Direct determination of the equilibrium constant and thermodynamic parameters in the reaction of pentadienyl radicals with O2

Reaction of pentadienyl radicals (C₅H₇) with O₂ has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals could be generated either by the photolysis of 1,3-pentadiene or by the two-step reaction of carbon tetrachloride photolysis followed by the H-atom abstraction reaction of Cl atom with 1,4-pentadiene. The equilibrium between pentadienyl radicals, O₂ and pentadienylperoxy radicals could be observed over the range 268–308 K. An analysis of the temporal signal of pentadienyl radicals was used to evaluate the equilibrium constant. Third-law analysis was used to evaluate the enthalpy change for the reaction C₅H₇ + O₂ ⇌ C₅H₇O₂. The observed CO bond energy in the C₅H₇O₂ adduct was found to be 56.0 ± 2.2 kJ·mol^–1, which is lower than the values of peroxy radicals formed with allyl and cyclohexenyl radicals which have an allylic resonance structure.     

Optimization of the chemoenzymatic mono-epoxidation of linoleic acid using D-optimal design

Mono-epoxied linoleic acid 9(12)-10(13)-monoepoxy 12(9)-octadecanoic acid (MEOA) was synthesized and optimized by immobilized Candida antarctica lipase (Novozym 435®) using D-optimal design. For optimizing the reaction, response surface methodology (RSM) was employed with four reaction variables such as the effect of amount of hydrogen peroxide (μL), amount of enzyme (w) and reaction time (h). At optimum conditions the experiment to obtain a higher yield% with a medium OOC% of MEOA was predicted at an amount of H₂O₂ μL of 15, Novozym 435® of 0.12 g and 7 h of reaction time. At this condition, the yield of MEOA was 82.14%, 4.91% of OOC and 66.65 mg/g of iodine value (IV). The observed value was reasonably close to the predicted value. Hydrogen peroxide was found to have the most significant effect on the degree of epoxidation OOC% and yield%. The epoxy ring opening (–C–O–C–) has been observed by Fourier Transform Infrared Spectroscopy (FTIR) at 820 cm⁻¹ and the double band (–CC–) at 3009 cm⁻¹. ¹H NMR analyses confirmed that the oxirane ring (–CH–O–CH–) of MEOA at 2.92–3.12 ppm and four signals of methane (–CHCH–) was at 5.38–5.49 ppm while the ¹³C NMR showed the oxirane ring (–C–O–C–) at 54.59–57.29 ppm and the olefinic carbons at 124.02–132.89 ppm.     

Phase diagram and local environment of Sn and Te: SnTeBi and SnTeBi2Te3 systems

From thermal analyses and X-ray diffraction the phase diagram of the BiSnTe and SnTeBi₂Te₃ sections was determined. The local environment of Sn and Te atoms was studied by ¹¹⁹Sn and ¹²⁵Te Mössbauer spectroscopy. The BiSnTe section showed a eutectic reaction at 267 °C and 20 % mole SnTe–80 % mole Bi. No intermediate compound was detected. The SnTeBi₂Te₃ section is characterized by a eutectic reaction at 585 °C and 40 % mole SnTe–60% mole Bi₂Te₃ and a peritectic reaction at 600 °C and 50 % mole SnTe–50% mole Bi₂Te₃. It corresponds to the compound SnBi₂Te₄, which has a rhombohedral layered structure with unit cell parameters a=4.3954(4) Å and c=41.606(1) Å. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASSnTe / Bi / Bi₂Te₃ / phase diagram / Mössbauer     

The effects of hydrogen on KOH activation of petroleum coke

KOH activation of petroleum coke (PC) was conducted with 30 vol%H₂ + 70 vol% N₂ as carrier gas. TG-DTG, FTIR, elemental analysis, N₂ adsorption, GC and XRD techniques were used to investigate the effects of hydrogen on the activation. During the initial stage of the activation, i.e. the carbonization of the PC, additional CH and CH₂ species were formed due to the chemisorption of hydrogen on the nascent sites of the PC created by the removal of the surface heteroatom groups. The formation of the CH and CH₂ species increased the quantity of ‘active sites’ which is favorable to the further activation reaction, and developed the porous structure of the activated carbons. The micropore volume and BET surface areas of the activated carbon prepared under 30 vol% H₂ + 70 vol% N₂ and with a relatively low KOH/PC weight ratio of 2:1 have been increased from 0.78 cm³/g and 1936 m²/g to 0.97 cm³/g and 2477 m²/g, respectively, compared to that prepared in pure N₂ atmosphere with the same KOH/PC ratio.     

