PHOSPHOLES WITH REDUCED PYRAMIDAL CHARACTER FROM STERIC CROWDING .2. PHOTOELECTRON SPECTRAL EVIDENCE FOR SOME ELECTRON DELOCALIZATION IN ,4-DI-TERT-BUTYL-6-METHYLPHENYL)-3-METHYLPHOSPHOLE

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a lar ge, sterically demanding P-substituent. Earlier PE spectra had shown n o difference in ionization energies (IE) for simple phospholes and the ir tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of t he Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholan e showed that, as is known for nitrogen, planarization at phosphorus m arkedly reduced the ionization energy value (8.74 to 6.29 eV). A reduc tion in IE also occurred on planarizing 1-methylphosphole, but to a le sser extent, being offset by increased electron delocalization (8.93 t o 7.16 eV). This suggests that experimental comparison of IE for the u nsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determ ined for the crowded ,4-di-tert-butyl-6-methylphenyl)-3-methylphosphol e was 7.9 eV, the lowest ever recorded for a phosphole. The correspond ing phospholane had IE 7.55 eV. The difference in the values is attrib uted to electron delocalization in the phosphole. Calculations perform ed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)-phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6- 31G level; 57.6 degrees at the HF/6-31* level) with respect to the C- 2-P-C-5 plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respective ly).