Thiophene, fundamental vibrational modes

Fig. 7.36 The fundamental vibrational modes of thiophene involving significant hydrogen atom displacement. A plus ((0) sign represents a displacement above, and a minus (0) sign below, the plane of the ring arrows represent displacements in the plane of the ring ip, in-plane op, out-of-plane.

Thiophene has 2 symmetry and the 21 fundamental vibrations are distributed between four symmetry species as follows iA + iA2+lB + 3B2. All vibrations are active in the Raman spectrum, but those of A2 symmetry are inactive in the infrared spectrum. The INS spectrum of thiophene has been modelled by ab initio and force field calculations [141]. The calculated spectrum and the experimental spectra of pure solid thiophene and thiophene adsorbed by a M0/AI2O3 catalyst are shown in Fig. 7.35. The vibrational modes involving significant hydrogen atom displacements are shovm in Fig.7.36. 

The complete vapor phase assignment including IR vapor, Raman vapor, and liquid spectra, have been determined to obtain a complete set of vibrational frequencies in the vapor and liquid states of thiophene <94SA(A)765>. The results confirm the assignments made earlier by Rico et al. <65SA689> and, for several of the fundamental modes, the vapor frequency has now been measured or improved. 

The temperature dependence on the IR absorption and Raman scattering bandwidths of some fundamental modes of Aj, B, and B2 symmetries of thiophene has been investigated in the region 400-1600 cm <81CPH251>. The spectroscopic study was carried out in the liquid phase which requires simultaneous investigation of the bands. Results show that the IR and anisotropic Raman bandwidths increase with increasing temperature and more so in the latter case. The rotational diffusion coefficients obtained from the bandwidths indicate that the rotational and vibrational relaxation phenomena occur simultaneously. The IR and isotropic and anisotropic Raman band profiles of the Aj symmetry mode have also been studied in the liquid phase <81CPH265>. 

FT-IR and FT-Raman spectroscopies combined with DFT quantum chemical calculations have been used to probe the fundamental modes of vibrations of many thiophene-containing molecules. The majority of work in this field was performed by Casado, Herndndez, Ldpez Navarrete and co-workers. 

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