Hydrogen-bond transition from the vibration mode of ordinary water to the (H,Na)I hydration states: Molecular interactions and solution viscosity

With the aid of differential phonon spectrometrics (DPS) and surface stress detection, we show that HI and NaI solvation transforms different fractions of the HO stretching phonons from the mode of ordinary water centred at ∼3200 to the mode of hydration shell at ∼3500 cm⁻¹. Observations suggest that an addition of the H ↔ H anti-hydrogen-bond to the Zundel notion, [H(H₂O)₂]⁺, would be necessary as the HO bond due H₃O⁺ has a 4.0 eV energy, and the H ↔ H fragilization disrupts the solution network and the surface stress. The I⁻ and Na⁺ ions form each a charge centre that aligns, stretches, and polarize the O:HO bond, resulting in shortening the HO bond and its phonon blue shift in the hydration shell or at the solute-solvent interface. The solute capabilities of bond-number-fraction transition follow: f_H = 0, f_Na ∝ C, and f_I ∝ 1 − exp(−C/C₀) toward saturation, with C being the solute molar concentration and C₀ the decay constant. The f_H = 0 evidences the non-polarizability of the H⁺ because of the H ↔ H formation. The linear f_Na(C) suggests the invariance of the Na⁺ hydration shell size because of the fully-screened cationic potential by the H₂O dipoles in the hydration shell but the nonlinear f_I(C) fingerprints the I⁻ ↔ I⁻ interactions at higher concentrations. Concentration trend consistency between Jones–Dole’s viscosity and the f_NaI(C) coefficient may evidence the same polarization origin of the solution viscosity and surface stress.     

Photochemistry of 1,3,5-trithianes in solution: Steady-state and laser flash photolysis studies

In this work, we characterized the direct photochemistry of a set of five structurally-related 1,3,5-trithianes. The compounds were 1,3,5-trithiane, the α- and β-isomers of the 2,4,6-trimethyl derivatives, and the α- and β-isomers of the 2,4,6-triphenyl derivatives. Under steady-state, 254-nm irradiation of acetonitrile solutions of all five trithianes, dithioesters of the form RC( = S)SCH(R)SCH₂R were identified and shown to be primary photoproducts (R = H, CH₃, or C₆H₅). Shorter dithioesters, RC( = S)SCH₂R, were also identified and shown to be secondary products. The presence of the dithioesters could be monitored by their strong absorption bands in the region of 310 nm. This same band was evident following the laser flash photolysis of the five trithianes. The laser-induced transient spectra showed another absorbing species (I) in all five trithianes. This species was not stable and showed a complementary decay that matched the growth of the stable photoproducts at 310 nm. This suggested that the intermediates (I) are the precursors of the corresponding dithioesters, RC( = S)SCH(R)SCH₂R. These correlated processes were related to monophotonic events. However, in the laser flash photolysis experiments in the triphenyl derivatives, there was an additional pathway for the formation of the dithioesters, and this was biphotonic. When the biphotonic formation of products was compensated for, RC( = S)SCH(R)SCH₂R formation quantum yields from steady-state and laser flash photolysis matched within experimental error. The absorption band of (I) varied systematically with substituents, 320 nm in 1,3,5-trithiane, 340 nm in the 2,4,6-trimethyl derivatives, and 420 nm in the 2,4,6-triphenyl derivatives. The nature of these intermediates (I) were discussed as resulting from CS bond cleavage, probably heterolytic.     

A general route for the stereoselective synthesis of (E)-(1-propenyl)phenyl esters by catalytic CC bond isomerization

A general and efficient procedure for the stereoselective synthesis of (E)-(1-propenyl)phenyl esters from readily accessible allylphenols has been developed. The process involves a two-step sequence consisting of the initial acylation of the allylphenols with an acid chloride, followed by catalytic CC bond isomerization in the resulting allylphenyl esters. The latter step was performed in methanol at 80 °C using catalytic amounts (0.5 mol %) of the commercially available bis(allyl)-ruthenium(IV) dimer [{RuCl(μ-Cl)(η³:η³-C₁₀H₁₆)}₂] (C₁₀H₁₆=2,7-dimethylocta-2,6-diene-1,8-diyl). Reactions proceeded in high yields (68–93%) and short times (4–9 h) with complete E-selectivity.     

