Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Transferts Protoniques de Sels d'Ammonium Substitues—VIII: Sels de Pyridinium 4-Substitues

The deprotonation rate 1/τ of the title compounds, [4 – R – Py H]⁺, where R = NH₂, t-Bu, Me, Cl, Br or CN, is measured using the coalescence of the pyridinic α-protons, in a mixture CF₃COOH/H₂O/HClO₄ of variable acidity Ho, at 38°C. 1/τ is a linear function k/ho of the acidity 1/ho. k is approximately proportional to the water content and independent of the salt concentration, which seems to be evidence for an exchange with an intermediate pyridine hydrate, according to: . After a preliminary ionisation step: k values, like K_A, fit a Hammett relationship (ρ = 5,05), except for R ? NH₂, and are very sensitive to the nature of R (k = 3,44 × 10² for R = NH₂ and k = 3,14 × 10⁸ M^?1 s^?1 for R ? CN), while k_H values (10¹⁰ s^?1) are not.     

Naked Eye Detection of Carbonate,Hydroxide, and Cyanide Ions with 1,4′‐Diazaflavonium Bromides: A Simple Spectrophotometric Method for Cyanide Determination

Anion sensor properties of N‐alkyl‐substituted 1,4′‐diazaflavonium bromides in methanol–water were evaluated by UV–vis spectrometry. Pronounced changes were observed in the absorption spectra of all compounds for only OH^?, CO₃^2?, and CN^? among F^?, Cl^?, Br^?, I^?, OH^?, CO₃^2?, NO₃^?, PO₄^3?, CN^?, SO₄^2?, HSO₄^?, HCO₃^?, SCN^?, NO₂^?, and P₂O₇^2? ions. Two new absorption bands at 385 and 685 nm accompanying the distinct color change for OH^?, CO₃^2?, and CN^? ions were observed in case of all compounds. The color changes were from pink to blue for CO₃^2? and OH^? ions and from pink to purple for CN^? ion. Thanks to the distinct color change, the compounds can be used as selective colorimetric anion sensors. Linear changes of absorbance of N‐heptyl‐substituted compound at 385 nm as a function of the ion concentration were used to determine CN^? ion in water samples. Detection and quantification limits of the proposed method were 0.94 and 2.82 mg/L, respectively.     

Action de nucléophiles simples sur un bromoénurononitrile,précurseur et équivalent synthétique partiel d'un ynurononitrile

Reactions of Mononucleophiles with a Bromoenurononitrile, Precursor and Partial Synthetic Equivalent of an Ynurononitrile Several mononucleophiles (bases) have been reacted with one or the other of the geometrical isomers of the bromoenurononitrile 1. Depending on the nucleophile and the conditions, many different mechanistic pathways were followed, f. ex.: with OH^?, stereospecific elimination from (Z)- 1 leading to 2 , with N^?₃ and F^?, stereospecific E-A_N reactions leading from (Z)- 1 to (Z)- 8 and (Z)- 12 respectively, with PhCH₂SH, conjugate nucleophilic addition to 7, with Me₂NH, conjugate nucleophilic addition followed by a S_N2 to 11 , as well as several cases of nonstereoselective, probably A_N-E, reactions leading to 3,6,9 and 10. In spite of their diversified reactivity, bromoenurononitriles like 1 , partial synthetic equivalent of 2 , constitute useful synthetic intermediates.     

Intrinsic Surface Reaction Constant in 1-pK Model of Mg-Fe Hydrotalcite-like Compounds

The relationship among intrinsic surface reaction constant (K) in 1-pK model, point of zero net charge (PZNC) and structural charge density (σst) for amphoteric solid with structural charges was established in order to investigate the effect of σst on pK. The theoretical analysis based on 1-pK model indicates that the independent PZNC of electrolyte concentration (c) exists for amphoteric solid with structural charges. A common intersection point (CIP) should appear on the acid-base titration curves at different c, and the pH at the CIP is pHPZNC. The pK can be expressed as pK=-pHPZNC log[(1 2αPZNC)/(1-2αPZNC)], where αPZNC≡σst/eNANs, in which e is the elementary charge, NA the Avogadro‘s constant and Ns the total density of surface sites. For solids without structural charges, pK=-pHPZNC. The pK values of hydrotalcite-like compounds (HTlc) with general formula of [Mg1-xFex(OH)2](Cl,OH)x were evaluated. With increasing x, the pK increases, which can be explained based on the affinity of metal cations for H^- or OH^- and the electrostatic interaction between charging surface and H^- or OH^-.     

