Rate constants and transition‐state geometry of reactions of alkyl,alkoxyl, and peroxyl radicals with thiols

Enthalpy, activation energy, and rate constant of 9 alkyl, 3 acyl, 3 alkoxyl, and 9 peroxyl radicals with alkanethiols, benzenethiol, and L ‐cysteine are calculated. The intersection parabolas model is used for activation energy calculations. Depending on the structure of attacking radical, the activation energy of reactions with alkylthiols varies from 3 to 43 kJ mol^?1 for alkyl radicals, from 7 to 9 kJ mol^?1 for alkoxyl, and from 18 to 35 kJ mol^?1 for peroxyl radicals. The influence of adjacent π‐bonds on activation energy is estimated. The polar effect is found in reactions of hydroxyalkyl and acyl radicals with alkylthiols. The steric effect is observed in reactions of alkyl radicals with tert‐alkylthiols. All these factors are characterized via increments of activation energy. Quantum chemical calculations of activation energy and geometry of transition state were performed for model reactions: C^?H₃ + CH₃SH, CH₃O^? + CH₃SH, and HO₂^? + CH₃SH with using density functional theory and Gaussian‐98. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 284–293, 2009     

Solvolyse des Tetrathiocyanato-dianilino-chromat(III)-ions in Aceton—Wasser-Mischungen

The solvolysis of the [Cr(NCS)₄(aniline)₂]^? complex ion has been studied in various acetone-water mixtures in the presence of perchloric acid (10^?3–10^?1 mole/l). In acid solutions the first two NCS^? ions are exchanged, presumably, for water molecules and in parallel an aniline molecule for acetone, the latter in a second order reaction, accelerated by hydrogen ions. The exchange of the amine is followed by the substitution of the first two NCS^? ions. The third and fourth NCS^? ions are substituted only in neutral or weakly acid solutions. Kinetic parameters have been derived for the reactions mentioned above. The influence of solvent composition and of acidity is discussed.     

The kinetics and mechanisms of the gas-phase unimolecular eliminations of halogeno esters. Participation of the carbomethoxy group

The kinetics of the gas-phase elimination of several chloroesters were determined in a static system over the temperature range of 410–490°C and the pressure range of 47–236 torr. The reactions in seasoned vessels, and in the presence of a free-radical inhibitor, are homogeneous, unimolecular, and follow a first-order law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for methyl 3-chloropropionate, log k₁(s^?1) = (13.22 ± 0.07) - (231.5 ± 1.0) kJ/mol/2.303RT; for methyl 4-chlorobutyrate, log k₁(s^?1) = (13.31 ± 0.25) - (221.5 ± 3.4) kJ/mol/2.303RT; and for methyl 5-chlorovalerate, log k₁(s^?1) = (13.12 ± 0.25) - (221.7 ± 3.2) kJ/mol/2.303RT. Rate enhancements and lactone formation reveal the participation of carbonyl oxygen of the carbomethoxy group. The order COOCH₃-5 > COOCH₃-6 > COOCH₃-4 in assistance is similar to the sequence of group participation in solvolysis reactions. The partial rates for the parallel eliminations to normal dehydrohalogenation products and lactones have been estimated and reported. The present results lead us to consider that an intimate ion-pair mechanism through participation of the carbomethoxy group may well be operating in some of these reactions.     

Synthesis,characterization, and mechanism of trans-dichlorido-bis-(ethylenediamine)cobalt(III) with L-cystine in the presence of chloride

Substitution reactions of trans-[CoCl₂(en)₂]Cl (where en?=?ethylenediamine) with L-cystine has been studied in 1.0?×?10^?1?mol?dm^?3 aqueous perchlorate at various temperatures (303–323?K) and pH (4.45–3.30) using UV-Vis spectrophotometer on various [Cl^?] from 0.05 to 0.01?mol?L^?1. The products have been characterized by their physico-chemical and spectroscopic data. Trans-[CoCl(en)₂(H₂O)]²⁺, from the hydrolysis of trans-[CoCl₂(en)₂]⁺ in the presence of Cl^?, formed a complex with L-cystine at all temperatures in 1?:?1 molar ratio. L-cystine is bidentate to Co(III) through Co–N and Co–S bonds. Product formation and reversible reaction rate constants have been evaluated. The rate constants for SN_i mechanism have been evaluated and activation parameters E _a, ΔH ^#, and ΔS ^# are determined.     

