Kinetic Isotope Effect Evidence for a Concerted Hydrogen Transfer Mechanism in Transfer Hydrogenations Catalyzed by [p-(Me2CH)C6H4Me]Ru- (NHCHPhCHPhNSO2C6H4-p-CH3)

2003 ◽  
Vol 68 (5) ◽  
pp. 1998-2001 ◽  
Author(s):  
Charles P. Casey ◽  
Jeffrey B. Johnson
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Hydrogen Transfer ◽  
Transfer Mechanism ◽  
Kinetic Isotope

Article

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1085-1096 ◽  
Author(s):  
A M Kuznetsov ◽  
Jens Ulstrup
Keyword(s):  
Proton Transfer ◽  
Isotope Effect ◽  
Rate Constants ◽  
Bovine Serum ◽  
Kinetic Isotope Effect ◽  
Hydrogen Transfer ◽  
Low Frequency ◽  
Specific Rate ◽  
Adiabatic Limits ◽  
Kinetic Isotope

We discuss a broad theoretical frame for hydrogen transfer in chemical and biological systems. Hydrogen tunnelling, coupling between the tunnel modes and the environment, and fluctuational barrier preparation for hydrogen tunnelling are in focus and given precise analytical forms. Specific rate constants are provided for three limits, i.e., the fully diabatic, the partially adiabatic, and the fully adiabatic limits. These limits are all likely to represent real chemical or biological hydrogen transfer systems. The rate constants are referred particularly to the driving force and temperature dependence of the kinetic isotope effect (KIE). The origin of these correlations is different in the three limits. It is rooted in the tunnel factor and weak excitation of the heavier isotopes in the former two limits, giving a maximum for thermoneutral processes. A new observation is that the adiabatic limit also accords with a KIE maximum for thermoneutral processes but the KIE is here reflected solely in the activation Gibbs free energy differences, in this case rooted in the low-frequency environmental nuclear dynamics. Three systems of biological hydrogen tunnelling, viz. lipoxygenase, yeast alcohol dehydrogenase, and bovine serum amine oxygenase, offer unusual new cases for analysis and have been analysed using the theoretical frames. All the systems show large KIEs and strong indications of hydrogen tunnelling. They also represent different degrees of fluctuational barrier preparation, with lipoxygenase as the most rigid and bovine serum amine oxygenase as the softest system.Key words: generalized Born-Oppenheimer scheme, kinetic isotope effect, gated proton transfer, partially adiabatic proton transfer, proton tunnelling in enzyme catalysis.

scholarly journals Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

2015 ◽  
Vol 112 (14) ◽  
pp. 4218-4220 ◽  
Author(s):  
Liang Cheng ◽  
Charles Doubleday ◽  
Ronald Breslow
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Hydrogen Transfer ◽  
Hydrogen Atom Transfer ◽  
Hydride Shift ◽  
Kinetic Isotope ◽  
Isomerization Mechanism ◽  
The Difference ◽  
Intrinsic Kinetic ◽  
Quantitative Hplc

Hydrogen atom transfer reactions between the aldose and ketose are key mechanistic features in formose chemistry by which formaldehyde is converted to higher sugars under credible prebiotic conditions. For one of these transformations, we have investigated whether hydrogen tunneling makes a significant contribution to the mechanism by examining the deuterium kinetic isotope effect associated with the hydrogen transfer during the isomerization of glyceraldehyde to the corresponding dihydroxyacetone. To do this, we developed a quantitative HPLC assay that allowed us to measure the apparent large intrinsic kinetic isotope effect. From the Arrhenius plot of the kinetic isotope effect, the ratio of the preexponential factors AH/AD was 0.28 and the difference in activation energies Ea(D) − Ea(H) was 9.1 kJ·mol−1. All these results imply a significant quantum-mechanical tunneling component in the isomerization mechanism. This is supported by multidimensional tunneling calculations using POLYRATE with small curvature tunneling.

Kinetic isotope effect for the thermally-induced migration of hydrogen in cyclopentadienes

1969 ◽  
Vol 47 (9) ◽  
pp. 1555-1559 ◽  
Author(s):  
Stewart McLean ◽  
C. J. Webster ◽  
R. J. D. Rutherford
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Hydrogen Transfer ◽  
Arrhenius Parameters ◽  
Thermally Induced ◽  
Methyl Group ◽  
Hydrogen Migration ◽  
Kinetic Isotope ◽  
Non Linear

The Arrhenius parameters have been obtained for the rearrangement of 5-methylcyclopentadiene by a 1,2-hydrogen migration and for the corresponding rearrangement of 5-methylpentadeuteriocyclopentadiene. The results, taken in conjunction with those reported previously, allow the effect of the methyl group on the rate of hydrogen migration to be assessed. The kinetic isotope effect is of particular interest because the hydrogen transfer takes place through a transition state that is unequivocally non-linear.

scholarly journals Selective deuteration illuminates the importance of tunneling in the unimolecular decay of Criegee intermediates to hydroxyl radical products

2017 ◽  
Vol 114 (47) ◽  
pp. 12372-12377 ◽  
Author(s):  
Amy M. Green ◽  
Victoria P. Barber ◽  
Yi Fang ◽  
Stephen J. Klippenstein ◽  
Marsha I. Lester
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Hydrogen Transfer ◽  
Decay Rates ◽  
Equation Modeling ◽  
Oh Radicals ◽  
Atom Transfer ◽  
Atmospheric Conditions ◽  
Kinetic Isotope ◽  
Criegee Intermediates

Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the atmosphere, proceeds through unimolecular decay of Criegee intermediates. Here, we report a large kinetic isotope effect associated with the rate-limiting hydrogen-transfer step that releases OH radicals for a prototypical Criegee intermediate, CH3CHOO. IR excitation of selectively deuterated syn-CD3CHOO is shown to result in deuterium atom transfer and release OD radical products. Vibrational activation of syn-CD3CHOO is coupled with direct time-resolved detection of OD products to measure a 10-fold slower rate of unimolecular decay upon deuteration in the vicinity of the transition state barrier, which is confirmed by microcanonical statistical theory that incorporates quantum mechanical tunneling. The corresponding kinetic isotope effect of ∼10 is attributed primarily to the decreased probability of D-atom vs. H-atom transfer arising from tunneling. Master equation modeling is utilized to compute the thermal unimolecular decay rates for selectively and fully deuterated syn methyl-substituted Criegee intermediates under atmospheric conditions. At 298 K (1 atm), tunneling is predicted to enhance the thermal decay rate of syn-CH3CHOO compared with the deuterated species, giving rise to a significant kinetic isotope effect of ∼50.

Sign in / Sign up

Export Citation Format

Share Document