Kinetic analysis of the disproportionation of aqueous glyoxal

Rates of disproportionation of 0.015–0.4 mM aqueous glyoxal toglycolic acid were measured at 0.24–75 mM NaOH and constant ionic strength, leading to the empirical rate expression r = (a₁[OH^?] + a₂[OH^?]²) [G_T]/(1 + a₃[OH^?]), where [G_T] is the total glyoxal concentration. These results were confirmed in bicarbonate/carbonate buffer and at 2–20 mM [G_T]. The rate form is in contradiction to earlier work on glyoxal, which suggested a second-order dependence on [OH^?], but agrees with the rate equation for phenylglyoxal disproportionation. The kinetic data can be explained by a mechanism postulating the presence of monohydrated and dihydrated forms of glyoxal in equilibrium, with the rate-limiting steps being intramolecular hydride ion transfers to the unhydrated carbonyl carbon of the mono- and divalent anions of glyoxal monohydrate.     

Mechanistic Origin of Antagonist Effects of Usual Anionic Bases (OH−, CO32−) as Modulated by their Countercations (Na+, Cs+, K+) in Palladium‐Catalyzed Suzuki–Miyaura Reactions

The mechanism of the reaction of trans‐ArPdBrL₂ (Ar=p‐Z‐C₆H₄, Z=CN, H; L=PPh₃) with Ar′B(OH)₂ (Ar′=p‐Z′‐C₆H₄, Z′=H, CN, MeO), which is a key step in the Suzuki–Miyaura process, has been established in N,N‐dimethylformamide (DMF) with two bases, acetate (nBu₄NOAc) or carbonate (Cs₂CO₃) and compared with that of hydroxide (nBu₄NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M⁺ (e.g., M⁺OH^?), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M⁺ (Na⁺, Cs⁺, K⁺) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L₂, partly inhibiting its transmetalation with Ar′B(OH)₂. A decreasing reactivity order is observed when M⁺ is associated with OH^?: nBu₄N⁺> K⁺> Cs⁺> Na⁺. Acetates lead to the formation of trans‐ArPd(OAc)L₂, which does not undergo transmetalation with Ar′B(OH)₂. This explains why acetates are not used as bases in Suzuki–Miyaura reactions that involve Ar′B(OH)₂. Carbonates (Cs₂CO₃) give rise to slower reactions than those performed from nBu₄NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH^?. The mechanism of the reaction with carbonates is then similar to that established for nBu₄NOH, involving ArPd(OH)L₂ in the transmetalation with Ar′B(OH)₂. Due to the low concentration of OH^? generated from CO₃^2? in water, both transmetalation and reductive elimination result slower than those performed from nBu₄NOH at equal concentrations as Cs₂CO₃. Therefore, the overall reactivity is finely tuned by the concentration of the common base OH^? and the ratio [OH^?]/[Ar′B(OH)₂]. Hence, the anionic base (pure OH^? or OH^? generated from CO₃^2?) associated with its countercation (Na⁺, Cs⁺, K⁺) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L₂, acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar′B(OH)₃^? and by complexation of ArPd(OH)L₂ by M⁺.     

Kinetics and mechanism of oxidation of the drug mephenesin by bis(hydrogenperiodato)argentate(III) complex anion

Mephenesin is being used as a central‐acting skeletal muscle relaxant. Oxidation of mephenesin by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO₆)₂]^5?, has been studied in aqueous alkaline medium. The major oxidation product of mephenesin has been identified as 3‐(2‐methylphenoxy)‐2‐ketone‐1‐propanol by mass spectrometry. An overall second‐order kinetics has been observed with first order in [Ag(III)] and [mephenesin]. The effects of [OH^?] and periodate concentration on the observed second‐order rate constants k′ have been analyzed, and accordingly an empirical expression has been deduced: k′ = (k_a + k_b[OH^?])K₁/{f([OH^?])[IO^?₄]_tot + K₁}, where [IO^?₄]_tot denotes the total concentration of periodate, k_a = (1.35 ± 0.14) × 10^?2M^?1s^?1 and k_b = 1.06 ± 0.01 M^?2s^?1 at 25.0°C, and ionic strength 0.30 M. Activation parameters associated with k_a and k_b have been calculated. A mechanism has been proposed to involve two pre‐equilibria, leading to formation of a periodato‐Ag(III)‐mephenesin complex. In the subsequent rate‐determining steps, this complex undergoes inner‐sphere electron transfer from the coordinated drug to the metal center by two paths: one path is independent of OH^? whereas the other is facilitated by a hydroxide ion. In the appendix, detailed discussion on the structure of the Ag(III) complex, reactive species, as well as pre‐equilibrium regarding the oxidant is provided. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 440–446, 2007     

