Thermolysis of 3‐alkyl‐3‐methyl‐1,2‐dioxetanes: activation parameters and chemiexcitation yields

3‐Methyl‐3‐(3‐pentyl)‐1,2‐dioxetane 1 and 3‐methyl‐3‐(2,2‐dimethyl‐1‐propyl)‐1,2‐dioxetane 2 were synthesized in low yield by the α‐bromohydroperoxide method. The activation parameters were determined by the chemiluminescence method (for 1 ΔH‡ = 25.0 ± 0.3 kcal/mol, ΔS‡ = −1.0 entropy unit (e.u.), ΔG‡ = 25.3 kcal/mol, k₁ (60°C) = 4.6 × 10⁻⁴s⁻¹; for 2 ΔH‡ = 24.2 ± 0.2 kcal/mol, ΔS‡ = −2.0 e.u., ΔG‡ = 24.9 kcal/mol, k₁ (60°C) = 9.2 × 10⁻⁴s⁻¹. Thermolysis of 1–2 produced excited carbonyl fragments (direct production of high yields of triplets relative to excited singlets) (chemiexcitation yields for 1: ϕ^T = 0.02, ϕ^S ≤ 0.0005; for 2: ϕ^T = 0.02, ϕ^S ≤ 0.0004). The results are discussed in relation to a diradical‐like mechanism. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:176–179, 2001     

Thermolysis of 3‐alkyl‐3‐methyl‐1,2‐dioxetanes: Activation parameters and chemiexcitation yields

3‐Methyl‐3‐(3‐pentyl)‐1,2‐dioxetane 1 and 3‐methyl‐3‐(2,2‐dimethyl‐1‐propyl)‐1,2‐dioxetane 2 were synthesized in low yield by the α‐bromohydroperoxide method. The activation parameters were determined by the chemiluminescence method (for 1 ΔH‡ = 25.0 ± 0.3 kcal/mol, ΔS‡ = −1.0 entropy unit (e.u.), ΔG‡ = 25.3 kcal/mol, k₁ (60°C) = 4.6 × 10⁻⁴s⁻¹; for 2 ΔH‡ = 24.2 ± 0.2 kcal/mol, ΔS‡ = −2.0 e.u., ΔG‡ = 24.9 kcal/mol, k₁ (60°C) = 9.2 × 10⁻⁴s⁻¹. Thermolysis of 1–2 produced excited carbonyl fragments (direct production of high yields of triplets relative to excited singlets) (chemiexcitation yields for 1: ϕ^T = 0.02, ϕ ≤ 0.0005; for 2: ϕ^T = 0.02, ϕ^S ≤ 0.0004). The results are discussed in relation to a diradical‐like mechanism. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:459–462, 2001     

Thermolysis of 3-methyl-3-(o-aryl)-1,2-dioxetanes: Activation parameters and chemiexcitation yields

3-Methyl-3-(o-tolyl)-1,2-dioxetane 1 and 3-methyl-4-(o-bromophenyl)-1,2-dioxetane 2 were synthesized in low yield by the β-bromo hydroperoxide method. The activation parameters were determined by the chemilumin-escence method (for 1 ΔG^? = 24.7 ± 0.3 kcal/mol, ΔH^? = 25.4, ΔS^? = + 1.9 e.u., k₆₀ = 3.4 × 10^?4s^?1; for 2 ΔG^? = 24.7 ± 0.4 kcal/mol, ΔH^? = 24.7, ΔS^? = 0.0 e.u., k₆₀ = 4.1 × 10^?4s^?1). Thermolysis of 1–2 directly produced high yields of excited triplets as expected for this type of dioxetane [triplet chemiexcitation yields (?⁷) for 1 0.03; for 2 0.02; the ?^T/?^S ratios were estimated to be approximately 200 for both compounds]. The effect of ortho-aryl substituents was inconsistent with electronic effects. The ortho substitution in 1–2 resulted in a marked increase in stability of the dioxetanes. The results are discussed in relation to a diradical-like mechanism.     

