Secondary deuterium isotope effects on both rate and product determining steps in the thermolysis of 4-methylene-1-pyrazoline

1981 ◽  
Vol 59 (16) ◽  
pp. 2556-2567 ◽  
Author(s):  
Moon Ho Chang ◽  
Robert J. Crawford
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Bond Rupture ◽  
Cleavage Process ◽  
Kinetic Isotope ◽  
Product Distributions ◽  
Deuterium Isotope Effects ◽  
Allylic Radical

The secondary deuterium kinetic isotope effect in the thermolysis of 4-methylene-1-pyrazoline and four of its deuterated isomers has been studied. The value of δΔG≠/n is 142 cal mol−1 if it is a single C—N cleavage process or 71 cal mol−1 if it is a concerted rupture of two C—N bonds. The product distributions can only be rationalized in terms of a mechanism involving a rate determining C—N bond rupture to form a diazenyl diradical intermediate which then can close to methylenecyclopropanes by three possible modes. This requires a secondary isotope effect in the product determining step of 1.33 ± 0.05, for the rotation of CH2 (CD2) out of the plane of the allylic radical in the intermediate.

SOME DEUTERIUM KINETIC ISOTOPE EFFECTS: IV. β-DEUTERIUM EFFECTS IN WATER SOLVOLYSIS OF ETHYL, ISOPROPYL, AND tert-BUTYL COMPOUNDS

1960 ◽  
Vol 38 (11) ◽  
pp. 2171-2177 ◽  
Author(s):  
K. T. Leffek ◽  
J. A. Llewellyn ◽  
R. E. Robertson
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Alkyl Halides ◽  
Tert Butyl ◽  
Kinetic Isotope ◽  
Deuterium Isotope Effects ◽  
Intramolecular Forces

The secondary β-deuterium isotope effects have been measured in the water solvolytic reaction of alkyl halides and sulphonates for primary, secondary, and tertiary species. In every case the kinetic isotope effect was greater than unity (kH/kD > 1). This isotope effect may be associated with varying degrees of hyperconjugation or altered non-bonding intramolecular forces. The experiments make it difficult to decide which effect is most important.

Solvolysis of specifically deuterated 7-chloro- and 7-bromo-exo-2-norbornyl brosylates; unusually large γ-kinetic isotope effects in the absence of σ-participation

1980 ◽  
Vol 58 (16) ◽  
pp. 1738-1750 ◽  
Author(s):  
Nick Henry Werstiuk ◽  
George Timmins ◽  
Frank Peter Cappelli
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope

A series of specifically deuterated syn-7-chloro-, anti-7-chloro-, syn-7-bromo-, and anti-7-bromo-exo-2-norbornyl brosylates have been prepared and solvolyzed in NaOAc-buffered 80:20 EtOH–H2O. For solvolysis at 25 °C the γ-kinetic isotope effects (KIE's) for syn-7-chloro-exo-2-norbornyl brosylate-endo-6-d (1e), anti-7-chloro-exo-2-norbornyl brosylate-endo-6-d (2c), syn-7-bromo-exo-2-norbornyl brosylate-endo-6-d (1f), anti-7-bromo-exo-2-norbornyl brosylate-endo-6-d (2d), syn-7-chloro-exo-2-norbornyl brosylate-exo,exo-5,6-d2 (1g), anti-7-chloro-exo-2-norbornyl brosylate-exo,exo-5,6-d2 (2e) are 1.125 ± 0.007, 1.128 ± 0.005, 1.063 ± 0.008, 1.149 ± 0.020, 1.119 ± 0.011, and 1.115 ± 0.013, respectively. There is no detectable γ-kinetic isotope effect for solvolysis of anti-7-chloro-endo-2-norbornyl brosylate-endo-6-d(3a) and the β-KIE for anti-7-chloro-exo-2-norbornyl brosylate-exo-3-d(4a) is 1.111 ± 0.011. From a consideration of the possible sources of the unusually large secondary KIE's, we conclude that the exo-6-d and endo-6-d γ-KIE's likely are derived from a combination of effects rather than from participation of the C1—C6 bond in the ionization step.