Reactions of mer(exo)‐ and mer(endo)‐[Co(dien)(dapo)X]2+ Isomers (X=OH,Cl, ONO2, N3)

Properties indirectly determined, or alluded to, in previous publications on the titled isomers have been measured, and the results generally support the earlier conclusions. Thus, the common five‐coordinate intermediate generated in the OH^?‐catalyzed hydrolysis of exo‐ and endo‐[Co(dien)(dapo)X]²⁺ (X=Cl, ONO₂) has the same properties as that generated in the rapid spontaneous loss of OH^? from exo‐ and endo‐[Co(dien)(dapo)OH]²⁺ (40±2% endo‐OH, 60±2% exo‐OH) and an unusually large capacity for capturing (R=[CoN₃]/[CoOH][]=1.3; exo‐[CoN₃]/endo‐[CoN₃]=2.1±0.1). Solvent exchange for spontaneous loss of OH^? from exo‐[Co(dien)(dapo)OH]²⁺ has been measured at 0.04 s^?1 (k₁, 0.50M NaClO₄, 25°) from which similar loss from the endo‐OH isomer may be calculated as 0.24 s^?1 (k₂). The OH^?‐catalyzed reactions of exo‐ and endo‐[Co(dien)(dapo)N₃]²⁺ result in both hydrolysis of coordinated via an OH^?‐limiting process =153 M ^?1 s^?1; =295 M ^?1 s^?1; K_H=1.3±0.1 M ^?1; 0.50M NaClO₄, 25.0°) and direct epimerization between the two reactants =33 M ^?1 s^?1; =110 M ^?1 s^?1; 1.0M NaClO₄, 25.0°). Comparisons are made with other rapidly reacting Co^III‐acido systems.     

Halide‐Anion Binding by Singly and Doubly N‐Confused Porphyrins

The halide‐binding properties of N‐confused porphyrin (NCP, 1 ) and doubly N‐confused porphyrins (trans‐N₂CP ( 2 ), cis‐N₂CP ( 3 )) were examined in CH₂Cl₂. In the free‐base forms, cis‐N₂CP ( 3 ) showed the highest affinity to each anion (Cl^?, Br^?, I^?) with association constants K_a=7.8×10³, 1.9×10³, and 5.8×10² M ^?1, respectively. As metal complexes, on the other hand, trans‐N₂CP 2–Cu exhibited the highest affinity to Cl^?, Br^?, and I^? with K_a=9.0×10⁴, 2.7×10⁴, and 1.9×10³ M ^?1, respectively. The corresponding K_a values for cis‐N₂CP 3–Cu and NCP 1–Cu were about 1/10 and 1/2, respectively, of those of 2–Cu . With the help of density functional theory (DFT) calculations and complementary affinity measurements of a series of trisubstituted N‐confused porphyrins, the efficient anion binding of NCPs was attributed to strong hydrogen bonding at the highly polarized NH moieties owing to the electron‐deficient C₆F₅ groups at meso positions as well as the ideally oriented dipole moments and large molecular polarizability. The orientation and magnitude of the dipole moments in NCPs were suggested to be important factors in the differentiation of the affinity for anions.     

The kinetics and mechanism of alkaline hydrolysis of N-substituted phthalimides

The aqueous cleavage of N-(2-bromoethyl)phthalimide (NBEPH), N-(3-bromopropyl)phthalimide (NBPPH), and N-carbethoxyphthalimide (NCPH) have been studied within the [ōH] range of 5 × 10^?4 M to 2 × 10^?3 M, pH range of 8.82 to 10.62 and 8.06 to 8.66, respectively. The observed pseudo-first-order rate constants, k_obs, reveal a linear relationship with [ōH] with essentially zero intercept. The alkaline hydrolysis of N-(hydroxymethyl)phthalimide (NHMPH) has been studied within the [ōH] range of 5.64 × 10^?6 M to 2.0 M. The [OH]-rate profile reveals that both ionized and nonionized NHMPH are reactive toward ōH. The second-order rate constant, k_OH, for the reaction of ōH with non-ionized NHMPH is ca. 10⁴ times larger than that with ionized NHMPH. The values of k_OH obtained for NBEPH, NBPPH, NCPH, and nonionized NHMPH show a reasonable linear relationship with Taft substituent constants, and the slope (ρ*) of the plot is 1.01 ± 0.10. The low value of ρ* of 1.01 is attributed to nucleophilic attack as the rate-limiting. The k_OH value for ionized NHMPH reveals nearly 10³-fold negative deviation from the linear Taft plot.     

