1. R. H. Schlessinger Proc. Int. Symp. 158 1970
2. L. Field D. L. Tuleen “Seven-membered heterocyclic compounds containing oxygen and sulfur” A. Rosowsky Wiley-Interscience X 573
3. V. J. Traynelis 667
4. Ground states of conjugated molecules. XX. SCF MO treatment of compounds containing bivalent sulfur