Study of the planarization of the tricoordinate phosphorus in phospholes; photoelectron spectra and structure of partially planarized phospholes

The gradual flattening of the tricoordinate phosphorus in phosphole by the increasing steric bulk of the substituent group is shown on the HF/6-31G* optimized geometries of alkylarylphospholes. By the decreasing pyramidality, aromaticity indices show increase, as a result of the increased conjugation. The aromaticity of 1-(2,4,6-tri-tertiarybutyl)-phosphole is similar to that of furan according to the geometrical indices. In the photoelectron spectra of the alkylaryl substituted phospholes the phosphorus lone pair ionization energy also decreases along the decreasing pyramidality. 1-(2,4,6-tri-Tertiarybutyl-phenyl)-3-methylphosphole has the lowest ionization energy value ever reported for a phosphole. (C) 1998 Elsevier Science S.A. All rights reserved.