Thermolysis of hexasubstituted‐4,5‐dihydro‐3H‐pyrazoles: Kinetics and activation parameters

A kinetics study of the thermolysis of a series of hexasubstituted‐4,5‐dihydro‐3H‐pyrazoles (pyrazolines 1a: 3,3,4,4‐tetramethyl‐5‐phenyl‐5‐acetoxy; 1b: cis‐3,5‐diphenyl‐3,3,4‐trimethyl‐5‐acetoxy; 1c: cis‐3,5‐diphenyl‐3,4,4‐trimethyl‐5‐methoxy; 1d: 3,3,5‐triphenyl‐4,4‐dimethyl‐5‐acetoxy), which produced the corresponding hexasubstituted cyclopropanes 2a–d in quantitative yields was carried out. The first order rate constants (k₁) for thermal decomposition and activation parameters were determined. The relative reactivity series was found to be 1d >> 1b ∼ 1c > 1a. The activation parameters for thermolysis were found to be: for 1a ΔH‡ = 39.8 kcal/mol, ΔS‡ = 14 eu, k_150° = 6.8 × 10⁻⁵ s⁻¹; for 1b ΔH‡ = 33.5 kcal/mol, ΔS ‡ = 0.2 eu, k_150° = 1.7 × 10⁻⁴s⁻¹; for 1c ΔH‡ = 32.7 kcal/mol, ΔS‡ = −1.8 eu, k_150° = 1.2 × 10⁻⁴s⁻¹; for 1d ΔH‡ = 30.1 kcal/mol, ΔS‡ = −1.6 eu, k_150° = 8.8 × 10⁻³s⁻¹. The effect of variation of C3 substituents on the activation parameters for thermolysis paralleled the trend reported for acyclic analogs. The results are consistent with the formation of a (singlet) 1,3‐diradical intermediate with subsequent closure to yield the cyclopropanes. The mechanism of diradical formation appears to involve N₂‐C₃ bond cleavage as the rate determining step rather than simultaneous two bond scission. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:299–302, 2000     

Reaction of tetramethyl-1,2-dioxetane with triphenylphosphine: Activation parameters for the formation of 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane

The reaction of tetramethyl-1,2-dioxetane ( 1 ) and triphenylphosphine ( 2 ) in benzene-d₆ produced 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane ( 3 ) in ?90% yield over the temperature range of 6–60°. Pinacolone and triphenylphosphine oxide ( 4 ) were the major side products [additionally acetone (from thermolysis of 1 ) and tetramethyloxirane ( 5 ) were noted at the higher temperatures]. Thermal decomposition of 3 produced only 4 and 5 . Kinetic studies were carried out by the chemiluminescence method. The rate of phosphorane was found to be first order with respect to each reagent. The activation parameters for the reaction of 1 and 2 were: Ea ? 9.8 ± 0.6 kcal/mole; ΔS^≠ = ?28 eu; k_30° = 1.8 m^?1sec^?1 (range = 10–60°). Preliminary results for the reaction of 1 and tris (p-chlorophenyl)phosphine were: Ea ? 11 kcal/mole, ΔS^≠ = ?24 eu, k_30° = 1.3 M^?1sec^?1 while those for the reaction of 1 and tris(p-anisyl)phosphine were: Ea ? 8.6 kcal/mole, ΔS^≠ = ?29 eu, k_30° = 4.9 M^?1 sec^?1.     

Nucleophilic and electrophilic promotion of ligand substitution reactions in oxo‐bridged,dinuclear chromium(III) complexes

