共查询到20条相似文献,搜索用时 140 毫秒
1.
Kinetic studies of reactions of the MoMo bonded complex (h5-C5H5)2Mo2(CO)6 in decalin show that it undergoes reversible homolytic fission and that the activation enthalpy required to break the MoMo bond is 135.9 ± 2.2 kJ mol?1. 相似文献
2.
D. Lalage S. Brown Joseph A. Connor Henry A. Skinner 《Journal of organometallic chemistry》1974,81(3):403-409
From measurements of the heats of iodination of CH3Mn(CO)5 and CH3Re(CO)5 at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔHof[CH3Mn(CO)5, c] = ?189.0 ± 2 kcal mol?1 (?790.8 ± 8 kJ mol?1), ΔHof[Ch3Re(CO)5,c] = ?198.0 ± kcal mol?1 (?828.4 ± 8 kJ mo?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn2(CO)10 and Re2(CO)10, values are derived for the dissociation energies: D(CH3Mn(CO)5) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol?1 and D(CH3Re(CO)5) = 53.2 ± 2.5 kcal mol?1. In general, irrespective of the value accepted for D(MM) in M2(CO)10, the present results require that, D(CH3Mn) = D(MnMn) + 18.5 kcal mol?1 and D(CH3Re) = D(ReRe) + 30.8 kcal mol?1. 相似文献
3.
The rate constant of the primary decomposition step was determined for four symmetrical and four unsymmetrical azoalkanes. From the experimental activation energies and some literature enthalpy data, the following enthalpies of formation of radicals and group contributions were calculated: ΔH? (CH3N2) = 51.5 ± 1.8 kcal mol?1, ΔH? (C2H5N2) = 44.8 ± 2.5 kcal mol?1, ΔH? (2?C3H7N2) = 37.9 ± 2.2 kcal mol?1, [NA-(C)] = 27.6 ± 3.7 kcal mol?1, [NA-(?A) (C)] = 61.2 ± 3.1 kcal mol?1. 相似文献
4.
ICR trapped ion techniques are used to examine the kinetics of proton transfer from MnH+ (formed as a fragment ion from HMn (CO)5 by electron impact) to bases of varying strength. Deprotonation is rapid with bases whose proton affinity exceeds 196±3 kcal mol?1. This value for PA (Mn) yields the homolytic bond dissociation energy D0(Mn+-H) = 53±5 kcal mol?1. 相似文献
5.
Overtone absorption spectra are reported for ethanol vapor (10150–19900 cm?1) measured by intracavity photoacoustic spectroscopy. The OH overtones are composed of two sub-bands which are assigned as the transitions of two conformers of the OH bond in the trans or gauche position with respect to the methyl group. From the temperature dependence of the OH overtone intensity we determine the enthalpy difference between the conformers to be 0.7 ± 0.1 kcal/mole. 相似文献
6.
The ironiron bond energy in [C5H5Fe(CO)2]2 (I) has been determined by measuring the rate of disproportionation of the monoacetyl complex (AcC5H4)(C5H5)Fe2(CO)4 (II) to I and [AcC5H4Fe(CO)2]2 (III). The reaction follows first order kinetics in benzene solution in the temperature range of 60–100°C with activation parameters calculated as: ΔH≠ = 26.9 ± 2.7 kcal mol?1 and ▽s≠ = 2.0 ± 3.2 cal mol?1 deg?1. 相似文献
7.
S. A. I. Cazut E. H. Ramírez Maisuls M. R. Delfino J. M. Romero N. L. Jorge E. A. Castro 《Russian Journal of General Chemistry》2009,79(10):2187-2190
Thermal decomposition of formaldehyde diperoxide (1,2,4,5-tetraoxane) in aqueous solution with an initial concentration of 6.22 × 10?3 M was studied in the temperatures range from 403 to 439 K. The reaction was found to follow first-order kinetic law, and formaldehyde was the major decomposition product. The activation parameters of the initial step of the reaction (ΔH ≠ = 15.25 ± 0.5 kcal mol?1, ΔS ≠ = ?47.78 ± 0.4 cal mol?1K?1, E a = 16.09 ± 0.5 kcal mol?1) support a mechanism involving homolytic rupture of one peroxide bond in the 1,2,4,5-tetraoxane molecule with participation of the solvent and formation of a diradical intermediate. 相似文献
8.
