Applications of proton magnetic resonance to rotational isomerism in halotoluene derivatives. IV. α,α,2,4,6-Pentachlorotoluene in carbon disulfide, an exchanging ABC system

1970 ◽  
Vol 48 (24) ◽  
pp. 3877-3881 ◽  
Author(s):  
Helen G. Gyulai ◽  
B. J. Fuhr ◽  
H. M. Hutton ◽  
T. Schaefer
Keyword(s):  
Carbon Disulfide ◽  
Activation Parameters ◽  
Frequency Factor ◽  
Kcal Mole ◽  
Hindered Rotation ◽  
Line Shapes ◽  
Line Shape Analysis ◽  
Enthalpy Of Activation ◽  
Carbon Carbon Bond ◽  
Entropy Of Activation

The p.m.r. of α,α,2,4,6-pentachlorotoluene in carbon disulfide solution displays hindered rotation about the sp2–sp3 carbon–carbon bond, causing broadening and collapse of the proton resonance spectrum. A full line-shape analysis using the computer program DNMR gives an activation energy of 14.2 ± 0.3 kcal/mole, a frequency factor of 11.9 ± 0.2, an enthalpy of activation of 13.7 ± 0.3 kcal/mole, and an entropy of activation of −4.4 ± 1 e.u. These data are compared with the activation parameters obtained in methylcyclohexane and toluene-d8 solutions from different line shapes.

Steroids and steroidases. IX. Activation parameters and the mechanism of base- and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione. The nature of the active center of the Δ5 → Δ4-3-ketosteroid isomerase of Pseudomonas testosteroni

1969 ◽  
Vol 47 (23) ◽  
pp. 4459-4466 ◽  
Author(s):  
J. Bryan Jones ◽  
Donald C. Wigfield
Keyword(s):  
Amino Acids ◽  
Active Center ◽  
Activation Parameters ◽  
Circumstantial Evidence ◽  
Kcal Mole ◽  
Acid Base ◽  
Ketosteroid Isomerase ◽  
Enzyme Action ◽  
Enthalpy Of Activation

Determination of the activation parameters for the acid-, base-, and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione has revealed that the facility of the enzymic process is mainly due to an extremely low enthalpy of activation of 5.0 kcal mole−1. Further circumstantial evidence regarding the nature of the reacting groups at the active center has also been obtained, and a mechanism of enzyme action is proposed employing tyrosine and histidine as the principal amino acids responsible for catalyzing the isomerization.

Application of proton magnetic resonance to rotational isomerism in halotoluene derivatives. III. α,α-Dichloro-2,4,6-tribromotoluene

1970 ◽  
Vol 48 (18) ◽  
pp. 2839-2842 ◽  
Author(s):  
J. Peeling ◽  
T. Schaefer ◽  
C. M. Wong
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Room Temperature ◽  
Proton Magnetic Resonance Spectrum ◽  
Methine Proton ◽  
Proton Spectrum ◽  
Kcal Mole ◽  
Ring Proton ◽  
Line Shapes ◽  
Carbon Carbon Bond

At room temperature the proton magnetic resonance spectrum of α,α-dichloro-2,4,6-tribromotoluene is ABX where the methine proton in the sidechain is X and is lying in the plane of the aromatic ring. At higher temperatures the ring proton spectrum, AB, broadens and eventually collapses to yield an A2X spectrum. From an analysis of the ring proton line shapes the barrier to rotation of the dichloromethyl group about the sp2–sp3 carbon–carbon bond is obtained; ΔG* = 17.5 ± 0.1 kcal/mole at 304°K, ΔH* = 15.67 ± 0.08 kcal/mole, ΔS* = −7 e.u., Ea = 16.38 ± 0.08 kcal/mole, log A = 11.78 ± 0.23 where the least squares errors given should probably be multiplied by a factor of from 3 to 5 to take possible systematic errors into account. The barrier is about 2 kcal/mole higher than in α,α,2,4,6-pentachlorotoluene. The barrier to rotation arises from the conformation in which chlorine and bromine atoms are eclipsed.

