Benzene vibrational assignments

Detailed vibrational assignments have been carried out on arene chromium and arene molybdenum tricarbonyl complexes (18, 19). Splitting of the E band in the carbonyl region was observed for substituted benzene chromium tricarbonyl complexes but not in (CgHg)Cr(CO)3 itself, showing that the concept of local symmetry (Cg ) is of very restricted validity when discussing the C—O stretching vibrations in such complexes (18). 

Notation. The exceedingly convenient scheme introduced by CDM is used in this and later discussions of benzene spectra. Assignments of vibrational structure in the spectra are given by notation such as 6jl0 li. This indicates that three vibrational modes Vg, v q, and rj are involved in the transition with d = 0 in both electronic states for all others. The superscript indicates the quanta of each active mode in the upper state (v e = 1, v lo = 0, and = 1) and subscripts are for the lower electronic... 

Figure 4.6 also shows the first combination spectral region of benzene and pyridine near 4500-4700 cm. The distinctive set of four large peaks and several smaller ones are considerably shifted for pyridine, relative to benzene. The assignments for these peaks are given in Table 4.1. All are combinations involving a C-H stretch and another vibration. 

Fluorine Compounds.—In the last ten years measurements have been made on a number of fluorine-containing benzene derivatives. 4-Fluorotoluene was studied by Scott et al. and a vibrational assignment consistent with the calorimetric results was obtained 1,2-difluorobenzene was similarly studied. Benzene derivatives richer in fluorine were studied at the National Physical Laboratory thus, vapour heat capacities and enthalpies of vaporization were measured for hexafluorobenzene, pentafluorobenzene, pentafluorochlorobenzene, and 2,3,4,5,6-pentafluorotoluene. ... 

For phenylthiazoles the assignments are complicated by the presence of the benzene ring whose vibrations fall in the same region as those of the thiazole ring. The resulting spectra are analogous to those described above [126]. 

G. Varsanyi, Assignments for vibrational spectra of seven hundred benzene derivatives L. Lang, Ed., Adam Hilger London, 1974, Vol. 1. p. 1. 

Angus WR, Bailey CR, Hale JB, Ingold CK, Leckie AH, Raisin CG, Thompson JW, Wilson CL (1936) Structure of benzene. Part VIII. Assignment of vibration frequencies of benzene and hexadeuterobenzene. J Chem Soc (London) 971-987... 

The basis for the further assignment of the bands to specific vibrations is the assignment of the benzene bands given by Whiffen (1955) on the basis of an exact molecular model calculation. 

Accordingly the ratio vh/ d i criterion for the nature of the vibrations. In the attempted interpretation given in Table 4, the procedure was first to look for the various possible assignments on the basis of the expected spectral regions, and then to select from these those assignments in which the intensity of the bands of benzene and of the d -... 

The first study belongs to Brown and Rogers", who measured the IR spectra of methyl-and ethyllithium as Nujol mulls, aud of ethyllithium in benzene solution, and assigned the bands observed at 880 and 925 cm to the C—Li stretching vibrational modes of the monomeric and polymeric species, respectively. 

Treatment of 5-ethoxy-l,2,3,4-thiatriazolium tetrafluoroborates (161) with aromatic amines in boiling benzene affords yellow crystalline 5-arylamino-l,2,3,4-thiatriazolium salts (166) in good yields (60-70%) (Scheme 35). These are deprotonated with warm ethanolic aqueous sodium hydroxide to give the red crystalline mesoionic l,2,3,4-thiatriazolium-5-aminides (20). The mesoionic compounds show a strong IR absorption in the region 1580-1590 cm which can be assigned to the exocyclic (7=N stretching vibration. UV and visible spectra are reported e.g. (20a) 279 (e... 

The three aromatic amino acids (Phe, Tyr, Trp) have side-chain groups corresponding to the benzene, phenol, and indole chromophores, respectively. The spectroscopic properties of the rat transitions in these chromophores have been reviewed.136-381 Coupling of aromatic transitions among themselves and with peptide transitions can give rise to CD bands in the near and far (k < 250 nm) UV. Near-UV CD bands are useful indicators of the environment of the aromatic chromophores and can frequently be assigned to specific types of side chain, based upon band position, presence of vibrational fine structure, etc. Far-UV CD bands due to aromatic side chains, except for the La band of Tyr (-230 nm) and the Bb band of Trp (-225 nm), are generally difficult to resolve from peptide CD bands and can complicate the conformational analysis of peptides. 

Varsanyi, G., Assignments for Vibrational Spectra of 700 Benzene Derivatives, Hilger, London (1974). 

---