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Density functional theory (DFT) calculations of δ(13C) and δ(1H) chemical shifts and 3J(13COO1H) coupling constants of three model hydroperoxides of the naturally occurring cis‐11‐OOH and trans‐9‐OOH isomers of oleate and 9‐cis, 11‐trans‐16‐OOH endo hydroperoxide of methyl linolenate are reported. The computational δ(OOH) for various functionals and basis sets were found to be nearly identical for the cis/trans geometric isomers. The chemical shifts of the methine CHOOH protons and carbons, on the contrary, are highly diagnostic for the identification of cis/trans geometric isomerism. The chemical shifts of the olefinic protons and carbons strongly depend on the orientation of the hydroperoxide unit relative to the double bond and, thus, of importance in conformational analysis. The results are in very good agreement with the available experimental data. For the various diastereomeric pairs of the model endo‐hydroperoxide, the strongly deshielded OOH resonances, due to the presence of an intramolecular hydrogen bond between the hydroperoxide proton and an oxygen of the endo‐peroxide ring, along with the δ(CHOOH), are highly diagnostic for identification and structure elucidation of complex erythro‐ and threo‐ diastereomeric pairs of endo‐hydroperoxides; the computational results are in very good agreement with the available experimental data. The 3J(13COO1H) coupling constants were found to be < 2 Hz for the cis–trans geometric models and < 0.5 Hz for the endo‐hydroperoxide and, thus, unimportant in stereochemical analysis. Sharp resonances of the hydroperoxide protons, with Δν1/2 < 3 Hz, are required for the successful implementation of the 1H13C heteronuclear multiple bond correlation (HMBC) technique. [ABSTRACT FROM AUTHOR]

Parent radical cation of nonpolar solvent (alkanes and alkyl chlorides) ionize 9-(trimethylsilyl)xanthenes and 9-(trimethylsilyl)fluorenes in a diffusion-controlled electron transfer. A mobility restriction of the benzylic silane group in positions vertical to the phenyl plane stabilizes the radical cation and accounts for a dissociative radical cations, which undergo C-Si bond fragmentation in the order benzylsilane > xanthenylsilane > fluorenylsilane.

Results reveal that in polar media, 4-diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline photoionizes to two types of radical cations via a two-photon process and the triplet state. In nonpolar solvents, only one type of radical cation is formed.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An ab-initio CASSCF/CASMP2 study on the structures and energies of the classical and bridged forms of the vinyl cation is presented. Our calculations which are in agreement with the experimental results predict that the bridged form of the vinyl cation (C2H3+) is more stable than its classical counterpart and is the unique species on the potential energy surface. A quantum mechanical explanation based in the notion of electron correlation energy and in Boltzmann distribution shows that the classical form is considered to be a transient species having a fleeting existence. [ABSTRACT FROM AUTHOR]

The exact knowledge of hydrogen atomic positions of O-H…O hydrogen bonds in solution and in the solid state has been a major challenge in structural and physical organic chemistry. The objective of this review article is to summarize recent developments in the refinement of labile hydrogen positions with the use of: (i) density functional theory (DFT) calculations after a structure has been determined by X-ray from single crystals or from powders; (ii) 1H-NMR chemical shifts as constraints in DFT calculations, and (iii) use of root-mean-square deviation between experimentally determined and DFT calculated 1H-NMR chemical shifts considering the great sensitivity of 1H-NMR shielding to hydrogen bonding properties. [ABSTRACT FROM AUTHOR]