Molecular and vibrational structure of 1,6,6a lambda(4)-trithiapentalene. Infrared linear dichroism spectroscopy and ab initio normal-mode analyses

The vibrational structure of the title compound (TTP) was studied by experimental and theoretical methods. IR absorption spectra were recorded in argon matrix and in stretched polyethylene at 12 K. The linear dichroism (LD) observed in the latter solvent provided experimental symmetry assignments of the observed vibrational states. Molecular geometries and harmonic force fields were calculated ab initio with the 6-311G** basis set using three different procedures: restricted Hartree-Fock theory (HF), second-order Moller-Plesset perturbation theory (MP2), and density functional theory (DFT). In the latter, Becke's gradient-corrected exchange functional was combined with Perdew and Wang's correlation functional (BPW91), leading to excellent agreement with observed IR transitions. The combined experimental and theoretical evidence enabled an essentially complete assignment of the fundamental vibrations. Of particular importance is the assignment for the first time of the long-sought ''bell-clapper'' mode associated with the unique S-S-S structural element of TTP, giving rise to an intense, long-axis polarized transition in the far-IR (153 cm(-1)).