No abstract

Chcriiicd KineticsThc 90 M H z ' H N M R spectra of '5N,'sN'-dipentadeutcrophenylformamidine (DPFA), a nitrogen analogue of formic acid, dissolved in tetrahydrofuran-d8 (THF) have been measured as a function of concentration, deuterium fraction in the 'H-I5N sites, and of temperature. The spectra show characteristic changes, from which thermodynamic and kinetic information on hindered rotation, hydrogen bond association and proton exchange of DPFA in THF are obtained by NMR lineshape analysis. DPFA forms two conformers A and B in THF, to which s-trans and s-cis structures have been assigned. At low concentrations both DPFA conformers are located in a hydrogen bond to the solvent molecules. However, as a result of the different structure, only A is able to selfassociate to any observable extent. This effect leads to concentration dependent A/B populations. Higher selfassociates or mixed AB associates are not observed. Whereas B is not able to exchange protons, A is subject to a very fast proton transfer. By measuring proton lifetimes as a function of concentration and of the deuterium fraction in the labile proton sites, it was established that two protons are transferred in every exchange process. Thermodynamic data of the association process obtained by the analysis of the chemical shifts, of the A/B populations and the proton lifetimes as a function of concentration agreed very well. These results are evidence that A forms only selfassociated hydrogen bonded dimers with a cyclic structure in which the double proton transfer takes place. The observation of a kinetic HH/HD isotope effect of 20 at 178 K establishes this transfer as the rate limiting step of the overall proton exchange. Rate constants of the double proton transfer in the cyclic dimer were obtained as a function of temperature from which an activation energy of about 19 kJ mol-' was obtained. In addition, the rates of interconversion between the two conformers were determined as a function of temperature. Details of the reaction mechanism and differences to the related carboxylic acids are discussed. Thus, it is shown that dynamic NMR spectroscopy can be a useful tool for elucidating elementary steps of complex reaction networks.

No abstract

Using Dynamic NMR Spectroscopy we have detected a novel intramolecular double hydrogen transfer in the oxalamidine (OA) system. OA is, therefore, so far the smallest known molecular unit capable of such a transfer. Because of solubility problems, experiments were performed on tetraphenyloxalamidine (TPOA, Ia) and the isotopically labeled compound TPOA‐15N4‐d20 (Ib) dissolved in CD2CI2. At low temperatures TPOA exists in a number of different conformations, of which only one conformation with two intramolecular hydrogen bonds is capable of an intramolecular hydrogen transfer which shows up in the 1H‐15N signal of this conformer. At low temperatures a 1H‐15N doublet (88 Hz coupling constant) is observed which changes into a triplet (44 Hz coupling constant) as the temperature is raised. The position of this signal is independent of temperature and concentration. Two further conformations were detected, of which one formed strong intermolecular hydrogen bonds and underwent fast intermolecular proton exchange. Above room temperature all NH signals coalesce into an exchange‐broadened singlet. 13C experiments prove that this effect is due to a fast internal conversion of all conformers. IR experiments on TPOA in CCl4 did not reveal the complicated conformational behavior of TPOA. By lineshape analysis we obtained the rate constants of the HH migration, which are given by kHH = 1010.8 exp(−43 kJ mol−1/RT). An upper limit for the HD exchange rates were found for a sample deuterated to 90% in the labile proton sites: kHD = exp(−53 kJ mol −1/RT), i.e. kHH/kHD = 2.1 at 298 K. The data are compared to previous results on the azophenine rearrangement.

Dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday 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 polariza tion 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 'H-15N sites. Using 1H NM R 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.