Base hydrolysis reactions of [Cr(tmpa)(NCSe)]₂O²⁺, [Cr(tmpa)(N₃)]₂O²⁺, [Cr₂(tmpa)₂(μ−O)(μ−PhPO₄)]⁴⁺ and [Cr₂(tmpa)₂(μ−O)(μ−CO₃)]²⁺ follow the pseudo‐first‐order relationship (excess OH⁻): k_obsd=k_o+k_bQ_p[OH⁻]/(1+Q_p[OH⁻]). For the CO₃²⁻ complex, k_b(60°C)=(1.50±0.03)×10⁻² s⁻¹; ΔH‡=61±2 kJ/mol, ΔS‡=−99±7 J/mol K; Q_p(60°C)=(3.8±0.3)×10¹ M⁻¹; ΔH°=67±2 kJ/mol, ΔS°=230±7 J/mol K (I=1.0 M). An isokinetic relationship among k_OH(=k_bQ_p) activation parameters for five (tmpa)CrOCr(tmpa) complexes shows that all follow essentially the same pathway. Activated complex formation is thought to require nucleophilic attack of coordinated OH⁻ at the chromium‐leaving group bond in the k_b step, accompanied by reattachment of a tmpa pyridyl arm displaced by OH⁻ in the Q_p preequilibrium. Abstraction of both thiocyanate ligands was observed upon mixing [Cr(tmpa)(NCS)]₂O²⁺ with [Pd(CH₃CN)₄]²⁺ in CH₃CN solution. The proposed mechanism requires rapid complexation of both reactant thiocyanate ligands by Pd(II) (K_p(25°C)=(4.5±0.2)×10⁸ M⁻²; ΔH°=−32±6 kJ/mol, ΔS°=59±19 J/mol K) prior to rate‐limiting Cr NCS bond‐breaking (k₂(25°C)=(1.17±0.02)×10⁻³ s⁻¹; ΔH‡=98±2 kJ/mol, ΔS‡=27±5 J/mol K). Pd(II)‐assisted NCS⁻ abstraction is not driven by weakening of the Cr( )NCS⁻ bond through ligation of the sulfur atom to palladium, but rather by a favorable ΔS‡ resulting from the release of Pd(NCS)⁺ fragments and weak solvation of the activated complex in CH₃CN solution. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 351–356, 1999     

Dynamics of a conformational change in aqueous 18-crown-6 by an ultrasonic absorption method

A temperature dependence study of the ultrasonic amplitudes, velocities, and relaxation times for a presumed conformational transition of noncomplexed aqueous 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is discussed. At all temperatures a single relaxation was observed within a 15–255-MHz frequency range. The equilibrium constant for the presumed conformational transition \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CR}_1 \mathop \rightleftarrows\limits^{K_{12} } {\rm CR}_2 $\end{document} was determined to be K₂₁ = (2 ± 2) × 10^?2. The activation parameters are ΔH₂₁^≠ = 10.2 ± 1.0 kcal/mol, ΔS₂₁^≠ = 7.7 ± 0.2 cal/(mol·deg), ΔH₁₂^≠ = 7.4 ± 1.0 kcal/mol, and ΔS₁₂^≠ = 7.7 ± 0.2 cal/(mol·deg), while the thermodynamic enthalpy and entropy were found to be ?2.6 ± 1.0 kcal/mol and 0 ± 0.2 cal/(mol·deg), respectively. The rate constants at 25.0°C for the presumed conformational transition are k₂₁ = (1.0 ± 0.3) × 10⁷ sec^?1 and k₁₂ = (6.2 ± 0.2) × 10⁸ sec^?1.     

Preparation and Diels-Alder Reactivity of 2,3,5,6,7,8-Hexamethylidenebicyclo [2.2.2]octane (‘[2.2.2]Hericene’). Force-field calculations of exocyclic dienes as a moiety of bicyclic skeletons

