On the molecular and vibrational structure of 1,6,6aλ 4-trithiapentalenes. Analysis of the “ bell-clapper” asymmetrical S–S–S stretching mode

The molecular and vibrational structure of the 1,6,6aλ4-trithiapentalene (TTP) ring system was studied by experimental and theoretical procedures. IR absorption spectra were recorded of 2,5-dimethyl-1,6,6aλ4-trithiapentalene (DMTTP) in liquid solution, in a stretched polyethylene matrix, and in solid state tablet samples. The linear dichroism observed in the stretched polymer sample provided experimental symmetry assignments of the observed vibrational states. The results of B3LYP and B3PW91 density functional theoretical calculations were in good agreement with the observed molecular geometries and vibrational transitions for TTP and DMTTP. The computed molecular structures were characterized by sulfonium ylide-like Mulliken charge distributions (positively charged, three-coordinated sulfur center in position 6a, negatively charged carbons in positions 2, 3a and 5), consistent with the large dipole moments reported for these species. Of particular interest was the strong vibrational transition observed around 187 cm−1 in the far-IR spectrum of DMTTP, similar to the transition previously observed at 153 cm−1 for TTP. These transitions must be assigned to the asymmetrical S–S–S stretching vibration, the so-called “bell-clapper” mode. According to B3LYP and B3PW91 calculations the potential is U-shaped, corresponding to a negative anharmonicity constant xe in the order of − 0.025. Anharmonic effects are predicted to increase the frequency of the fundamental transition by about 5%. Hartree–Fock (HF) theory predicts a double-minimum potential for this mode, while post-HF Møller–Plesset second-order perturbation theory (MP2) predicts a single-minimum potential with a complicated shape and a positive anharmonicity.