Transition State For Decomposition Research Articles

Quaternary ammonium fluorides and difluorosilicates as nucleophilic fluorination reagents.

TBAT (tetrabutylammonium difluorotriphenylsilicate) is an excellent homogeneous nucleophilic fluorination reagent, but a high excess of the reagent was reported to be essential. We hence optimized the reaction conditions and compared its nucleophilic fluorination reactivity with that of other common commercial nucleophilic fluorination reagents, such as anhydrous TBAF and TASF (tris(dimethylamino)sulfonium difluorotrimethylsilicate). As the substrates, we employed a standard set of primary and secondary octyl substrates under identical conditions. To eliminate the possibility of hydrogen fluoride elimination in the above reagents, we prepared four quaternary ammonium fluorides lacking β-elimination possibility in the hydrocarbon chain, transformed them to the corresponding difluorotriphenylsilicates, and compared their reactivity with that of the commercial reagents. Furthermore, attempts to isolate analogous tetrabutylammonium difluoromethyldiphenylsilicate or difluorodimethylphenylsilicate failed, as was confirmed by comparison of the published experimental data with computed 19F NMR spectra. Finally, we studied the transition states of decomposition of various tetramethylammonium methylphenyldifluorosilicates by DFT methods and found that their relative energies increase with an increasing number of phenyl groups. The formation of difluorosilicates is a nearly barrierless process.

Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

AbstractC2H4−O2 reactions form ethylene oxide (EO) on Ag nanoparticles dispersed on α‐Al2O3 and promoted with alkali and other elements, with Re and Cs most frequently used in practice. Traces of alkyl chlorides (e. g., C2H5Cl) and alkanes in much larger amounts are added to reactants to balance the rate of deposition and removal (and the coverage) of Cl adatoms (Cl*) on Ag surfaces. Such Cl adlayers retain Ag surface ensembles that form O2‐derived intermediates that favor EO synthesis, but typically decrease O2 activation rates. A series of Ag/α‐Al2O3 catalysts (with and without Re or Cs) are used here to examine the role of promoters and Cl moderators through an analysis of the effects of reaction conditions and C2H5Cl levels using convection‐reaction constructs and a mechanistic formalism that considers O2 (as chemisorbed O2*) as the reactant in EO synthesis through one electrophilic O‐atom, with the second O‐atom (O*) acting as a unselective nucleophile that must be scavenged by a sacrificial reductant (C2H4 or EO); such channels are precluded by any intervening O2 dissociation events (that form two O*). O2 consumption rates in C2H4−O2 reactions decrease 10‐fold upon exposing Ag/α‐Al2O3 to C2H5Cl, evincing persistent Cl* species that block active sites and require >104 C2H4 oxidation turnovers to be removed. Activation barriers do not change but relative rates of epoxidation versus combustion increase in the presence of this refractory Cl* adlayer, which reduces the number and shrinks the size of available site ensembles without altering their intrinsic reactivity for O2 activation to form O2* but attenuating rates of its subsequent dissociation. Refractory O* adlayers also form on Ag surfaces, as shown by the accumulation of persistent O* species upon exposing reduced Ag particles to N2O. Inter‐atom repulsive forces weaken O* binding and destabilize N2O decomposition transition states to eventually allow stable N2 and O2 evolution for catalytic N2O decomposition, which occurs at landing ensembles formed at interstices of refractory adlayers. Cl* forms denser adlayers than O*, as evidenced by a 10‐fold decrease not only to C2H4−O2 rates but also to N2O stoichiometric and catalytic rates and O* uptakes. The occasional evolution of O2, which forms larger landing ensembles that more readily form 2O* from O2, occurs with higher frequency from O* than mixed O*/Cl* adlayers, and this effect of Cl* to inhibit (O−O)* dissociation is demonstrated in CO probe reaction studies, which show that highly reactive bound dioxygen species are retained to a greater extent when adlayers comprise both Cl* and O*. Re and Cs do not influence the nature of such adlayers but increase and decrease, respectively, the number of acid sites and selectivity losses via EO combustion. Their respective domains may also block a modest fraction of Ag surfaces without consequences on intrinsic O2 activation rates or EO selectivities. The effects of Re beyond those reported here may emerge upon synthetic protocols that alter Re location and Ag−Re intimacy to enable channels which utilize both O‐atoms in epoxidation.

