Vibrational spectra substituted benzenes

The spectrum of strychnine shown in Fig. 4.1-19C exhibits bands representing CH bonds at saturated and unsaturated C atoms, an amide 1 vibration, and an ortho di substituted benzene ring. The large number of sharp bands and the extensive coincidences between all IR and Raman spectra shown in Fig. 4.1-19 point to multiatomic, only moderately symmetric rigid molecules. 

The Raman spectrum of PS shows a couple of distinctive doublets at 1603 and 1584 cm and at 1033 and 1002 cm , both ring-mode vibrations, and both characteristic of a monosubstituted aromatic compound. The Raman band at 622 cm is also indicative of the substituted benzene ring. In addition, there are a few IR absorption peaks due to functional groups such as terminal vinyl group at 907 and 980 cm (refer to 910 and 990 cm peaks of PE) and the irons C=C double bond at 967 cm" resulting from termination reactions. Note also that these bands can also be assigned to trace amounts of butadiene that are sometimes added to polystyrene when it is used for making thin films. This addition adds some flexibility and reduces the brittle nature of the polystyrene film. Atactic polystyrene has bands related to its atactic nature at 1370, 1328, 1306, 1070, and 943 cm and the isotactic form exhibits bands at [62] 1364, 1314, 1297 cm and a doublet at 1075 and 1056 cm (see Fig. 32). As isotactic polystyrene crystallizes, the doublet is seen to shift to 1080 and 1048 cm, and a new band appears at 985 cm The crystallinity may be determined from the intensity of the 985 cm band, and the ratio 566/ 543 is used to evaluate the content of isotactic sequence. For refer-... 

In para-substituted benzenes with identical substituents the 1175 cm vibration (2,5 vs. 3,6) in centro-symmetric and is infrared inactive but is Raman active, " whereas the 1013 and 1117 cm centro-antisymmetric vibrations are Raman inactive. For the other isomers in Table 8.7 the vibrations are active (but not necessarily strong) in the Raman " as well as the infrared spectrum. Two especially prominent Raman bands are the 1027 cm" band for mono and the 1033 cm band for ortho which show up at 1018-1030 cm" and 1020-1060 cm" respectively. " As was mentioned before, mono-, meta-y and 1,3,5-trisubstituted benzenes have a very strong Raman band at 990-1010 cm (see Fig. 8.11). 

As it is now suspected that a substituted benzene ring is likely to be present, attention is turned to the region 900-650 cm where the out-ofplane C—H bends and the ring puckering vibration occur. There are two remaining intense bands which appear in the spectrum in this region near 770 and 695 cm . These bands are characteristic of mono or meta substitution of the ring. 

The corresponding difference IR-LD spectrum, depicted in Figure 2.14, is characterized by the positive IR band at 2229 cm, belonging to the stretching vibration of the C=N group (Vcn)- In parallel vibrational modes, the bands at 1605 cm and 1502 cm" (Figure 2.14) are also positive. These bands correspond to in-plane 8a and 19a modes of the substituted benzene ring in the molecule of the NLC and possess A, symmetry class (in the terms of the local Cj, symmetry). The... 

Carbon-hydrogen stretching vibrations with frequencies above 3000 cm are also found m arenes such as tert butylbenzene as shown m Figure 13 33 This spectrum also contains two intense bands at 760 and 700 cm which are characteristic of monosub stituted benzene rings Other substitution patterns some of which are listed m Table 13 4 give different combinations of peaks... 

Pyridine. Pyridine and its methyl substituted derivatives (picolines and lutidines) were found to polymerize electrochemically and, under certain circumstances, catalytically. This behavior was not expected because usually pyridine undergoes electrophilic substitution and addition slowly, behaving like a deactivated benzene ring. The interaction of pyridine with a Ni(100) surface did not indicate any catalytic polymerization. Adsorption of pyridine below 200 K resulted in pyridine adsorbing with the ring parallel to the surface. The infrared spectrum of pyridine adsorbed at 200 K showed no evidence of either ring vibrations or CH stretches (Figure 5). Desorption of molecular pyridine occurred at 250 K, and above 300 K pyridine underwent a... 

Silabenzene 24 reveals a characteristic Si—H stretching vibration at 2217 cm-1, as expected for hydrogen attached to a sp2-hybridized silicon atom. Compound 24 shows a typical benzene-type UV spectrum with absorptions at X = 217, 272 and 320 nm, which fit into the series of the already known donor-substituted heterobenzenes30. An additional structural proof was the partially reversible photochemical conversion of 24 into Dewar... 

Substitution of benzene with polar groups containing unshared electrons (auxochromes like OH or NH2). shifts the absorption bands to longer wavelengths and also intensifies them. The spectra of phenol and aniline in heptane solution are reproduced ifl Figwes 5.1 and 5.2. The vibrational structure of the 260 m/x band is not seen in the aniline spectrum. In general, the fine structure of the 260 m/x band disappears in polar solvents when the... 

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