Telomerisation de l'isoprene en catalyse redox: Influence de la nature du telogene

There has been study of the redox telomerization of isoprene with various telogens (chloromethanes and functional telogens) using FeCl₃ as catalyst. The telomers have been identified. The reactivities of the telogens have been compared for fixed operating conditions.     

Telomerisation par catalyse redox—III. Etude de la reactivite des telogenes du type r-cc13, selon la nature du substituant r vis-a-vis du styrene

Among the various chloromethanes and chloroethanes, only compounds of the type R-CCl₃ are reactive telogens for styrene in catalysed telomerizations of the type due to Vofsi. We have found evidence for a new type of breakdown of CCl₃Br to CCI₂Br and CI in addition to the known breakdown to CC1₃ and Br'. A Hammet equation has been established for the telomerization involving styrene and telogens of the type RCCl₃. Excellent correlation has been found between the logarithm of the reactivity of a telogen and the Taft constant of the group R.     

Formation of C1/C2-bromo-/chloro-hydrocarbons by haloperoxidase reactions

Summary We report here the first results of a study of the enzymatic formation of halogenated C₁/C₂-hydrocarbons from natural, biochemically relevant molecules using enzymes such as chloroperoxidase (CPO) or horseradish peroxidase (HRP). As halogen sources KBr, NaCl or a sea salt solution were utilized. The haloform reaction was used as a subsequent reaction to yield the halogenated methanes and ethanes from the halogenated substrate molecules. The products of the incubations were analyzed by HRGC/ECD. With these in vitro reactions we attempted to investigate possible biochemical pathways for the formation of some volatile halogenated organic compounds which are assumed to be of biogenic origin. Most of the incubations with KBr formed bromoform as the main product together with dibromo-chloro-methane and 1,1,2,2-tetrachloroethane as by-products. Incubations with NaCl result in chloroform as the main product in analogy to the formation of bromoform. The reaction with sea salt yields no major product but a spectrum of halogenated C₁/C₂-hydrocarbons. Incubations of a water extract of algae meal without any further source of Cl^–/Br^–-ions yield CHCl₃ as the main product and other mixed halogenated methanes as side products. Blank reactions were carried out without enzyme, H₂O₂ or substrate to show that the products of the incubations are formed enzymatically and to exclude the possibility of normal hypohalogenic reactions of the halide, H₂O₂ and the substrate.     

Substituent effects in second row molecules: Molecular orbital studies of phosphorus(III) compounds

Substituent effects in directly bonded P(III) compounds are investigated by ab initio MO calculations of relative energies and the results compared with those for the corresponding nitrogen species. The investigation covers substitution by X = BH₂, CH₃, NH₂, OH, F in PHX^-, PH₂X, and PH₃X⁺ series molecules with some attention also to PX₃ and PX₃H⁺ species. Except for compounds containing the π-acceptor substituent BH₂, σ-interactions dominate substitution behaviour but the second row species tolerate electron withdrawal better than their first row analogues, the severe destabilization of NH₂X and NH₃X⁺ by σ-electron withdrawal being absent from PH₂X and PH₃X⁺. In contrast to the σ-withdrawing NH₂ group, the PH₂ group is characterized as a mild σ-donor. PH^- is a σ-donor and PH⁺₃ a σ-acceptor. π-Bonding to the second row atom is an important means of maintaining electroneutrality in the PH₃X⁺ series, where dπ functions have a bigger role than pπ functions.     

Empirical relations between σI, σoR and taft σ* values of alkyl,acyl, benzoyl and substituted phenyl groups

The X(X) values¹ of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ_I and σ^o_R according to the regression equation

and η*_R=η(X)?η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*]^X_R=αη^X_R where α equals ?10.6 and ?10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*]^X_Rand η^x_r represent Taft σ* and [σ_I(X)?σ_oR(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ_I(X) and σ^o_R(X) according to the equations v(OH) = ?423.0 σ_I(X) + 3654.7 v(OH) = ?270.0 σ⁰_R(X) + 3586.7 where X = Ph, PhCO, CH₂=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp² hybridized carbon atom are proportional to σ⁰_R according to the equation σ*(X) = 3.97 σ⁰_R(X) + 1 New σ_I σ^o_R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.     

