Dynamic NMR study of the kinetic HH/HD/DD isotope effects on the double proton transfer in cyclic bis(p-fluorophenyl)formamidine dimers

Autor:	Hans-Heinrich Limbach, Ludger Meschede
Rok vydání:	1991
Předmět:	Arrhenius equation Proton Stereochemistry Chemistry Dimer General Engineering Nuclear magnetic resonance spectroscopy Tautomer symbols.namesake chemistry.chemical_compound Reaction rate constant Kinetic isotope effect symbols Physical chemistry Stepwise reaction Physical and Theoretical Chemistry
Zdroj:	The Journal of Physical Chemistry. 95:10267-10280
ISSN:	1541-5740 0022-3654
DOI:	10.1021/j100178a009
Popis:	KA2 = exp[(-29 & 1 J mol-I K-')/R] expt(5.7 f 0.3 kJ mol-')/RT] L mol-l, 170 K < T < 254 K For the tautomerism in the cyclic dimer the following isotopic rate constants were obtained: kAIHH = 109,9'0.3 exp[(-18.9 f 0.3 kl mol-')/RT] S-'; kAIHD = 10'0~7M0.3 exp[(-26.7 f 0.3 kJ mOl-')/RT] S-'; kA2DD = 10i',5M.3 eXp[(-33.4 f 0.3 kJ mol- )/ s-,, 164.2 K < T < 260.9 K; with the kinetic isotope effects of kA2HH/kA,HD = 23 * 4, kA2HD/kA,DD = 10 f 2, kAFd/kE2 = 237 f 20 at 189.2 K. The Arrhenius curves of all isotopic reactions could be simulated in terms of a concerted proton transfer that proceeds by thermally activated tunneling at low temperatures. This result is in contrast to previous findings for intramolecular degenerate double proton transfer reactions where stepwise reaction pathways have been established. With a three-dimensional model potential surface it is shown, however, that the difference between the concerted and the stepwise reaction pathway disappears when the hydrogen bond length is decreased during the course of the reaction. It follows that the observed kinetic isotope effects are in agreement with both a concerted or a stepwise reaction pathway in a compressed
Databáze:	OpenAIRE
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