MOLECULAR AND VIBRATIONAL STRUCTURE OF 1,6,6A-LAMBDA(4)-TRITHIAPENTALENE - INFRARED LINEAR DICHROISM SPECTROSCOPY AND AB-INITIO NORMAL-MODEANALYSES

The vibrational structure of the title compound (TTP) was studied by e xperimental and theoretical methods. IR absorption spectra were record ed in argon matrix and in stretched polyethylene at 12 K. The linear d ichroism (LD) observed in the latter solvent provided experimental sym metry assignments of the observed vibrational states. Molecular geomet ries and harmonic force fields were calculated ab initio with the 6-31 1G* basis set using three different procedures: restricted Hartree-Fo ck 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 corr elation functional (BPW91), leading to excellent agreement with observ ed IR transitions. The combined experimental and theoretical evidence enabled an essentially complete assignment of the fundamental vibratio ns. Of particular importance is the assignment for the first time of t he long-sought ''bell-clapper'' mode associated with the unique S-S-S structural element of TTP, giving rise to an intense, long-axis polari zed transition in the far-IR (153 cm(-1)).