A Mechanistic Study of the Spontaneous Hydrolysis of Glycylserine as the Simplest Model for Protein Self‐Cleavage
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Tzvetan T. Mihaylov Prof. Dr. Tatjana N. Parac‐Vogt Prof. Dr. Kristine Pierloot 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(2):456-466
A common feature of several classes of intrinsically reactive proteins with diverse biological functions is that they undergo self‐catalyzed reactions initiated by an N→O or N→S acyl shift of a peptide bond adjacent to a serine, threonine, or cysteine residue. In this study, we examine the N→O acyl shift initiated peptide‐bond hydrolysis at the serine residue on a model compound, glycylserine (GlySer), by means of DFT and ab initio methods. In the most favorable rate‐determining transition state, the serine ?COO? group acts as a general base to accept a proton from the attacking ?OH function, which results in oxyoxazolidine ring closure. The calculated activation energy (29.4 kcal mol?1) is in excellent agreement with the experimental value, 29.4 kcal mol?1, determined by 1H NMR measurements. A reaction mechanism for the entire process of GlySer dipeptide hydrolysis is also proposed. In the case of proteins, we found that when no other groups that may act as a general base are available, the N→O acyl shift mechanism might instead involve a water‐assisted proton transfer from the attacking serine ?OH group to the amide oxygen. However, the calculated energy barrier for this process is relatively high (33.6 kcal mol?1), thus indicating that in absence of catalytic factors the peptide bond adjacent to serine is no longer a weak point in the protein backbone. An analogous rearrangement involving the amide N‐protonated form, rather than the principle zwitterion form of GlySer, was also considered as a model for the previously proposed mechanism of sea‐urchin sperm protein, enterokinase, and agrin (SEA) domain autoproteolysis. The calculated activation energy (14.3 kcal mol?1) is significantly lower than the experimental value reported for SEA (≈21 kcal mol?1), but is still in better agreement as compared to earlier theoretical attempts. 相似文献
9.
The kinetics of the reactions of hydroxy radicals with cyclopropane and cyclobutane has been investigated in the temperature range of 298–492 K with laser flash photolysis/resonance fluorescence technique. The temperature dependence of the rate constants is given by k1 = (1.17 ± 0.15) × 10?16 T3/2 exp[?(1037 ± 87) kcal mol?1/RT] cm3 molecule?1 s1 and k2 = (5.06 ± 0.57) × 10?16 T3/2 exp[?(228 ± 78) kcal mol?1/RT] cm3 molecule?1 s?1 for the reactions OH + cyclopropane → products (1) and OH + cyclobutane → products (2), respectively. Kinetic data available for OH + cycloalkane reactions were analyzed in terms of structure-reactivity correlations involving kinetic and energetic parameters. 相似文献
10.
The heat of formation of benzophenone oxide, Ph2CO2, was measured using photoacoustic calorimetry. The enthalpy of the reaction Ph2CN2 + O2 → Ph2CO2 + N2 was found to be ?48.0 ±0.8 kcal mol?1 and ΔHf(Ph2CN2) was determined by measuring the reaction enthalpy for Ph2CN2 + EtOH → Ph2CHOEt + N2 (?53.6 ±1.0 kcal mol?1). Taking ΔHf(PhCHOEt) = ?10.6 kcal mol?1 led to ΔHf(Ph2CN2) = 99.2 ± 1.5 kcal mol?1 and hence to ΔHf(Ph2CO2) = 51.1 ± 2.0 kcal mol?1. The results imply that the self-reaction of benzophenone oxide i.e., 2Ph2CO2 → 2Ph2CO + O2 is exothermic by ?76.0 ±4.0 kcal mol?1. 相似文献
11.
E. Taskinen 《Tetrahedron》1976,32(19):2327-2329
The relative thermodynamic stability of the monoalkoxy- and 1,2-dialkoxyethylene systems [OCCH(C) and OCCO, respectively] has been studied by chemical equilibration of suitable isomeric compounds. Although a single alkoxy substituent stabilizes the CC bond by about 25 kJ mol?1, the 1,2-dialkoxyethylene system is no more stable than the monoalkoxyethylene (“ordinary” vinyl ether) system. On the contrary, the MeOCCOMe system was found to be about 4 kJ mol?1 (on an enthalpy basis) less stable than the system MeOCCH. 相似文献
12.