Fragmentation of the enolate ion of 2,3-butanedione. Characterization of the acetyl anion by charge inversion mass spectrometry

The unimolecular and collision-induced fragmentation reactions of the enolate ion of 2,3-butanedione, [CH₃COCOCH₂]^?, have been studied, Unimolecular fragmentation on the metastable ion time-scale forms [HCCO]^?, [C₂H₃O]^?, [C₃H₅O]^? and [CH₃CO₂]^?. Charge inversion mass spectrometry shows that the [C₂H₃O]^? ion is the acetyl anion while the [C₃H₅O]^? product is the acetone enolate ion; formation of the latter product involves a large release of kinetic energy (T _1/2 = 0.99 eV). The fragmentation reactions occurring following collisional activation have been determined for 8 keV collisions and over the range 1.5–30 eV center-of-mass collision energy. Formation of [HCCO]^? and [CH₃CO]^? are of the most important reactions following collisional activation and it is concluded that the two reactions have similar critical reaction energies even though formation of [HCCO]^? is favored thermochemically.     

Multi‐Pathway Consequent Chemoselectivities of CpRuCl(PPh3)2/MeI‐Catalysed Norbornadiene Alkyne Cycloadditions

Chemoselectivities of five experimentally realised CpRuCl(PPh₃)₂/MeI‐catalysed couplings of 7‐azabenzo‐norbornadienes with selected alkynes were successfully resolved from multiple reaction pathway models. Density functional theory calculations showed the following mechanistic succession to be energetically plausible: (1) CpRuI catalyst activation; (2) formation of crucial metallacyclopentene intermediate; (3) cyclobutene product ( P2 ) elimination (ΔG_Rel(RDS)≈11.9–17.6 kcal mol^?1). Alternative formation of dihydrobenzoindole products ( P1 ) by isomerisation to azametalla‐cyclohexene followed by subsequent CpRuI release was much less favourable (ΔG_Rel(RDS)≈26.5–29.8 kcal mol^?1). Emergent stereoselectivities were in close agreement with experimental results for reactions a , b , e . Consequent investigations employing dispersion corrections similarly support the empirical findings of P1 dominating in reactions c and d through P2 → P1 product transformations as being probable (ΔG≈25.3–30.1 kcal mol^?1).     

Rate constants for the reactions of conjugated olefins with NO2 in the gas phase

Gas-phase rate constants for the reaction of NO₂ with 16 conjugated olefins were determined at room temperature by either conventional methods for bimolecular processes or by competitive reactions. It was found that the rate constants for conjugated olefins were larger than those for simple mono-olefins by factors of 10³–10⁴. Temperature dependence studies reveal that the difference in the rate constants for the two types of reactions can primarily be attributed to differences in their activation energies: k_{1,3-cyclohexadiene} = 5.8 × 10^?14 exp[?(6.1 ± 1.6)/RT] cm³ molecule^?1 s^?1; k_cis-2-butene = 4.68 × 10^?14 exp(?11.2/RT) cm³ molecule^?1 s^?1 [2]. A linear free energy relationship between the reactions of OH and NO₂ with conjugated diolefins was observed.     

