Bond Characterization of a Unique Thiathiophthene Derivative: Combined Charge Density Study and X‑ray Absorption Spectroscopy

Thiathiophthene (TTP),
a planar molecule with two fused heterocyclic
five-membered rings and an essentially linear S–S–S
bond, is a molecule of great interest due to its unique chemical bondings.
To elucidate the remarkable bonding nature, a combined experimental
and theoretical study on the electron density distribution of 2,5-dimethyl-3,4-trimethylene-6a-TTP
(1) is investigated based on a multipole model through
high-resolution X-ray diffraction data experimentally and on the density
functional calculations (DFT) theoretically. In addition, S K-edge
X-ray absorption spectroscopy (XAS) is measured to verify the chemical
bonding concerning the sulfur atoms. The molecule can be firmly described
as 10π electron with aromatic character among the eight atoms,
S3C5, of the two fused five-membered rings plus
three-center four-electron σ character along the S–S–S
bond. Such bonding description is verified with the calculated XAS
spectrum, where the pre-edge absorption for transitions from S 1s
to π* and σ* are located. The three-center four-electron
S–S–S σ bond makes the terminal S atoms richer
in electron density than the central one.