Search

NMR and DFT studies of phenol compounds as molecular sensors were carried out to investigate H2O/DMSO eutectic mixtures at a molecular level. The experimental 1H NMR chemical shifts of the OH groups, δexp(OH), of phenol, paracoumaric acid, and vanillic acid show maximum deshielding and, thus, hydrogen bond interactions in the range of mole fractions 0.20 < χ(DMSO) < 0.33. In the mole fractions χ(DMSO) < 0.2, a progressive decrease in δexp(OH) was observed which demonstrates a decrease in hydrogen bond interactions at infinite dilution in H2O, despite the increase in the number of available hydrogen bond acceptor and donor sites. DFT calculated δcalc(OH) of minimum energy solvation clusters were shown to be in reasonable agreement with the pattern in experimental δexp(OH) data. The chemical shift deshielding and, thus, increased hydrogen bond interactions in the natural product + DMSO + nH2O (n = 2, 3) solvation clusters, relative to complexes in DMSO or H2O solutions, cannot be attributed to a single structural parameter of the cooperative interactions between H2O and DMSO molecules with the phenol OH groups of the natural products. The minimum energy conformers of phenol compounds + 2H2O + DMSO complexes are in excellent agreement with a recent low temperature neutron diffraction experiment of 3D2O + DMSO and demonstrate a general structural motif of solvation complexes. The combined use of 1H NMR and DFT studies with emphasis on δ(OH) of phenol compounds, as molecular sensors, can provide an effective method for the study of solute–solvent interactions at the atomic level.

Direct activation of aromatic C-H bonds in polyphenolic compounds in a single step, without the use of late transition metals, is demonstrated with the use of D2O and common phosphate buffer at neutral pD and near ambient temperatures. Detailed variable temperature and pD 1H NMR studies were carried out to investigate, for the first time, the Gibbs activation energy (ΔG‡), the activation enthalpy (ΔH‡), and activation entropy (TΔS‡) of H/D exchange reactions of the natural product catechin and the model compounds resorcinol and phloroglucinol. NMR and DFT calculations support a catalytic cycle comprising two water molecules in a keto-enol tautomeric process. The reduction of ΔG‡ values due to the catalytic role of two molecules of water by a factor of 20-30 kcal mol-1 and the resulting acceleration of the H/D exchange rate by a factor of 1020-1030 should be compared with a minor reduction in ΔG‡ of 0.4 to 4.5 kcal mol-1 due to the effect of an additional electron donating oxygen group and the deprotonation of OH groups. It can therefore be concluded that although the H/D exchange process can be accelerated by a small amount of an acid or a base to break a C-H bond, water as a catalyst plays the major role. This approach opens a new vistas for the combined use of NMR and DFT studies as tools to understand the molecular basis of the catalytic role of water.

The rotation motion of a larger substituent of an aromatic ring is accompanied by the electron density fluctuation of the highest occupied molecular orbitals. For benzyltrimethylsilanes p-R3–C6H4––CR1R2(SiMe3) [R3 = H,PhCO; R1, R2 = H,H; H,Me; Me,Me; H,Ph; Ph,Ph] the silyl containing substituent rotates on the axis between the aromatic moiety and the benzylic carbon as indicated in the formula. In the course of this rotation a diversity of various conformers is passed. For the sake of simplicity, we reduce this diversity to two (extreme) borderline structures, one with the substituent in plane with the aromatic ring (transient), and one where the substituent is twisted by 90° (stable structure). In the very rapid and non-hindered electron transfer from the conformer mixture to n-butyl chloride parent radical cations, the singlet ground state conformers are converted into two kinds of radical cations. The cations tending to the planar type are stable whereas those of the twisted type dissociate immediately into benzyl type radicals (p-R3–C6H4–C˙R1R2) and trimethylsilyl cations. The above mentioned product pattern and quantum-chemical calculations on characteristic parameters of the ground state conformers and the product cations, show that the electron transfer occurs completely unhindered after diffusional encounter, i.e. after each approach of the reactants. Therefore the statistics of the rotation conformers mixture is reflected by the pattern and ratio of the product transients such as metastable radical cations and radicals derived from a dissociative radical cation. Up to now this is the only case where a diffusion-controlled electron transfer reaction proceeding in the nanosecond scale is influenced by intramolecular rotation motions within the electron donor.