The formation and decay mechanisms of HCO in the photodissociation of gas phase acetaldehyde

The kinetic mechanism for the formation and decay of HCO(0,0,0) following flashlamp excitation (10 μs pulse width) into the ¹A″ → ¹A′ absorption transition of gas phase acetaldehyde (0.2 Torr) was examined by time-resolved intracavity laser detection (TRMD) and by phosphorescence lifetime measurements. The HCO radical was found to appear primarily in the vibrationless level reaching a maximum concentration about 250 μs after the excitation of acetaldehyde. The formation rate of HCO(0,0,0) was observed to be insensitive to an order of magnitude change in the number of collisions of excited-state acetaldehyde with either argon, cyclohexane, or the cell wall. Contrastingly, the decay rate of HCO exhibited a strong dependence on the collisional environment. The rate constants for HCO(0,0,0) decay by collisions with acetaldehyde, argon, and cyclohexane and by reaction with O₂ were measured by TRILD. The rate constant for O₂, quenching of ³A″ phosphorescence was also obtained.The potential for HCO(0,0,0) being either a primary or secondary dissociation product is considered in the formulation of a kinetic mechanism describing both the formation and decay behavior observed. Evidence is presented in support of a mechanism in which (1) HCO(0,0,0) is formed by the thermal reaction between acetyl radicals. CH₃CO, and ground-state acetaldehyde after excited-state acetaldehyde undergoes primary dissociation to CH₃CO, and (2) HCO(0,0,0) decays principally by collisionally-induced dissociation at the cell wall.     

Excess enthalpies of binary and ternary mixtures of acetonitrile with methanol,ethanol and benzene

Excess molar enthalpies are measured for the binary mixtures methanol—acetonitrile and ethanol—acetonitrile at 25 and 35°C and for the ternary mixtures methanol—acetonitrile—benzene and ethanol—acetonitrile—benzene at 25°C using an isothermal dilution calorimeter. The binary results are well reproduced with an association model which contains four equilibrium constants for the association of alcohol, two equilibrium constants for that of acetonitrile, and two solvation equilibrium constants between alcohol and acetonitrile molecules. The ternary results are compared with those calculated from the model with binary parameters.     

Hydrogen abstraction and dimerisation reactions of some organo-transition metal free radicals

The 17-electron species [M(CO)_5χL_χ] (M  Mn, Re, χ  0; M  Mn, Re; L  Ph₃P, χ  1, 2; M  Mn, Re; L  (o-MeC₆H₄O)₃P, χ  2; M  Mn; L  (p-ClC₆H₄O)₃P, (PhO)₃P, χ  2; M  Mn; L  P(OMe)₃, χ  3) have been generated by one electron oxidation of the corresponding anions and show typical radical reactivity, undergoing dimerisation or hydride abstraction in reactions controlled by steric effects. Evidence is presented for the source of the hydrogen atom. The 19-electron species [M(CO)₃(η⁷-C₇H₇)]^? (M  Cr, Mo) and [Fe(CO)₃(η⁵-C₆H₇)]^?, generated by reduction of the corresponding cations, undergo dimerisation at the organic ligand. Similar treatment of [Fe(CO)₂-L(η-cp)]⁺ (L  CO, PPh₃, P(OPh)₃, Me₂CO) yields [Fe₂(CO)₄(η-cp)₂] and these reduction reactions are rationalised in terms of the nature of the HOMO in the intermediate radical. Similar reduction of [Rh(diphos)₂]⁺ yield the 17-electron intermediate [Rh(diphos)₂] and this also undergoes hydrogen abstraction.     

Reaction of benzyne with cyclic olefins—II : Ene addition to some cycloalkenes

Addition of benzyne to 1-methylcyclohexene, (+)-carvomenthene, (+)-limonene, α- and β-pinenes and δ³-carene has been investigated. Structures to the ene products formed are assigned on the basis of spectroscopic evidence. Arguments are advanced in favour of a concerted ene mechanism for the addition of benzyne to these olefins.     

