A nuclear magnetic resonance study of the intramolecular hydrogen bond in acetylacetone

Crystal structure, Hirshfeld surface analysis and spectroscopic characterization of the di-enol tautomeric form of the compound 3,3'-[(2-sulf-an-yl-idene-1,3-di-thiole-4,5-di-yl)bis-(sulfane-di-yl)]bis-(pentane-2,4-dione)

The reaction between [TBA]₂[Zn(dmit)₂] and 3-chloro-2,4-penta-nedione yielded single crystals of the title compound, (3E,3'E)-3,3'-[(2-sulfanylidene-1,3-dithiole-4,5-diyl)bis(sulfanediyl)]bis(4-hydroxypent-3-en-2-one), C₁₃H₁₄O₄S₅, after solvent evaporation. The title compound crystallizes in the triclinic space group P with two mol-ecules related by an inversion center present in the unit cell. The central thione ring moiety contains a carbon-carbon double bond covalently linked to two sulfoxide substituents located outside of the plane of the ring. The S-C-C-S torsion angles are -176.18 (8) and -0.54 (18)°. Intra-molecular hydrogen bonds occur within the two dione substituents (1.67-1.69 Å). Adjacent asymmetric units are linked by C-H⋯S (2.89-2.90 Å), S⋯S [3.569 (1) Å] and O⋯H [2.56-2.66 Å between non-stacked thione rings] short contacts.

Excited state dynamics of bis-dehydroxycurcumin tert-butyl ester, a diketo-shifted derivative of the photosensitizer curcumin

Bis-dehydroxycurcumin tert-butyl ester (K2T23) is a derivative of the natural spice curcumin. Curcumin is widely studied for its multiple therapeutic properties, including photosensitized cytotoxicity. However, the full exploitation of curcumin phototoxic potential is hindered by the extreme instability of its excited state, caused by very efficient non radiative decay by means of transfer of the enolic proton to the nearby keto oxygen. K2T23 is designed to exhibit a tautomeric equilibrium shifted toward the diketo conformers with respect to natural curcumin. This property should endow K2T23 with superior excited-state stability when excited in the UVB band, i.e., in correspondence of the diketo conformers absorption peaks, making this compound an interesting candidate for topical photodynamic therapy of, e.g., skin tumors or oral infections. In this work, the tautomeric equilibrium of K2T23 between the keto-enolic and diketo conformers is assessed in the ground state in several organic solvents by UV-visible absorption and by nuclear magnetic resonance. The same tautomeric equilibrium is also probed in the excited-state in the same environments by means of steady-state fluorescence and time-correlated single-photon counting measurements. These techniques are also exploited to elucidate the excited state dynamics and excited-state deactivation pathways of K2T23, which are compared to those determined for several other curcuminoids characterized in previous works of ours. The ability of K2T23 in photosensitizing the production of singlet oxygen is compared with that of curcumin.

Proton transfer in acetylacetone and its α-halo derivatives

A two-dimensional potential energy function has been applied to study the bent intramolecular H-bonds within acetylacetone and its α-halo derivatives. The theoretically predicted proton transfer barrier heights correlate very well with geometrical parameters and electronic properties related to the H-bond strength.

Diketone Radical Cations: Ketonic and Enolic Forms As Revealed by Matrix EPR Studies and DFT Calculations

Radical cations of 2,3-butanedione, 2,4-pentanedione, 3-methylpentane-2,4-dione, 2,5-hexanedione, and 2,3-pentanedione were investigated by electron paramagnetic resonance (EPR) spectroscopy in a solid Freon matrix and density functional theory (DFT) quantum chemical calculations. All the diketone radical cations in ketonic form show small proton hyperfine couplings (typically unresolved in the EPR spectra). In the cases of 2,4-pentanedione and 3-methylpentane-2,4-dione, enolic forms of the radical cations (pi-type species with main spin population at carbon atom) were characterized. Preferential stabilization of the enolic form of 3-methylpentane-2,4-dione radical cation was explained by trap-to-trap positive hole migration rather than monomolecular relaxation of the ionized ketonic form through H atom transfer.

