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1.
Emilia Iglesias 《国际化学动力学杂志》2012,44(10):668-679
The keto–enol tautomerism of 3‐chloro‐pentane‐2,4‐dione (ClPD) was studied in aqueous micellar solutions of cationic, anionic, and nonionic surfactants. The enol of ClPD tautomerizes rapidly in water to the equilibrium proportions of the keto form, KE=0.55; whereas the keto–enol conversion of 3‐ethyl‐pentane‐2,4‐dione (EPD) is a much slower reaction than the enol nitrosation. Kinetics of enol –nitrosation of both ClPD and EPD in aqueous acid medium using nitrous acid shows first‐order dependence upon [ketone] and linear or curve relationships of the observed rate constant, ko, as a function of [nitrite] or [H+]; the observed behavior depends on the molecular structure of diketone and varies with the experimental conditions. The reaction is strongly catalyzed by Cl?, Br?, or SCN?, and the observed rate constant shows a curve dependence on [Br?] or [SCN?], which is more pronounced at high acidity. The results are consistent with a reaction mechanism in which the nitrosation occurs initially on the enol–oxygen and releasing a proton to form a chelate–nitrosyl complex intermediate in steady state. Fine differences on the mechanistic spectrum of enols nitrosation are considered on the basis of the molecular structure of the diketone. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 668–679, 2012 相似文献
2.
Xu‐Xia Sun Shu‐Lan Ma Hai‐Bo Huang Chuan‐Min Qi 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(2):o87-o88
In the crystal structure of the title compound, C21H21NO2, strong N—H⋯O and O—H⋯O hydrogen bonds exist. The keto–amine form is favoured over the enol–imine form in the tautomerism. Six‐membered chelate rings formed by intramolecular hydrogen bonds increase the stability of the whole molecule. Intermolecular hydrogen bonds link adjacent units together, forming an infinite one‐dimensional chain parallel to the a axis. 相似文献
3.
Timothy J. Brown Dr. Atsushi Sugie Dr. Marina G. D. Leed Prof. Ross A. Widenhoefer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(22):6959-6971
A family of seven cationic gold complexes that contain both an alkyl substituted π‐allene ligand and an electron‐rich, sterically hindered supporting ligand was isolated in >90 % yield and characterized by spectroscopy and, in three cases, by X‐ray crystallography. Solution‐phase and solid‐state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two‐term rate laws of the form rate=k1[complex]+k2[complex][allene] consistent with allene‐independent and allene‐dependent exchange pathways with energy barriers of ΔG≠1=17.4–18.8 and ΔG≠2=15.2–17.6 kcal mol?1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG≠=8.9–11.4 kcal mol?1) intramolecular exchange of the allene π faces through η1‐allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)2o‐binaphthyl}Au(η2‐4,5‐nonadiene) ]+SbF6? ( 5 ), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG≠298K=17.4 kcal mol?1), which ruled out the participation of a η1‐allylic cation species in the low‐energy π‐face exchange process for this complex. 相似文献
4.
Kate J. Akerman Orde Q. Munro 《Acta Crystallographica. Section C, Structural Chemistry》2013,69(3):258-262
The Schiff base enaminones (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C13H17NO4S, (I), and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C15H21NO2, (II), were studied by X‐ray crystallography and density functional theory (DFT). Although the keto tautomer of these compounds is dominant, the O=C—C=C—N bond lengths are consistent with some electron delocalization and partial enol character. Both (I) and (II) are nonplanar, with the amino–phenol group canted relative to the rest of the molecule; the twist about the N(enamine)—C(aryl) bond leads to dihedral angles of 40.5 (2) and −116.7 (1)° for (I) and (II), respectively. Compound (I) has a bifurcated intramolecular hydrogen bond between the N—H group and the flanking carbonyl and hydroxy O atoms, as well as an intermolecular hydrogen bond, leading to an infinite one‐dimensional hydrogen‐bonded chain. Compound (II) has one intramolecular hydrogen bond and one intermolecular C=O...H—O hydrogen bond, and consequently also forms a one‐dimensional hydrogen‐bonded chain. The DFT‐calculated structures [in vacuo, B3LYP/6‐311G(d,p) level] for the keto tautomers compare favourably with the X‐ray crystal structures of (I) and (II), confirming the dominance of the keto tautomer. The simulations indicate that the keto tautomers are 20.55 and 18.86 kJ mol−1 lower in energy than the enol tautomers for (I) and (II), respectively. 相似文献
5.