The [2.2.2]hericene ( 6 ), a bicyclo[2.2.2]octane bearing three exocyclic s-cis-butadiene units has been prepared in eight steps from coumalic acid and maleic anhydride. The hexaene 6 adds successively three mol-equiv. of strong dienophiles such as ethylenetetracarbonitrile (TCE) and dimethyl acetylenedicarboxylate (DMAD) giving the corresponding monoadducts 17 and 20 (k₁), bis-adducts 18 and 21 (k₂) and tris-adducts 19 and 22 (k₃), respectively. The rate constant ratio k₁/k₂ is small as in the case of the cycloadditions of 2,3,5,6-tetramethylidene-bicyclo [2.2.2]octane ( 3 ) giving the corresponding monoadducts 23 and 27 (k₁) and bis-adducts 25 and 29 (k₂) with TCE and DMAD, respectively. Constrastingly, the rate constant ratio k₂/k₃ is relatively large as the rate constant ratio k₁/k₂ of the Diels-Alder additions for 5,6,7,8-tetramethylidenebicyclo [2.2.2]oct-2-ene ( 4 ) giving the corresponding monoadducts 24 and 28 (k₁) and bis-adducts 26 and 30 (k₂). The following second-order rate constants (toluene, 25°) and activation parameters were obtained for the TCE additions: 3 +TCE→ 23 : k₁ = 0.591±0.012 mol^?1·l·s^?1, ΔH^≠=10.6±0.4 kcal/mol, and ΔS^≠ = ?24.0±1.4 cal/mol·K (e.u.); 23 +TCE→ 25 : k₂=0.034±0.0010 mol^?1·l·s^?1, ΔH^≠ = 10.6±0.6 kcal/mol, and ΔS^≠ = ?29.7±2.0 e.u.; 4 +TCE→ 26 : k₁ = 0.172±0.035 mol^?1·l·s^?1, ΔH^≠ 11.3±0.8 kcal/mol, and ΔS^≠ = ?24.0±2.8 e.u.; 24 +TCE→ 26 : k₂ = (6.1±0.2)·10^?4 mol^?1·l·s^?1, ΔH^≠ = 13.0±0.3 kcal/mol, and ΔS^≠ = ?29.5±0.8 e.u.; 6 +TCE→ 17 : k₁ = 0.136±0.002 mol^?1·l·s^?1, ΔH^≠ = 11.3±0.2 kcal/mol, and ΔS^≠ = ?24.5±0.8 e.u.; 17 +TCE→ 18 : k₂ = 0.0156±0.0003 mol^?1·l·s^?1, ΔH^≠ = 10.9±0.5 kcal/mol, and ΔS^≠ = ?30.1 ± 1.5 e.u.; 18 +TCE→ 19 : k₃=(5±0.2) · 10^?5 mol^?1 mol^?1 ·l·s^?1, ΔH^≠ = 15±3 kcal/mol, and ΔS^≠ = ?28 ± 8 e.u. The following rate constants were evaluated for the DMAD additions (CD₂Cl₂, 30°): 6 +DMAD→ 20 : k₁ = (10±1)·10^?4 mol^?1 · l·s^?1; 20 +DMAD→ 21 : k₂ = (6.5±0.1) · 10^?4 mol^?1 ·l·^?1; 21 +DMAD→ 22 : k₃ = (1.0±0.1) · 10^?4 mol^?1 ·l·s^?1. The reactions giving the barrelene derivatives 19, 22, 26 and 30 are slower than those leading to adducts that are not barrelenes. The former are estimated less exothermic than the latter. It is proposed that the Diels-Alder reactivity of exocyclic s-cis-butadienes grafted onto bicycle [2.2.1]heptanes and bicyclo [2.2.2]octanes that are modified by remote substitution of the bicyclic skeletons can be affected by changes inthe exothermicity of the cycloadditions, in agreement with the Dimroth and Bell-Evans-Polanyi principle. Force-field calculations (MMPI 1) of 3, 4, 6 and related exocyclic s-cis-butadienes as a moiety of bicyclo [2.2.2]octane suggested single minimum energy hypersurfaces for these systems (eclipsed conformations, planar dienes). Their flexibility decreases with the degree of unsaturation of the bicyclic skeleton. The effect of an endocyclic double bond is larger than that of an exocyclic diene moiety.     

Theoretical and experimental studies of the diketene system: Product branching decomposition rate constants and energetics of isomers

The kinetics and mechanism for the thermal decomposition of diketene have been studied in the temperature range 510–603 K using highly diluted mixtures with Ar as a diluent. The concentrations of diketene, ketene, and CO₂ were measured by FTIR spectrometry using calibrated standard mixtures. Two reaction channels were identified. The rate constants for the formation of ketene (k₁) and CO₂ (k₂) have been determined and compared with the values predicted by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory for the branching reaction. The first-order rate constants, k₁ (s⁻¹) = 10^{15.74 ± 0.72} exp(−49.29 (kcal mol⁻¹) (±1.84)/RT) and k₂ (s⁻¹) = 10^{14.65 ± 0.87} exp(−49.01 (kcal mol⁻¹) (±2.22)/RT); the bulk of experimental data agree well with predicted results. The heats of formation of ketene, diketene, cyclobuta-1,3-dione, and cyclobuta-1,2-dione at 298 K computed from the G2M scheme are −11.1, −45.3, −43.6, and −40.3 kcal mol⁻¹, respectively. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 580–590, 2007     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

Thermal isomerization and decomposition of 1,2-butadiene in shock waves

1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density range of 1100–1600 K and 1.36 × 10^?5 ? 1.75 × 10^?5 mol/cm³. The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C₂H₆, C₂H₄, CH₄, and benzene, which were analyzed by gas chromatography. The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K. In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers. The present data were successfuly modeled with a 82 reaction mechanism. From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k₃ = 2.5 × 10¹³ exp(?63 kcal/RT) s^?1 and for the decomposition 1,2-butadiene = C₃H₃ + CH₃ to be k₆ = 2.0 × 10¹⁵ exp(?75 kcal/RT) s^?1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed. These rate constants are only applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

