Infrared inactive bond

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 cm". By IR spectroscopy, Kiibler et al. located the symmetric C-N stretching vibration at 1223 cm" in E- and at 866 cm in Z-azobenzene. The N=N vibration in Z-azobenzene is at 1511 cm" (in KBr pellets). These numbers are confirmed by newer work Biswas and Umapathy report 1439 and 1142 cm for the N=N and C-N vibrations (in CCE), and Fujino and Tahara found nearly identical results (1440 cm" and 1142 cm ). A thorough vibrational analysis of the E-isomer is given by Amstrong et al. The vibrations in the (n,7t ) excited state are very similar 1428 cm" and 1130 cm"h... 

The third class of compounds that are not prone to increase their coordination numbers are the hexaalkyldistannanes, R3SnSnR3, and the related oligostannanes. The Sn-Sn stretch is infrared inactive, but Raman active, and Me3SnSnMe3 shows v(SnSn) 192 cm-1. If the phenyl groups in hexaphenylditin are alkylated in the ortho positions, steric hindrance weakens the Sn-Sn bond, and the vibration frequency and force constant are reduced (see Table 18-2). 

Dimethyl-2-butene, in contrast, is a symmetrical molecule, so its C = C bond has no dipole moment. When the bond stretches, it still has no dipole moment. Since stretching is not accompanied by a change in dipole moment, no absorption band is observed. The vibration is infrared inactive. 2,3-Dimethyl-2-heptene experiences a very small change in dipole moment when its C = C bond stretches, so only an extremely weak absorption band (if any) will be detected for the stretching vibration of the bond. 

Molecules like 02H take the nonplanar Q structure (twisted about the 0-0 bond by ca. 90"), whereas N2F2 and [N203] exist in two forms /mm-planar (C2/,) and cis-planar (C21.). Figure 11-7 shows the six normal modes of vibration for the C2, and C2 structures. The selection rules for these two structures are different only in the vibration, which is infrared inactive and Raman depolarized in the planar model but infrared active and Raman polarized in the nonplanar model. 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 By IR spectroscopy, Kiibler et al. located the symmetric... 

Raman spectroscopy yields analogous information, but is complementary to infrared absorption, in that vibrations which are infrared inactive are generally Raman active, and vice versa. For example, carbon-sulfur bonds are easily detectable via Raman measurements [5,10]. Since the detected hght is scattered from the sample, spectra are readily obtained on crosslinked specimens. Interferences due to fluorescence are avoided by using a longer wavelength source [37]. Raman microscopy has potential for spatially resolved analyses [38]. 

For a molecule to show infrared absorptions it must possess a specific feature, i.e. an electric dipole moment of the molecule must change during the vibration. This is the selection rule for infrared spectroscopy. Figure 1.4 illustrates an example of an infrared-active molecule, a heteronuclear diatomic molecule. The dipole moment of such a molecule changes as the bond expands and contracts. By comparison, an example of an infrared-inactive molecule is a homonuclear diatomic molecule because its dipole moment remains zero no matter how long the bond. 

Characteristic group frequencies of alkenes are shown in Figure 11. In alkenes, the =CH stretching vibration generally occurs above 3000 cm". In symmetrical irans or symmetrical tetrasubstituted double-bond compounds, the C=C stretching frequency near 1640cm, usually a medium intensity band, is infrared inactive because, in this... 

In conjugated polyenes containing conjugated trans- and cw-substituted double bonds, mechanical interaction can affect the 965 cm" region. The in-phase trans-CH wag vibration and the infrared inactive out-of-phase cis CH wag vibration have about the same frequency, 965 cm. This means that both kinds of groups will be excited at the same time, but with various phase relationships. The expected frequencies and relative intensities of a few examples are listed in Table 7.4 where, for example, the last entry indicates that a cis-trans-cis conjugated triene has bands within 10 cm" of 994 and 936 cm", each about one-third as intense as an isolated trans group band at 965 cm". ... 

Steady decrease in intensity as the bonding between metal and ring ligand becomes more and more ionic. Thus in the saltlike alkali metal cyclo-pentadienyls, it disappears completely (57). However, this behavior can be explained by assuming that the deviation from Ds symmetry of the anion to Cs of the covalently bonded ring becomes smaller and smaller when going, for example, from osmocene, CpaOs, to CpK. With Dj symmetry, this n.v. is infrared-inactive and therefore vanishes. 

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