Further studies on the dissociation of HBr behind shock waves

Previously reported shock tube studies of the dissociation of HBr in the temperature range of 2100–4200°K have been extended to lower temperatures (1450–2300°K) in pure HBr. The course of reaction was followed by monitoring the radiative recombination emission in the visible spectrum from Br atoms. The results imply that, in the lower range of temperatures, the activation energy of dissociation, E in the expression AT^?2e^?E/RT, can be approximated by the HBr bond energy (88 kcal/mole). It was also found that, in this temperature range, the rate of HBr dissociation is sensitive to the Br₂ dissociation rate and the HBr + Br exchange rate. When these rates were adjusted to bring computed reaction profiles into agreement with experimental ones, it was found that the higher-temperature data could also be fitted reasonably well with an HBr dissociation activation energy of 88 kcal/mole, contrary to the conclusions of our previous work, which favored an activation energy of 50 kcal/mole. The “best value” for k₁^Ar, the rate coefficient for HBr dissociation in the presence of Ar as chaperone, appears to be 10^{21.78 ± 0.3} T^?2 10^?88/θ cc/mole sec, where θ = 2.3 RT/1000; that for k₁^HBr, is 10^22.66T^?210^?88/θ. A detailed review is given of the rate coefficients for the other pertinent reactions in the H₂–Br₂ system, viz., Br₂ dissociation and reactions of HBr with H and Br.     

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.     

Kinetics of reactions of OBrO with NO,O3, OClO,and ClO at 240–350 K

Kinetics for the reactions of OBrO with NO, O₃, OClO, and ClO at 240–350 K were investigated using the technique of discharge flow coupled with mass spectrometry. The Arrhenius expression for the OBrO reaction with NO was determined to be k₁ = (2.37 ± 0.96) × 10^?13 exp[(607 ± 63)/T] cm³ molecule^?1 s^?1. The reactions of OBrO with O₃, OClO, and ClO are slow chemical processes at 240–350 K. Upper limit rate constants for the OBrO reactions with O₃, OClO, and ClO at 240–350 K were estimated to be k₂ < 5.0 × 10^?15 cm³ molecule^?1 s^?1, k₃ < 6.0 × 10^?14 cm³ molecule^?1 s^?1, and k₄ < 1.5 × 10^?13 cm³ molecule^?1 s^?1, respectively. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 430–437, 2002     

The gas‐phase elimination kinetics of ethyl 2‐furoate and ethyl 2‐thiophenecarboxylate

The gas‐phase elimination kinetics of ethyl 2‐furoate and 2‐ethyl 2‐thiophenecarboxylate was carried out in a static reaction system over the temperature range of 623.15–683.15 K (350–410°C) and pressure range of 30–113 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: ethyl 2‐furoate, log k₁ (s^?1) = (11.51 ± 0.17)–(185.6 ± 2.2) kJ mol^?1 (2.303 RT)^?1; ethyl 2‐thiophenecarboxylate, log k₁ (s^?1) = (11.59 ± 0.19)–(183.8 ± 2.4) kJ mol^?1 (2.303 RT)^?1. The elimination products are ethylene and the corresponding heteroaromatic 2‐carboxylic acid. However, as the reaction temperature increases, the intermediate heteroaromatic carboxylic acid products slowly decarboxylate to give the corresponding heteroaromatic furan and thiophene, respectively. The mechanisms of these reactions are suggested and described. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 145–152, 2009     

Reactivity of Oxygen Radical Anions Bound to Scandia Nanoparticles in the Gas Phase: CH Bond Activation

The activation of C?H bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O^?.) is an important species in C?H activation. The mechanistic details of C?H activation by O^?. radicals can be well understood by studying the reactions between O^?. containing transition metal oxide clusters and alkanes. Here the reactivity of scandium oxide cluster anions toward n‐butane was studied by using a high‐resolution time‐of‐flight mass spectrometer coupled with a fast flow reactor. Hydrogen atom abstraction (HAA) from n‐butane by (Sc₂O₃)_NO^? (N=1–18) clusters was observed. The reactivity of (Sc₂O₃)_NO^? (N=1–18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc₈O₁₃^?) and 12 (Sc₂₄O₃₇^?). Larger (Sc₂O₃)_NO^? clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc₂O₃)_NO^? (N=1–5) clusters, which were found to contain the O^?. radicals as the active sites. The local charge environment around the O^?. radicals was demonstrated to control the experimentally observed size‐dependent reactivity. This work is among the first to report HAA reactivity of cluster anions with dimensions up to nanosize toward alkane molecules. The anionic O^?. containing scandium oxide clusters are found to be more reactive than the corresponding cationic ones in the C?H bond activation.     