Single‐Crystal X‐ray Diffraction Structure of the Stable Enol Tautomer Polymorph of Barbituric Acid at 224 and 95 K 下载免费PDF全文
Madalynn G. Marshall Valerie Lopez‐Diaz Bruce S. Hudson 《Angewandte Chemie (International ed. in English)》2016,55(4):1309-1312
The thermodynamically stable enol crystal form of barbituric acid, previously prepared as powder by grinding or slurry methods, has been obtained as single crystals by slow cooling from methanol solution. The selection of the enol crystal was facilitated by a density‐gradient method. The structure at 224 and 95 K confirms the enol inferred on the basis of powder data. The enol has bond lengths that are consistent with the expected bond order and with DFT calculations that include treatment of hydrogen bonding. In isolation, the enol is higher in energy than the tri‐keto form by 50 kJ mol?1 which must be more than compensated by enhanced hydrogen bonding. Both crystal forms have four normal H‐bonds; the enol has two additional H‐bonds with O–O distances of 2.49 Å. Conversion into the enol form occurs spontaneously in the solid state upon prolonged storage of the commercial tri‐keto material. Slurry conversion of tri‐one to enol in ethanol is reversed in direction in ethanol‐D1. 相似文献
6.
Oleg Ya. Borbulevych Igor R. Golding Alexander N. Shchegolikhin Zinaida S. Klemenkova Mikhail Yu. Antipin 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(8):996-998
Although the two polymorphic modifications, (I) and (II), of the title compound, C13H10N2O, crystallize in the same space group (P21/c), their asymmetric units have Z′ values of 1 and 2, respectively. These are conformational polymorphs, since the molecules in phases (I) and (II) adopt different rotations of the phenyl ring with respect the central 2‐cyanocarboxyaminoprop‐2‐enyl fragment. Calculations of crystal packing using Cerius2 [Molecular Simulations (1999). 9685 Scranton Road, San Diego, CA 92121, USA] have shown that (I) is more stable than (II), by 1.3 kcal mol?1 for the crystallographically determined structures and by 1.56 kcal mol?1 for the optimized structures (1 kcal mol?1 = 4.184 kJ mol?1). This difference is mainly attributed to the different strengths of the hydrogen bonding in the two forms. 相似文献
7.
First Principles (DFT) Characterization of RhI/dppp‐Catalyzed CH Activation by Tandem 1,2‐Addition/1,4‐Rh Shift Reactions of Norbornene to Phenylboronic Acid 下载免费PDF全文
Prof. Dr. Eric Assen B. Kantchev Surya R. Pangestu Dr. Feng Zhou Dr. Michael B. Sullivan Prof. Hai‐Bin Su 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15625-15634
The C?H activation in the tandem, “merry‐go‐round”, [(dppp)Rh]‐catalyzed (dppp=1,3‐bis(diphenylphosphino)propane), four‐fold addition of norborene to PhB(OH)2 has been postulated to occur by a C(alkyl)?H oxidative addition to square‐pyramidal RhIII?H species, which in turn undergoes a C(aryl)?H reductive elimination. Our DFT calculations confirm the RhI/RhIII mechanism. At the IEFPCM(toluene, 373.15 K)/PBE0/DGDZVP level of theory, the oxidative addition barrier was calculated to be 12.9 kcal mol?1, and that of reductive elimination was 5.0 kcal mol?1. The observed selectivity of the reaction correlates well with the relative energy barriers of the cycle steps. The higher barrier (20.9 kcal mol?1) for norbornyl–Rh protonation ensures that the reaction is steered towards the 1,4‐shift (total barrier of 16.3 kcal mol?1), acting as an equilibration shuttle. The carborhodation (13.2 kcal mol?1) proceeds through a lower barrier than the protonation (16.7 kcal mol?1) of the rearranged aryl–Rh species in the absence of o‐ or m‐substituents, ensuring multiple carborhodations take place. However, for 2,5‐dimethylphenyl, which was used as a model substrate, the barrier for carborhodation is increased to 19.4 kcal mol?1, explaining the observed termination of the reaction at 1,2,3,4‐tetra(exo‐norborn‐2‐yl)benzene. Finally, calculations with (Z)‐2‐butene gave a carborhodation barrier of 20.2 kcal mol?1, suggesting that carborhodation of non‐strained, open‐chain substrates would be disfavored relative to protonation. 相似文献
8.
