Visible spectra interpretation

Helf White (Ref 2) interpret the above behavior of the nitrocompds in inhibiting the scintillation process as one of simple light absorption rather than as a true chemical quenching (ae-excitation process). To substantiate this, the UV and near-visible spectrum of each of the light compds in toluene—PPO soln was measured using the 50% extinction concn for each nitrocompd (as determined from Fig 1). 

TMB (42) was first generated by Roth el al. by photochemical decarbonyla-tion of the ketone 44 in a low-temperature matrix. This preparation was intensely colored, with a main transition at 490 nm and several subsidiary absorptions. Earlier ti-CI quantum chemical computations had predicted ultraviolet-visible (UV-vis) is transitions for the singlet and triplet states of TMB, and the bands observed by the Roth group were in better agreement with the predictions for the triplet. The preparation also showed a narrow ESR spectrum interpreted by the authors as that of a triplet species with D = 0.0042 cm and E = 0.0009 cm, which gave a linear Curie plot. The authors assumed that the carriers of the UV-vis and ESR spectra were the same species, namely, triplet TMB. They concluded that TMB is a ground-state triplet, contrary to the disjoint theory and to the computational results described above. 

Up to this point we have considered two central issues involved in interpreting electronic spectra of transition metal complexes—the number and intensities of spectral lines. There is a third important spectral feature, the widths of observed bands, which we have not yet discussed. Consider again the visible spectrum for... 

Spectroscopy has not proven to be very conclusive in solving this problem. Similarities between the visible spectrum of the calcined catalyst and that of bulk dichromates have been noted (5,12-14). In the end, however, there is always doubt about the interpretation of spectra because no adequate reference data exist for these surface bound species (76). Krauss and coworkers have carefully studied the luminescence of Cr/silica and concluded that at least a portion of the chromium is present as chromate (75). 

When a sample of TS-1 interacts with a stoichiometric amount of H202, a strong band appears at 25,000 cm-1 in the UV-visible spectrum upon addition of a stoichiometric amount of ammonia, the band shifts to 27,500 cm-1 and slowly declines. This sequence has been interpreted as evidence of formation of a mixed complex in which both NH3 and the peroxo group are coordinated to the TiIv. This mixed complex could be the precursor of NH2OH (Geobaldo et al., 1992). 

The six-coordinate species is spin-admixed (S = 3/2,5/2) while the predominantly five-coordinate species (1-OH) is high spin (S = 5/2). The spin states have significance, it will be shown in interpreting some of the kinetics results. Reversible binding of NO to 1-H20 leads to formation of the linearly bonded diamagnetic porphyrin nitrosyl, (TMPS)Fen(NO + )(H20), (1-NO). The product of reaction of 1-OH with NO has a noticeably different UV/visible spectrum from that of 1-NO, and this was ascribed to the species (TMPS)Fen(NO + )(OH). 

A number of physical studies have been performed on Mn111 porphyrins in an attempt to understand their electronic structure. The visible spectra of such compounds have been of particular interest. For most metal porphyrins the visible spectrum is insensitive to the nature of the coordinated metal and this has been interpreted as indicating little interaction between the metal and the porphyrin 7r-orbitals in such compounds. However this is not the case for Mn111 porphyrins which exhibit metal-dependent charge transfer absorptions. This dependence appears to reflect significant n orbital-Mn"1 interaction. Resonance Raman and linear dichroism spectral studies are also consistent with this conclusion.667... 

In many instances UV-visible spectroscopy is a valuable tool in the identification of chromophoric functional groups in discrete organic molecules. But even a simple, two-component mixture may make the interpretation of a UV-visible spectrum difficult from a functional group point of view. The following example from Olsen (1975) illustrates this point well ... 

For this overall reaction, the components of A G° are A77° = —16.8 kJ mol-1 and (at 298 K) 7 A. S 0 = +36.2 kJmol-1. Both contribute to the overall negative A G°, but the contribution from the entropy-based term 7 A5° is larger. The smaller AH0 contribution is in part associated with increased crystal field stabilization, seen experimentally as a change in the maximum in the visible spectrum. We can interpret this reaction as dominated by a favourable entropy change associated with chelation (i.e. an increase in disorder and particles or participating molecules), as exemplified in (5.13) and (5.14). 

A shift to higher frequencies of the absorption maximum in the visible spectrum of the CU2C02SOD is observed on copper reduction. This blue shift is analogous to that shown by the cobalt-substituted SOD with an empty Cu site. This observation has been interpreted as proof of the protonation of the bridging histidine on copper reduction (284). 

Mn " in water was well reviewed in 1968, and nothing published since appears to add significantly to the basic data. Interpretations were given in terms of Mn, with hydrolysis to MnOH which translates, not unreasonably to [Mn(H20)5] and [Mn(OH)(H20)5] . The aqueous solutions are described as green and rather unstable. All data referring to (MnfHjO) ] were obtained by extrapolating to some ideal, either of time or concentration. But, even so, the extinction coefficient obtained for the visible spectrum of this species is ten times that obtained for it in... 

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