Solvolysis process of organophosphorus compound P-[2-(dimethylamino)ethyl]-N,N-dimethylphosphonamidic fluoride with simple and α-nucleophiles: a DFT study

Density functional theory (DFT) has been used to study the solvolysis process of the organophosphorus compound P-[2-(dimethylamino)ethyl]-N,N-dimethylphosphonamidic fluoride (GV) with simple nucleophile [hydroxide (HO^?)] and α-nucleophiles [hydroperoxide (HOO^?) and hydroxylamine anion (NH₂O^?)]. The lowest energy conformer of GV used for the solvolysis process was identified with Monte Carlo conformational search (MCMM) algorithm employing MMFFs force field followed by DFT calculations. The profound effect was found for α-nucleophiles toward the solvolysis of GV compared to normal alkaline hydrolysis. Incorporation of solvent (water) employing SCRF (PCM) model at B3LYP/6-31+G* showed that solvolysis of GV with hydroperoxide (activation energy = 7.6 kcal/mol) is kinetically more favored compared to hydroxide and hydroxylamine anion (activation energy = 11.0 and 9.2 kcal/mol, respectively). The faster solvolysis of GV with hydroperoxide is achieved due to strong intermolecular hydrogen bonding in the transition state geometry compared to similar α-nucleophile hydroxylamine anion. Assistance of a water molecule in solvolysis of GV affects the activation barriers; however, the hydroperoxidolysis remains the preferential process. The topological properties of electron density distributions for (–X–H···O, X = O, N) intermolecular hydrogen bonding bridges have been analyzed in terms of Bader theory of atoms in molecules (AIM). Further, the analysis was extended by natural bond orbital (NBO) methods for the strength of intermolecular hydrogen bonding in the transition state geometries. This study showed that the reactivity of these α-nucleophiles toward the solvolysis of GV is a delicate balance between the nucleophilicity and hydrogen-bond strength. Solvation governs the overall thermodynamics for the destruction of GV, which otherwise is unfavored in the gas phase studies.     

Kinetics of the gas‐phase reactions of chlorine atoms with CH2F2, CH3CCl3, and CF3CFH2 over the temperature range 253–553 K

Relative rate techniques were used to study the title reactions in 930–1200 mbar of N₂ diluent. The reaction rate coefficients measured in the present work are summarized by the expressions k(Cl + CH₂F₂) = 1.19 × 10^?17 T² exp(?1023/T) cm³ molecule^?1 s^?1 (253–553 K), k(Cl + CH₃CCl₃) = 2.41 × 10^?12 exp(?1630/T) cm³ molecule^?1 s^?1 (253–313 K), and k(Cl + CF₃CFH₂) = 1.27 × 10^?12 exp(?2019/T) cm³ molecule^?1 s^?1 (253–313 K). Results are discussed with respect to the literature data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 401–406, 2009     

Kinetics of hydroxyl radical reactions with formaldehyde and 1,3,5-trioxane between 290 and 600 K

