scholarly journals Highly Precise Measurement of Kinetic Isotope Effects Using 1H-Detected 2D [13C,1H]-HSQC NMR Spectroscopy

2012 ◽  
Vol 134 (51) ◽  
pp. 20589-20592 ◽  
Author(s):  
Kathryn A. Manning ◽  
Bharathwaj Sathyamoorthy ◽  
Alexander Eletsky ◽  
Thomas Szyperski ◽  
Andrew S. Murkin
Keyword(s):  
Nmr Spectroscopy ◽  
Precise Measurement ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope

NMR Stratagems for the Study of Multiple Kinetic Hydrogen/Deuterium Isotope Effectsof Proton Exchange. Example: Di-p-fluorophenylformamidine/THF

1989 ◽  
Vol 44 (5) ◽  
pp. 459-472
Author(s):  
Hans-Heinrich Limbach ◽  
Ludger Meschede ◽  
Gerd Scherer
Keyword(s):  
Nmr Spectroscopy ◽  
Magnetization Transfer ◽  
Isotope Effects ◽  
Geometric Mean ◽  
Kinetic Isotope Effects ◽  
Proton Exchange ◽  
Exchange Reactions ◽  
H Nmr ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium

Stratagems are presented for the determination of kinetic isotope effects of proton exchange reactions by dynamic NMR spectroscopy. In such experiments, lineshape analyses and/or polarization transfer experiments are performed on the exchanging protons or deuterons as well as on remote spins, as a function of the deuterium fraction in the mobile proton sites. These methods are NMR analogs of previous proton inventory techniques involving classical kinetic methods. A theory is developed in order to derive the kinetic isotope effects as well as the number of transferred protons from the experimental NMR spectra. The technique is then applied to the problem of proton exchange in the system 15N,15N′-di-p-fluorophenylibrmamidine, a nitrogen analog of formic acid, dissolved in tetrahydrofuran-d8 (THF). DFFA forms two conformers in THF to which s-trans and s-cis structures have been assigned. Only the s-trans conformer is able to dimerize and exchange protons. Lineshape simulations and magnetization transfer experiments were carried out at 189,2 K, at a concentration of 0.02 mol l-1, as a function of the deuterium fraction D in the 1H-15N sites. Using 1H NMR spectroscopy, a linear dependence of the inverse proton lifetimes on D was observed. From this it was concluded that two protons are transported in the rate limiting step of the proton exchange. This result is expected for a double proton transfer in an s-trans dimer with a cyclic structure. The full kinetic HH/HD/DD isotope effects of 233:11:1 at 189 K were determined through 19F NMR experiments on the same samples. The deviation from the rule of geometric mean, although substantial, is much smaller than found in previous studies of intramolecular HH transfer reactions. Possible causes of this effect are discussed.

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