H-transfers Coupled to Conformational Changes

Let us first discuss the intramolecular degenerate double proton transfers in azo-phenine [21] and in oxalamidines [22a[. By liquid state NMR of the labeled compounds the intramolecular pathways of the transfer processes were established and the rate constants fenn, k = and when possible k were measured. The Arrhenius diagrams are depicted in Fig. 6.40. In all cases, the reactions 

These findings supported the formation of a zwitterionic intermediate according to the stepwise mechanism of Fig. 6.11. The small dependence of the kinetic isotope effects on temperature is confirmed for OA7 as the Arrhenius curves of the isotopic reactions are almost parallel. Note that a quantitative discussion of these parameters is difficult as the temperature range of the experimental data was limited. Therefore, the parameter sets obtained are not unique. 

In all cases, the molecular structures do not allow for a simultaneous compression of both hydrogen bonds which would require a very high energy. Therefore, the transfers are stepwise, as indicated by Fig. 6.39(c). 

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 

In solution the aryl groups of a cyclic dimer will not, therefore, be the same as lead to an asymmetry of the double well for the HH-transfer, as illustrated in Fig. 

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H-transfers Coupled to Conformational Changes and Hydrogen Bond Pre-equilibria... 

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