable protonated product is, in all cases, (ROH)(2)H(+). The trends observed with lead are compared with Cu(II), which is capable of forming stable gas-phase complexes with water and all of the alcohols considered here.     

Trace analysis for sulphur and phosphorus in aqueous solution by a carbon filament atom reservoir technique

The chemiluminescent emissions of S₂ and HPO are excited around a resistively heated carbon filament in a hydrogen-nitrogen diffusion flame. Detection limits of 5 · lO^-10 g and 4 · 10^-9 g were obtained, respectively, for sulphur and phosphorus for 1-μl samples of sulphuric and phosphoric acid solutions. Interference effects were studied; there are numerous cationic interferences but few anionic interferences.     

Perhaloketones XIX . Derivatives of benzo-N-heterocycles

A study was made of the behavior of imidazole and six benzo-N-heterocyclic compounds upon direct treatment with anhydrous and hydrated hexafluoro- and sym-tetrafluorodichloroacetones. Two of the compounds did not react, and four gave addition compounds. Indole reacted with both ketones in the 3-position. Heterocyclic precursors were made by reacting o-phenylenediamine, o-aminophenol, and o-aminobenzenethiol with hexafluoroacetone hydrate, thus introducing the 2-hydroxyhexafluoro-2-propyl moiety into the ring para to the amino group. A similar product was made from o-phenylenediamine and tetrafluorodichloroacetone. These compounds were then converted to twenty-eight heterocyclic derivatives. Four of the latter underwent haloform cleavage of the hydroxyhexahalopropyl group, a novel, but not widely applicable, preparative approach to the corresponding heterocyclic carboxylic acids.     

Loop-length-dependent folding of G-quadruplexes

Guanine-rich DNA sequences can form a large number of structurally diverse quadruplexes. These vary in terms of strand polarity, loop composition, and conformation. We have derived guidelines for understanding the influence of loop length on the structure adopted by intramolecular quadruplex-forming sequences, using a combination of experimental (using CD and UV melting data) and molecular modeling and simulation techniques. We find that a parallel-stranded intramolecular quadruplex structure is the only possible fold when three single residue loops are present. When single thymine loops are present in combination with longer length loops, or when all loops are longer than two residues, both parallel- and antiparallel-folded structures are able to form. Multiple conformations of each structure are likely to coexist in solution, as they were calculated to have very similar free energies.     

Electron paramagnetic resonance of ultraviolet-irradiated polyolefins

If films of polyolefins are ultraviolet-irradiated at liquid nitrogen temperature, alkyl radicals which can be examined by EPR are produced. No EPR spectra are observed from polyolefins irradiated at room temperature in air. Ultraviolet irradiation of polyolefins containing small alkyl side chains generally produces radicals corresponding to the pendant group or methyl radicals if the side chains contain a methyl branch. For some polymers the radical species could not be identified with certainty. Stabilization studies indicate that an optimum concentration of ultraviolet stabilizer is necessary for maximum stabilization of polyolefins. Preliminary results of EPR studies of the ultraviolet irradiation of various polyolefins are given, and some possible radical species are discussed.     

Unimolecular rate constants for HX or DX elimination (X = F, Cl) from chemically activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: threshold energies for HF and HCl elimination

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).