Effect of added salts on the rates of dissociation and racemization of tris(1,10-phenanthroline)iron(II) ion in aqueous methanol solutions

The kinetics of dissociation and racemization of [Fe(phen)₃]²⁺ have been studied in aqueous methanol solutions containing perchlorate, chloride, and thiocyanate ions. The racemization rate was decreased by ClO^?₄ and increased by SCN^?, while the dissociation rate was decreased by ClO^?₄ and increased slightly by Cl^? and remarkably by SCN^?. The effect of anions on the reaction rates became remarkable with the increase in methanol content of the solutions. The results were reasonably explained in terms of ion association. The dissociation rate of the complex ion in the ion-pair increased in the order, ClO^?₄ < Cl^? < SCN^?, of associated anions, suggesting the ion-pair interchange mechanism for the dissociation. The ion-association constants were determined to be 11 ± 4, 18 ± 4, and 25 ± 15 (I = 0.1, 25°C) for ClO^?₄, Cl^?, and SCN^?, respectively, in 0.64 mole-fraction (0.8 volume-fraction) aqueous methanol.     

Selectivities in 1,2,3-triazolide displacements of halides and additions to alkynes

4,5-Dicarbomethoxy-1,2,3-triazolide or 4-phenyl-1,2,3-triazolide displace chloride from ethyl chloroacetate or β-chloropropionate to give both 1-N and 2-N alkylated products. Our highest 2-N to 1-N selectivity was ca 5/1 and was found with the base triethylamine in DMF. The same triazolides and others add to alkynes, e.g. ethyl propiolate, methyl acetylenedicarboxylate, phenylpropiolaldehyde, ethyl phenylpropiolate, etc, to give Michael adducts at the 2-N position exclusively. Here the usual preference holds, i.e., the anti adduct is favored, but anti to syn isomerization usually sets in. On the basis of the available data for nucleophilic substitutions and additions, a limited directioselectivity pattern emerges for H-1,2,3-triazoles (T) and their anions (T^?): neutral T almost invariably leads with 1-N; T^t-- usually adds to unsaturates at 2-N; unsubstituted, 4-substituted and 4,5-disubstituted T^? attack organic halides at both 1-N and 2-N. Compared to phenyl, 2-triazolyl exerts a greater deshielding effect on proton chemical shifts; these and other patterns in the PMR spectra of the Michael adducts are discussed. CNDO calculations indicate that the 1-H is more stable than the 2-H-1,2,3-triazole and that in both neutral triazole and in triazolide, the 1-nitrogen position should lead nucleophilic attacks-this directioselectivity prediction is only partly (and probably fortuitously) correct.     

ESR studies on the preparation of the sulphite radical anion,SO3−., in aqueous solution

The sulphite radical anion, SO₃^?., was generated from the redox reactions between the bisulphite ion, HSO₃^?, and transition-metal i     

Influence of some metal ions on oxidation of nadh and on formation of the superoxide anion radical (O(2)(-)), during enzymatic catalysis by E.C. 1.2.3.2 xanthine oxidase

The inhibitory effect of selected metal ions [Ag(I), Hg(II), Cu(II), Cr(VI), V(V), Au(III), T1(I) and Zn(II)], on the xanthine oxidase (XOD) catalysis of xanthine oxidation, has been investigated with reference to the XOD catalysis of oxidation of NADH. Hg(II), Ag(I), Zn(II) and Au(III) act as inhibitors, T1(I) has no effect and Cu(II), Cr(VI) and V(V) act as activators. The formation of O(2)(-) during XOD catalysis of oxidation of either xanthine or NADH has also been studied. All the metal ions considered act as inhibitors with respect to O(2)(-) production when the reducing substrate is xanthine, but only a few of them when the substrate is NADH, the others showing no effect whatsoever whether or not they activate NADH oxidation in the course of the same reaction. Vanadium (V) has an anomalous effect: it inhibits xanthine oxidation but considerably increases NADH oxidation, and thus appears to modify the catalytic properties of the enzyme. This behaviour appears promising as the basis for a kinetic method for determination of V(V).     

