A VALENCE-SHELL AND INNER-SHELL ELECTRONIC AND PHOTOELECTRON SPECTROSCOPIC STUDY OF THE FRONTIER ORBITALS OF 2,1,3-BENZOTHIADIAZOLE, C6H4SN2, 1,3,2,4-BENZODITHIADIAZINE, C6H4S2N2, AND 1,3,5,2,4-BENZOTRITHIADIAZEPINE, C6H4S3N2

HeI photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, C1s and N1s edges, and S1s synchrotron radiation photoabsorption spectroscopy have been used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole, 1,3,2,4-benzodithiadiazine, and 1,3,5,2,4-benzotrithiadiazepine. The term values obtained from the S1s, S2s and N1s oscillator strength spectra are sensitive to the aromatic/anti-aromatic character in the thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively constant. The combination of valence photoelectron and core excitation results, aided by semiempirical MNDO) molecular orbital calculations, provides a useful probe of the frontier molecular orbitals of these aromatic/anti-aromatic molecules.