ORGANOPHOSPHORUS COMPOUNDS .93. AROMATICITY OF THIAPHOSPHOLES AND SELENAPHOSPHOLES - A PHOTOELECTRON SPECTROSCOPIC AND QUANTUM-CHEMICAL STUDY

The Hel photoelectron spectra of 2-tert-butyl-5,6,7,8-tetrahydro-4H-cyclohepta[d]-1,3-thiaphosphole, 2-tert-butyl-4,5,6,7-tetrahydrocyclohexa[d]-1,3-selenaphosphole and 3,5-di-tert-butyl-1,2,4-selenadiphosphole were recorded and interpreted by using Koopmans' theorem and correlations with the respective spectral data of similar compounds. The positions of the pi-ionization energies have been found to be similar to those of the corresponding carbon-containing compounds. The MP2/6-31G* optimized geometries of the investigated unsubstituted parent compounds are planar and exhibit significant bond length equalization, thus indicating that the systems are aromatic. Since the bonding angles at the ring carbon atoms are nearest to 120 degrees for the systems containing two heavy atoms (i.e. the thia- and selena-phospholes), the ring strain should be the smallest in these compounds. The stabilization of the ring compounds may also be concluded from appropriate isodesmic reactions. Although, at the correlated level used in the present work, the stabilization is considerably larger for all the compounds investigated than at the HF level of the theory, the corresponding compounds containing P-C and C=C bonds exhibit similar stabilization, reflecting the similarities of the P=C and C=C bonds. The slightly higher stabilization of the P=C bond-containing compounds at the correlated levels is likely to be attributed to the overall stability of the ring system and not to the different conjugation of the P=C and C=C units.