Coordination,oligomerisation and transfer hydrogenation of acetylenes by some ruthenium and osmium carboxylates: Crystal and molecular structures of bis(trifluoroacetato)carbonyl-(methanol) bis(triphenylphosphine)ruthenium(II) and (1,4-diphenylbut-1-en-3-yn-2-yl)trifluoroacetato-(carbonyl) bis(triphenylphosphine)ruthenium(II)

The hydrides [MH(O₂CCF₃)(CO)(PPh₃)₂] (M = Ru or Os) react with disubstituted acetylenes PhCCPh and PhCCMe to afford vinylic products [M{C(Ph)CHPh}(O₂CCF₃)(CO)(PPh₃)₂] and [M{C(Ph)CHMe}(O₂CCF₃)(CO) (PPh₃)₂]/[M{C(Me)CHPh}(O₂CCF₃)(CO)(PPh₃)₂] respectively. Acidolysis of these products with trifluoroacetic acid in cold ethanol liberates cis-stilbene and cis-PhHCCHMe respectively thus establishing the cis-stereochemistry of the vinylic ligands. The complexes [M(O₂CCF₃)₂(CO)(PPh₃)₂] formed during the acidolysis step undergo facile alcoholysis followed by β-elimination of aldehyde to regenerate the parent hydrides [MH(O₂CCF₃)(CO)(PPh₃)₂] and thereby complete a catalytic cycle for the transfer hydrogenation of acetylenes. The molecular structure of the methanol-adduct intermediate, [Ru(O₂CCF₃)₂(MeOH)(CO)(PPh₃)₂] has been determined by X-ray methods and shows that the coordinated methanol is involved in H-bonding with the monodentate trifluoroacetate ligand [MEO-H---OC(O)CF₃; O...O = 2.54 Å]. The hydrides [MH(O₂CCF₃)(CO) (PPh₃)₂]react with 1,4-diphenylbutadiyne to afford the complexes [M{C(CCPh)CHPh} (O₂CCF₃)(CO)(PPh₃)₂]. The ruthenium product, which has also been obtained by treatment of [RuH(O₂CCF₃)(CO)(PPh₃)₂] with phenylacetylene, has been shown by X-ray diffraction methods to contain a 1,4-diphenylbut-1-en-3-yn-2-yl ligand. The osmium complexes [Os(O₂CCF₃)₂(CO)(PPh₃)₂], [OsH(O₂CCF₃)(CO)(PPh₃)₂] and [Os{C(CCPh)CHPh}(O₂CCF₃)(CO)(PPh₃)₂] all serve as catalysts for the oligomerisation of phenylacetylene. Acetylene reacts with [Ru(O₂CCF₃)₂(CO)(PPh₃)₂] in ethanol to afford the vinyl complex [Ru(CHCH₂)(O₂CCF₃)(CO)(PPh₃)₂].     

Comparison of free and metal coordinated 1,4-disubstituted-1,4-diaza-1,3-butadienes : Crystal and molecular structures of 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene and trans-[dichloro(triphenylphosphine)(1,4-di-tert-butyl- 1,4-diaza-1,3-butadiene)palladium(II)]

The crystal and molecular structures of c-Hex-DAB (c-hexyl-NC(H)C(H)N-c-hexyl; DAB = 1,4-diaza-1,3-butadiene) and of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are reported. Crystals of c-Hex-DAB are monoclinic with space group C_2/c and cell constants: a = 24.70(1), b = 4.660(2), c = 12.268(3)Å, β = 107.66(4)°, Z = 4. The molecule has a flat E-s-trans-E structure with bond lengths of 1.258(3)Å for the CN double bond and 1.457(3)Å for the central CC′ bond. These bond lengths and the NC-C′ angle of 120.8(2)° indicate that the C- and N-atoms are purely sp²-hybridized and that there is little or no conjugation within the central DAB skeleton. Crystals of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are triclinic with space group P-1 and cell constants: a = 17.122(3), b = 18.279(3), c = 10.008(5)Å, α = 96.77(2), β = 95.29(3), γ = 109.79(2). Z = 4. The t-Bu-DAB ligand is coordinated to the metal via one lone pair only. In this 2e; σ-N coordination mode the E-s-trans-E conformation of the free DAB-ligand is still present and the bonding distances within the DAB-ligand are hardly affected. [CN: 1.261(10)Å; CC′: 1.479(10)Å (mean).] The PdN-, NC- and central CC′-bond lengths are compared with those found in other metal -R-DAB complexes.     