Thermal rearrangement of α,β- to β,γ-unsaturated esters. Evidence for a 1,5-hydrogen transfer mechanism

1968 ◽  
Vol 46 (13) ◽  
pp. 2225-2232 ◽  
Author(s):  
Donald E. McGreer ◽  
Norman W. K. Chiu
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Hydrogen Transfer ◽  
Activation Parameters ◽  
Transfer Mechanism ◽  
Thermal Rearrangement ◽  
Kcal Mole ◽  
Isopropyl Group ◽  
First Order ◽  
Group A ◽  
Entropy Of Activation

Methyl cis-4-methyl-2-pentenoate (1) has been found to rearrange at 252 °C by a simple first-order and presumably unimolecular mechanism to methyl 4-methyl-3-pentenoate (3) with a rate constant k of 4.28 × 10−5 s−1. The heat of activation ΔH†, was determined to be 37.5 ± 2 kcal/mole and the entropy of activation, ΔS†, was determined to be −8.2 ± 4 e.u. These activation parameters and the steric requirements of the reaction support a mechanism involving a cyclic 1,5-transfer of hydrogen. Studies on the isomerization of methyl cis-2-pentenoate, methyl cis-2,4-dimethyl-2-pentenoate, and methyl cis-2-methyl-2-pentenoate to the corresponding 3-pentenoates suggest that the 1,5-hydrogen transfer mechanism could well be general for the equilibration of α,β- and β,γ-unsaturated esters.An isopropyl group cis to a methyl or a carbomethoxy group on a double bond is shown by nuclear magnetic resonance to prefer a conformation with the methyls of the isopropyl group held away from the methyl or carbomethoxy group. A cis hydrogen does not restrict the conformation of the isopropyl group.

Kinetics in Solution

2008 ◽  
Author(s):  
José A. Martinho Simões ◽  
Manuel Minas da Piedade
Keyword(s):  
Analytical Method ◽  
Activation Parameters ◽  
Analytical Techniques ◽  
Frequency Factor ◽  
Nonequilibrium Kinetics ◽  
Entropy Of Activation ◽  
Concentration Changes ◽  
Enthalpy And Entropy ◽  
Reaction In Solution

The main equations used to extract thermochemical data from rate constants of reactions in solution were presented in section 3.2. Here, we illustrate the application of those equations with several examples quoted from the literature. First, however, recall that the rate constant for any elementary reaction in solution, defined in terms of concentrations, is related to the activation parameters through equations 15.1 or 15.2. Equation 15.1 yields the enthalpy and the entropy of activation respectively from the slope and the intercept of a ln(k/T) versus 1/T plot (an Eyring plot). Equation 15.2 leads to the Arrhenius activation energy and the frequency factor, respectively, from the slope and the intercept of a ln k versus 1/T plot (an Arrhenius plot). All the parameters refer to the mean temperature of the plot, and Δ‡Ho is related to Ea by equation 15.3. Finally, recall that if the activation parameters are available for the forward (subscript 1) and the reverse (subscript −1) reaction, the enthalpy of this reaction is calculated by equation 15.4. In the preceding chapter on equilibrium in solution, it was pointed out that any analytical method suitable for determining equilibrium compositions of a reaction mixture at several temperatures can be used to obtain the enthalpy and entropy of that reaction. A similar statement can be made here: Any analytical method suitable for monitoring concentration changes with time at several temperatures can be used to derive the activation parameters of a reaction. Therefore, the analytical techniques used in equilibrium experiments are also applied in nonequilibrium (kinetics) studies. However, in this case, the choice of the analytical method will have an additional and important restriction, for it must consider the reaction rate. An instrumental technique suitable for determining the concentration of a given species under equilibrium conditions may be inappropriate for determining a fast concentration change of the same species.