Theoretical Studies of the Photochemical Dynamics of Acetylacetone: Isomerzation, Dissociation, and Dehydration Reactions

The potential energy surfaces of the C-O cleavage, rotational isomerization, keto-enolic tautomerization, and dehydration reactions of acetylacetone in the lowest triplet and ground states have been determined using the complete active space self-consistent field and density functional theory methods. The main photochemical mechanism obtained indicates that the acetylacetone molecule in the S(2)((1)pipi*) state can relax to the T(1)((3)pipi*) state via the S(2)-S(1) vibronic interaction and an S(1)/T(1)/T(2) intersection. The C-O fission pathway is the predominant dissociation process in the T(1)((3)pipi) state. Rotational isomerization reactions proceed difficultly in the ground state but very easily in the T(1)((3)pipi*) state. Keto-enolic tautomerization takes place with little probability for acetylacetone in the gas phase.

The C2v Structure of Enolic Acetylacetone

It has often been postulated that the lowest energy enolic form of Acetylacetone (AcAc) assumes C(s) symmetry, i.e., has a double-minimum potential possibly exhibiting a low barrier to internal proton transfer and not a single minimum, C(2v). Recent theoretical calculations and experimental work support the C(s) hypothesis but the literature on this fascinating molecule is divided. Toward this objective, the high-resolution rotational spectra of enolic acetylacetone and 3 isotopologues have been obtained, revealing C(2v) symmetry. The two methyl groups exhibit a very low barrier to internal rotation, thus making AcAc internally highly dynamic.

Direct determination of hydrogen-bonded structures in resonant and tautomeric reactions using ultrafast electron diffraction

We elucidate the keto-enol tautomeric equilibrium in acetylacetone, the structure of both keto and enol forms, and the nature of the intramolecular O-H...O HB in enolic acetylacetone using our ultrafast electron diffraction apparatus, thereby shedding new light on the nature of the hydrogen bond in resonant tautomeric structures. The enolic structure exhibits some pi-resonance delocalization; however, this delocalization is not strong enough to give a symmetric skeletal geometry. The long O...O distance in the refined structure renders the homonuclear O-H...O hydrogen bond in acetylacetone localized and asymmetric.

NMR Spectra and Magnetic Exchange Interactions in Nickel(II) Complexes with a Chelated Nitronyl Nitroxide Radical

1H or (2)H NMR contact shifts of the beta-diketonate methine in the radical complexes [Ni(beta-diketonato)(2)(NIT2-py)] and [Ni(beta-diketonato)(tmen)(NIT2-py)](+) (NIT2-py = 2-(2'-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl and tmen = N,N,N',N'-tetramethylethylenediamine) were found to correlate to a product of the respective (1)H or (2)H NMR contact shifts of the methine in the nonradical complexes [Ni(beta-diketonato)(2)(tmen)] and the fractional contribution to the Ni(II) moiety arising from the magnetic interaction between nickel(II) ion and NIT2-py radical, proposing a new method to estimate the exchange coupling constant J value from the NMR contact shifts. Variable temperature (2)H NMR measurements of the C-D signals in [Ni(acac-d)(2)(NIT2-py)], [Ni(acac-d)(tmen)(NIT2-py)](+), and [Ni(acac-d)(2)(tmen)] suggested the topological discrimination of the acetylacetonates.

"Strong" hydrogen bonds in chemistry and biology

Hydrogen bonds are a key feature of chemical structure and reactivity. Recently there has been much interest in a special class of hydrogen bonds called "strong" or "low-barrier" and characterized by great strength, short distances, a low or vanishing barrier to hydrogen transfer, and distinctive features in the NMR spectrum. Although the energy of an ordinary hydrogen bond is ca 5 kcal mol-1, the strength of these hydrogen bonds may be > or = 10 kcal mol-1. The properties of these hydrogen bonds have been investigated by many experimental techniques, as well as by calculation and by correlations among those properties. Although it has been proposed that strong, short, low-barrier hydrogen bonds are important in enzymatic reactions, it is concluded that the evidence for them in small molecules and in biomolecules is inconclusive.