Rate coefficients for the gas‐phase reaction of OH radicals with selected dihydroxybenzenes and benzoquinones

Rate coefficients have been determined for the gas‐phase reaction of the hydroxyl (OH) radical with the aromatic dihydroxy compounds 1,2‐dihydroxybenzene, 1,2‐dihydroxy‐3‐methylbenzene and 1,2‐dihydroxy‐4‐methylbenzene as well as the two benzoquinone derivatives 1,4‐benzoquinone and methyl‐1,4‐benzoquinone. The measurements were performed in a large‐volume photoreactor at (300 ± 5) K in 760 Torr of synthetic air using the relative kinetic technique. The rate coefficients obtained using isoprene, 1,3‐butadiene, and E‐2‐butene as reference hydrocarbons are k_OH(1,2‐dihydroxybenzene) = (1.04 ± 0.21) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,2‐dihydroxy‐3‐methylbenzene) = (2.05 ± 0.43) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,2‐dihydroxy‐4‐methylbenzene) = (1.56 ± 0.33) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,4‐benzoquinone) = (4.6 ± 0.9) × 10⁻¹² cm³ s⁻¹, k_OH(methyl‐1,4‐benzoquinone) = (2.35 ± 0.47) × 10⁻¹¹ cm³ s⁻¹. This study represents the first determination of OH radical reaction‐rate coefficients for these compounds. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 696–702, 2000     

Molecularly imprinted polymers as artificial steroid receptors

The polymers selective to six different steroids (testosterone, Δ⁴-androstene-3,17-dione, 1,4-androstadiene-3,17-dione, β-estradiol, progesterone, testosterone propionate) have been synthesized using molecular imprinting based on noncovalent interactions. Analysis of the influence of structural features of the steroids under study has shown that molecules with a relatively rigid structure and the OH group at C-17 position are the most efficient templates for methacrylic acid-containing imprinted polymers. The chromatographic study of the polymers synthesized has demonstrated a strong dependence of the selectivity and intensity of interaction with analytes on the composition of solvents used both as porogen and chromatographic mobile phase. To obtain polymers with highly selective recognition sites and to create the optimal conditions for molecular recognition, all possible interactions (between template and functional monomer, template and solvent, solvent and functional monomer) should be taken into account. <?TF="palat-i"> The batch rebinding study of testosterone by the imprinted polymer in acetonitrile has revealed some heterogeneity of recognition sites, and permitted determination of K_ass = 1.05 × 10⁴ M ⁻¹, ΔG° = −5.4 kcal/mol and N = 1.2 μmol/g for high-affinity sites and K_ass = 0.33 × 10⁴ M ⁻¹, ΔG° = −4.8 kcal/mol and N = 2.2 μmol/g for low-affinity sites. <?TF="palat-i"> The results obtained show how it is possible to regulate in different modes the molecular recognition by imprinted polymers as well as to fabricate polymers possessing the necessary properties depending on their practical application.© 1998 John Wiley & Sons, Ltd.     

Kinetics of iodination of hydrogen sulfide by iodine and the heat of formation of the SH radical

The kinetics of the gas-phase thermal iodination of hydrogen sulfide by I₂ to yield HSI and HI has been investigated in the temperature range 555–595 K. The reaction was found to proceed through an I atom and radical chain mechanism. Analysis of the kinetic data yields log k (l/mol·sec) = (11.1 ± 0.18) – (20.5 ± 0.44)/θ, where θ = 2.303 RT, in kcal/mol. Combining this result with the assumption E_?1 = 1 ± 1 kcal/mol and known values for the heat of formation of H₂S, I₂, and HI, ΔH_f,298⁰(SH) = 33.6 ± 1.1 kcal/mol is obtained. Then one can calculate the dissociation energy of the HS? H bond as 90.5 ± 1.1 kcal/mol with the well-known values for ΔH_f,298⁰ of H and H₂S.     

