STRUCTURES AND MOLECULAR-PROPERTIES OF CHARGE-PERTURBED MOLECULES .53. THIANTHRENE RADICAL-CATION TETRACHLOROALUMINATE

One-electron oxidation of thianthrene, which exhibits a reversible half-wave potential at + 1.3 V and an irreversible one at + 1.65 V, under aprotic conditions with AlCl3/H2CCl2 yields the violet radical cation salt [H4C6S2C6H4+][AlCl4-]. Its single-crystal structure analysis at 90 K reveals an interplanar angle between the phenyl rings of 174-degrees and demonstrates that the neutral sulfur heterocyclic molecule, bent by 128-degrees, is considerably flattened upon one-electron removal. Semiempirical enthalpy of formation hypersurfaces allow to rationalize the experimental results by a double-minimum potential for the bent neutral molecule with a rather low inversion barrier and a single minimum one for a planar radical cation. Other radical cation properties such as the spin distribution deduced from its ESR spectrum or the visible absorption maxima near nuBAR = 11000 and at 18250 cm-1 corresponding to pi --> pi* and pi --> sigma* transitions are also satisfactory reproduced by PM3 calculations partly including configuration interaction based on the experimental structure.