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes.

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts due to the lack of deactivating d-d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp2Ti(C2Ar)2·MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp2Ti(C2Ph)2. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3LMCT in all cases except Cp2Ti(C2Ph)2·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only Cp2Ti(C2Ph)2 and Cp2Ti(C2Ph)2·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10-4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φrxn, between 0.25 and 0.99. The coordination of MX decreases Φrxn by two to three orders of magnitude. There is a clear trend that Φrxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the 3LMCT state and the singlet transition state for decomposition.

Radical decomposition reactions: factors affecting transition-state energetics and geometries

The enthalpies, activation energies, and geometries of transition states of the decomposition of organic radicals, viz., alkyl, alkoxyl, acyl, peroxyl, etc., are reviewed. The results of semiempirical and quantum chemical calculations are compared with experimental data. Special attention is given to an analysis of physicochemical factors affecting the activation barriers and geometry of transition states of radical decomposition.

Is it possible to synthesize organic Ar compound? a theoretical study

The currently synthesized noble gas (Ng) molecules are mostly xenon or krypton compounds. HArF is the only experimentally prepared compound containing a light Ng atom. In this work, a new argon compound with the formula HArC4CN, was predicted to be theoretically stable (5.66 kcal/mol at ROCCSD(T)/6-311++g(2d,2p) level of theory). Two decomposition transition states were found, i.e. the 3-body and 2-body decomposition, which produces H, Ar and C4CN or HC4CN and Ar respectively. The HArC4CN molecule is meta-stable, but the low energy barrier between the stable molecule and the 3-body transition state makes it possible to synthesize from the three fragments and the high energy barrier between the minimum and the 2-body transition state prevented it from decomposing. Natural bond orbital (NBO) and electron localization function (ELF) analysis show a strong ionic bond between Ar and C atoms whereas covalent between Ar and H. Compared with previous work, it is the conjugation effect of −C4CN as well as the its electronegativity that activates the noble gas atom and results in a stable noble gas compound.

DFT Calculations of Adsorption and Decomposition of N2O on Rh(100)

Adsorption and decomposition of N2O on Rh(100) surface has been investigated by density functional theory (DFT) methods with full geometry optimization, transition state search (synchronous transit methods and constrained geometry optimization), and molecular dynamics. More stable adsorption forms (lying-flat form with all three atoms of the N2O molecule interacting with the surface and “horse-like” form with two N atoms attached to the surface) have been found in the present work besides the two forms reported previously (an upright form with the terminal N atom bonding to the surface and a lying form attaching with both terminal N and O atoms to atop sites). Decomposition pathways of N2O with and without coadsorbed oxygen atoms have been identified and compared with the experimental data. The present results suggest the necessity of internal energy partitioning measurements of desorbing species to discuss the transition states of decomposition more definitely.

Synthesis and reactivity of (μ-η 2:η 2-peroxo)dicopper(II) complexes with dinucleating ligands: Hydroxylation of xylyl linker with a NIH shift