scholarly journals Kinetic Oxidation Studies of Pentoxifylline by N-Chlorosuccinimide in Acidic Medium Using Iridium(III) Chloride as Inhibitor

2021 ◽  
Vol 34 (1) ◽  
pp. 162-168
Author(s):  
Rakesh Patel ◽  
Ravi Prakash ◽  
Ritu Swamini Bala ◽  
Brijesh Kumar Prajapati ◽  
Rupam Yadav
Keyword(s):  
Activation Energy ◽  
Activation Parameters ◽  
First Order Kinetics ◽  
Free Energy Of Activation ◽  
Enthalpy Of Activation ◽  
Rate Of Reaction ◽  
Entropy Of Activation ◽  
Acidic Conditions ◽  
Kinetic Oxidation

In present study, the kinetics and mechanism of oxidation of pentoxifylline (PTX) by N-chlorosuccinimide (NCS) in acidic conditions at 40 ± 0.1 ºC is reported. The reaction depicts first-order kinetics in regard to [NCS], [PTX] and [HClO4]. The reaction rate goes on decreasing as the concentration of iridium(III) chloride is increased. This shows that iridium(III) chloride plays the role of an inhibitor in the reaction under investigation. Nil impact of [Hg(OAc)2], [NHS] and dielectric constant (D) of the medium on the rate of oxidation of pentoxifylline have been observed. This reaction has been investigated from 308-323 K and the monitored rate of reaction suggests a direct relationship between temperature and the rate of reaction. From the graph between log k and 1/T, value of activation energy (Ea) was numerated and more activation parameters like enthalpy of activation (ΔH#), entropy of activation (ΔS#) and free energy of activation (ΔG#) were calculated with the help of activation energy (Ea). On account of experimentally determined the kinetic orders and activation parameters, a most plausible reaction path has been suggested for the oxidation of pentoxifylline in presence of Ir(III) as an inhibitor.

Determination of the Accurate Values of the Rate Constant and Thermodynamic Parameters for the Rotation About the C(sp2)-C(aryl) Bond; Adamantan-1-yl 3-Bromo-2,4,6-trimethylphenyl Ketone

1998 ◽  
Vol 63 (7) ◽  
pp. 955-966
Author(s):  
Eva Přibylová ◽  
Miroslav Holík
Keyword(s):  
Thermodynamic Parameters ◽  
Shape Analysis ◽  
Rate Constants ◽  
Line Shape ◽  
Microsoft Excel ◽  
Hindered Rotation ◽  
H Nmr ◽  
Line Shape Analysis ◽  
Coalescence Temperature

Four programs for the 1H NMR line shape analysis: two commercial - Winkubo (Bruker) and DNMR5 (QCPE 165) and two written in our laboratory - Newton (in Microsoft Excel) and Simtex (in Matlab) have been tested in order to get highly accurate rate constants of the hindered rotation about a single bond. For this purpose four testing criteria were used, two of them were also developed by us. As supplementary determinations the rate constants obtained for the coalescence temperature and for the thermal racemization of chromatographically separated enantiomers were used which fitted well the temperature dependence of the rate constants determined by the line shape analysis. As a test compound adamantan-1-yl 3-bromo-2,4,6-trimethylphenyl ketone was prepared and studied. It was shown that supermodified simplex method used in our algorithm (Simtex), though time consuming, gives the most accurate values of the rate constants and consequently the calculated thermodynamic parameters Ea, ∆H≠, and ∆S≠ lay in relatively narrow confidence intervals.