W.V. Steele 《The Journal of chemical thermodynamics》1979,11(12):1185-1188
The standard enthalpy of combustion of cyclohexylamine has been measured in an aneroid rotating-bomb calorimeter. The value ΔHoo(c-C6H11NH2, 1) = ?(4071.3 ± 1.3) kJ mol?1 yields the standard enthalpy of formation ΔHfo(c-C6H11NH2, 1) = ?(147.7 ± 1.3) kJ mol?1. The corresponding gas-phase standard enthalpy of formation for cyclohexylamine is ΔHfo(c-C6H11NH2, g) = ?(104.9 ± 1.3) kJ mol?1. The standard enthalpy of formation of cyclohexylamine hydrochloride, ΔHfo(c-C6H11NH2·HCl, c) = ?(408.2 ± 1.5) kJ mol?1, was derived by combining the measured enthalpy of solution of the salt in water, literature data, and the ΔHco measured in this study. Comment is made on the thermochemical bond enthalpy H(CN). 相似文献
13.
Restricted rotation about the naphthalenylcarbonyl bonds in the title compounds resulted in mixtures of cis and trans rotamers, the equilibrium and the rotational barriers depending on the substituents. For 2,7-dimethyl-1,8-di-(p-toluoyl)-naphthalene (1) ΔH° = 3.66 ± 0.14 kJ mol?1, ΔS° = 1.67 ± 0.63 J mol?1 K?1, ΔH≠ct = 55.5 ± 1.3 kJ mol?1, ΔH≠ct = 51.9 ± 1.3 kJ mol?1, ΔS≠ct = ?41.3±4.1 J mol?1 K?1 and ΔS≠ct = ?42.9±4.1 J mol?1 K?1. The rotation about the phenylcarbonyl bond requires ΔH≠ = ?56.9±4.4 kJ mol?1 and ΔS≠ = ?20.5±15.3 J mol?1 K?1 for the cis rotamer, and ΔH≠ = 43.5Δ0.4 kJ mol?1 and ΔS≠ =± ?22.4Δ1.3 J mol?1 K?1 for the trans rotamer. The role of electronic factors is likely to be virtually the same for both these rotamers but steric interaction between the two phenyl rings occurs in the cis rotamer only. Hence, the difference of the activation enthalpies obtained for the cis and trans rotamers, ΔΔH?1 = 13.4 kJ mol?1, provides a basis for the estimation of the role of steric factors in this rotation. For the tetracarboxylic acid 2 and its tetramethyl ester 3 the equilibrium is even more shifted towards the trans form because of enhanced steric and electrostatic interactions between the substituents in the cis form. The barriers for the rotation around the phenylcarbonyl bond and the cis-trans isomerization are lowered; an explanation for this result is presented. 相似文献
14.
L. P. Ogorodova I. A. Kiseleva 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2011,85(10):1857-1859
The standard dehydroxylation enthalpy of natural talc Mg3[Si4O10](OH)2 (87.8 ± 9.0 kJ/mol at 298.15 K) and the enthalpy of formation of dehydrated talc from the elements (Δf
H
elo (298.15 K) = −5527.0 ± 9.0 kJ/mol) were determined for the first time using Hess’s law, based on the total values of the
enthalpy increments in heating a sample from room temperature to 973 K and the enthalpies of dissolution at 973 K for dehydrated
talc measured in this work and those previously determined for talc and corresponding oxides. 相似文献
15.
Dr. Sarolta Pilbák Dr. Ödön Farkas Prof. Dr. László Poppe 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(25):7793-7802
Quantum mechanics/molecular mechanics calculations in tyrosine ammonia lyase (TAL) ruled out the hypothetical Friedel–Crafts (FC) route for ammonia elimination from L ‐tyrosine due to the high energy of FC intermediates. The calculated pathway from the zwitterionic L ‐tyrosine‐binding state (0.0 kcal mol?1) to the product‐binding state ((E)‐coumarate+H2N? MIO; ?24.0 kcal mol?1; MIO=3,5‐dihydro‐5‐methylidene‐4H‐imidazol‐4‐one) involves an intermediate (IS, ?19.9 kcal mol?1), which has a covalent bond between the N atom of the substrate and MIO, as well as two transition states (TS1 and TS2). TS1 (14.4 kcal mol?1) corresponds to a proton transfer from the substrate to the N1 atom of MIO by Tyr300? OH. Thus, a tandem nucleophilic activation of the substrate and electrophilic activation of MIO happens. TS2 (5.2 kcal mol?1) indicates a concerted C? N bond breaking of the N‐MIO intermediate and deprotonation of the pro‐S β position by Tyr60. Calculations elucidate the role of enzymic bases (Tyr60 and Tyr300) and other catalytically relevant residues (Asn203, Arg303, and Asn333, Asn435), which are fully conserved in the amino acid sequences and in 3D structures of all known MIO‐containing ammonia lyases and 2,3‐aminomutases. 相似文献
16.