New hexadentate dinucleating ligands having a xylyl linker, X–L–R, were synthesized, where X–L–R = 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]-2,4,6-trimethybenzene (Me 2–L–Me) and 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]-2-fluorobenzene (H–L–F). They form dinuclear copper(I) complexes, [Cu 2(X–L–R)] 2+ (Me 2–L–Me ( 1) and H–L–F ( 2)). The copper(I) complexes in acetone at −78 °C react with O 2 to produce intra- and intermolecular (μ-η 2:η 2-peroxo)dicopper(II) species depending on the concentrations of the complexes: both complexes generate intramolecular (μ-η 2:η 2-peroxo)dicopper(II) species [Cu 2(O 2)(X–L–R)] 2+ ( 1-O 2 and 2-O 2) at the concentrations below ∼5 mM, whereas at ∼60 mM, both complexes produce intermolecular (μ-η 2:η 2-peroxo)dicopper(II) species, which were confirmed by the electronic and resonance Raman spectroscopies. The electronic spectrum of 1-O 2 in acetone at concentrations below ∼5 mM showed an absorption band at ( λ max = 442 nm, ε = 5600 M −1 cm −1) assignable to the π σ ∗ (O–O)-to-Cu(II) ( ( d x 2 - y 2 + d x 2 - y 2 ) component) LMCT transition in addition to an intense band attributable to the π σ ∗ (O–O)-to-Cu(II) ( ( d x 2 - y 2 - d x 2 - y 2 ) component) LMCT transition ( λ max = 359 nm, ε = 21000 M −1 cm −1), indicating that the (μ-η 2:η 2-peroxo)Cu(II) 2 core of 1-O 2 takes a butterfly structure. Decomposition of 1-O 2 resulted in hydroxylation of the 2-position of the xylyl linker with 1,2-methyl migration (NIH shift), suggesting that the hydroxylation reaction proceeds via a cationic intermediate as proposed for closely related (μ-η 2:η 2-peroxo)Cu(II) 2 complexes having a xylyl linker. Kinetic study of the decomposition (hydroxylation of the xylyl linker) of 1-O 2 suggests that a stereochemical effect of the methyl group in the 2-position of the xylyl linker has a significant influence on a transition state for decomposition (hydroxylation of the xylyl linker).

Theoretical predictions of the decomposition mechanism of 1,3,3-trinitroazetidine (TNAZ)

A theoretical study of the decomposition pathways of 1,3,3-trinitroazetidine (TNAZ) is described. Possible decomposition transition-states, intermediates, and products are identified and structures, energies, and vibrational frequencies are determined at the B3LYP/6-31G(d,p) level for these species. Four major pathways are apparent. Two pathways are initiated by the fission of the N–NO2 and C–NO2 bonds to yield radical intermediates, while the other two pathways involve the molecular elimination of HONO. Energy profiles for the pathways and possible routes to some of the experimentally observed species of TNAZ decomposition are presented. The energy required to initiate the NO2 bond fission pathways are 4–8 kcal/mol lower than the HONO elimination pathways. In the gas phase, the NO2 elimination pathways will be the dominant routes for TNAZ decomposition. In the condensed phase, however, this trend may be reversed.

Thermal decomposition of a series of 1,2-diazetines.

A homologous series of tricyclic diazetines (6a-c), differing by the number of methylene groups in the saturated bridges of the fused carbon bicycles, was synthesized. The DeltaH++ of decomposition for each of the diazetines to afford N2 and the corresponding alkene was determined experimentally: 6a, 31.7; 6b, 39.3; 6c, 38.8 kcal/mol. The ground-state strain energy of each diazetine was estimated utilizing computationally obtained DeltaHf's for each of the experimentally investigated diazetines as well as several other diazetines whose DeltaH++'s had been previously reported in the literature. The sum of the ground-state strain energies and DeltaH++'s of decomposition for all of the diazetines was nearly constant, with an average value of 59 kcal/mol, suggesting that all of the diazetines decompose via the same mechanism. Generally, the higher the ground-state strain energy of the diazetine, the less the DeltaH++ for decomposition. The decomposition transition states for 6a-c and 7 were modeled computationally at the RB3LYP/6-311+G(3df,2p)//UB3LYP/6-31+G(d,p) level. The agreement of the experimentally determined DeltaH++ values with transition-state energies obtained computationally supports the reaction mechanism originally proposed by Yamabe that the elimination process occurs by an unsymmetrical, yet concerted, transition state with strong biradical character.