Investigation of the water exchange mechanism of the Plutonyl(VI) and Uranyl(VI) ions with quantum chemical methods

The water exchange reactions of [PuO₂(OH₂)₅]²⁺ and [UO₂(OH₂)₅]²⁺ were investigated with density functional theory (DFT) and wave function theory (WFT). Geometries and vibrational frequencies were calculated with DFT and CPCM hydration. The electronic energies were evaluated with general multiconfiguration quasi-degenerate second-order perturbation theory (GMC-QDPT2). Spin-orbit (SO) effects, computed with SO configuration interaction (SO–CI), are negligible. Both Actinyl(VI) ions react via an associative exchange mechanism, most likely I_a. The Gibbs activation energies (ΔG^?) at 25 °C are 33–34 and 30–37 kJ mol^?1 for [PuO₂(OH₂)₅]²⁺ and [UO₂(OH₂)₅]²⁺, respectively. ΔG^? for dissociative mechanisms (D, I_d) is higher by more than 15 kJ mol^?1.     

A study on the hydroxylation of 20-membered hexaaza macrocyclic bicopper complexes

A study was made on the hydroxylation of six macrocyclic bicopper complexes with bridge of SCN^?, Cl^?, Br^?, I^?, N₃^? or OH^? respectively in aqueous solution. Electronic spectra and data of titration by ion selective electrode show that the basic structure of the complexes remained unchanged during hydroxylation and hydroxyl group bound to copper atom only at the axial direction of the complexes. pH titration was made by auto-titration and data acquisition system controlled by microcomputer. Data of pH titration was pq-analysed by program LEMIT which show that 1-4 hydroxyl groups bind to copper atom stepwisely. Twenty four stepwise hydroxylation constants of the complexes were calculated and concentration distribution of various species during pH changing and hydroxylation were obtained. The six complexes except that with bridge N₃^? formed dihydroxo-complexes mainly at about pH 9 and formed monohydroxo-complexes mainly at about pH 7.5.     

How difficult are anion-molecule SNAr reactions of unactivated arenes in the gas phase,dimethyl sulfoxide,and methanol solvents?

Nucleophilic substitution on the aromatic ring (S_NAr) is a very important reaction for organic transformations. This kind of reaction is usually difficult to take place, requiring organometallic catalysis or activation of the ring by electron withdrawing groups to turn the nucleophilic attack possible. In this work, the relative importance of intrinsic gas phase barrier and the solvent effect on several S_NAr reactions using theoretical calculations were investigated. The reactions of the anions OH^?, CN^?, and CH₃O^? and the enolates CH₃COCH₂^? and CH₃COCHCOCH₃^? with bromobenzene and (o, m, p)-methoxy bromobenzene in methanol and dimethyl sulfoxide as solvents were considered. The OH^? and CH₃O^? ions are highly reactive in the gas phase. However, the solvent effect induces a high activation barrier in solution, turning the reaction difficult, although feasible. The CN^? and CH₃COCHCOCH₃^? ions have high activation barriers even in the gas phase. The interesting CH₃COCH₂^? ion has a moderate barrier in the gas phase, although the free energy barrier in DMSO solution reaches 33 kcal mol^?1. Our analysis suggests that decreasing the solvent effect, arylation of enolates with unactivated arenes could become possible.

     

Kinetics and mechanisms of oxidation by metal ions,part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

The stopped-flow measurements on the disappearance of alkaline osmium(VIII) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant k_obs ([HCHO] ? [Os(VIII)]) decreased with increasing [OH^?]. The ionic strength, however, had no effect on k_obs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between OsO₄(OH) and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant K_ha for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OH^? are evaluated. The equilibrium constant K_ha, within the experimental limits, is independent of temperature. The pK_a value, calculated from K_ha, is 13.62 ± 0.05 which is in good agreement with the pK_a value 13.27 for formaldehyde. The activation parameters, ΔH? = 40 ± 2 kJ mol^?1 and ΔS? = ?51 ± 6 JK^?1 mol^?1, for the rate limiting constant k are determined.     

High-Pressure Stopped-Flow Study of the Kinetics of Base Hydrolysis of the Dichromate Ion by Hydroxide Ion,Ammonia, Water,and 2,6-Lutidine in Aqueous Solution

Volumes of activation for the base hydrolysis of the dichromate anion have been measured at 298.2 K, using high-pressure stopped-flow spectrophotometry. The values of ΔV* (cm³ · mol^?1), ? 17.9 ± 0.6, ? 19.2 ± 0.9, ? 24.9 ± 0.9 and ? 26.0 ± 0.7 for OH^?, NH₃, H₂O and 2,6-lutidine, respectively, are consistent with an interchange mechanism with associative activation mode (I^a).     

