Proton transfer dynamics deuterium

Kelley and co-workers [70, 71] measured the dynamics of the excited-state intramolecular proton transfer in 3-hydroxyflavone and a series of its derivatives as a function of solvent (Scheme 2.9). The energy changes associated with the processes examined are of the order of 3 kcal/mol or less. The model they employed in the analysis of the reaction dynamics was based upon a tunneling reaction path. Interestingly, they find little or no deuterium kinetic isotope effect, which would appear to be inconsistent with tunneling theories. For 3-hydroxy-flavone, they suggest the lack of an isotope effect is due to a very large... 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

The effects of small changes in the molecular structure can be observed in the case of the related diarylamidines which are the nitrogen analogs of formic acid and which represent models for nucleic acids. In tetrahydrofuran, for N,N -di-(p-F-phenyl)amidine (DFFA) three forms were observed by NMR, a solvated s-cis-form and a solvated s-trans-form which is in fast equilibrium with a cyclic dimer in which a HH-transfer takes place [24] as illustrated in Fig. 6.43. Fortunately, at low temperatures, the s-cis- and the s-trans-forms were in slow exchange. The rate constants of the HH, HD and DD reactions were determined by dynamic H and i F NMR as a function of concentration, deuterium fraction in the mobile proton sites and of temperature. The dependence of the observed rate constants of the s-trans-form on concentration is depicted in Fig. 6.44. The solid lines were calculated using Eq. (6.39) from which the rate constants in the dimer as well as the equilibrium constants of the dimer formation could be obtained. The Arrhenius... 

Using dynamic solid-state CP MAS NMR spectroscopy, the kinetics of the degenerate intermolecular double and quadruple proton and deuteron transfers in the cyclic dimer of N labelled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP) and in the cyclic tetramer of N labelled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied in a wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants were measured on a millisecond time scale by line shape analysis of the doubly N labelled eompounds and by magnetisation transfer experiments on a second timescale of the singly N labelled compounds in order to minimise the effeets of proton-driven N spin diffusion. The Arhenius curves of all processes were found to be nonlinear and indicated tunneling processes at low temperatures. In a preliminary analysis, they were modelled in terms of the Bell-Limbach tuimeling model. 

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