Radiation Induced cis and trans Isomerization of Polyisoprenes and Temperature Dependence of the Equilibria

1967 ◽  
Vol 40 (4) ◽  
pp. 1222-1229
Author(s):  
Jitsuo Tsurugi ◽  
Tsugio Fukumoto ◽  
Masayuki Yamagami ◽  
Hiroshi Itatani
Keyword(s):  
Transition State ◽  
Infrared Absorption ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Azo Compounds ◽  
Γ Radiation ◽  
Gutta Percha ◽  
Radiation Induced ◽  
Aromatic Azo Compounds ◽  
Cis And Trans

Abstract The cis and trans interconversion of polyisoprenes in solutions induced by γ-radiation in the presence of a sensitizer, which is any one of organic bromides or n-butyl mercaptan, was studied by using hevea and gutta percha as starting substances. The percentage cis remaining or converted after irradiation was determined by the infrared absorption. The equilibrium constants for the interconversion at 22, 60, and 100° C were found to be 3.00, 5.25, and 7.33, respectively. The firstorder rate constants for as cis → trans and trans → cis isomerizations at 22° C were calculated to be 9.05 and 2.91, respectively. The results were interpreted by the mechanism proposed by Golub, according to which the double bonds from π complexes with radiolytic fragments from sensitizers give a radical transition state capable of interconversion. However, our results showing that heating shifts the equilibrium toward trans isomer are not in accord with the mechanisms of the radiation-induced isomerization of polybutadiene of Golub and those for photoisom-erization of aromatic azo compounds.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

scholarly journals Mechanism of Allosteric Modulation of Rod Cyclic Nucleotide–gated Channels

1999 ◽  
Vol 113 (5) ◽  
pp. 601-620 ◽  
Author(s):  
Elizabeth R. Sunderman ◽  
William N. Zagotta
Keyword(s):  
Transition State ◽  
Rate Constant ◽  
Rate Constants ◽  
Cyclic Nucleotide ◽  
Equilibrium Constants ◽  
Rod Photoreceptor ◽  
Open Probability ◽  
Allosteric Transition ◽  
Cng Channel ◽  
Single Channel Currents

The cyclic nucleotide–gated (CNG) channel of retinal rod photoreceptor cells is an allosteric protein whose activation is coupled to a conformational change in the ligand-binding site. The bovine rod CNG channel can be activated by a number of different agonists, including cGMP, cIMP, and cAMP. These agonists span three orders of magnitude in their equilibrium constants for the allosteric transition. We recorded single-channel currents at saturating cyclic nucleotide concentrations from the bovine rod CNG channel expressed in Xenopus oocytes as homomultimers of α subunits. The median open probability was 0.93 for cGMP, 0.47 for cIMP, and 0.01 for cAMP. The channels opened to a single conductance level of 26–30 pS at +80 mV. Using signal processing methods based on hidden Markov models, we determined that two closed and one open states are required to explain the gating at saturating ligand concentrations. We determined the maximum likelihood rate constants for two gating schemes containing two closed (denoted C) and one open (denoted O) states. For the C ↔ C ↔ O scheme, all rate constants were dependent on cyclic nucleotide. For the C ↔ O ↔ C scheme, the rate constants for only one of the transitions were cyclic nucleotide dependent. The opening rate constant was fastest for cGMP, intermediate for cIMP, and slowest for cAMP, while the closing rate constant was fastest for cAMP, intermediate for cIMP, and slowest for cGMP. We propose that interactions between the purine ring of the cyclic nucleotide and the binding domain are partially formed at the time of the transition state for the allosteric transition and serve to reduce the transition state energy and stabilize the activated conformation of the channel. When 1 μM Ni2+ was applied in addition to cyclic nucleotide, the open time increased markedly, and the closed time decreased slightly. The interactions between H420 and Ni2+ occur primarily after the transition state for the allosteric transition.

Hydride transfer reactions. Substituent effects in oxidation of N-benzylacridans by π acceptors

1983 ◽  
Vol 61 (11) ◽  
pp. 2544-2551 ◽  
Author(s):  
Allan K. Colter ◽  
Charles C. Lai ◽  
Terry W. Williamson ◽  
Raymond E. Berry
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Substituent Effects ◽  
Second Order ◽  
Face To Face ◽  
Order Rate ◽  
Linear Free Energy ◽  
Donor And Acceptor ◽  
Kinetics Of