Substituenteneffekte auf die C–C-Bindungsstärke, 8. Standardbildungsenthalpien und Spannungsenthalpien von 1,1,2,2-Tetraethylethylenglykol-dimethylether und D,L-1,2-Dimethyl-1,2-diphenylethylenglykol-dimethylether

Effects of Substituents on the Strength of C - C Bonds, 8¹. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol ^-1, s. d. in parentheses) ΔH°_c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔH^v = 14.3 (± 0.3) and the heat of sublimation of 2 ΔH^sub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°_t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (_s) of 15.9 and 8.0 kcal mol^-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group.     

Kinetics and mechanism of the reaction of sulphur(IV) with N,N′‐ethylene‐bis(sali‐cylidiniminato)manganese‐(III) in aqueous solution

The reaction of (diaqua)(N,N′‐ethylene‐bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, [Mn(Salen)(SO₃)]⁻ (S‐bonded isomer) via three distinct paths: (i) Mn(Salen)(OH₂)₂⁺ + HSO₃⁻ → (k₁); (ii) Mn(Salen)(OH₂)₂⁺ + SO₃²⁻ → (k₂); (III) Mn(Salen)(OH₂)(OH) + SO₃²⁻ → (k₃) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO₃²⁻/HSO₃⁻ may be attributed to the strong trans‐activating influence of the S‐bonded sulphite. The values of the rate constants (10⁻²k_i/dm² mol⁻¹ s⁻¹ at 25°C, I = 0.3 mol dm⁻³), ΔH_i^#/kJ mol⁻¹ and ΔS_i^#/J K⁻¹ mol⁻¹ respectively are: 2.97 ± 0.27, 42.4 ± 0.2, −55.3 ± 0.6 (i = 1); 11.0 ± 0.8, 33 ± 3, −75 ± 10 (i = 2); 20.6 ± 1.9, 32.4 ± 0.2, −72.9 ± 0.6 (i = 3). The trend in reactivity (k₂ > k₁), a small labilizing effect of the coordinated hydroxo group (k₃/k₂ < 2), and substantially low values of ΔS^# suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua‐hydroxo complexes is most likely associative interchange (I_a). No evidence for the formation of the O‐bonded sulphito complex and the ligand isomerization in the sulphito complex, (Mn^III‐OSO₂ → Mn^III‐SO₃), ensures the selectivity of the Mn^III centre toward the S‐end of the S^IV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce Mn^II, S₂O₆²⁻ and SO₄²⁻. The formation of dithionate is a consequence of the fast dimerization of the SO₃₋. generated in the rate determining step and also SO₄²⁻ formation is attributed to the fast scavenging of the SO₃₋. by the Mn^III species via a redox path. The internal reduction of the Mn^III centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO₃₋ and the other SO₃²⁻. The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (ΔS^# = −(1.3 ± 0.2) × 10² and −(1.6 ± 0.2) × 10² J K⁻¹ mol⁻¹ for the paths involving HSO₃₋ and SO₃²⁻ respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 627–635, 1999     

Ammonia–dimethylchloramine system: Kinetic approach in an aqueous medium and comparison with the mechanism involving liquid ammonia

After an exhaustive study of the system ammonia–dimethylchloramine in liquid ammonia, it was interesting to compare the reactivity of this system in liquid ammonia with the same system in an aqueous medium. Dimethylchloramine prepared in a pure state undergoes dehydrohalogenation in an alkaline medium: the principal products formed are N-methylmethanimine, 1,3,5-trimethylhexahydrotriazine, formaldehyde, and methylamine. The kinetics of this reaction was studied by UV, GC, and HPLC as a function of temperature, initial concentrations of sodium hydroxide, and chlorinated derivative. The reaction is of the second order and obeys an E2 mechanism (k₁ = 4.2 × 10⁻⁵ M⁻¹ s⁻¹, ΔH^○# = 82 kJ mol⁻¹, ΔS^○# = −59 J mol⁻¹ K⁻¹). The oxidation of unsymmetrical dimethylhydrazine by dimethylchloramine involves two consecutive processes. The first step follows a first-order law with respect to haloamine and hydrazine, leading to the formation of an aminonitrene intermediate (k₂ = 150 × 10⁻⁵ M⁻¹ s⁻¹). The second step corresponds to the conversion of aminonitrene into formaldehyde dimethylhydrazone at pH 13). This reaction follows a first-order law (k₃ = 23.5 × 10⁻⁵ s⁻¹). The dimethylchloramine–ammonia interaction corresponds to a SN2 bimolecular mechanism (k₄ = 0.9 × 10⁻⁵ M⁻¹ s⁻¹, pH 13, and T = 25°C). The kinetic model formulated on the basis of the above reactions shows that the formation of the hydrazine in an aqueous medium comes under strong competition from the dehydrohalogenation of dimethylchloramine and the oxidation of the hydrazine formed by the original chlorinated derivative. A global model that explains the mechanisms both in an anhydrous and in an aqueous medium was elaborated. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 340–351, 2008     