Absolute rate constants are measured for the reactions: OH + CH₂O, over the temperature range 296–576 K and for OH + 1,3,5-trioxane over the range 292–597 K. The technique employed is laser photolysis of H₂O₂ or HNO₃ to produce OH, and laser-induced fluorescence to directly monitor the relative OH concentration. The results fit the following Arrhenius equations: k (CH₂O) = (1.66 ± 0.20) × 10^?11 exp[?(170 ± 80)/RT] cm³ s^?1 and k(1,3,5-trioxane) = (1.36 ± 0.20) × 10^?11 exp[?(460 ± 100)/RT] cm³ s^?1. The transition-state theory is employed to model the OH + CH₂O reaction and extrapolate into the combustion regime. The calculated result covering 300 to 2500 K can be represented by the equation: k(CH₂O) = 1.2 × 10^?18 T^2.46 exp(970/RT) cm³ s^?1. An estimate of 91 ± 2 kcal/mol is obtained for the first C? H bond in 1,3,5-trioxane by using a correlation of C? H bond strength with measured activation energies.     

Temperature‐dependent kinetics study of the gas‐phase reactions of atomic chlorine with acetone, 2‐butanone,and 3‐pentanone

A laser flash photolysis–resonance fluorescence technique has been employed to study the kinetics of the reactions of atomic chlorine with acetone (CH₃C(O)CH₃; k₁), 2‐butanone (C₂H₅C(O)CH₃; k₂), and 3‐pentanone (C₂H₅C(O)C₂H₅; k₃) as a function of temperature (210–440 K) and pressure (30–300 Torr N₂). No significant pressure dependence is observed for any of the reactions studied. Arrhenius expressions (units are 10^?11 cm³ molecule^?1 s^?1) obtained from the data are k₁(T) = (1.53 ± 0.19) exp[(?594 ± 33)/T], k₂(T) = (2.77 ± 0.33) exp[(+76 ± 33)/T], and k₃(T) = (5.66 ± 0.41) exp[(+87 ± 22)/T], where uncertainties are 2σ and represent precision only. The accuracy of reported rate coefficients is estimated to be ±15% over the entire range of pressure and temperature investigated. The room temperature rate coefficients reported in this study are in good agreement with a majority of literature values. However, the activation energies reported in this study are in poor agreement with the literature values, particularly for 2‐butanone and 3‐pentanone. Possible explanations for discrepancies in published kinetic parameters are proposed, and the potential role of Cl + ketone reactions in atmospheric chemistry is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 259–267, 2008     

Reaction of the NO3 radical with some thiophenes: Kinetic study and a correlation between rate constant and EHOMO

The rate constants and activation energies for the reactions of some thiophenes with the NO₃ radical were measured using the absolute fast‐flow discharge technique at 263–335 K and low pressure. The proposed Arrhenius expressions for 2‐ethylthiophene, 2‐propylthiophene, 2,5‐dimethylthiophene, and 2‐chlorothiophene are k = (4.2 ± 0.28) ×10^?16 exp[(2280 ± 70)]/T, k = (7.0 ± 2) × 10^?18 exp[(3530 ± 70)]/T, k = (1 ± 1) × 10^?14 exp[(1648 ± 240)]/T, and k = (8 ± 2) × 10^?17 exp[(2000 ± 200)]/T (k = cm³ molecule^?1 s^?1), respectively. The reactions of this radical with 2‐chlorothiophene and 3‐chlorothiophene were also studied by a relative method in a Teflon static reactor at room temperature and atmospheric pressure. The effect of substitution on thiophene reactivity is discussed, and a relationship between the rate constants and the ionization potential (IP = ?E_HOMO) has been proposed. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 570–576, 2006     

The gas-phase elimination kinetics of 2-hydroxy-2-methylbutyric acid and 2-ethyl-2-hydroxybutyric acid