Strongly hindered rotation in α,α,2,6-terrachlorotoluene. Stereospecific five-bond coupling. Hyperconjugative contributions to long-range sidechain-ring proton coupling constants

1968 ◽  
Vol 46 (12) ◽  
pp. 2187-2188 ◽  
Author(s):  
T. Schaefer ◽  
R. Schwenk ◽  
C. J. Macdonald ◽  
W. F. Reynolds
Keyword(s):  
Free Energy ◽  
Resonance Spectrum ◽  
Coupling Constants ◽  
Proton Resonance ◽  
Kcal Mole ◽  
Hindered Rotation ◽  
Ring Proton ◽  
Meta Position ◽  
Proton Coupling ◽  
Free Energy Of Activation

At −40 °C the C—H bond of the dichloromethyl group of α,α,2,6-tetrachlorotoluene lies in the plane of the ring. The proton resonance spectrum demonstrates a stereospecific five-bond coupling between the C—H proton and the ring proton in the meta position. The coupling to the para proton is essentially zero as expected from a hyperconjugative mechanism. The free energy of activation of rotation of the dichloromethyl group is about 15 kcal/mole at 25 °C.

scholarly journals Poly(N-isopropylacrylamide-co-methacrylic acid) microgel stabilized copper nanoparticles for catalytic reduction of nitrobenzene

2015 ◽  
Vol 33 (3) ◽  
pp. 627-634 ◽  
Author(s):  
Zahoor H. Farooqi ◽  
Zonarah Butt ◽  
Robina Begum ◽  
Shanza Rhauf Khan ◽  
Ahsan Sharif ◽  
...  
Keyword(s):  
Copper Nanoparticles ◽  
Methacrylic Acid ◽  
Catalytic Reduction ◽  
Aqueous Media ◽  
Activation Parameters ◽  
Reducing Agent ◽  
Different Temperatures ◽  
Entropy Of Activation ◽  
Micro Reactors

Abstract Poly(N-isopropylacrylamide-co-methacrylic acid) microgels [p(NIPAM-co-MAAc)] were synthesized by precipitation polymerization of N-isopropylacrylamide and methacrylic acid in aqueous medium. These microgels were characterized by dynamic light scattering and Fourier transform infrared spectroscopy. These microgels were used as micro-reactors for in situ synthesis of copper nanoparticles using sodium borohydride (NaBH4) as reducing agent. The hybrid microgels were used as catalysts for the reduction of nitrobenzene in aqueous media. The reaction was performed with different concentrations of cat­alyst and reducing agent. A linear relationship was found between apparent rate constant (kapp) and amount of catalyst. When the amount of catalyst was increased from 0.13 to 0.76 mg/mL then kapp was increased from 0.03 to 0.14 min-1. Activation parameters were also determined by performing reaction at two different temperatures. The catalytic process has been discussed in terms of energy of activation, enthalpy of activation and entropy of activation. The synthesized particles were found to be stable even after 14 weeks and showed catalytic activity for the reduction of nitrobenzene.

13C NMR-spektroskopische Untersuchungen an Äthylaluminiumverbindungen / 13C NMR Spectroscopic Investigations of Ethylaluminium Compounds

1976 ◽  
Vol 31 (6) ◽  
pp. 730-736 ◽  
Author(s):  
R. Rottler ◽  
C. G. Kreiter ◽  
G. Fink
Keyword(s):  
Temperature Dependence ◽  
Exchange Reaction ◽  
13C Nmr ◽  
Line Shape ◽  
Nmr Spectra ◽  
Exchange Process ◽  
Kcal Mole ◽  
Line Shapes ◽  
Calculated Line ◽  
C Nmr

The 13C NMR spectra of the ethylaluminium compounds [Al(C2H5)xCl3_x]2 x = 1, 1,5, 2 and 3 are presented and factors governing the temperature dependence of the line shape are discussed. The exchange reaction of terminal ethyl groups for chlorine ligands and ethyl ligands, resp., in ethylaluminium-sesquichloride was investigated by fitting the calculated line shapes to the observed spectra.The energy of activation of this exchange process was determined as to be 12,3 ‡ 1,5 kcal/mole. The synthesis of 13C2-[Al(C2H5)Cl2]2 is described.

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