Double absorption band

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Figure 9.33 shows examples of SVLF spectra obtained by tuning a frequency-doubled dye laser to the Og absorption band of the system of pyrazine (1,4-diazabenzene)... 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

It is likely that initially the open-chain adducts 353 and 354 are formed by the addition of an amino group either to the carbonyl function or to the triple bond, whereupon these intermediates close up to the azepines 355 and their bis-imine tautomers 356. In the H NMR spectra, the methylene protons of 356 are at 2.85-2.97 ppm, whereas the methyl protons are fixed at 2.20-2.27 ppm. The IR spectra show absorption bands corresponding to the aromatic ring (1600 cm ) and to the diazepine cycle C=N double bonds (1580 cm ). However, there are no bands of... 

Alongside this, IR spectra of M3NbOF6 type compounds display a strong absorption band at 920 cm"1 indicating the presence of a double niobium-oxygen bond, Nb=0 [57, 115, 155, 156]. Kaidalova [156] explained this contradiction as a phenomenon that is related to the hampered rotation of the NbOF63 polyhedron in the equatorial plane. 

Four IR absorption bands have been identified in the spectrum of the hydroxysulfonyl radical (HOSO 2) which has been obtained by the reaction of hydroxyl radicals with sulfur dioxide in argon matrix at 11 K16. The observed bands at 3539.9 and 759.5 cm 1 have been assigned to O—H and S—OH stretching modes while the bands at 1309.2 and 1097.3 cm-1 have been assigned to the asymmetric and symmetric stretching modes of the double bonded S02 moiety. These data are consistent with the theoretical prediction on the geometry of the hydroxysulfonyl radical12. 

Liquid crystalline main chain polymers with siloxane spacer groups were obtained by the hydrosilation of (Si—H) terminated polydimethylsiloxane oligomers and mesogenic groups with terminal double bonds as shown in Reaction Scheme XVII-(a). Reactions were usually carried out in THF with the Wacker Oil catalyst 255). Completion of the reactions was followed by the disappearance of the strong (Si—H) absorption band at 2140 cm-1 using IR spectroscopy. 

Microstmcmral changes on irradiation have been observed by IR and UV spectroscopy. Changes in absorption bands due to vinyhdene double bonds [356,357], substituted double bonds, and ethyl and methyl groups give a measure of modifications in the presence of radiation. The ratio of the double bonds (located mainly at the end of a polymer chain) and scission is reported by some investigators [356-358] and found to be independent of temperature and dose. This is beheved to be due to the reaction of the methyl radical side group with hydrogen atoms on the backbone of the parent chain. 

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

Infrared spectra of unsaturated organotin epoxides show absorption bands at 1235 and 850 cm-1, which are typical of epoxy groups, 3085 and 1640 cm-1 (double bonds) and in the range 510-585 cm-1 (Sn—C bonds). Symmetrical deformation vibrations of the CH3 or CH2 group that is closest to the tin atom are related to changes of H—C—H angles and appear in the 1190-1175 cm-1 range 31). 

Because the extension of the polaron in polyene radical cations is finite (10-20 double bonds depending on the type of calculation), its electronic structure is independent of the number of double bonds attached to either side of it, so that the two lines in Figure 29 must bend at some point to meet the abscissa horizontally, as indicated by the dashed curves. Apparently, the point of inflection has not been reached for n = 15, but it is of interest that the curve for the first excited state could well extrapolate to 0.35 eV, which happens to be where the absorption of a polaron in polyacetylene has been observed300. If this is true, a second, more intense absorption band should occur between 0.5 and 0.7 eV, but the corresponding experiments have not yet been carried out. 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

Substituted ethylenes, with hydrogen atoms replaced by various alkyl groups, have a common feature in their electronic spectra, i.e. an absorption band at ca. 164 — 180 nm. This band is interpreted to show that the delocalization of the electron pair is largely confined to the vicinity of the unsaturated centre, commonly referred to as the C=C double bond. If the de-localization is assumed not to exceed a linear distance of one bond length on either side of the double bond, the electron pair remains in a linear potential box of width 3d, with allowed energy levels of... 

Optical Properties. The double-decker complexes of porphyrazines have characteristic electronic absorption spectra (Table V). The intense Soret bands of the double-decker complexes are blue shifted with respect to the single pz ligand as a consequence of the strong n-n interactions. Another characteristic of sandwich compounds is the additional appearance of absorption bands shifted to the red (termed Q ) and to the blue (termed Q") of the normal g-band region. These new transitions are thought to result from orbitals delocalized over the two macrocyclic ligands (33, 82). 

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