Phenol Blue, resonance

A third example can be taken from analytical chemistry. Absorption and resonance Raman spectra of phenol blue were measured in liquid and supercritical solvents to determine the solvent dependence of absorption bandwidth and spectral shifts. Good correlation between absorption peak shift and resonance Raman bands and between Raman bands and bandwidth of C-N stretching mode were observed while anomalous solvent effect on the absorption bandwidth occnrred in liquid solvents. Large band-widths of absorption and resonance Raman spectra were seen in supercritical solvents as compared to liquid solvents. This was dne to the small refractive indices of the supercritical solvents. The large refractive index of the liqnid solvents only make the absorption peak shifts withont broadening the absorption spectra (Yamaguchi et al., 1997). 

A further interesting confirmation of these theoretical considerations was provided by the behaviour of Phenol Blue. The compound establishes a resonance hybrid of (19) and (20) in which (20) is a zwitterion, in other... 

T Yamaguchi, Y Kimura, M Hirota. Solvent and solvent density effects on the spectral shifts and the bandwidths of the absorption and the resonance raman spectra of phenol blue. J Phys Chem 101 9050, 1997. 

T Yamaguchi, Y Kimura, N Hirota. Solvation state selective excitation in resonance Raman spectroscopy. 1. Experimental stndy of the C=N and C=0 stretching modes of phenol blue. J Chem Phys 109 9075, 1998. 

Spiropyran merocyanine specffa shift markedly to the blue as the solvent polarity increases [4,25], as shown in Fig. 7a for 6,8-dinitro-BIPS merocyanine. This is generally accepted to imply that they have a zwitterionic character caused by the donation of electron density from the indoline nitrogen to the phenolic C9 oxygen [4,25]. The rational behind this assignment to the zwitterion is based simply on the fact that if the zwitterion is in the highest occupied molecular orbital (HOMO) state, then the corresponding quinoidal resonant form is in the lowest unoccupied molecular orbital (LUMO) state and changing the solvent to a more polar one... 

The visible spectrum exhibits a maximum at 430 nm, blue-shifted from that of the native enzyme, and similar to the spectra of the phenolate complexes. The addition of aniline to native enzyme does not result in spectral changes, suggesting that o-ami-nophenol binds to the iron through the oxygen. This is further supported by the resonance Raman spectra which exhibit both tyrosinate and phenolate modes, like the phenolate complexes. Interestingly, there is a significant increase in the background fluorescence of the Raman spectra as in the ES complexes. So, this complex appears to have features of both the phenolate complexes and the ES complexes. Mechanistic postulates will be discussed in the next section. 

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