Raman spectroscopic study of CuO–V2O5–P2O5–CaO glass system

In order to evidence the structural changes induced by CuO and V₂O₅ in the phosphate glass network and their modifier or former role, x(CuO·V₂O₅)(100 − x)[P₂O₅·CaO] glass system was prepared and investigated using Raman spectroscopy (0 ≤ x ≤ 40 mol%).Raman spectra of the studied glasses present the specific bands of the phosphate glasses at low concentration of transition metal (TM) ions, but at higher concentration (x > 7 mol%) a strong depolymerization of the phosphate network appears; non-bridging oxygen atoms are involved in VOP and CuOP bonds and new short units are formed. For a high concentration of V₂O₅ (x > 10 mol%) the Raman bands of V₂O₅ prevail in the spectra; this fact suggests that vanadium oxide imposes its structural units in the network acting thus as a network glass former.2D correlation analysis was also applied for the concentration-dependent Raman spectra in order to verify the assignments of the vibration modes and to find correlations in the changes induced by TM ions content. 2D correlation maps indicate a good correlation between the bands at ∼705 cm⁻¹ assigned to POP stretching vibration and at ∼1175 cm⁻¹ assigned to PO₂ groups which suggest the depolymerization of the phosphate network. The correlation between the 1270 cm⁻¹ and 930 cm⁻¹ bands also suggests that V₂O₅ oxide is responsible for PO bonds breaking and POV formation.     

Reaction of isocyanides with iminophosphorane-based carbene ligands: Synthesis of unprecedented ketenimine–ruthenium complexes

The RuC bond of the bis(iminophosphorano)methandiide-based ruthenium(II) carbene complexes [Ru(η⁶-p-cymene)(κ²-C,N-C[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et (1), Ph (2)) undergoes a C–C coupling process with isocyanides to afford ketenimine derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNR′)[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et, R′ = Bz (3a), 2,6-C₆H₃Me₂ (3b), Cy (3c); R = Ph, R′ = Bz (4a), 2,6-C₆H₃Me₂ (4b), Cy (4c)). Compounds 3–4a–c represent the first examples of ketenimine–ruthenium complexes reported to date. Protonation of 3–4a with HBF₄ · Et₂O takes place selectively at the ketenimine nitrogen atom yielding the cationic derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNHBz)[P{NP(O)(OR)₂}Ph₂]₂)][BF₄] (R = Et (5a), Ph (6a)).     

Large electron correlation effect leading to BeBe bond

The bonding of the beryllium diatomic molecule (Be₂) in the ground state is exclusively made from the electron correlation effect. Unlike the ordinary van der Waals bond, where the electron correlation of the dispersion type makes weak bond energy (D_e) at large bond distance (R_e), the BeBe bond is surprisingly strong with D_e = 830 cm^?1 and R_e = 245 pm. This paper presents in an analytical way the different electron correlation effects with the corresponding spectroscopic data.     

Cationic organoiron mixed-sandwich hydrazine complexes: Reactivity toward aldehydes,ketones, β-diketones and dioxomolybdenum complexes

This review covers comprehensively the authors work during the present decade based on the chemistry of ionic organometallic hydrazines formulated as [(η⁵-Cp′)Fe(η⁶-Ar-NHNH₂)]⁺PF₆^? (Cp′ = C₅H₅, C₅Me₅; Ar = aryl), that could be considered as a new generation of hydrazines owing to the changes provoked by the coordination of the 12-electron Cp′Fe⁺ fragment both in the electronic properties of the aromatic ring and in the hydrazine group. The reactivity of this new class of hydrazine is obviously centered, as in the classic Fischer's organohydrazines, Ar-NHNH₂, on the –NHNH₂ functional unit which is able to react with aldehydes, RCH(O) (R = alkyl, aryl, ferrocenyl (Fc)) and ketones, RR′CO (R = alkyl, aryl; R′ = alkyl, aryl, Fc), to afford ionic organometallic hydrazones. Likewise, the mixed-sandwich hydrazine precursors react with β-diketones Me–C(O)–CH₂–C(O)–Me to afford ionic organometallic pyrazoles, and with cis-dioxo-molybdenum complexes, e.g. [MoO₂(S₂CNEt₂)₂], to afford ionic organometallic mono-organodiazenido complexes in which the two metal centers are connected by a μ,η⁶:η¹-aryldiazenido bridge. While some ionic hydrazones exhibit NLO properties, the ionic organodiazenido hybrid complexes exhibit charge-transfer features.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.     

