Gas phase ESCA studies of trithiapentalene and its 2,5-dimethyl derivative

1,6,6a-trithiapentalene and its 2,5-dimethyl derivative have been studied in the gas phase by means of ESCA in order to elucidate the unusual type of S---S bonding in these molecules. The high resolution spectra show very wide S 2p lines from the terminal sulfur atoms due to vibrational broadening. The effect of core ionization on the S---S bond lengths in trithiapentalene has been estimated using the equivalent core approximation and the CNDO/2 method. Ground and final state potential curves have been calculated and explain qualitatively the differences in line widths for the terminal and central sulfur atoms. The observed chemical shifts agree with predictions from CNDO/2 and ab initio calculations.