Multi‐Pathway Consequent Chemoselectivities of CpRuCl(PPh3)2/MeI‐Catalysed Norbornadiene Alkyne Cycloadditions 下载免费PDF全文
Dr. Wei‐Hua Mu Prof. De‐Cai Fang Shu‐Ya Xia Rui‐Jiao Cheng Prof. Gregory A. Chass 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(43):15396-15403
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 (ΔGRel(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 (ΔGRel(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). 相似文献
9.
Viktor Kettmann Jan Svtlík Christoph Kratky 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(5):590-592
The title compound, C14H11NO4, consists of a methoxy‐substituted coumarin skeleton fused to a 2‐methyl‐4‐pyridone ring. The ring system of the molecule is approximately planar and the methoxy group is roughly coplanar with the ring plane. The 4‐pyridone ring exists in a 4‐hydroxy tautomeric form and is stabilized by an intramolecular hydrogen bond between the O—H and C=O groups. Comparison of the results with those found for other structures containing the 4‐pyridone substructure reveals a substantial effect of the nature of the substituents bonded to the pyridine ring on the keto–enol tautomerism. 相似文献
10.
Angiebelk Monsalve Felix Rosas María Tosta Armando Herize Rosa M. Domínguez Doris Brusco Gabriel Chuchani 《国际化学动力学杂志》2006,38(2):106-114
The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k1(s?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol?1 (2.303 RT)?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k1(s?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol?1 (2.303 RT)?1; ethyl piperidine‐3‐carboxylate, log k1(s?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol?1 (2.303 RT)?1; and 3‐piperidine carboxylic acid, log k1(s?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol?1 (2.303 RT)?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006 相似文献
11.
Dr. Peter Friedrich Dr. Ulrich Baisch Dr. Ross W. Harrington Fredrick Lyatuu Kai Zhou Dr. Felix Zelder Prof. Dr. William McFarlane Prof. Dr. Wolfgang Buckel Prof. Dr. Bernard T. Golding 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(50):16114-16122
Coenzyme B12 can assist radical enzymes that accomplish the vicinal interchange of a hydrogen atom with a functional group. It has been proposed that the Co? C bond homolysis of coenzyme B12 to cob(II)alamin and the 5′‐deoxyadenosyl radical is aided by hydrogen bonding of the corrin C19? H to the 3′‐O of the ribose moiety of the incipient 5′‐deoxyadenosyl radical, which is stabilized by 30 kJ mol?1 (B. Durbeej et al., Chem. Eur. J. 2009 , 15, 8578–8585). The diastereoisomers (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin were used as models for coenzyme B12. A downfield shift of the NMR signal for the C19? H proton was observed for the (R)‐isomer (δ=4.45 versus 4.01 ppm for the (S)‐isomer) and can be ascribed to an intramolecular hydrogen bond between the C19? H and the oxygen of CHOH. Crystal structures of (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin showed C19? H???O distances of 3.214(7) Å (R‐isomer) and 3.281(11) Å (S‐isomer), which suggest weak hydrogen‐bond interactions (?ΔG<6 kJ mol?1) between the CHOH of the dihydroxypropyl ligand and the C19? H. Exchange of the C19? H, which is dependent on the cobalt redox state, was investigated with cob(I)alamin, cob(II)alamin, and cob(III)alamin by using NMR spectroscopy to monitor the uptake of deuterium from deuterated water in the pH range 3–11. No exchange was found for any of the cobalt oxidation states. 3′,5′‐Dideoxyadenosylcobalamin, but not the 2′,5′‐isomer, was found to act as a coenzyme for glutamate mutase, with a 15‐fold lower kcat/KM than 5′‐deoxyadenosylcobalamin. This indicates that stabilization of the 5′‐deoxyadenosyl radical by a hydrogen bond that involves the C19? H and the 3′‐OH group of the cofactor is, at most, 7 kJ mol?1 (?ΔG). Examination of the crystal structure of glutamate mutase revealed additional stabilizing factors: hydrogen bonds between both the 2′‐OH and 3′‐OH groups and glutamate 330. The actual strength of a hydrogen bond between the C19? H and the 3′‐O of the ribose moiety of the 5′‐deoxyadenosyl group is concluded not to exceed 6 kJ mol?1 (?ΔG). 相似文献
12.
Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism 下载免费PDF全文
Bojana Ginovska Dr. Tom Autrey Dr. Kshitij Parab Dr. Mark E. Bowden Dr. Robert G. Potter Dr. Donald M. Camaioni 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(44):15713-15719
We report that 2,6‐lutidine?trichloroborane (Lut?BCl3) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl2. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl3 was calculated to react with H2 and form the ion pair, [LutH+][HBCl3?], with a barrier of ΔH≠=24.7 kcal mol?1 (ΔG≠=29.8 kcal mol?1). Metathesis with a second molecule of Lut?BCl3 produced Lut?BHCl2 and [LutH+][BCl4?]. The overall reaction is exothermic by 6.0 kcal mol?1 (ΔrG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl2+) and the four‐membered boracycle [(CH2{NC5H3Me})BCl2]. Barriers for addition of H2 across the Lut/LutBCl2+ pair and the boracycle B?C bond are substantially higher (ΔG≠=42.1 and 49.4 kcal mol?1, respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle B?N bond is comparable (ΔH≠=28.5 and ΔG≠=32 kcal mol?1). Conversion of the intermediate 2‐(BHCl2CH2)‐6‐Me(C5H3NH) to Lut?BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3. Intramolecular protodeboronation, which could form Lut?BHCl2 directly, is prohibited by a high barrier (ΔH≠=52, ΔG≠=51 kcal mol?1). 相似文献
13.
Enrique Saldívar‐Guerra Jos Bonilla Gregorio Zacahua Martha Albores‐Velasco 《Journal of polymer science. Part A, Polymer chemistry》2006,44(24):6962-6979
Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (kh)]. At higher TEMPO concentrations ([N?] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition kad, also becomes relevant. From previous experimental data and simulations, initial estimates of kh ≈ 1 and kad ≈ 6 × 10?7 L mol?1 s?1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression kth[M]3 (where [M] is the monomer concentration and kth is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006 相似文献
14.
Cross‐Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation 下载免费PDF全文
Dr. Mikhail A. Kabeshov Dr. Ondřej Kysilka Dr. Lubomír Rulíšek Dr. Yury V. Suleimanov Dr. Marco Bella Prof. Dr. Andrei V. Malkov Prof. Dr. Pavel Kočovský 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(34):12026-12033
Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin ( 1 a ) and acetone, catalyzed by L ‐leucinol ( 3 a ), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn‐enamine 6 , catalyzed by isatin, was identified as the rate‐determining step by both the calculations (ΔG≠=26.1 kcal mol?1 for the analogous L ‐alaninol, 3 b ) and the kinetic isotope effect (kH/kD=2.7 observed for the reaction using [D6]acetone). The subsequent reaction of the syn‐enamine 6 with isatin produces (S)‐ 2 a (calculated ΔG≠=11.6 kcal mol?1). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin‐catalyzed formation of the syn‐enamine 6 , which is thermodynamically favored over its anti‐rotamer 7 by 2.3 kcal mol?1; and 2) the high preference of the syn‐enamine 6 to produce (S)‐ 2 a on reaction with isatin ( 1 a ) rather than its enantiomer (ΔΔG≠=2.6 kcal mol?1). 相似文献
15.
Bagrat A. Shainyan Mikhail Yu. Moskalik Matthias Heydenreich Erich Kleinpeter 《Magnetic resonance in chemistry : MRC》2014,52(8):448-452
Restricted rotation about the N–S partial double bonds in a bis‐N‐triflyl substituted 3,8‐diazabicyclo[3.2.1]octane derivative 1 has been frozen at low temperature (ΔG≠ = 11.6 kcal mol?1), and the existence of all four rotamers about the two N–S bonds, 3‐in,8‐in, 3‐in,8‐out, 3‐out,8‐in, and 3‐out,8‐out, respectively, proved experimentally by NMR spectroscopy and theoretically by DFT and MP2 calculations. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
16.
