Far-UV laser flash photolysis in solution. A study of the chemistry of 1,1-dimethylsilene in hydrocarbon solvents

The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ 10 μs) and exhibits a UV absorption spectrum with λ_max 255 nm. Characteristic rapid quenching is observed for the silene with methanol (k_McOH = (4.9 ± 0.2) × 10⁹ M⁻¹ s⁻¹), tert-butanol (k_BuOH = (1.8 ± 0.1) × 10⁹ M⁻¹ s⁻¹) and oxygen (k_O2 = (2.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹). The Arrhenius activation parameters for the reaction with methanol have been determined to be E_a = −2.6 ± 0.6 kcal mol⁻¹ and log A = 7.7 ± 0.3.     

Thermochemistry of organosilicon compounds VII. Permethylcyclosilazanes and 1,1,3,3-tetramethyldisilazane

The enthalpy of formation (ΔH_f⁰), enthalpy of evaporation (ΔH_v⁰) and enthalpy of atomization (ΔH_a) of permethylcyclosilazanes (Me₂SiNH)_n (n = 3, 4) and 1,1,3,3-tetramethyldisilazane (Me₂SiH)₂NH have been determined. The enthalpies of formation of these compounds were compared with those calculated by the Benson-Buss-Franklin and Tatevskii additive schemes. In higher permethylcyclosilazanes the energy of the endocyclic Si---N bond is 306 ± 2 kJ mol⁻¹ (73 kcal mol⁻¹), that is 12 ± 2 kJ mol⁻¹ (3 kcal mol⁻¹) lower than the energy of the acyclic Si---N bond. The strain energy of the cyclotrisilazane ring is estimated to be 10.5 kJ mol⁻¹ (2.5 kcal mol⁻¹), whereas the energy of the ring Si---N bond is 295 kJ mol⁻¹ (70.5 kcal mol⁻¹).

The thermochemical data for permethylcyclosilazanes were compared with the corresponding values for permethylcyclosiloxanes calculated from the results of previously reported studies.     

Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

One-electron oxidation of mitomycin C and its corresponding peroxyl radicals. A steady-state and pulse radiolysis study

The one-electron oxidation of Mitomycin C (MMC) as well as the formation of the corresponding peroxyl radicals were investigated by both steady-state and pulse radiolysis. The steady-state MMC-radiolysis by OH-attack followed at both absorption bands showed different yields: at 218 nm G_i (-MMC) = 3.0 and at 364 nm G_i (-MMC) = 3.9, indicating the formation of various not yet identified products, among which ammonia was determined, G(NH₃) = 0.81. By means of pulse radiolysis it was established a total κ (OH + MMC) = (5.8 ± 0.2) × 10⁹ dm³ mol⁻¹ s⁻¹. The transient absorption spectrum from the one-electron oxidized MMC showed absorption maxima at 295 nm (ε = 9950 dm³ mol⁻¹ cm^t-1), 410 nm (ε = 1450 dm³ mol⁻¹ cm⁻¹) and 505 nm ( ε = 5420 dm³ mol⁻¹ cm⁻¹). At 280–320 and 505 nm and above they exhibit in the first 150 μs a first order decay, κ₁ = (0.85 ± 0.1) × 10³ s⁻¹, and followed upto ms time range, by a second order decay, 2κ = (1.3 ± 0.3) × 10⁸ dm³ mol^-1 s⁻¹. Around 410 nm the kinetics are rather mixed and could not be resolved.

The steady-state MMC-radiolysis in the presence of oxygen featured a proportionality towards the absorbed dose for both MMC-absorption bands, resulting in a G_i (-MMC) = 1.5. Among several products ammonia-yield was determined G(NH₃) = 0.52. The formation of MMC-peroxyl radicals was studied by pulse radiolysis, likewise in neutral aqueous solution, but saturated with a gas mixture of 80% N₂O and 20% O₂. The maxima of the observed transient spectrum are slightly shifted compared to that of the one-electron oxidized MMC-species, namely: 290 nm (ε = 10100 dm³ mol⁻¹ cm⁻¹), 410 nm (ε = 2900 dm³ mol⁻¹ cm⁻¹) and 520 nm (ε = 5500 dm³ mol⁻¹ cm⁻¹). The O₂-addition to the MMC-one-electron oxidized transients was found to be at 290 to 410 nm gk(MMC·OH + O₂) = 5 × 10⁷ dm³ mol⁻¹ s⁻¹, around 480 nm κ = 1.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and at 510 nm and above, κ = 3 × 10⁸ dm³ mol⁻¹ s⁻¹. The decay kinetics of the MMC-peroxyl radicals were also found to be different at the various absorption bands, but predominantly of first order; at 290–420 nm κ₁ = 1.5 × 10³ s⁻¹ and at 500 nm and above, κ = 7.0 × 10³ s⁻¹.

