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Solvent and solvent density effects on the spectral shifts and the bandwidths of the absorption and the resonance Raman spectra of phenol blue

We have measured the absorption and the resonance Raman spectra of a solvatochromic dye, phenol blue, in liquid and supercritical solvents. We have found anomalous solvent dependence of the absorption bandwidth in liquid solvents: the width has apparently no correlation with the absorption peak shift. On the other hand, we have found good linear correlation between the absorption peak shift and the peak position of the resonance Raman bands (the C=N and the C=O stretching modes). The relative intensities of the Raman bands and the bandwidth of the C=N stretching mode also show correlation with the absorption peak shift. Incorporating these Raman data, the anomalous bandwidth of the absorption spectrum is explained by the change of the intramolecular vibrational contribution to the absorption bandwidth due to the electronic structure change by the solvent, which cancels the change of the solvent contribution. We have estimated the solvent reorganization energy assuming linear dependence of the intramolecular contribution on the absorption band center and neglecting the solvent reorganization energy in alkanes such as ethane and cyclohexane. In liquid solution, the estimated solvent reorganization energy is correlated fairly well with the absorption peak shift. Solvent dependence of the Raman bandwidth of the C=N stretching mode resembles the solvent dependence of the solvent reorganization energy estimated in this way. Relatively large bandwidths of both absorption and resonance Raman spectra have been observed in supercritical solvents compared with those in liquid solvents of a similar absorption peak shift. We interpreted this as due to the small refractive indices of the supercritical solvents relative to the liquid solvents; the large refractive indices of the liquid solvents only make the absorption peak shifts without broadening the absorption spectra.

Insight into the preferential N-bindingversusO-binding of nitrosoarenes to ferrous and ferric heme centers

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-bindingversusO-binding of ArNOs to hemes. Our X-ray structural results favored N-binding of ArNO to ferrous heme centers, and O-binding to ferric hemes. Results of the DFT calculations rationalize this preferential binding on the basis of the energies of associated spin-states, and reveal that the dominant stabilization forces in the observed ferrous N-coordination and ferric O-coordination are dπ-pπ* and dσ-pπ*, respectively. Our results provide, for the first time, an explanation whyin situoxidation of the ferrous-ArNO compound to its ferric state results in the observed subsequent dissociation of the ligand.

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O2 activation by copper(I). The reaction of the copper(I) complex of N,N,N′,N′-tetramethypropylenediamine with a series of para-substituted nitrosobenzene deriva