The ionization constants of 2‐substituted 4,6‐diamino‐s‐triazines: The applicability to the hammett and taft equations

The ionization (or basicity) constants (pK_b) were determined for many 2‐substituted 4,6‐diamino‐s‐tri‐azines ( I ) by means of the electrometric titration. I includes 2‐alkoxy or aryloxy‐( Ia ), 2‐alkyl‐ or 2‐aryl‐( Ib ), and 2‐alkylamino‐ or 2‐arylamino‐4,6‐diamino‐s‐triazines ( Ic ). For the series with the same alkyl or aryl group, the order of the basicity was found to be Ic < Ib < Ia . A study was made of relationships between the pK_b, values of I , and the substituent constants, σ_p, σ_m, σ_p⁺, σ_m⁺, σ_p^O, σ_m^o, σ_I, σⁿ, and σ*. The Hammett relationships were observed between the pK_a values of I, and the substituent constants σ_m, (or the combination ones, [0.97σ_m + 0.03σ_p] as well as another [0.77σ_I + 0.23σ_R]). The Taft relationships were also found between the pK_a values of Ia , Ib , and Ic and the constants σ_*, respectively. Furthermore, in the case of Ic a linear relationship was observed between the pK_a values and Σσ⁸.     

Correlations of coupling constants with alkyl substituent constants

The one-bond coupling constants (¹J_XY) of nine series of organometallic compounds with the part structure R-X-Y (R is an alkyl group attached directly to the metal atom X) have been correlated with two sets of alkyl substituent constants, TAFT's σ* and our σ_a. The latter gives better correlations and is more consistent with the experimental data.     

19F Chemical shift tensor in fluorobenzene compounds

Fluorine-19 NMR multiple-pulse experiments have been applied to a series of meta-, para- and ortho-substituted fluorobenzene compounds in the solid state. The principal elements of the ¹⁹F chemical shift tensor (σ₁₁, σ₂₂, σ< ₃₃) were determined and the orientation of the tensor axes was inferrred from secondary information like molecular motion, related compounds and liquid crystal studies. Comparison with anisotropies obtained from molecules dissolved in liquid crystals in the nematic phase is discussed where data are available. Using the Gicrke-Flygarc approach we were able to exctract the spin-rotation interaction tensor elements C_??, C_tcy;tcy; and C_zz of ¹⁹F in sever; fluorobenzene compounds.     

Correlation analysis of reactivity in the addition of substituted benzylamines to ethyl α-cyanocinnamate

The kinetics of addition of a number of ortho-, meta-, and para-substituted benzylamines to ethyl α-cyanocinnamate (ECC) in acetonitrile have been studied. The reaction is first-order with respect to the amine and ECC. The rates of reaction of meta- and para-substituted benzylamines showed excellent correlations with Taft's σ₁ and σ_R⁰, and with σ₁ and σ_R^BA values, respectively. The reaction of the ortho-compounds showed a very good correlation with Charton's triparametric equation. The reaction is subject to steric hindrance by the ortho-substituents. A mechanism involving formation of a zwitterionic intermediate in a slow step followed by a fast proton transfer has been proposed. © 1996 John Wiley & Sons, Inc.     