The thermal dehydrations of formate dihydrates of Mg(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) were studied by means of thermogravimetry, differential thermal analysis and differential scanning calorimetry in air.The reaction orders of dehydration obtained by the dynamic and the static methods were found to be 2/3 for all the salts examined, which indicated that the rate of dehydration was controlled by a chemical process at a phase boundary. This was confirmed by microscopic observation.The values of activation energy, frequency factor and the enthalpy change of dehydration for all salts examined, were 21–30 kcal mole?1, 1010-1012 sec?1 and 28–31 kcal mole?1, respectively.The temperature at which the dehydration occurred was regarded as a measure of the strength of the metalOH2 bond, and this temperature increased with increasing the reciprocal of the radius of the metallic ion. 相似文献
17.
V. D. Kiselev H. A. Kashaeva L. N. Potapova D. A. Kornilov A. I. Konovalov 《Russian Chemical Bulletin》2014,63(1):280-283
The rates of an ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene were studied in a temperature range of 15–40 °C and in a pressure range of 1–2013 bar. The enthalpy of reaction in 1,2-dichloroethane (?158.2±1.0 kJ mol?1), the enthalpy (51.3±0.5 kJ mol?1), entropy (122±2 J mol?1 K?1), and volume of activation (?31.0±1.0 cm3 mol?1), and the volume of this reaction (?26.6±0.3 cm3 mol?1) were determined. The high exothermic effect of the reaction suggests its irreversibility. 相似文献
18.
The dehydroxylation of gibbsite into boehmite was investigated by means of DSC analysis under non-isothermal conditions in the temperature range 453–673 K at heating rates from 2.5 to 20.0 K min?1. Mathematical analysis of the experimental DSC curves revealed the mechanism and kinetics of the gibbsite dehydroxylation process. The kinetic curvesα=f(t) andα=f(T) are sigmoidal in shape; their inflection points and the νm point of the curvesν=f(T) andν=f(T) are interrelated and are defined by the concept of a stationary point. The activation energy for the first stage of gibbsite dehydroxylation in the temperature range 453–673 K is 132.92±8.33–142.26±8.33 kJ mol?1. 相似文献
19.
We report variational transition‐state theory calculations for the OH + O3→ HO2 + O2 reaction based on the recently reported double many‐body expansion potential energy surface for ground‐state HO4 [Chem Phys Lett 2000, 331, 474]. The barrier height of 1.884 kcal mol?1 is comparable to the value of 1.77–2.0 kcal mol?1 suggested by experimental measurements, both much smaller than the value of 2.16–5.11 kcal mol?1 predicted by previous ab initio calculations. The calculated rate constant shows good agreement with available experimental results and a previous theoretical dynamics prediction, thus implying that the previous ab initio calculations will significantly underestimate the rate constant. Variational and tunneling effects are found to be negligible over the temperature range 100–2000 K. The O1? O2 bond is shown to be spectator like during the reactive process, which confirms a previous theoretical dynamics prediction. © 2007 Wiley Periodicals, Inc. 39: 148–153, 2007 相似文献
20.
Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO2 (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO2 isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO2 are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed‐ and open‐shell reference molecules. Three high‐level composite methods G3, CBS‐QB3, and CBS‐APNO are used for the computation of enthalpy. The enthalpy of formation, ΔHof(298 K), for HONO is determined as ?18.90 ± 0.05 kcal mol?1 (?79.08 ± 0.2 kJ mol?1) and as ?10.90 ± 0.05 kcal mol?1 (?45.61 ± 0.2 kJ mol?1) for nitryl hydride (HNO2), which is significantly higher than values used in recent NOx combustion mechanisms. H‐NO2 is the weakest bond in isonitrous acid; but HNO2 will isomerize to HONO with a similar barrier to the HO? NO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O2 system is presented, and an analysis of the triplet surface is initiated. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378–398, 2007 相似文献