A new form of the equation of state for pure substances and its application to oxygen

Schmidt, R. and Wagner, W., 1985. A new form of the equation of state for pure substances and its application to oxygen. Fluid Phase Equilibria, 19: 175–200.A new wide range equation of state is presented and expressed analytically in the form of the free energy as a function of density and temperature. This fundamental equation contains, in addition to pure polynomial and “BWR”-terms, new exponential functions especially convenient for the critical region. To guarantee an effective structure, the combination of the terms of the equation was found by using an optimization method recently developed. As a result, the optimized function for the free energy is capable of representing the thermodynamic surface of oxygen in the range 54 ≤ T ≤ 300 K, 0 < p ≤ 818 bar and 0 < ρ ≤ 41 mol dm^?3 within the experimental uncertainty of the data available. With the exception of very few items of data, this statement is also valid for the whole coexistence curve and the critical region. Extrapolations of this new equation beyond the range of data yield physically meaningful results.     

A convenient synthetic route to new bis(arenediazo) complexes of molybdenum and tungsten

Complexes of the type [(C₅H₅)Mo(N₂Ar)(N₂Ar′)(PPh₃)] [PF₆] have been prepared and shown to be useful starting materials for the synthesis of a variety of neutral, cationic or anionic compounds containing [cis-Mo(N₂Ar)(N₂Ar′)]²⁺ units.     

Mechanisms of addition of primary aromatic amines and cyclohexylamine to tricarbonyl(cycloheptatrienyl)tungsten cation

X-substituted anilines (X = H, 2-Me, 4-Me or 2-Cl) and cyclohexylamine are shown to add to the tropylium ring of cation [(η)-C₇H₇)W(CO)₃]⁺ to give the corresponding ring adducts of tricarbonyl(cyclohepta-1,3,5-triene)tungsten. Kinetic studies have demonstrated that the anilines form the triene ring adducts via the rapid pre-equilibrium formation of a π-complex, which rearranges in a rate-determining fashion to form a cationic triene intermediate [(1–6-η-C₆H₇.NH₂R)-W(CO)₃]⁺ (R = C₆H₅, 2-MeC₆H₄, 4-MeC₆H₄ or 2-ClC₆H₄); from this the final product is rapidly formed via amine- or solvent-assisted proton loss. With the aliphatic cyclohexylamine, the cationic triene intermediate is formed directly, followed by competing rate-determining solvent- and amine-assisted proton removal.     

New routes to halogenated B8 and B9 boron cages

B₈Br₈ and B₉Br₉ are formed when B₈Cl₈ and B₉Cl₉ are heated with aluminium tribromide. Cage-size reduction occurs on heating B₁₀Cl₁₀ and B₁₁Cl₁₁ with hydrogen to give B₉Cl₈H and B₉Cl₇H₂, respectively. At least six bromine atoms in B₉Br₉ can be substituted for methyl groups using SnMe₄.     

The electrochemistry of organotransition-metal complexes : II. Formation and reactivity of the radical cations [trans-Fe(CO3(PR3)2]+ and [Fe(CO)3 (Ph2 PCH2 CH2 PPh2)]+

[Fe(CO)₃ L₂] (L = PPh₃, PPh₂Me, P(OPh)₃ or P(NMe₂)₃; L₂ = Ph₂ PCH₂ CH₂ PPh₂⁺) undergo reversible one-electron oxidations to give the radical cations [Fe(CO)₃L₂]⁺ which have been studied by IR and ESR spectroscopy.     

ESR studies on the reactions of sulphite radical anion,SO3.̄−, with nitroalkane compounds in aqueous solutions

The reactions of the sulphite radical anion, SO₃^{$\text{.}\text{?}$?} (generated from the Ti³⁺-H₂O₂-Na₂SO₃ system at pH 9), in aqueous solutions with some nitroalkane compounds were investigated by using a rapid-mixing flow technique coupled with electron spin resonance (ESR) which can detect the radicals having a lifetime of 5–100 ms. The SO₃^{$\text{.}\text{?}$?} radical added to the double bond of CN in the nitroalkane aci-anions which are the main form of nitroalkanes in aqueous alkaline solutions. From the observed hyperfine splitting constants of the SO₃^{$\text{.}\text{?}$?} adducts of nitroalkane aci-anions, the preferred conformation of the adducts was deduced.