Unimolecular decomposition of nitrooxyalkyl radicals from NO3–isoprene reaction

The unimolecular decomposition of the nitrooxyalkyl radicals arising from the NO3-initiated reaction of isoprene is investigated. Density functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energies of the transition states of decomposition of the nitrooxylalkyl radicals and the corresponding oxirane products. Geometry optimizations were performed using density functional theory at the B3LYP/6-31G(d,p) level and the single-point energies were computed using second-order Møller–Plesset perturbation theory and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G(d) level of theory, the decomposition barriers range from 13 to 21 kcal mol−1, and the decomposition products (i.e., the separated oxirane and NO2) are 9 to 25 kcal mol−1 more stable than the nitrooxylalkyl radicals. The rate constants of decomposition of the nitrooxylalkyl radicals and the pressure-dependent oxirane yields have been calculated using the transition state theory and master equation formalism.

Copolymers of carbon dioxide and nitrogen: an AM1 and ab initio computational study

Extending work by various groups on possible dimers, trimers, etc. of dinitrogen and of carbon dioxide, the authors have studied analogous copolymers of N2 and CO2 computationally. Twelve cyclic structures were examined with the AM1, HF/3-21G, HF/6-31G* and MP2(FC)/6-31G* methods, and the acyclic “monomer” to “tetramer” HO(C(O)O–N N–)nH, n=1–4, were studied at the AM1 and HF/3-21G levels; the cyclic species included 2-oxa-3,4-diazacyclobut-3-ene-1-one, 2-oxa-3,4,5,6-tetraazacyclohexa-3,5-diene-1-one, and various aza/oxa bicyclo[2.2.0] and bicyclo[2.2.2] systems. For the cyclic species, it was concluded that only the MP2(FC)/6-31G* results, which differ considerably from those at the other three levels, are likely to be reliable. These MP2 calculations indicate that only seven of the 12 cyclic structures studied are stationary points (one is a transition structure), and none of them is kinetically stable at room temperature. Although some have high energy densities (ca. 7–10kJg−1), their expected low kinetic stabilities seems to make this of little practical value. The acyclic “copolymers” were all relative minima at the AM1 and HF/3-21G levels; unlike the cyclic species, their kinetic stabilities were not investigated directly by comparing the energies of reactants and decomposition transition states. The energy density of the infinite acyclic polymer was found by extrapolation to be 5.1 (AM1) or 5.6 (3-21G)kJg−1. The calculated vibrational spectra of the MP2 stationary points and of the acyclic molecules gave some indication of instability by the presence of low-frequency modes leading in the limit to decomposition.

Unimolecular Decomposition of n-C4H9 and iso-C4H9 Radicals

The kinetics of the unimolecular decomposition of the n-C4H9 radical has been studied experimentally in a heated tubular flow reactor coupled to a photoionization mass spectrometer. Rate constants for the decomposition were determined in time-resolved experiments as a function of temperature (560−620 K) and bath gas density ((3−18) × 1016 molecules cm-3) in two bath gases, He and N2. The rate constants are in the falloff region under the conditions of the experiments. Structures, vibrational frequencies, and barriers for internal rotations of n-butyl and iso-butyl radicals and their decomposition transition states were obtained by ab initio calculations using UHF/6-31G* and MP2/6-31G* methods. The results of ab initio calculation, together with the reanalysis of earlier studies of the reverse reactions, were used to create transition-state models of the reactions of unimolecular decomposition of n-butyl (1) and iso-butyl (2) radicals. Falloff behavior of reaction 1 was reproduced using master equation modeling with the energy barrier height for decomposition obtained from optimization of the agreement between experimental and calculated rate constants. The values of 〈ΔE〉all = −28 cm-1 (He) and −40 cm-1 (N2) for the average energy loss per collision were obtained using an exponential-down model. The resulting models of the reactions provide the high-pressure limit rate constants for the decomposition reactions ( (n-C4H9 → C2H5 + C2H4) = 1.06 × 1013 exp(−14005 K/T), (iso-C4H9 → CH3 + C3H6) = 2.14 × 1012T0.65 exp(−15529 K/T) s-1) and the reverse reactions ( (C2H5 + C2H4 → n-C4H9) = 6.59 × 10-21T2.44 exp(−2697 K/T), (CH3 + C3H6 → iso-C4H9) = 1.66 × 10-20T2.57 exp(−3879 K/T) cm3 molecule-1 s-1). Parametrization of the temperature and pressure dependence of the unimolecular rate constants for the temperature range 298−900 K and pressures 0.001−10 atm in He and N2 is provided using the modified Lindemann−Hinshelwood expression.