The kinetics of oxidation of a series of eight N-(substituted benzyl)acridans (3, NBA's) by 1,4-benzoquinone (BQ), p-chloranil (CA), 2,3-dicyano-1,4-benzoquinone (DCBQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE) in acetonitrile (AN) and by BQ in 50:50 (v/v) AN-water were measured at 25 °C. Equilibrium constants for pseudobase formation, [Formula: see text], from the corresponding acridinium ions (4) were measured in water at 25 °C. Hammett correlations of the second-order rate constants for reaction of the NBA's without ortho substitutents (3a–e) led to ρ values of −0.29 (BQ, AN), −0.55 (CA), −0.56 (DCBQ), −0.64 (TCNQ), −0.41 (TCNE), and −0.47 (BQ, 50:50 AN–water). The second-order rate constants for 3a–e also give good linear free energy correlations with the [Formula: see text] values of 4a–e. These correlations and the [Formula: see text] values for the ortho-substituted acridinium ions (4f, g, h) are used to calculate rate constants for oxidation of the corresponding ortho-substituted NBA's (3f, g, h). The rate constants calculated in this way are 4.7 to 6.6 times and 6.4 to 12 times larger, respectively, than the observed rate constants for N-(2,4,6-trimethylbenzyl) and N-(2,6-dichlorobenzyl)acridan (3g and h). The variations in ρ values are attributed mainly to differences in the amount of electrostatic stabilization in the transition state resulting from differences in the separation of donor and acceptor and the degree of delocalization of the negative charge. The rate retarding effect of a pair of ortho substituents is attributed to sterie effects in a preferred face-to-face transition state.

Electrostatic effects of substrate charge on base-catalyzed proton exchange in purines. Implications for the involvement of zwitterions

1980 ◽  
Vol 58 (1) ◽  
pp. 86-91 ◽  
Author(s):  
Spencer E. Taylor
Keyword(s):  
Aqueous Solution ◽  
Transition State ◽  
Exchange Reaction ◽  
Rate Constants ◽  
Electrostatic Interactions ◽  
Equilibrium Constants ◽  
Proton Exchange ◽  
Net Charge ◽  
Hydroxide Ion ◽  
Rate Behaviour

Rates of detritiation from the C-8 position of various ionized purine molecules have been shown to reflect electrostatic interactions occurring within the transition state of the exchange reaction. The purines are capable of existing as protonated, anionic, neutral, and zwitterionic species in aqueous solution, each of which have been shown previously to undergo specific hydroxide ion-catalyzed detritiation. The effects of substrate charge are quantified as the protonactivatingfactor, aniondeactivatingfactor, and zwitterionactivatingfactor. Localized charge and not net charge is shown to govern the reactivities of the various species, especially the zwitterions.Estimation of rate constants for the zwitterionic species (hitherto only postulated to account for the observed rate behaviour) has enabled the calculation of the corresponding zwitterionic equilibrium constants (Kzw). Kzw values calculated in this manner agree with those estimated from suitable literature pKa data.

Oxidation of (1-hydroxybenzyl)ferrocene and its derivatives substituted in phenyl ring with bis(triphenylsilyl) chromate

1982 ◽  
Vol 47 (12) ◽  
pp. 3375-3380 ◽  
Author(s):  
Jaroslav Holeček ◽  
Karel Handlíř ◽  
Milan Nádvorník ◽  
Milan Vlček
Keyword(s):  
Linear Dependence ◽  
Rate Constants ◽  
Oxidation Rate ◽  
Phenyl Ring ◽  
Equilibrium Constants ◽  
Aprotic Solvents ◽  
Protonation Equilibrium ◽  
Oxidation Of Alcohols ◽  
Hammett Σ Constants ◽  
Sulphuric Acid Medium

Kinetics have been studied of oxidation of (1-hydroxybenzyl)ferrocenes substituted in phenyl ring with bis(triphenylsilyl) chromate in benzene solutions as well as protonation of these alcohols in sulphuric acid medium. Logarithms of the oxidation rate constants (kobs, 20-40 °C) and those of the protonation equilibrium constants (KR+, 25 °C) show linear dependence on the Hammett σ constants, the ρ constant values being -0.86 to -0.40 and -2.50, respectively. These negative values suggest that the both processes are influenced by the same effects and confirm the mechanism proposed earlier for oxidation of alcohols with ferrocenyl substituent by action of bis(triphenylsilyl) chromate in aprotic solvents.

Bifunctional Au/Co3O4‐Cs (Basic and Redox)‐ Catalyzed Oxidative Synthesis of Aromatic Azo Compounds from Anilines

2021 ◽  
Author(s):  
Charles O. Oseghale ◽  
Dele Peter Fapojuwo ◽  
Oyekunle Azeez Alimi ◽  
Christianah Aarinola Akinnawo ◽  
Batsile M. Mogudi ◽  
...  
Keyword(s):  
Azo Compounds ◽  
Aromatic Azo Compounds
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