Spectrum, optical solvent

The electronic interaction between the two metal centers leads to the appearance of IT bands which can be observed in a region ranging from the ultraviolet to the near infrared part of the spectrum. Optical T transitions ace due to electron transfer from one metal center M to the other M. The energy of the IT transition strongly depends on the redox asyirmetry of the metal centers as well as the dielectric properties of the solvent. The general behavior of mixed-valence corrpounds has been reviewed excellently very recently (20). 

Transparent solid samples can be analyzed directly by placing them in the IR beam. Most solid samples, however, are opaque and must be dispersed in a more transparent medium before recording a traditional transmission spectrum. If a suitable solvent is available, then the solid can be analyzed by preparing a solution and analyzing as described earlier. When a suitable solvent is not available, solid samples may be analyzed by preparing a mull of the finely powdered sample with a suitable oil. Alternatively, the powdered sample can be mixed with KBr and pressed into an optically transparent pellet. 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

Diphenylmethylene is certainly the most exhaustively studied of the aromatic carbenes. Low temperature epr spectroscopy (Trozzolo et al., 1962) clearly established the ground state of this carbene as the triplet. The optical spectrum of the triplet was recorded first in a 1,1-diphenylethylene host crystal (Closs et al., 1966) and later in frozen solvents (Trozzolo and Gibbons, 1967). 

The dependence of this phenomenon on temperature and concentration has been studied in detail (70,71,87) and treated mathematically (87). In principle any compound capable of self-association might be capable of self-induced nonequivalence. These cases should be sufficient to suggest due caution on the part of those who would establish the identity of a racemate (e.g., a synthetic natural product ), by comparison of its NMR spectrum with that of the naturally derived optically pure substance. This phenomenon is not restricted to solutes with aromatic substituents, as evidenced by Table 12. Self-induced nonequivalence may be eliminated by addition of polar solvents or by dilution of the sample. Under these conditions, as has been shown for dihydroquinine (14), spectra of racemic, optically pure, and enriched material become identical. 

The absorbance spectrum of a liquid or solution can be readily measured by using quartz or sapphire cuvettes and fiber-optic probes of variable path lengths. The most suitable solvents for this purpose are those not containing 0-H, N-H and C-H groups which exhibit little or no absorption in this spectral region, such as CHCI3, CCI4 or CS2. 

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