The elimination kinetics of the title compounds have been examined over the temperature range of 270–320°C and pressure range of 19–117 torr. The reactions, carried out in seasoned vessels, with the free-radical suppressor toluene always present, are homogeneous, unimolecular, and follow a first-order rate law. The products of 2-hydroxy-2-methylbutyric acid are 2-butanone, CO, and H₂O; while of 2-ethyl-2-hydroxybutyric acid are 3-pentanone, CO, and H₂O. The rate coefficient is expressed by the following Arrhenius equation: for 2-hydroxy-2-methylbutyric acid, log k₁(s^?1 = (12.87 ± 0.19) ? (171.2 ± 2.1) kJ mol^?1 (2.303 RT)^?1; and for 2-ethyl 2-hydroxybutyric acid, log k₁s^?1) = (12.13 ± 0.34) ? (159.4 ± 3.7) kJ mol^?1 (2.303 RT)^?1. Augmentation of alkyl bulkiness at the 2-position of the 2-hydroxycarboxylic acids showed an increase in the rate of dehydration. The electron release of alkyl groups, rather than steric acceleration, appears to enhance the pyrolysis decomposition of these substrates. These reactions are believed to proceed through a semi-polar five-membered cyclic transition type of mechanism. © 1995 John Wiley & Sons, Inc.     

Analysis of linear free-energy relationships combined with activation parameters assigns a concerted mechanism to alkaline hydrolysis of x-substituted phenyl diphenylphosphinates

A kinetic study is reported for alkaline hydrolysis of X‐substituted phenyl diphenylphosphinates ( 1 a – i ). The Brønsted‐type plot for the reactions of 1 a – i is linear over 4.5 pK_a units with β_lg=?0.49, a typical β_lg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σ^o and σ^? constants are linear but exhibit many scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with ρ=1.42 and r=0.35. The r value of 0.35 implies that leaving‐group departure is partially advanced at the rate‐determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO^? ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (D_N + A_N) mechanism is also ruled out on the basis of the small β_lg value. As the substituent X in the leaving group changes from H to 4‐NO₂ and 3,4‐(NO₂)₂, ΔH ^≠ decreases from 11.3 kcal mol^?1 to 9.7 and 8.7 kcal mol^?1, respectively, while ΔS ^≠ varies from ?22.6 cal mol^?1 K^?1 to ?21.4 and ?20.2 cal mol^?1 K^?1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a – i .     

Kinetics of styrylquinolinc formation

A comprehensive kinetic investigation of reactions occurring in the formation of styryl-quinolines has been conducted. Specific rate data such as rate equations, rate constants, and thermodynamic activation values have been determined and utilized in a study of which factors are of greatest importance in the reactions forming 2-styrylquinolines. A mechanism has been proposed for the condensation reaction which agrees with rate relationships found. Gas-liquid partition chromatography was used to follow the kinetics of the condensation reactions. A rate constant of 5.41 × 10^?2M^?1min^?1 was found for the reaction of benzaldehyde with 2-methyl-quinoline using zinc chloride as a catalyst at 104.0°. Rate constants of 1.28 × 10^?2 MT^?1 min^?1 and 1.05 × 10^?2 M^?1 min^?1 were found for the reactions of p-methylbenzaldehyde and p-methoxybenzaldehyde with quinaldine to form 2-(p-methylstyryl)quinoline and 2-(p-methoxystyryl)-quinoline, respectively at 92.4°. A linear relationship was found using the Hammett equation. An Arrhenius plot was constructed from rate constants determined at five different temperatures for the reaction of benzaldehyde and quinaldine to form 2-styrylquinoline, using zinc chloride as a catalyst. The energy of activation, E_a, was found to be 22.2 kcal/mole for this reaction. The enthalpy of activation, ΔH?, free energy of activation, ΔF?, and entropy of activation, ΔS?, were found to be 21.4 kcal/mole, 27.7 kcal/mole and -16.7 eu/mole, respectively, at 104.0°. The mechanism proposed in the formation of 2-styrylquinoline involves the fast formation of a carbanion-zinc chloride complex, which then attacks, in the rate determining step, the aldehyde utilized in the reaction. The lack of reaction of certain methylquinolines is attributed to the inadequacy of the carbanion formed and not to the difficulty involved in the initial formation of the carbanion.