Rate constants for reactions of OH radicals with a series of asymmetrical ethers and tert-Butyl alcohol

Absolute rate constants for the reactions of OH radicals with butyl ethyl ether (k₁), methyl tert-butyl ether (k₂), ethyl tert-butyl ether (k₃) tert-amyl methyl ether (k₄) and tert-butyl alcohol (k₅) have been measured over the temperature range 230–372 K using a pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k₁ − k₅ when expressed in Arrhenius form gave: k₁ = (6.59 ± 0.66) × 10 ⁻¹² exp|(362 ± 60)/T|, k₂ = (5.03 ± 0.27) × 10⁻¹² exp|&minus(133 ± 30)/T|, k₃ = (4.40 ± 0.24) × 10⁻¹² exp|(210 ± 37)/T|,k⁴ = (4.7 ± 0.7) × 10⁻¹² exp|(82 ± 85)/T|, and k₅ = (2.66 ± 0.48) × 10⁻¹² exp| −(270 ± 130)/T|. However, the Arrhenius plots for k₁–k₅, were slightly curved and are best fitted by the three parameter fits which are given in the article. The room temperature values of k₁, k₂, k₃, k₄, and k₅ are (2.08 ± 0.23) × 10⁻¹¹, (3.13 ± 0.36) × 10⁻¹², (8.80 ± 0.50) × 10⁻¹², (6.28 ± 0.45) × 10⁻¹², and (1.08 ± 0.10) × 10⁻¹², respectively, in cm³ molecule⁻¹ s⁻¹. © 1996 John Wiley & Sons, Inc.     

Efficiency and excited state processes in a three-component system,based on thioxanthene derived dye/amine/additive,usable in photopolymer plates

The excited state interactions occurring when a three-component system of thioxanthene derived dye TXD/amine/additive (diphenyliodonium salt, CBr₄, benzoyl peroxide, cumene hydroperoxide) is subjected to sensitization processes in the visible range, were investigated through time-resolved absorption spectroscopy, spectrofluorometry, and photolysis. The rate constants of the various operative processes were measured together with the values of the fluorescence quantum yields (e.g. ϕ _f = 0.75 ± 0.07 in methanol) and the lifetimes of the singlet excited state of the dye (e.g. 6 ns in methanol). Singlet state quenching by methyldiethanolamine (MDEA) occurs with a rate constant k = 10⁹ M⁻¹ s⁻¹ in methanol. The reactivity of the triplet excited state of the dye is very low (k = 5.6 × 10⁴ M⁻¹s⁻¹ for the TXD/MDEA interaction). The ketyl radicals that arise from the interaction of the singlet state of the dye with the amine, are quenched by such additives as CBr₄ (k = 6.7 × 10⁵M⁻¹s⁻¹), or the onium salts (k = 5.7 × 10⁵M⁻¹ s⁻¹). The calculations of the yields of interaction of the singlet state of the dye with the two components of the system demonstrate that the process of quenching by the amine is the main one (ϕ_int = 0.5) compared to that by, e.g., an onium salt (ϕ_int = 0.07). Sensitivity of 0.3 mJ cm⁻² obtained in a laser scanning system is also reported. © 1996 John Wiley & Sons, Inc.     

Visible absorption spectral measurements of the formation of the cobalt porphyrin dioxygen complex using a high-pressure cell ag

The 1:1 complex formation between dioxygen and the meso-tetraphenylporphinatocobalt(II)pyridine complex in toluene was followed spectrophotometrically using a high-pressure cell. The values of the equilibrium constant K_p obtained are 0.0278, 0.0513, 0.0840, 0.157 and 0.278 atm⁻¹ at −30, −36, −42, −48 and −54dgC, respectively. The thermodynamic data are ΔH^θ = −10.1±0.6 kcal mol⁻¹ and gDS^θ = −49±3 eu at a standard pressure of 1 atm.