Organic peroxy radicals: Kinetics, spectroscopy and tropospheric chemistry

For the experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 (1), the process of methane oxidation has been studied over the temperature range of 706–786 K. The concentration of CH3O2 has been measured by the radical freezing method, and that of CH3 from the rate of accumulation of ethane, assuming that C2H6 is produced by the reaction CH3 + CH3 C2H6 (2). The equilibrium constant of reaction (1) has been obtained at four temperatures. For the heat of the reaction the value Δ˚H298 = -32.2 ± 1.5 kcal/mol is recommended.

Experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 during the gas-phase oxidation of methane

Several low‐lying potential energy surfaces of the methoxy radical have been studied at C3v conformations using ab initio quantum chemical techniques. A double‐zeta quality basis set augmented with polarization and diffuse functions has been used throughout. Eight doublet and quartet states have been characterized as a function of C–O distance. The equilibrium conformation of the 2E ground state is found (RCO = 1.405 A, RCH = 1.112 A, ϑHCO = 111.3°), and the harmonic vibrational frequencies of the three a1 modes are calculated (3118, 1330, and 1017 cm−1). The ? 2A1←? 2E vertical excitation energy has been estimated to be 4.01 eV. The wave functions for the ? and ? states have been analyzed in terms of their dipole moments, population analyses, and orbital contour plots.

A theoretical potential energy surface study of several states of the methoxy radical

For Co(III) amine complexes, there is a large body of evidence that points to spontaneous substitution reactions occurring via interchange mechanisms in which bond breaking substantially precedes bond making in the transition state. The mechanisms of substitution of the Rh(III), Ir(III), Ru(III), Os(III) and Cr(III) analogues depend very much on a number of factors, such as the nature of the leaving group and steric factors, as well as the electronic configuration of the metal ion. On the balance of evidence, substitution reactions of the d3 and d5 complexes can proceed via interchange mechanisms in which either bond breaking precedes bond making or vice versa. For the spontaneous substitution reactions of [M(NH3)5Cl]2+ complexes, the very large ionic solvation terms of the [M(NH3)5]3+ and Cl- components with respect to [M(NH3)5Cl]2+ strongly favor a transition state in which there is substantial charge separation, i.e., dissociative interchange mechanisms are energetically more accessible than a...

New Insights into the Mechanisms of Spontaneous and Base-Catalysed Substitution Reactions of the Inert Metal Amine Complexes

Publisher Summary The terms ‘labile’ and ‘inert’ represent the two extremes of kinetic reactivity. The solvent molecules in the vicinity of the ion differ in properties from those in the bulk solution. A series of progressively ordered and electro-stricted hydration sheaths surround the ion. This chapter focuses on traditional and simple inert metal-ion complexes, mainly octahedral, in which the rate of displacement of at least one ligand permits it to be defined as labile relative to the normal reactivity of complexes of that metal ion. This reactivity may be an inherent property of the ligand as a consequence of poor nucleophilicity. Alternatively, lability may be induced as a result of effects, such as the influence of other nonleaving donors, reactions of the ligand with reagents external to the complex ion, or solvation phenomena. It is convenient to ‘scale’ lability in terms of the rate of substitution of a ligand. A ligand-based order of lability derived for a particular d3 or d6 metal ion can be largely transposed to another, at least as regards the more reactive ligands. Studies of water-exchange reactions of aqua transition metal ions have implied that the structure of the ion in the ground state is an important factor in defining lability. Ion charge plays a role, because more highly charged ions are less likely to exchange rapidly. A great deal of information exists about pentaamine complexes with one leaving group, particularly for cobalt(III) but also for chromium(III) and other metal ions.

Leaving Groups on Inert Metal Complexes with Inherent or Induced Lability

The most energetically stable alkoxyl radicals take the following configuration: r(C–O)=1.36 A, r(C–H)=1.12 A, and ∠OCH=109°28′ for CH3O·, and r(C–O)=1.36 A, r(C–C)=1.47 A, and ∠OCC=108° for C2H5O·. The pseudo π orbital is observed in the C–O bond.

Estimation of the Structures and Electronic States of Radicals Using the INDO–SCF MO Method. I. Alkoxyl Radicals

The kinetics of aquation and anation reaction have been studied with pentaamineruthenium(III) complexes containing halogenoacetates (fiuoro-, monochloro-, dichloro-, trichloro-, bromo-, and iodoace...

Studies of the Ruthenium(III) Complex. V. Aquation and Anation Reactions of Halogenoacetatopentaammineruthenium(III) Complexes in Aqueous Solutions

An MO-simulation of the geometrical change in the bimolecular methane oxidation, CH3+O2→CH3O2, was performed using the CNDO/2 approximation. The geometry of CH3O2 was predicted to take rOO=1.19 A and ∠COO=111° with fixed distances of rCO=1.44 A and rCH=1.09 A. The coupling reaction proceeds smoothly, without any appreciable activation energy, with the geometric transformation of the momentarily-living CH3--O2, with rCO=2.36∼1.44 A, rOO=1.132∼1.19 A, and ∠COO=90∼111°. The magnitude of the electron migration between CH3 and O2 throughout the reaction was estimated.

An MO-simulation of Elementary Reactions in Hydrocarbon Oxidation. I. A Bimolecular Coupling Reaction of the Methyl Radical and Molecular Oxygen

The energetically most stable CH3OO and CH3CH2OO radicals have been found to have the following geometric values: r(Oα–Oβ)=112 A, r(Oα–C)=138 A for the former and 139 A for the latter, ∠COαOβ=112° for the former and 115° for the latter, and r(C–C)=147 A for the latter

Estimation of Structures and Electronic States of Radicals Using INDO-SCFMO Method. II. Alkylperoxyl Radicals

The mechanism of the acid-catalyzed hydrolysis of (NH3)5RuO2CR2+ (R=H, CH3, C2H5, (CH3)2CH, CH2OH, and/or CH2NH2) has been established by the trapezoidal simulation analyses with a computer. The proposed reaction sequences of SN1 combined with SN2 mechanisms involving a quasi-stable intermediate (NH3)5Ru3+ have proved to be plausible, and the kinetic parameters (rate constants, ΔH\eweq, ΔS\eweq, etc.) have been given for all of the elementary reactions. The aquation rate has been determined by the solvent-assisted, heterolytic dissociation of (NH3)5RuO2CR2+ and follows this order: formato>acetato>propionato>isobutyrato>glycolato>glycinato.

Futoshi Kitagawa

Papers

Estimation of the Structures and Electronic States of Radicals Using the INDO–SCF MO Method. I. Alkoxyl Radicals

Studies of the Ruthenium(III) Complex. V. Aquation and Anation Reactions of Halogenoacetatopentaammineruthenium(III) Complexes in Aqueous Solutions

An MO-simulation of Elementary Reactions in Hydrocarbon Oxidation. I. A Bimolecular Coupling Reaction of the Methyl Radical and Molecular Oxygen

Estimation of Structures and Electronic States of Radicals Using INDO-SCFMO Method. II. Alkylperoxyl Radicals

Simulation Analysis of the Acid–catalyzed Hydrolysis of Carboxylatopentammineruthenium(III) Complexes