The presented results are of interest for the radiation behaviour of MMC as well as for its application as an antitumor drug in the combined radiation-chemotherapy of patients.     

Semiempirical and ab initio calculations on the alkaline hydrolysis of the β-lactam ring Influence of the solvent

We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol⁻¹ due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol⁻¹. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol⁻¹. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol⁻¹.     

Theoretical study of the N---H tautomerism in free base porphyrin

The N---H tautomerism of free base porphyrin is investigated at the semiempirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary structures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensure that realistic delocalized structures are obtained. Both the semiempirical and the ab initio calculations predict that the interconversion between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized as minima in the potential energy surface and are 8–10 kcal mol⁻¹ higher in energy. The activation energy for the trans → cis interconversion is calculated to be approximately 23 kcal mol⁻¹ at the 3-21G level. The activation energy for the synchronous trans → trans interconversion is higher and has a value of 30.5 kcal mol⁻¹. The activation energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values.     

Kinetic study of trifluoromethylation with s-(trifluoromethyl) dibenzothiophenium salts

The kinetic parameters were determined for C-trifluoromethylation of aniline with S-(trifluoromethyl)dibenzothiophenium triflate (1), its 3,7-dinitro derivative (2) and S-(trifluoromethyl)diphenylsulfonium triflate (3) in DMF-d₇. The higher reactivity of heterocyclic 1 compared with non-heterocyclic 3 could be explained on the basis of its greatly enhanced activation entropy (ΔS^≠: −11.2 cal mol ⁻¹ K⁻¹ for 1; −47.1 for 3), but not its enhanced activation enthalpy (ΔH^≠: 21.2 kcal mol⁻¹ for 1; 12.1 for 3). The aromatic delocalization of the heterocyclic ring may thus be only slightly disturbed by the S-trifluoromethyl substituent. The high reactivity of 2 was attributed to the great electron deficiency caused by two nitro groups in addition to the heterocyclic salt system (ΔH^≠ 17.0 kcal mol⁻¹, ΔS^≠ −9.1 cal mol⁻¹ K⁻¹ for 2). The reaction mechanism is discussed; the conventional S_N2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF₃-S bond may possibly be applicable.     

MNDO study of the proton affinity of fluorinated formaldehydes and acetones

The MNDO molecular orbital method is employed to calculate the proton affinities of fluorinated formaldehydes and acetones. Agreement with experimentally reported proton affinities is good. In the acetone series a decrease in proton affinity is calculated for each successive fluorine substituent. The calculated strength of the intramolecular hydrogen bond in the protonated fluoro-formaldehydes and acetones is 0.6—2.7 kcal mol⁻¹, in good agreement with the experimental value of 2—3 kcal mol⁻¹ in the protonated fluoroacetones. Examination of the calculated charge distribution shows that the trends in proton affinity can be understood qualitatively both in terms of initial-state and final-state effects caused by the fluorine substituents. Protonation at the fluorine atom is less stable by about 25 kcal mol⁻¹ than protonation at the oxygen atom for monofluoroacetone.     

Selective spectrophotometric determination of palladium(II) with 2(5-nitro-2-pyridylazo)-5-(N-propyl-N-3-sulfopropylamino)phenol(5-NO(2).PAPS) and tartaric acid with 5-NO(2).PAPS-niobium(V) complex

Spectrophotometric determinations of palladium(II) and tartaric acid were respectively investigated by using the color reactions between 2(5-nitro-2-pyridylazo)-5-(N-propyl-N-3-sulfopropylamino)phenol(5-NO₂.PAPS) and palladium(II) in strong acidic media, and between 5-NO₂.PAPS, niobium(V) tartaric acid in weak acidic media. The calibration graphs were linear in the range of 0–25 μg/10 ml palladium(II), with an apparent molecular coefficient () of 6.2×10⁴ l mol⁻¹ cm⁻¹ at 612 nm, and 0–23 μg/10 ml tartaric acid with =1.08×10⁶ l mol⁻¹ cm⁻¹ at 612 nm, respectively. The proposed methods were selective and sensitive in comparison with other chelating pyridylazo dyes–palladium(II) or metavanadic acid–tartaric acid method, and the effect of foreign ions such as copper(II) was negligible for the assay of palladium(II) with 5-NO₂.PAPS.     