Pulse radiolysis of aqueous diphenyloxide

Pulse radiolysis of aqueous diphenyloxide (DPO) has been performed under various experimental conditions. The OH radicals react with DPO on various positions of the molecule with a rate constant, k=2.1×10¹⁰ l mol⁻¹ s⁻¹. The major reaction step appears to be a cleavage of the C–O bond of DPO resulting into C₆H₄OH (λ=285 nm) and C₆H₅O(λ=325 nm) radicals in addition to DPO–OH adducts. They disappear according to a second-order reaction. In the presence of air or in a gas mixture of N₂O:O₂=4:1 the DPO–OH adducts are scavenged by oxygen, resulting into peroxyl radicals, which are long-lived species. For the reaction of e_aq⁻ with DPO a rate constant, k=2×10¹⁰ l mol⁻¹ s⁻¹ was found.     

Nickel-catalysed asymmetric SN2′ substitution chemistry of Baylis–Hillman derived allylic electrophiles

Unsymmetrical PhCHCH(CH₂X)(CO₂Me) (X = Cl, OAc) undergoes regioselective α-substitution with AlMe₃ to afford (E)-PhCHCH(Et)(CO₂Me) under Ni(acac)₂ catalysis. On the addition of planar chiral Ferrophite ligands [(R)-CpFe(1,2-C₅H₃Ar{P(OR)₂}) (Ar = Ph, 4-CF₃Ph, 3,5-Me₂Ph, 1-naphthyl; (OR)₂ = 1,1′-binaphthylene, 1,1′-biphenylene)] regioselective methylation γ to the leaving group is possible. It is proposed that the bulky Ferrophite ligand leads to an intermediate nickel allyl species Ni^II(Me)(Ferrophite){η³-PhCHCHCH(CO₂Me)} that adopts a configuration whereby the PhCH terminus of the π-allyl and the Ni–Me are syn leading to good regio (up to 6.4:1) and stereo (up to 94% ee) selectivities.     

Insertion of ketenes into lanthanocene n-butylamide and imidazolate complexes

Reaction of Cp₂LnNHⁿBu with 1 equiv. of Ph₂CCO in toluene affords dimeric complexes [Cp₂Ln(OC(CHPh₂)NⁿBu)]₂ [Ln = Yb (1), Dy (2)], derived from a formal insertion of the CC bond of the ketene into the N–H bond. Treatment of CpErCl₂ with 2 equiv. of LiNHⁿBu followed by reacting with Ph₂CCO affords a rearrangement product [Cp₂Er(OC(CHPh₂)NⁿBu)]₂ (3). Treatment of [Cp₂Ln(μ-Im)]₃ (Im = imidazolate) with PhRCCO gives [Cp₂Ln(μ-OC(Im)CPhR)]₂ [R = Et, Ln = Yb (4); R = Ph, Ln = Yb (5), Er (6)]. In contrast to the previous observations that [Cp₂ErNⁱPr₂]₂ and [Cp₂ErNHEt]₂ react with ketenes to give di-insertion products, in the present cases the presence of excess of ketenes has no influence on the final product even with prolonged heating and only monoinsertion products are isolated. All these complexes were characterized by elemental analysis, IR and mass spectroscopies. The structures of complexes 1 and 3–6 were also determined through X-ray single crystal diffraction analysis.     

Electronic characteristics of an extensive series of ruthenium complexes with the non-innocent o-benzoquinonediimine ligand: A pedagogical approach

Density functional theory (DFT) calculations are carried out on an extensive series of ruthenium complexes with the non-innocent (redox active) o-benzoquinonediimine (bqdi) ligand, namely [Ru(WXYZ)(bqdi)]ⁿ⁺ where WXYZ are a range of spectator ligands including ammonia, phosphines, 2,2′-bipyridine, 2,2′,2″-terpyridine, carbon monoxide, water, halide, acetonitrile, triazacyclononane, nitrosyl, cyclam, etc. In addition, a smaller series, Ru(acac)₂(R-bqdi) is explored, where acac = 2,4-pentanedionate, and R = H, Cl, Me, NO₂ and N-SO₂Me. A range of properties including Mulliken and Natural population analysis (NPA) charges, Mayer bond orders (Ru–N, CN, CC, etc.), net σ-donation and net π-back donation, and percentage Ru 4dπ in the LUMO, are derived and correlated with experimental properties including oxidation and reduction potentials and ligand electrochemical parameters, E_L(L). The various properties are understood in terms of the primary involvement of π-back donation to the π*-LUMO of bqdi. Net π-back donation is derived from the contribution of the π*-LUMO (and higher virtual orbitals) of bqdi, to filled molecular orbitals of the complex. The question of whether these species should be considered exclusively as being represented as [Ru^IIL₄(bqdi)] or [Ru^IIIL₄(sqdi)] (sqdi = o-benzosemiquinonediimine) is briefly considered and evidence presented for the former electronic structure. This is written as a pedagogical treatise rather than a detailed research discussion of the electronic properties of these molecules.     

