Delocalization, electronic spectra

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

Methods of electron spectroscopy are widely used to follow the electron-transfer process. Thus, the progress of electron transfer from naphthalene anion-radical to cup-stacked carbon nanotubes is easily detected by monitoring the UV absorption spectrum. Namely, the absorption band around 500-900 nm due to naphthalene anion-radical completely disappears after reduction of the nanotubes. At the same time, the reduced nanotubes exhibit ESR spectrum characterized with g-factor of 2.0025 (Saito et al. 2006). This g-value is close to the free spin g-factor of 2.0023 that is diagnostic of the delocalized electron on carbon nanomaterials (Stinchcombe et al. 1993). It should be parallelly... 

The electronic spectrum of 1,5-diphenylpentadienyllithium in thf shows the presence of contact (tight) and.solvent-separated (loose) ion pairs. The smaller the cation and the more delocalized the anion, the greater is the tendency to form loose ion pairs. They are also favored as the temperature is lowered (64,65). 13C-NMR studies (66,67) have been interpreted in terms of appreciable covalency, but the present view is that organolithiums are predominantly ionic (68). Schlosser and Stahle have analyzed coupling constants 2J(C—H) and 3J(H—H) in the 13C- and H-NMR spectra of allyl derivatives of Mg, Li, Na, and K and of pentadienyls of Li and K (69). They conclude that considerable pleating of the allyl and pentadienylmetal structures occurs. The ligand is by no means planar, and the metal binds to the electron-rich odd-numbered sites. The lithium is considered to be q3 whereas the potassium is able to reach t]5 coordination with a U-shaped ligand. 

The absence of HFS in the EPR spectrum of 10 min activated mixture points to the appearance of weak exchange interaction between V ions (localized centres), probably, through electron gas (delocalized electrons) - C-S-C relaxation [80,81]. 

Biferrocene 1 (M = Fe) oxidizes in two separate one-electron steps (6, 7) (Ei/2 = 0.31 and 0.64 V), of which only the first is chemically reversible (i.e., the monocation is stable but the dication is not) mild oxidation yields the monocation [l]" (M = Fe) as a mixed-valence Fe(II)Fe(III) species (8-10). Bis(fulvalene)diiron 3 (M = Fe) similarly undergoes two successive one-electron oxidations (the dication is also stable), but in this case the Mdssbauer spectrum of the monocation shows the iron atoms to be equivalent, implying a delocalized electronic structure and an oxidation state of 2.5 for each metal (9,11). The separation of... 

Consistent with the dithiolene structure proposed for the oxidized derivative of molybdopterin (3), no nonexchangeable H resonances are observed in the region expected for the CH protons of a dithiolate structure (5b). The mass spectrum of 3 is also consistent with the proposed dithiolene structure (19, 34). Resonance Raman spectra of DMSO reductase show bands at 1575 cm (oxidized form) and 1568 cm (reduced form) that are assigned to the C=C stretch of the dithiolene unit of 2 (40). However, the delocalized electronic structure of dithiolene ligands makes it difficult to assign the C=C stretch with certainty. As Rajagopalan notes (19), ultimate proof of the structure [of molybdopterin] will have to await either X-ray studies on a molyb-doenzyme or unequivocal chemical synthesis of the molecule. ... 

However, the electron spectrum in nanometer cavity depends strongly on the water loading. The red spectral shift with decreasing size of the confined water clusters presumably demonstrates a decreased solvation of the electron and an increased delocalization. This interpretation was confirmed by recent mixed quantum-classical simulations that predict larger electron gyration radius for low water loading. The chemical implications of this decreased solvation of the electron are still to be investigated. 

Subphthalocyanines contain a delocalized 14 -electron conjugated pathway, and are brightly colored compounds, both in the solid state and in organic solution. As such, they exhibit fairly strong absorption bands in their visible electronic spectrum. For instance, subphthalocyanine 2.284 exhibits a Soret-like absorbance band at ca. 305 nm and a more intense Q-like transition at 565 nm (e = 50 100 M cm and 89 100 M cm , respectively, in CHCI3). These bands are blue-shifted relative to those of the metallophthalocyanines (by c. 20-30 nm and 120-130 nm in the Soret and Q-band regions, respectively), and exhibit absorption coefficients that typically are smaller than those of the metallophthalocyanines. ... 

Although the two benzene rings in 56 are linked by almost single bonds, there is ample evidence to demonstrate that they are not independent and that substantial delocalization is present over the entire jr-system. Thus, the electronic spectrum has two sets of hands at 235-260 and 330—370 nm, significantly different from biphenyl which has only one absorption band at 250 nm. A charge-transfer complex of 56 with tetracyanoethylene, presumably of structure 57, was shown to be more stable than the corresponding fluorene complex73. The infra-red absorptions for... 

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