Arthur G. Anderson

Magnetic circular dichroism of cyclic π-electron systems: Part 30. Thialene and isothialene

Magnetic circular dichroism (MCD) spectra of thialene (1) and isothialene (2) were recorded and interpreted in terms of an aromatic (4N + 2) -electron [9] annulenide perimeter perturbed by a cross-link and a heteroatom. The B term sign sequence is the same for both heterocycles, B (L1) 0, suggesting that the frontier orbital energy differences are such that ΔHOMO ΔLUMO for 2. Isothialene thus represents a very rare example of an aromatic π system whose MCD signs for the L bands are predicted incorrectly at the usual PMO level. The correct answer, ΔHOMO < ΔLUMO, results when the PMO approach is carried beyond first order.