ESR-Spektren von CuN4-Chromophoren II: Komplexe mit Ammoniak, Äthylendiamin,Biguanid, N-Trifluoracetimidoyl-trifluoracetamidin,N-Trichloracetimidoyl-trichloracetamidin,Tetraaza-undecan,Hexamethyl-tetraaza-cyclotetradecan und -cyclotetra decadien in Einkristallen

The ESR spectra of eight copper complexes with two groups of nitrogen ligands have been measured in the solid state. The first group includes the σ-bonding ligands ammonia, ethylenediamine, 1,4,8,11-tetraaza-undecane, and the macrocyclic hexamethyl-1,4,8,11-tetraazacyclotetradecane of CURTIS . Ligands of the second group are biguanide, the CF₃ and CCl₃ derivatives of acetimidoyl-acetamidine, and hexamethyl-1, 4,8,11-tetraazacyclotetradecadiene. Single crystals of the parent Pt^II, Ni^II, or Zn^II compounds were used as host lattices and magnetic diluents. The spectra of the chromophores CuN₄, showing resolved hyperfine structure due to copper and nitrogen nuclear spins, are fitting a Spin-Hamiltonian on the basis of virtual site symmetry D_4h. The parameters g∥, g?, A∥, and A? were used to derive delocalization coefficients in terms of the LCAO-MO model introduced by MAKI and MC GARVEY . The comparative study clearly reveals that no reliable information on π-bonding is provided by application of this model. The results on σ-bonding ligands are particularly valuable in showing that the simple LCAO-MO model cannot account for the attenuation of spin orbit coupling due to σ-bonding in a physically meaningful way. Our results are qualitatively in agreement with AMMETER'S explanation of this phenomenon in terms of a virtual expansion of the d-shell in antibonding orbitals.     

Enolization of the phosphoryl group

Enolization of the phosphoryl group has been studied where Y = PPh₃, CN, Ts, COOEt, CONEt₂; R and R′ = Et, Bu, Ph, EtO, BuO, PhO; and X = Cl, Br, ClO₄, BF₄. It has been established that substances with are phosphaenols, but in substances with Y = CONEt₂ the phosphoryl group cannot be enolized under any conditions. Phosphaenolization is favored by a high acidifying ability of the Y group, the ability of the X anion to stabilize the phosphaenolic form due to formation of a hydrogen bond OH…X with the anion, and a low electronegativity of R and R′ groups. To evaluate the acidifying ability of Y, this article defines specific σ⁻ constants dependent on the number of substituents at the α-carbon atom: σ_CH3⁻, σ_CH2⁻ and σ_CH⁻. Their sums characterize the enolization ability of the phosphoryl group. The enolic structure in the solid state is possible if ∑_CHn⁻ > 2. If this sum lies in the range of 2 < ∑_CHn⁻ < 2.6 the phosphoryl–phosphaenol tautomerism can be expected in appropriate solutions. Acidic properties of the investigated compounds in MeNO₂ and EtOH (absolute) have been determined. Calculations of the acidity of the phosphoryl CH forms (A) and of the phosphaenol OH forms (B) have been carried out.     

Magnetic shielding tensor components in a highly strained phosphorus molecule: 1,2,3-triphenylphosphirene

Using the magic-angle spinning (MAS) technique the principal values of the ³¹P NMR magnetic shielding tensor were measured in 1,2,3-triphenylphosphirene: σ₁₁ = −58; σ₂₂ = 23 and σ₃₃ = 636 ppm. The anisotropy is quite large for a three-coordinate phosphorus compound. The high-field component σ₃₃ corresponds to the direction perpendicular to the ring, and is responsible for the high-field resonance in the isotropie phase.     

Proton magnetic shielding in malonic acid by multiple pulse techniques

The proton NMR in single crystals of malonic acid has been studied by multiple pulse line narrowing techniques. The nuclear magnetic shielding tensors σ⁽ⁱ⁾ of all protons in malonic acid could be determined from the spectra. There are two magnetically distinct carboxyl protons. The principal components of their shielding tensors are found to be σ⁽¹⁾_ZZ= ?0.8 ppm, σ⁽¹⁾_YY = ?19.2 ppm, σ⁽¹⁾_XX = ?21.8 ppm, and σ⁽²⁾_ZZ = ?1.0 ppm, σ⁽²⁾_ZZ = ?21.3 ppm relative to adamantane. The error limits are estimated to be ± 1 ppm. The most shielded directions lie along the hydrogen bond directions to within 8 degrees. The least shielded directions are essentially perpendicular to the plane of the carboxyl groups. Within experimental accuracy the shielding of the aliphatic protons is axially symmetric about the CH bond axes. The anisotropy Δσ = σ_? ? σ_⊥ is (4 ± 1) ppm. The gross features of the anisotropy of the carboxyl protons are shown to be governed by the diamagnetic effect.     