?-Substituted cyclopropyl r�dicals grafted to silica surface. Methods of preparation, structure, and properties

Cyclopropyl type radicals\(X - _\triangledown \cdot (X = \begin{array}{*{20}c} \backslash \\ --- \\ / \\ \end{array} C, \begin{array}{*{20}c} \backslash \\ --- \\ / \\ \end{array} Si, \begin{array}{*{20}c} \backslash \\ --- \\ / \\ \end{array} Si---O)\) were identified on a surface of activated silica by ESR. The possibility of their preparation by two methods,i.e., by photocyclization of substituted allyl radicals and by interaction of cyclopropane molecules with particular surface defects, was demonstrated. The effects of β-substituents on radiospectroscopic characteristics and thermal stabilities of the radicals were studied. Rate constants and activation energies of decomposition of substituted cyclopropyl radicals were estimated. The experimental data obtained were compared with the results of quantum-chemical calculations of model systems. The mechanism of photocyclization of allyl radicals was discussed. The transition states of decomposition of cyclopropyl and β-fluorine-substituted cyclopropyl radicals were calculated. Electronegative substitution was found to result in a considerable decrease in the activation energy of the cyclopropane ring opening.

Factors affecting the rates of thermal decomposition of azidothiophenes

The thermolysis rate of 4-methoxy-1 -azidobenzene reveals that the methoxy group increases the rate much less than the endocyclic sulfur does in 3-azidothiophene. With 3-azidothiophene, the neighbouring groups 2-acetyl and 2-nitro enhanced the rate in decalin solution only 5- and 17-fold, respectively, whereas in azidobenzene the corresponding enhancements are 413 and 1060. It is argued that the preferred electron distribution in the transition state for decomposition of 3-azidothiophene is not consistent with that required for effective neighbouring group participation. 2-Cyano and 2-methoxycarbonyl substituents, which are not known as effective neighbouring groups in thermal decompositions of azides, have very little effect on the rate for 3-azidothiophene.

Studies on the kinetics of decomposition of diacyl peroxides: III. Kinetics and mechanism of decomposition of 3, 5, 5-trimethylhexanoyl peroxide in benzene

The kinetics of decomposition of 3,5,5-trimethylhexanoyl peroxide (1) in benzene at 30, 40, and 50°C respectively have been studied and the cage effect, main products and the products obtained in scavenging experiment with galvinoxyl determined and characterized. The results show that the decomposition of 1, with initial concentration between 0.04 to 0.43 mol. L−1, followed first plus three halves order kinetics with cage effect of 0.6, and the mechanism of decomposition is basically the same as previously proposed for lauroyl peroxide (2).4 In comparison with 2, the decomposition of 1 at the same temperature showed a faster rate for spontaneous decomposition, a larger cage effect and lesser induced decomposition, which is attributed to the branching in molecule of 1, especially to the presence of β-methyl group, which caused a larger entropy increase between the peroxide molecule and the transition state for decomposition.

Reactions of radicals. XXVI. On the use of activation parameters as a test for concerted decomposition of initiators

AbstractIn 1958, Bartlett and Hiatt [1] suggested that the number of bonds which are rotationally “frozen” at the transition state for decomposition of peresters can be determined from the magnitude of the values of ΔH‡ and ΔS‡. In recent years, other workers have extended this hypothesis to include the use of the magnitude of ΔH‡ and ΔS‡ values to determine the number of bonds which undergo synchronous scission in the decomposition of initiators. This criterion for separating one‐bond from two ‐or three‐bond initiators is discussed in the light of all of the data now reported in the literature. It is probable that the correlation of ΔH‡ and ΔS‡ in an isokinetic plot cannot be used to distinguish either the number of bonds which are rotationally frozen at the transition state or the number that are undergoing scission, except in limited and rather imprecise ways. However, the insights and principles of Bartlett and Hiatt still are extremely useful as qualitative tools.