RESUMEN
Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d8 and in CD2Cl2. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d8 and CD2Cl2, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD2Cl2. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell-Limbach tunneling model.
RESUMEN
Octa(aminopropylsilsesquioxane) Si8O12[(CH2)3NH2]8 is a very important precursor for many other hybrid organic/inorganic polyhedral oligomeric silsesquioxanes (POSS) because of the reactivity of its primary amine groups. Unfortunately, it is unstable in water, which can lead to the cleavage of its siloxane cage. In the present work, such a degradation was confirmed using solid-state (29)Si NMR spectroscopy, and the molecular features at the basis of this instability were studied using molecular dynamics simulations (MD). It was also investigated whether replacing the primary amine end groups by secondary amines or by amides with long aliphatic chains could lead to an improvement in the water stability of the Si/O framework. In the pure bulk models, all POSS interdigitate with their pendant organic arms intertwined. Upon insertion of isolated molecules into water, the dimensions of the primary amine POSS remain close to those of the bulk, while the secondary amine and the amide POSS favor conformations that optimize the intramolecular chain-chain interactions. When there are several POSS molecules in water, they cluster with each other through both intra- and intermolecular chain-chain interactions. This tendency for the organic chains to intertwine whenever possible provides some protection to the siloxane cages from water, but also leaves some of the siloxane O exposed. As such, the latter are accessible to form transient hydrogen bonds with the water molecules, which could be a precursor step to hydrolysis and thus cage breakage. In the molecular models, a better protection was obtained in the amide POSS for two reasons: its chains tended to wrap efficiently around its cage, and its ketone O kept water from getting close to the siloxanes. The molecular modeling characterizations were found to agree very well with experimental evidence.
RESUMEN
Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline (15)N,(15)N'-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of (15)N NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal (15)N and (2)H relaxation measurements. (15)N CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline (15)N,(15)N'-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and (15)N,(15)N'-di-(4-fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.
