Thiophene, fundamental vibrational

The 21 fundamental vibrations of thiophene are composed of eight vibrations of Aj symmetry, seven of Bj symmetry, and three each of and B2. The assignments of all 21 vibrations are listed in Table 53. 

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. 

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.

Palmer24 discussed the electronic structure of 1,2-benzisothiazole in the context of the other aza derivatives of indole, benzofuran, and benzo-thiophene. The infrared and Raman spectra of 1,2- and 2,1-benzisothiazoles have been recorded, and a complete assignment of the fundamental vibrations proposed.25 The rates of N-methylation and the NMR chemical shifts of the resulting JV-methyl salts of isothiazoles, pyrazoles, isoxazoles, and their benzofused derivatives have been examined.26 The protonation equilibria of 1,2-benzisothiazoles have been measured.27... 

The 21 fundamental vibrations of thiophene are composed of eight vibrations of A, symmetry, seven of B, symmetry, and three each of A2 and B2. The assignments of all 21 vibrations have been documented by Rico et al. as early as 1965 <65SA689> and are listed in Table 8. Based on these frequencies, harmonic force constants, and mean amplitudes of vibration have been developed... 

Quartic force fields for thiophene have been generated using DFT to evaluate vibrational levels by second-order perturbation theory (PT) and also by the variational method. The results for the fundamental frequencies are in very good agreement with observation <2003SAA1881>. 

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. 

Finally, we will concentrate on the chemical reactivity of silyl derivatives of thiophene. The oxidative polymerization of various silyl monomers lead to polythiophene. The evaluation of this new polymerization reaction implies a precise characterization of the produced conjugated materials. Knowledge and the control of the pertinent parameters which direct the properties of the conjugated systems are essential. Also required is the development of methods which allow a precise characterization of the samples. The role of vibrational infrared and Raman spectroscopy is of fundamental importance in this field. Optical spectroscopy is one of the few tools for unravelling the structure of these materials and understanding their properties. First, new criteria based on infrared, Raman and photoluminescence spectroscopy which allow precise estimates of the conjugation properties will be reported. Then the synthesis and characterization of polythiophene samples arising from the oxidative polymerization of silyl thiophene will be presented. 

Ultraviolet and Infrared Spectra.—The effect of conjugation in the excited state of some silicon-substituted thiophens has been discussed. The u.v. and i.r. characteristic absorption spectra of some pairs of cis- and rra/75-isomers of 2 - and 3 -substituted 2-styrylthiophens have been determined. It is claimed that the spectra of the stereoisomers are sufficiently different to assign the configuration. The i.r. and Raman spectra of 4-chloro-, 4-bromo-, 4-iodo-, and 4-cyano-2-fluoro-thiophens have been studied in the region 500—540 cm . The fundamental frequencies have been assigned, and normal co-ordinate calculations have been performed. The force constants were adjusted to reproduce accurately the fundamental frequencies. Thermodynamic functions and mean amplitudes of vibration for these molecules were also calculated. Thiophen-2-aldehyde and 19 related aldehydes have been studied under high resolution in the i.r. C=0 region. 

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