Formal reduction potentials, E°′, of a series of 12 para‐substituted nitrobenzenes (R―C6H4―NO2, where R═NH(CH3), NH2, OCH3, CH3, H, OCO(CH3), Cl, CF3, CO(CH3), CO(H), CN, and NO2) were measured and related to experimental parameters and the calculated descriptors: the lowest unoccupied molecular orbital (LUMO) energy (ELUMO), electron affinity (EA), electrophilicity index (ω), and relative group electronegativity (χR). Excellent correlation was obtained in the linear relationship between E°′ and both ELUMO (R2 > 0.99) and ω, followed by EA, χR, experimental δ1H nuclear magnetic resonance (NMR), infrared asymmetric NO2 stretching frequency (νas), and lastly, δ13C NMR. Calculated ELUMO was found to provide the most convenient theoretical means for predicting experimental E°′ while, together with ω, also giving the highest degree of accuracy. Copyright © 2011 John Wiley & Sons, Ltd.

Air oxidation of ortho-fluorodithizone resulted in the first X-ray resolved structure of a disulfide of dithizone, validating the last outstanding X-ray structure in the oxidation of dithizone, H(2)Dz, which proceeds via the disulfide, (HDz)(2), to the deprotonated dehydrodithizone tetrazolium salt, Dz. Density functional theory calculations established the energetically favored tautomers along the entire pathway; in gas phase and in polar as well as nonpolar solvent environments. DFT calculations using the classic pure OLYP and PW91, or the newer B3LYP hybrid functional, as well as MP2 calculations, yielded the lowest energy structures in agreement with corresponding experimental X-ray crystallographic results. Atomic charge distribution patterns confirmed the cyclization reaction pathway and crystal packing of Dz. Time dependent DFT for the first time gave satisfactory explanation for the solvatochromic properties of dithizone, pointing to different tautomers that give rise to the observed orange color in methanol and green in dichloromethane. Concentratochromism of H(2)Dz was observed in methanol. Computed orbitals and oscillators are in close agreement with UV-visible spectroscopic experimental results.

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The initial photochromic reaction of dithizonatophenylmercury(II) in solution was investigated by femtosecond transient absorption spectroscopy. Ultrafast excitation within less than 100 fs caused a radiationless photoreaction with a time constant of 1.5 ps, which is interpreted as C=N isomerization through a conical intersection. The orthogonally twisted intermediate state was observed through its excited-state absorption. Bifurcation along pathways towards the ground states of the orange cis and blue trans configurations occurs below the funnel of the conical intersection. The photochromism of the title compound in a very polar solvent such as methanol is observed for the first time.

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