Bands in electronic spectra

H Norrstrom and A Teder. Absorption Bands in Electronic Spectra of Lignins. Part 1. Lignins from Alkaline Cooks on Spruce. Svensk Papperstidn. 74 85-93, 1971. 

Absorption bands in electronic spectra are usually broad the absorption of a photon of light occurs in sslO s whereas molecular vibrations and rotations occur more slowly. Therefore, an electronic transition is a snapshot of... 

The properties of the microenvironment of soluble synthetic polymers such as polymethacrylamide (PMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(2-vinylpyridine) (P-2VP), poly(4-vinylpyridine) (P-4VP), poly(methyl methacrylate) (PMMA), poly(butyl methacrylate) (PBMA), polystyrene (PS), poly(4[5]-vinylimidazole) (PVIm), and poly(N-2-hydroxypropyl methacrylamide) (PHPMA) and cross-lined polymers were studied by the shift and shape of the band in electronic spectra of a solvatochromic "reporter" molecule embedded in polymer chains. Preferential interaction of parts of the polymer molecule with a reporter and the shielding of interactions between solvent molecules and a reporter molecule of a polymer causes a shift and broadening of its solvatochromic band. This shift is mechanistically interpreted as a change in the polarity of the microenvironment of a polymer in solution in comparison with polarity of the solvent used. 4-(4-Hydroxystyryl)-N-alkylpyridinium-betaine, spiropyran-merocyanine, and l-dimethylamino-5-sulfonamidonaphthalene (Dansyl) reporters were used. In almost all cases the polarity of the polymer microenvironment was lower than that of the solvent. At the same time, the dependence of the nature of the environment on the distance of the reporters from the polymer chain was studied. 

To determine the polarity of the microenvironment of polymers, polymer labelled with solvatochromic reporters were prepared, either by (1) copolymerization with solvatochromic monomers 2 >3 or (2) polymer analogous reactions (e.g., the reaction of copolymer acUve esters with primary amino groups of the solvatochromic molecule or alkylation reaction of PVIm and cross-linked polymers with solvatochromic mole-cule.34i37 The properties of the microenvironment of polymers were studied by the shift and shape of band in electronic spectra of a solvatochromic reporter molecule embedded in polymers. 

The detailed electronic structure was elucidated by spectroelectrochemistry. The absorption bands in electronic spectra of 33a and 33e are basically similar. The absorption of 33a " at longer wavelength = 993 nm), however, appears more clearly than that in 33e = 883 nm). The spectrum of 33a ... 

Infrared absorption bands of S (n>2) complexes are usually of low intensity, while the Raman spectra often show intense and characteristic hnes. However, the assignments are difficult because of their complexity. The absorptions in electronic spectra are often assigned to intrahgand transitions, and it is known that the increasing length of the polysulfido chain leads to an increasing red shift of the absorption band. 

It should be noted that the existence of different centers Is also found in covalently-linked dimer 11. Moreover, the analysis of all data obtained for dimers points clearly towards the efficient transfer of the excited singlet state energy from two centers of compound 1 to two acceptor centers of compound 2 in dimers (14,30). Increase In the porphyrin concentration by 300-700 times (say, for compounds 1 and 2) does not cause additional changes In electronic spectra as against diluted solutions. If the results obtained from temperature experiments (Fig. 2b) and measurements of fluorescence lifetimes In different bands are taken Into account, one may conclude that the additional centers observed In... 

Table 1 Maxima (in nm) of the most intense absorption bands in electronic absorption spectra of substituted singlet and triplet phenylnitrenes (near UV and visible).

In IR spectra, the term quadratic in J is small [Eq. (4.118)] because Be is the same for the upper and lower levels of the transition. In electronic transitions, the dimensions of the molecule change substantially, and B e and Bf are generally quite different. For example, note the difference in Re and Be for the two states of CO listed in Table 4.1. The effect of the quadratic term is thus quite significant, and we get band-head formation for reasonably low values of J in electronic spectra band heads are a highly characteristic feature of such spectra. In the majority of cases, the... 

The beautiful colors associated with porphyrin and chlorophyll systems are manifest in their characteristic electronic absorption spectra. The most intense band in the spectra, around 400 nm, is known as the Soret band it is intrinsic to the large macrocyclic conjugated pathway and has molar extinction coefficients usually between 150 000-400 000. This extinction value is lower in chlorins than in porphyrins, and the band is absent in porphyrinogens (6) and ring-opened bile pigments. 

CTTS transitions in coordination compounds result in a radial movement of electron density from the metal to the surrounding solution medium. The energies of these transitions generally are very sensitive to environmental parameters such as solvent polarity, temperature and the presence of salts.104 This sensitivity has been used in a diagnostic sense to identify CTTS bands in the spectra of anionic cyanide complexes105 and 1,2-dithiolene complexes of Ni, Pd and Pt.106 Hydrated cations such as Cr2+(aq) and Fe2+ (aqj exhibit absorption bands that are sometimes referred to as CTTS in character. Since the solvent occupies the first coordination sphere of the metal, however, the distinction between CTTS and CTTL transitions in these systems becomes obscured. 

Nitro groups of nitroalkanes can be identified by strong infrared bands at about 1550 cm-1 and 1375 cm-1, whereas the corresponding bands in the spectra of aromatic nitro compounds occur at slightly lower frequencies. A weak n —> n transition occurs in the electronic spectra of nitroalkanes at around 270 nm aromatic nitro compounds, such as nitrobenzene, have extended conjugation and absorb at longer wavelengths ( 330 nm). 

For a quantitative treatment of establishing connections between vibronic coupling and vibrational progressions in electronic spectra, band profiles from vibronic wavefunctions must be calculated using common procedures of time-dependent perturbation theory and Fermi s golden rule [57], For emission, e.g., the transition rate which is the transition probability per unit time summed over... 

Redox Photochemistry of MLCT The existence of intense absorption bands in the spectra of MZL complexes where the redox potentials of the M1 /M1 couples, for example, Cu(I), Fe(II), and L/L , are not too negative was associated by spectroscopists with MLCT electronic transitions.125 128 The MLCT electronic transition causes a radial displacement of charge from the metal to the ligand. There... 

Electronic Absorption Spectroscopy. The absorption spectra of some polyenes in the crystalline state have been studied. On adsorption of certain gases on the crystallite surfaces, a new band appears on the low-energy side of the long-wavelength band in the spectra of these polyenes, e.g. at 536, 537, and 375 nm for all- trans-ft-carotene, 15-cis-/3-carotene, and retinyl compounds, respectively.92 The surface pressure vs. area isotherms and absorption spectra of all-trans-, 9-cis-, and 13-cis-retinal, all-trans-retinol, and all- trans-retinyl acetate have been studied at air-water and air-solid interfaces, respectively. The spectra of monolayers of the isomeric retinals showed a red shift of 15 lnm compared with the solution spectra, whereas... 

The characteristic electronic absorption spectra of lignin and lignin-related compounds may be interpreted, in principle, on the basis of the UV absorption properties of polysubstituted benzene. Using the designations of Braude (1955), the bands in the spectra are grouped into three types B band, E, band and E2 band. The details of these bands are discussed in the section on absorption characteristics. 

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