About the TATB assumption: effect of charge reversal on transfer of large spherical ions from aqueous to non-aqueous solvents and on their interfacial behaviour

We present a molecular dynamics study of the solvation properties of large spherical ions S⁺ and S⁻ of same size, in water, chloroform and acetonitrile solutions, and at a water–chloroform interface. According to the “extrathermodynamic” TATB hypothesis, such ions have identical free energies of transfer from water to any solvent. We find that this is not the case, because S⁻ interacts better than S⁺ with water (by about 20 kcal mol⁻¹), while S⁺ is better solvated by acetonitrile (by about 2 kcal mol⁻¹) and chloroform (about 8 kcal mol⁻¹) solvents. The importance of “long-range” electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate: (i) the electrostatic potential at the centre of the solute; (ii) the interaction energies between the ions and the solvents; and (iii) the free energies of charging the neutral sphere S⁰ to S⁺ and S⁻, respectively. These conclusions are obtained with several solvent models and simulation conditions. The question of ion pairing for the S⁺S⁻, S⁺Cl⁻ and S⁻Na⁺ pairs is also examined in the three solvents. Finally, simulations at a liquid–liquid water–chloroform interface represented explicitly, show that S⁺ and S⁻ are highly surface active, although they do not possess, like classical surfactants, an amphiphilic topology. Adsorption at the interface is found with different methodologies and at different ion concentrations. These results are important in the context of the “TATB hypothesis”, and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous liquid environments.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Transients and cooperative action of β-carotene, vitamin E and C in biological systems in vitro under irradiation

Using N^•₃ species as specific electron acceptor a defined ascorbate radical: AH^•↔A^•−+H⁺ (λ_max=360 nm, =3400 dm³ mol⁻¹ cm⁻¹) is observed. The attack of DMSO^•+ on vit.E results in a vit.E^• radical (k=1×10⁹ dm³ mol⁻¹ s⁻¹; λ_max=425 nm, =2400 dm³ mol⁻¹ cm⁻¹; 2k=4.7×10⁸ dm³ mol⁻¹ s⁻¹). Vit.E-acetate leads to the formation of a radical cation (vit.E-ac^•+). β-carotene reacts also with DMSO^•+ forming a radical cation, β-car^•+ (k=1.75×10⁸ dm³ mol⁻¹ s⁻¹; λ_max=942 nm, =14 600 dm³ mol⁻¹ cm⁻¹), which probably leads to the formation of a dimer radical cation, (β-car)^•+₂ (k=2.5×10⁷ dm³ mol⁻¹ s⁻¹).

Using E.coli bacteria (AB1157) as a model system in vitro it was found that all three vitamins are rather efficient radiation protecting agents. They can also increase the activity of cytostatica, e.g., mitomycin C (MMC), by electron transfer process. The mixture of vit.E-ac and β-car acts contradictory, but adding vit.C to it a strong cooperative enhancement of the MMC activity is observed once again. A relationship between the pulse radiolysis and the radiation biological data is found and discussed. A possible explanation of the previously reported trials concerning the role of vit.E and β-car on the increased occurence of lung and other types of cancer in smokers and drinkers is presented.     

Reactions of oxidizing radicals with 2- and 3-aminopyridines: a pulse radiolysis study

Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O₂ was determined to be 1.0×10⁸ dm³ mol⁻¹ s⁻¹. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the pK_a of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO₄^−√ radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a pK value for the OH adduct. pK_a of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen (k=7.6×10⁶ dm³ mol⁻¹ s⁻¹). Specific one-electron oxidants like N₃^√, Br₂^−√ C₂^−√ and SO₄^−√ were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy.     

Triplet excited-state characterization and determination of the photoionization mechanism of the antitumoral drug pazelliptine

The triplet properties of the excited triplet state of pazelliptine (PZE), an antitumoral drug derived from ellipticine, were investigated in dioxane, ethanol and buffer aqueous solutions using the laser flash photolysis technique. The triplet absorption spectra and the kinetic parameters associated with the excited state decay were quite similar in the different solvents. ³PZE reacted with unexcited PZE in deaerated solutions (k = 6 × 10¹⁰ M⁻¹ s⁻¹) and was quenched by oxygen (k ≈ 2 × 10⁷ s⁻¹). The extinction coefficients of the triplet transition were estimated and used to calculate the singlet-triplet intersystem crossing quantum yields of about 5%.

A biphotonic ionization of PZE in buffer aqueous solution has been demonstrated in a previous work. This process was also observed in ethanol but not in dioxane. Mixed yttrium aluminum garnet laser harmonics (355 nm + 532 nm) and delayed-pulse experiments were carried out in order to determine the intermediate excited state involved in this photoionization process. The results indicate that pazelliptine radical cation and e⁻_s are formed via a consecutive two-photon absoprtion in which the first excited singlet state is the only intermediate.     