Kinetics and mechanism of the iodine oxidation of hydroxylamine

Studies of the stoichiometry and kinetics of the reaction between hydroxylamine and iodine, previously studied in media below pH 3, have been extended to pH 5.5. The stoichiometry over the pH range 3.4–5.5 is 2NH₂OH + 2I₂ = N₂O + 4I^? + H₂O + 4H⁺. Since the reaction is first-order in [I₂] + [I₃^?], the specific rate law, k₀, is k₀ = (k₁ + k₂/[H⁺]) {[NH₃OH⁺]₀/(1 + K_p[H⁺])} {1/(1 + K_I[I^?])}, where [NH₃OH⁺]₀ is total initial hydroxylamine concentration, and k₁, k₂, K_p, and K_I are (6.5 ± 0.6) × 10⁵ M^?1 s^?1, (5.0 ± 0.5) s^?1, 1 × 10⁶ M^?1, and 725 M^?1, respectively. A mechanism taking into account unprotonated hydroxylamine (NH₂OH) and molecular iodine (I₂) as reactive species, with intermediates NH₂OI₂^?, HNO, NH₂O, and I₂^?, is proposed.     

Conformation et spectrométrie de R.M.N.: II

The four isomeric 3-dimethylamino-trans-2-decalols-1,1,4,4-d₄ have been synthesised. Examination of PMR spectra of these compounds allows us to confirm the ‘flattened-chair’ conformation for the cis N(CH₃)₂^a OH^e isomer, whereas the remaining three conserve the double chair conformation. The same type of flattening is also observed in the case of the diaxial quaternary ammonium salt and is even more marked in the cis N(CH₃)₃^a OH^e isomer resulting in a ‘twist-chair’ conformation.     

Relationship between kinetic chain length and radical polymerization rate

In the copolymerization of monomers M₁ and M₂ which form polymer radicals of chain length n of N¹_n with electron on a M₁ type and N²_n with one on a M₂ type, it is assumed that the specific rates of termination between N¹_n and N¹_n and N¹_s, N¹_n and N²_s, and N²_n and N²_s are k_α(ns)^?a, k_β(ns)^?a, and k_γ(ns)^?a, respectively, where k_α, k_β, and k_γ are the rate constants of reaction between segment radicals in the respective termination, and a is constant. The relation between kinetic chain length n? and polymerization rate R_p is derived as: 1/n? = 1/n?₀ + const. (R_p)^A(a), where n?₀ is the kinetic chain length of the polymer formed by transfer and A (a) is unity (predominance of transfer) and 1/(1–2a) (no transfer). In the copolymerization between methyl methacrylate (M₁) and styrene (M₂) at 60°C, when R_p → 0, k_r12/k₁₂ + k_r21/k₂₁ = 5.9× 10^?5 is obtained, where k_r12 and k_r21 are the rate constants of transfer of N¹ to M₂ and N² to M₁, and k₁₂ and k₂₁ are the rate constants of propagation of N¹ to M₂ and N² to M₁. In the absence of transfer, the a value is found to be 0.065 ± 0.008, from the relation between n? and R_p, regardless of the monomer composition. Such a value is also estimated by setting b = 0.72 in a = 0.153 (2b–1), where b is the constant in the Mark-Houwink equation. Further, the value of k_β is found to be 1.18 × 10⁹l./mole-sec, which is comparable with the diffusion-controlled rate of reaction between small molecules. The rate of reaction between segment radicals is fivefold larger than the polymer-polymer termination when transfer predominates.     

Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl^–, NH⁺₄, and N₂O. The rate law is given by ?[d[NH₂Cl]/dt] = k₊[NH₃OH⁺][NH₂Cl] + k₀[NH₂OH][NH₂Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k₊= (1.03 ± 0.06) ×10³ L · mol^?1 · s^?1 and k₀=91 ± 15 L · mol^?1 · s^?1. Isotopic studies demonstrate that both nitrogen atoms in the N₂O come from the NH₂OH/NH₃OH⁺. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH? = 36 ± 3 kJ · mol^–1 and Δ S? = ?66 ± 9 J · K^?1 · mol^?1, and Δ H? = 12 ± 2 kJ · mol^?1 and Δ S? = ?168 ± 6 J · K^?1 · mol^?1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006