Synthesis,molecular structure and reactivity of new π-conjugated diyne with ferrocenyl and phenyl units

New π-conjugated butadiynyl ligand FcC(CH₃)₂Fc′–CC–CC–Ph (L₁) has been synthesized and its reaction with Co₂(CO)₈ has been studied. New clusters [FcC(CH₃)₂Fc′–CC–CC–Ph][Co₂(CO)₆]_n [(1): n = 1; (2): n = 2] and [Fc–CC–CC–Ph][Co₂(CO)₆]_n [(3): n =  1; (4): n = 2] were obtained by the reaction of ligands FcC(CH₃)₂Fc′–CC–CC–Ph (L₁) and Fc–CC–CC–Ph (L₂) with Co₂(CO)₈ respectively and the composition and structure of the clusters and ligands have been characterized by elemental analysis, FTIR, ¹H and ¹³C NMR and MS. The crystal structures of compounds L₁, L₂, 2 and 4 have been determined by X-ray single crystal analysis.     

Phosphanes with bulky oligosilyl substituents

By reaction of dichloroheptasilane [(SiMe₃)₂MeSi]₂SiCl₂ with lithiumphosphanides LiPHR, the silylphosphanes [(SiMe₃)₂MeSi]₂SiClPHR with R = 2, 4, 6-tri-tert-butylphenyl ( = supermesityl, Mes1) (1) and Si(SiMe₃)₃ ( = hypersilyl, Hyp) (2) were prepared. Both compounds were characterized with X-ray diffraction, multinuclear NMR spectroscopy and elemental analysis. Compound 1 did not react with n-BuLi, but only with a large excess of tert-BuLi. Phosphasilene [(SiMe₃)₂MeSi]₂SiPMes¹ could be identified by a ³¹P NMR signal at +346 ppm. All attempts to separate it from the reaction mixture failed due to many by-products which had formed through SiSi and SiP bond cleavage. Lithiation of 2 was possible with 4.2 equiv. of tert-BuLi, and crystals of the lithiumphosphanide [(SiMe₃)₂MeSi]₂SiClPLiHyp (3) could be obtained from THF, albeit in a quality not sufficient for X-ray diffraction. All attempts to achieve LiCl elimination and formation of the phosphasilene [(SiMe₃)₂MeSi]₂SiPSi(SiMe₃)₃ failed due to the unusual stability of the lithiumphosphanide. Prolongued refluxing in toluene (110 °C) only led to complete loss of coordinated THF, and ³¹P⁷Li spin spin coupling could be observed in the ³¹P NMR spectrum (¹J_PLi = 84 Hz).Reaction of potassium phosphanide [(SiMe₃)₃Si]SiMe₃PK with SiCl₄ led to the formation of [(SiMe₃)₃Si](SiMe₃)P(SiCl₃) (4), which could be successfully characterized with X-ray diffraction and multinuclear NMR spectroscopy. SiP bond lengths vary between 218 pm (SiCl₃) and 230 pm (hypersilyl). Despite these differences, ³¹P²⁹Si coupling constants are nearly identical (92.4 Hz and 85.5 Hz, respectively).     

Comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In a recent paper (Radiation Physics and Chemistry, 2005, vol. 74, pp. 210) it was suggested that the anomalous increase of molecular hydrogen radiolysis yields observed in high-temperature water is explained by a high activation energy for the reaction H+H₂O→H₂+OH. In this comment we present thermodynamic arguments to demonstrate that this reaction cannot be as fast as suggested. A best estimate for the rate constant is 2.2×10³ M⁻¹ s⁻¹ at 300 °C. Central to this argument is an estimate of the OH radical hydration free energy vs. temperature, ΔG_hyd(OH)=0.0278t−18.4 kJ/mole (t in °C, equidensity standard states), which is based on analogy with the hydration free energy of water and of hydrogen peroxide.