Recombination of iodine atoms by flash photolysis over a wide temperature range. IV Kinetic isotope effect

1973 ◽  
Vol 59 (3) ◽  
pp. 1043-1044 ◽  
Author(s):  
Heh Won Chang ◽  
George Burns
Keyword(s):  
Isotope Effect ◽  
Wide Temperature Range ◽  
Kinetic Isotope Effect ◽  
Flash Photolysis ◽  
Temperature Range ◽  
Kinetic Isotope

Reactions of trifluoromethyl radicals. V. Hydrogen abstraction from methyl acetate and methyl (2H3)Acetate

1980 ◽  
Vol 33 (7) ◽  
pp. 1437
Author(s):  
NL Arthur ◽  
PJ Newitt
Keyword(s):  
Isotope Effect ◽  
Rate Constants ◽  
Methoxy Group ◽  
Methyl Acetate ◽  
Kinetic Isotope Effect ◽  
Hydrogen Abstraction ◽  
Activation Energies ◽  
Temperature Range ◽  
Kinetic Isotope ◽  
Acetyl Group

Hydrogen abstraction by CF3 radicals from CH3COOCH3 and CD3COOCH3 has been studied in the temperature range 78-242°, and data have been obtained for the reactions: CF3 + CH3COOCH3 → CF3H+[C3H5O2] �������������(3) CF3 + CH3COOCH3 → CF3H+CH2COOCH3������������ (4) CF3 + CD3COOCH3 → CF3D+CD2COOCH3������������ (6) CF3 + CD3COOCH3 → CF3H+CD3COOCH2������������ (7) The corresponding rate constants, based on the value of 1013.36 cm3 mol-1 S-1 for the recombination of CF3 radicals, are given by (k in cm3 mol-1 s-1 and E in J mol-1): logk3 = (11.52�0.05)-(35430�380)/19.145T ���� (3)logk4 = (11.19�0.07)-(34680�550)/19.145T ���� (4)logk6 = (11.34�0.06)-(46490�490)/19.145T ���� (6)logk7 = (11.26�0.05)-(36440�400)/19.145T ���� (7)At 400 K, 59% of abstraction occurs from the acetyl group, and 41 % from the methoxy group. The kinetic isotope effect at 400 K for attack on the acetyl group is 25, due mainly to a difference in activation energies.

Ketonization equilibria of phenol in aqueous solution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 605-613 ◽  
Author(s):  
Marco Capponi ◽  
Ivo G Gut ◽  
Bruno Hellrung ◽  
Gaby Persy ◽  
Jakob Wirz
Keyword(s):  
Aqueous Solution ◽  
Proton Transfer ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Flash Photolysis ◽  
Equilibrium Constants ◽  
Thermodynamic Cycle ◽  
Acidity Constant ◽  
Kinetic Isotope ◽  
Carbon Acids

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

Reactions of Methyl Radicals. III. Hydrogen Abstraction from Methyl Acetate and Methyl [2H3]Acetate

1979 ◽  
Vol 32 (8) ◽  
pp. 1697 ◽  
Author(s):  
NL Arthur ◽  
PJ Newitt
Keyword(s):  
Isotope Effect ◽  
Rate Constants ◽  
Methoxy Group ◽  
Methyl Acetate ◽  
Kinetic Isotope Effect ◽  
Hydrogen Abstraction ◽  
Methyl Radicals ◽  
Temperature Range ◽  
Kinetic Isotope ◽  
Acetyl Group

A study of hydrogen abstraction by CH3 radicals from CH3COOCH3 in the temperature range 116-224°, and from CD3COOCH3 in the range 117-234°, has yielded data on the reactions: CH3 + CH3COOCH3 → CH4 + [C3H5O2] (4) CH3 + CH3COOCH3 → CH4 + CH2COOCH3 (5) CH3 + CH3COOCH3 → CH4 + CH3COOCH2 (6) CH3 + CD3COOCH3 → CH3D + CD2COOCH3 (7) The corresponding rate constants, based dn the value of 1013.34 cm3 mol-1 s-1 for the recombination of CH3 radicals, are given by (k in cm3 mol-1 s-1 and E in J mol-1): logk4 = (11.56 ± 0.12) - (44430 ± 970)/19.145T (4) logk5 = (11.17 ± 0.22) - (42900 ± 1760)/19.145T (5) logk6 = (11.44 ± 0.16) - (46980 ± 1290)/19.145T (6) logk7 = (11.39 ± 0.04) - (52110 ± 330)/19.145T (7) At 400 K, 64% of abstraction occurs from the acetyl group, and 36% from the methoxy group. The kinetic isotope effect at 400 K for attack on the acetyl group is 9.6; this is mainly due to a difference in activation energies since the quotient of A factors is close to unity.

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms

2020 ◽  
Vol 22 (20) ◽  
pp. 11219-11243 ◽  
Author(s):  
Ken Sakaushi
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Effect Analysis ◽  
Kinetic Isotope

The fundamental aspects of quantum electrocatalysts are discussed together with the newly developed electrochemical kinetic isotope effect (EC-KIE) approach.

scholarly journals l-mandelate dehydrogenase from Rhodotorula graminis: comparisons with the l-lactate dehydrogenase (flavocytochrome b2) from Saccharomyces cerevisiae

1993 ◽  
Vol 290 (1) ◽  
pp. 103-107 ◽  
Author(s):  
O Smékal ◽  
M Yasin ◽  
C A Fewson ◽  
G A Reid ◽  
S K Chapman
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Lactate Dehydrogenase ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Dimensional Structure ◽  
Striking Difference ◽  
Kinetic Properties ◽  
Proton Abstraction ◽  
Rate Determining Step ◽  
Kinetic Isotope

L-Lactate dehydrogenase (L-LDH) from Saccharomyces cerevisiae and L-mandelate dehydrogenase (L-MDH) from Rhodotorula graminis are both flavocytochromes b2. The kinetic properties of these enzymes have been compared using steady-state kinetic methods. The most striking difference between the two enzymes is found by comparing their substrate specificities. L-LDH and L-MDH have mutually exclusive primary substrates, i.e. the substrate for one enzyme is a potent competitive inhibitor for the other. Molecular-modelling studies on the known three-dimensional structure of S. cerevisiae L-LDH suggest that this enzyme is unable to catalyse the oxidation of L-mandelate because productive binding is impeded by steric interference, particularly between the side chain of Leu-230 and the phenyl ring of mandelate. Another major difference between L-LDH and L-MDH lies in the rate-determining step. For S. cerevisiae L-LDH, the major rate-determining step is proton abstraction at C-2 of lactate, as previously shown by the 2H kinetic-isotope effect. However, in R. graminis L-MDH the kinetic-isotope effect seen with DL-[2-2H]mandelate is only 1.1 +/- 0.1, clearly showing that proton abstraction at C-2 of mandelate is not rate-limiting. The fact that the rate-determining step is different indicates that the transition states in each of these enzymes must also be different.

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