Andrea Mazzanti Dr. Lodovico Lunazzi Prof. Renzo Ruzziconi Prof. Sara Spizzichino Manfred Schlosser Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(30):9186-9192
By making use of a novel diastereotopicity probe, namely C(CF3)2OH, it has been possible to measure by very low temperature 19F NMR spectroscopy the elusive aryl–aryl rotation barriers of biphenyls bearing an OH or F group in one ortho position. The experimental values (5.4 and 4.4 kcal mol?1, respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol?1, respectively). 相似文献
17.
Pressure effects on the two‐site jumping of sodium and potassium cations in a 2,5‐di‐tert‐butyl‐1,4‐benzoquinone ion pair have been studied using a high‐pressure EPR technique. The rate constants of the intramolecular and intermolecular migrations for Na+ and K+ were determined from an EPR spectral simulation. The migration rates were found to be accelerated by increasing the external pressure. Using the pressure dependence of the migration rates, we estimated the activation volumes of the intramolecular (ΔV1?) and intermolecular (ΔV2?) processes for the Na+ and K+ migrations: ΔV1? = ?5.3 cm3 mol?1 and ΔV2? = ?29 cm3 mol?1 for Na+, and ΔV1? = ?8.3 cm3 mol?1 and ΔV2? = ?0.85 cm3 mol?1 for K+. Based on the results, the mechanisms for the two‐site jumping of Na+ and K+ are discussed in terms of volume. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 397–401, 2001 相似文献
18.
Florian Eweiner Shunsuke Senda Klaus Bergander Dr. Christian Mück‐Lichtenfeld Dr. Stefan Grimme Prof. Dr. Roland Fröhlich Dr. Michiko Aoyama Hiroyuki Kawaguchi Prof. Dr. Yasuhiro Ohki Dr. Tsuyoshi Matsumoto Dr. Gerald Kehr Dr. Kazuyuki Tatsumi Prof. Dr. Gerhard Erker Prof. Dr. 《化学:亚洲杂志》2009,4(12):1830-1833
Treatment of the salt [PPh4]+[Cp*W(S)3]? ( 6 ) with allyl bromide gave the neutral complex [Cp*W(S)2S‐CH2‐CH?CH2] ( 7 ). The product 7 was characterized by an X‐ray crystal structure analysis. Complex 7 features dynamic NMR spectra that indicate a rapid allyl automerization process. From the analysis of the temperature‐dependent NMR spectra a Gibbs activation energy of ΔG≠ (278 K)≈13.7±0.1 kcal mol?1 was obtained [ΔH≠≈10.4±0.1 kcal mol?1; ΔS≠≈?11.4 cal mol?1 K?1]. The DFT calculation identified an energetically unfavorable four‐membered transition state of the “forbidden” reaction and a favorable six‐membered transition state of the “Cope‐type” allyl rearrangement process at this transition‐metal complex core. 相似文献
19.
Sippakorn Wannakao Dr. Bundet Boekfa Dr. Pipat Khongpracha Prof. Dr. Michael Probst Prof. Dr. Jumras Limtrakul 《Chemphyschem》2010,11(16):3432-3438
The adsorption and the mechanism of the oxidative dehydrogenation (ODH) of propane over VO2‐exchanged MCM‐22 are investigated by DFT calculations using the M06‐L functional, which takes into account dispersion contributions to the energy. The adsorption energies of propane are in good agreement with those from computationally much more demanding MP2 calculations and with experimental results. In contrast, B3LYP binding energies are too small. The reaction begins with the movement of a methylene hydrogen atom to the oxygen atom of the VO2 group, which leads to an isopropyl radical bound to a HO? V? O intermediate. This step is rate determining with the apparent activation energy of 30.9 kcal mol?1, a value within the range of experimental results for ODH over other silica supports. In the propene formation step, the hydroxyl group is the more reactive group requiring an apparent activation energy of 27.7 kcal mol?1 compared to that of the oxy group of 40.8 kcal mol?1. To take the effect of the extended framework into account, single‐point calculations on 120T structures at the same level of theory are performed. The apparent activation energy is reduced to 28.5 kcal mol?1 by a stabilizing effect caused by the framework. Reoxidation of the catalyst is found to be important for the product release at the end of the reaction. 相似文献
20.
Liliany Espitia Ruby Meneses Rosa M. Dominguez Maria Tosta Armando Herize Jesus Lezama Jennifer Lafont Gabriel Chuchani 《国际化学动力学杂志》2009,41(3):145-152
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 相似文献