TRIPLET EXCITATION TRANSFER INVOLVING β-IONONE. A KINETIC STUDY BY LASER FLASH PHOTOLYSIS

Abstract— Employing nanosecond laser flash photolysis, β-ionone (BI) has been examined as an acceptor and as a donor of triplet excitation. In the limit of diffusion-control as well as below it, the rate constants for the quenching of a series of sensitizer triplets by BI are2–3 times smaller than those by 2,4-hexadienal (HD), although the triplet energies (spectroscopic) of the two carbonyl-containing dienes are estimated to be the same (∼55 kcal mol^-1). We attribute the difference to a steric effect arising from ground-state geometric distortion and heavy alkyl-substitution in BI. In spite of possible exothermic energy transfer, BI triplet is nearly nonquenchable by azulene and ferrocene; this is explainable by torsional relaxation to an equilibrium geometry at which the vertical energy gap is smaller than 40 kcal mol^-1. The singlet oxygen yield from the interaction of BI triplet with oxygen in benzene is estimated to be 0.5, suggesting that spin-exchange and energy-transfer may be involved to the same extent in the oxygen quenching process.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

Structures and energetics of C3H6S isomers: a Gaussian-3 ab initio study

Twenty-two isomers/conformers of C₃H₆S^+√ radical cations have been identified and their heats of formation (ΔH_f) at 0 and 298 K have been calculated using the Gaussian-3 (G3) method. Seven of these isomers are known and their ΔH_f data are available in the literature for comparison. The least energy isomer is found to be the thioacetone radical cation (4⁺) with C_2v symmetry. In contrast, the least energy C₃H₆O^+√ isomer is the 1-propen-2-ol radical cation. The G3 ΔH_f298 of 4⁺ is calculated to be 859.4 kJ mol⁻¹, ca. 38 kJ mol⁻¹ higher than the literature value, ≤821 kJ mol⁻¹. For allyl mercaptan radical cation (7⁺), the G3 ΔH_f298 is calculated to be 927.8 kJ mol⁻¹, also not in good agreement with the experimental estimate, 956 kJ mol⁻¹. Upon examining the experimental data and carrying out further calculations, it is shown that the G3 ΔH_f298 values for 4⁺ and 7⁺ should be more reliable than the compiled values. For the five remaining cations with available experimental thermal data, the agreement between the experimental and G3 results ranges from fair to excellent.

Cation CH₃CHSCH₂^+√ (10⁺) has the least energy among the eleven distonic radical cations identified. Their ΔH_f298 values range from 918 to 1151 kJ mol⁻¹. Nevertheless, only one of them, CH₂=SCH₂CH₂^+√ (12⁺), has been observed. Its G3 ΔH_f298 value is 980.9 kJ mol⁻¹, in fair agreement with the experimental result, 990 kJ mol⁻¹.

A couple of reactions involving C₃H₆S^+√ isomers CH₂=SCH₂CH₂^+√ (12⁺) and trimethylene sulfide radical cation (13⁺) have also been studied with the G3 method and the results are consistent with experimental findings.     

Persulphate quenching of the excited state of ruthenium(II) tris-bipyridyl dication: thermal reactions

The rate constant for the reaction between the sulphate radical (SO₄^√−) and the ruthenium (II) tris-bipyridyl dication (Ru(bipy)₃²⁺) is (3.3±0.2)×10⁹ mol⁻¹ dm³ s⁻¹ in 1 mol dm⁻³ H₂SO₄ and (4.9±0.5)×10⁹ mol⁻¹ dm³ s⁻¹ in 0.1 mol dm⁻³, pH 4.7 acetate buffer. The SO₄^√−radical produced by the electron transfer quenching of Ru(bipy)₃^2+* by S₂O₈²⁻ reacts rapidly with both acetate buffer and chloride ions. These side reactions result in a reduction in the overall quantum yield of Ru(bipy)₃³⁺ production and reduced reaction selectivity when Ru(bipy)₃^2+* is quenched by persulphate.     

Ab initio molecular orbital study on three feasible mechanisms for substitution of the vinylic carbon in F2C=C(OMs)BMe3−

A substitution on 2,2-difluorovinylic carbon was investigated by using ab initio molecular orbital calculations. Three feasible mechanisms, which are the S_N1-like, the S_N2-type and the addition-elimination mechanisms, were ex- amined for a model borate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borate. Four TSs were obtained depending on the position of Li⁺ around the vinylborate although activation energies in the gas phase are rather high (ca. 30–40 kcal mol⁻¹) in comparison with that expected from the experimental conditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the acti- vation energy of one S_N2-type mechanism very much (4. l kcal mol⁻¹ at the B3LYP/6-31+G^*//RHF/6-31+G^/s* level of theory) while those for the other mechanisms do not change very much. Therefore, the S_N2-type mechanism is applicable to the substitution reaction observed for the vinylborate.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.