scholarly journals Insights Into the Known 13C Depletion of Methane—Contribution of the Kinetic Isotope Effects on the Serine Hydroxymethyltransferase Reaction

2022 ◽  
Vol 9 ◽  
Author(s):  
Gerd Gleixner
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Serine Hydroxymethyltransferase ◽  
Chemical Degradation ◽  
Methyl Groups ◽  
Kinetic Isotope ◽  
Isotopic Pattern ◽  
Before And After ◽  
Metal Organic

We determined the kinetic isotope effect on the serine hydroxymethyltransferase reaction (SHMT), which provides important C1 metabolites that are essential for the biosynthesis of DNA bases, O-methyl groups of lignin and methane. An isotope effect on the SHMT reaction was suggested being responsible for the well-known isotopic depletion of methane. Using the cytosolic SHMT from pig liver, we measured the natural carbon isotope ratios of both atoms involved in the bond splitting by chemical degradation of the remaining serine before and after partial turnover. The kinetic isotope effect 13(VMax/Km) was 0.994 0.006 and 0.995 0.007 on position C-3 and C-2, respectively. The results indicated that the SHMT reaction does not contribute to the 13C depletion observed for methyl groups in natural products and methane. However, from the isotopic pattern of caffeine, isotope effects on the methionine synthetase reaction and on reactions forming Grignard compounds, the involved formation and fission of metal organic bonds are likely responsible for the observed general depletion of “activated” methyl groups. As metal organic bond formations in methyl transferases are also rate limiting in the formation of methane, they may likely be the origin of the known 13C depletion in methane.

ISOTOPE EFFECTS IN MIXTURES OF LIQUID H2O AND T2O

1966 ◽  
Vol 44 (6) ◽  
pp. 689-694 ◽  
Author(s):  
Mark Salomon
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Hydroxide Ion ◽  
Kinetic Isotope ◽  
Hydronium Ion ◽  
Mercury Cathode

Calculations are presented for the equilibrium tritium isotope effect involving water, hydronium ion, and hydroxide ion. The results are used to predict the kinetic isotope effect in the transfer of protons to a mercury cathode.

Kinetic evidence for the importance of solvent-separated ion pairs during the solvolyses of adamantylideneadamantyl derivatives

2004 ◽  
Vol 82 (9) ◽  
pp. 1336-1340
Author(s):  
Xicai Huang ◽  
Andrew J Bennet
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Salt Effects ◽  
Kinetic Isotope

The aqueous ethanolysis reactions of adamantylideneadamantyl tosylate, -bromide, and -iodide (1-OTs, 1-Br and 1-I) were monitored as a function of ionic strength. Special salt effects are observed during the solvolyses of both homoallylic halides, but not in the case of the tosylate 1-OTs. The measured α-secondary deuterium kinetic isotope effects for the solvolysis of 1-Br in 80:20 and 60:40 v/v ethanol–water mixtures at 25 °C are 1.110 ± 0.018 and 1.146 ± 0.009, respectively. The above results are consistent with the homoallylic halides reacting via a virtual transition state in which both formation and dissociation of a solvent-separated ion pair are partially rate-determining. While the corresponding transition state for adamantylideneadamantyl tosylate involves formation of the solvent-separated ion pair.Key words: salt effects, kinetic isotope effect, internal return, solvolysis, ion pairs.

scholarly journals Kinetic isotope effects for fast deuterium and proton exchange rates

2016 ◽  
Vol 18 (15) ◽  
pp. 10144-10151 ◽  
Author(s):  
Estel Canet ◽  
Daniele Mammoli ◽  
Pavel Kadeřávek ◽  
Philippe Pelupessy ◽  
Geoffrey Bodenhausen
Keyword(s):  
Exchange Rates ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Proton Exchange ◽  
Kinetic Isotope ◽  
Spin Pairs

By monitoring the effect of deuterium decoupling on the decay of transverse 15N magnetization in D–15N spin pairs during multiple-refocusing echo sequences, we have determined fast D–D exchange rates kD and compared them with fast H–H exchange rates kH in tryptophan to determine the kinetic isotope effect as a function of pH and temperature.

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