The absorption cross section of the HCO radical at 614.59 nm and the rate constant for HCO+HCO → H2CO+CO

Using a combination of XeCl exciplex laser flash photolysis of gas-phase glyoxal and formaldehyde and time-resolved cw dye laser absorption at 614.59 nm, we have determined the ratio k₁/σ for the reaction HCO+HCO → H₂CO+CO (1) at 295 ±2 K. Similar studies involving the 308 nm photolysis of a variety of aldehydes combined with a determination of the absolute yields of the resulting hydrocarbon products have allowed us to deduce the initial yields of HCO radicals and hence the absorption cross section for HCO at the monitoring wavelength. We find σ=(2.3±0.6) × 10⁻¹⁸ cm², giving k₁=(7.5±2.9)× 10⁻¹¹cm³ molecule⁻¹ s⁻¹. Our values are compared with previous results.     

Excitation of He—41 D-substates by He+- and Ne+-projectiles investigated by level-crossing techniques

Collisions of He⁺- and Ne⁺-projectiles with He at impact energies between 90 keV and 800 keV were investigated. Relative excitation cross sections for magnetic sublevels of He—4¹ D were determined using level-crossing techniques. Absolute excitation cross sections σ _m of the Zeeman-sublevels are given using He—4¹ D cross sections from earlier measurements. The results show strong variations of the cross sections σ₀ and σ_±1 with a quasi-oscillatory behaviour. σ_±2 is much smaller than σ₀ and σ_±1.     

Influence of the nature of the conduction band of transition metal oxides on catalytic activity for oxygen reduction

Catalytic activities for oxygen reduction of some transition metal oxides with metallic conductivity such as LaTiO₃, SrFeO₃, SrVO₃, SrRuO₃, V_0.2Ti_1.8O₃ and La_1-xSr_xMnO₃ were investigated, and they as well as the activities of other oxides reported already were compared with the nature of their conduction bands. It was found that the catalytic activity of oxides having a σ^* conduction band was high. The conclusion is drawn that in order for a transition metal oxide to have a high catalytic activity, (1) it must have a σ^* band and (2) the band must contain electrons. This conclusion will be useful to predict the catalytic activity for oxygen reduction of transition metal oxides.     

Fine structure excitation transfer between the potassium 42P states induced by collisions with caesium atoms

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4² P _1/2)+Cs(6² S _1/2)?K*(4² P _3/2)+Cs(6² S _1/2) have been measured. The values we have obtained are σ(1/2→3/2)=77 Ų and σ(3/2→1/2)=48 Ų. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.     

The electronic effect of positively charged substituents on an aromatic ring: a 13cmr and theoretical study

The effects of the substituents -N⁺Me₃, -CH₂N⁺Me₃ and -P⁺Me₃ on the relative ¹³C chemical shifts at meta and para positions accord with the σ_r° values obtained from IR intensities provided the correlation equation of Syrova et al. is used. SCF-MO calculations using a minimal STO-3G basis set suggest that the -N⁺H₃ substituent acts as a weak π-electron donor and the -P⁺H₃ substituent as a π-electron acceptor in accord with the σ_r° constants for the methylated poles. In these calculations, the action of -PH₃⁺ as a π-electron acceptor can be rationalised in terms of an hyperconjugative effect involving the empty π-type group MO, that in -PH₃⁺ is located at a significantly lower energy than in -NH₃⁺. This π-type group MO is formed by suitable combinations of the 3p AO's of P with the 1s AO's of the hydrogens and therefore there is no need to invoke 3d-orbitals participation in -PH₃⁺ to explain this differential behaviour.