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The theoretical basis for the calculation of acid dissociation constants in the lowest excited singlet or triplet state of organic compounds has been reexamined in light of a recent study on solvatochromism. A mathematical analysis based on the Onsager cavity model reveals that the absorption or emission frequency of an acid and its conjugate base, as they appear in the Fo¨rster equation, should be replaced with the averages of absorption and emission frequencies corresponding to the 0–0 transition of acid and base, respectively, in order to account for Franck–Condon effects on the free energy balance. The free energy of spontaneous medium relaxation is found to be the same for absorption and emission, and proportional to one-half of the Stokes shift.The main premise of the proposed method, besides the obvious requirement that both the acid and its conjugate base should fluoresce or phosphoresce, is that each excited state exists long enough for equilibrium with the medium to be established prior to emission. [Copyright &y& Elsevier]

Mikanolide [systematic names: 1,10:2,3-diepoxy-6,8-dihydroxy-11-vinylgermacr-4-ene 12,14-di-γ-lactone and 7,10a-dimethyl-1a,1b,2a,6a,7,9a,10,10a-octahydro-4H-6,3-methenofuro[3,2-c]bisoxireno[f,h]oxacycloundecin-4,8(6H)-dione], C15H14O6, derived from a variety of Mikania micrantha growing in Portland, Jamaica, contains a methylcyclodecane ring fused to an unsaturated planar α,γ-lactone, an envelope-type near-planar vinyl-β,γ-lactone and two epoxide moieties. The crystal packing shows stacks of mikanolide molecules interlocked via a network of non-classical C-H…O hydrogen bonds between the lactone units. [ABSTRACT FROM AUTHOR]

Abstract: The reaction between ZnCl2 and di-2-pyridyl ketone benzoyl hydrazone (dpkbh) in acetonitrile under ultrasonic or reflux conditions gave [ZnCl2(η3-N,N,O-dpkbh)] in good yield. The identity of the compound was established from the results of its elemental analysis and a number of spectroscopic measurements. Solid-state infrared spectra of [ZnCl2(η3-N,N,O-dpkbh)] reveal the coordination of dpkbh, the presence of the amide proton and binding of the oxygen atom of dpkbh. 1H NMR spectra of [ZnCl2(η3-N,N,O-dpkbh)] show sensitivity to solvent and temperature variations and confirm the coordination of dpkbh. The electronic absorption spectra of [ZnCl2(η3-N,N,O-dpkbh)] in non-aqueous media show high sensitivity to changes in their surroundings and divulge two interlocked intra-ligand-charge transfer (ILCT) transitions of the donor–acceptor type between 300 and 500nm. Optical measurements show reversible inter-conversion between two forms of [ZnCl2(η3-N,N,O-dpkbh)] that may be due to complex–substrate interactions and thermodynamic analysis gave changes in enthalpy (ΔH θ ) of −23.3 and +14.1kJmol−1, entropy (ΔS θ ) of −51 and +31Jmol−1K−1, and free energy (ΔG θ ) of −8.16 and +4.54kJmol−1 at 298.15K in DMF and DMSO, respectively. Substrates in concentrations as low as 10−5 M can be detected and determined using [ZnCl2(η3-N,N,O-dpkbh)] in polar solvents. A provisional model for the binding of substrate to [ZnCl2(η3-N,N,O-dpkbh)] is developed and a binding constant in the range 6000–8000M−1 is obtained in the case of ZnCl2 in DMSO. Electrochemical measurements on [ZnCl2(η3-N,N,O-dpkbh)] in DMF show irreversible metal and ligand-based redox processes, and electrochemical reactions of dpkbh with ZnCl2 show facile coordination of ZnCl2 and formation of [ZnCl2(η3-N,N,O-dpkbh)]. X-ray structural analysis done on a crystal grown from a DMF solution of [ZnCl2(η3-N,N,O-dpkbh)] confirmed the identity of [ZnCl2(η3-N,N,O-dpkbh)] and shows an extensive network of non-covalent interactions that connects all molecules. [Copyright &y& Elsevier]