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Vibrational Spectra of Triiodomesitylene: Combination of DFT Calculations and Experimental Studies. Effects of the Environment

“…The fact that the three different Me groups have different tunneling gaps demonstrates that differences in local environment in the triclinic cell induce not only differences in the rotation hindering potentials but also in the Me conformations (Figure 2). By using a single crystal on the backscattering spectrometer IRIS at RAL, it was possible to assign each excitation to a specific methyl group within the crystal, 32 as already done for TBM 16 and TCM. 30 Figure 4 shows two spectra obtained with a single crystal rotating around the crystallographic axis a* which is perpendicular to the100 plane (bc plane in Figure 2).…”

“…Our group is particularly interested in the study of substituted benzenes, [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] especially when the substitution is the same on both sides of a methyl group (Me): in this case, the Me is a weakly hindered rotor with a large tunnel splitting and low energy torsional modes. [14][15][16][17][21][22][23] In the case of molecules with methyl groups surrounded by heavy atom substituents (Cl, Br, or I), a specific difficulty appears in the assignment of the Me torsional frequencies because they appear in the same frequency range as the lattice vibrations and some internal modes.…”

“…Our group is particularly interested in the study of substituted benzenes, − especially when the substitution is the same on both sides of a methyl group (Me): in this case, the Me is a weakly hindered rotor with a large tunnel splitting and low energy torsional modes. − , − In the case of molecules with methyl groups surrounded by heavy atom substituents (Cl, Br, or I), a specific difficulty appears in the assignment of the Me torsional frequencies because they appear in the same frequency range as the lattice vibrations and some internal modes. ,,, Furthermore, for weakly hindered Me rotors, the Me protons are largely spread around the carbon atom; ,,, in consequence, the validity and accuracy of the theoretical calculations which are based on the harmonic approximation are subject to caution for all the motions implying Me groups. Second, in the solid state, Me conformation changes may happen because of the crystal field, and this will modify the Me torsion spectrum.…”

“…In consequence, the frequencies of the torsion modes would be indicative of the Me conformation and simultaneously of its hindering potential. Furthermore, because the torsion modes occur at low frequencies (below 200 cm −1 ), their coupling with other vibrations must also be investigated. − …”

“…Furthermore, because the torsion modes occur at low frequencies (below 200 cm -1 ), their coupling with other vibrations must also be investigated. [22][23][24][25] This work is a part of a systematic study of the 1,3,5trihalogeno-2,4,6-trimethyl-benzenes (trihalogeno-mesitylenes or single crystal neutron diffraction at 15 K, very accurate structures for hydrogeneous tribromo-(TBM) 18 and triiodomesitylenes (TIM) 19 have been obtained. Because of a phase change at 146 K, trichloro-mesitylene (TCM) 26 has been studied as a deuterated powder.…”