The Study of HOMO Energy for Showing the Basicity and the nu(C=O) Stretching Bands Obtained by the Semi-Empirical and ab initio Methods in Phosphorus Ylides

Relationships between the HOMO energy, the nu(C=O) stretching band, basicity(pKa), and sigma(p) in phosphorus ylides have been studied. A linear relationship between the HOMO energies obtained by the AM1, HF/3-21G, and HF/6-31G methods and pKa was found. The same linear relationship was found between the HOMO energies obtained by these methods and sigma(p)(indicated p-position). Thus, the HOMO energy can be adopted instead of pKa and sigma(p) for choosing the ylide as the donor in organic reactions. In addition, a linear relationship between the nu(C=O) stretching bands obtained by these methods and the parameters described above was found. Hence, as for the electron donation of the ylide, it is noted that the HOMO energy can be used instead of pKa as a new method for showing the basicity of ylides. Furthermore, the nu(C=O) stretching bands obtained by these methods can be estimated by the HOMO energy obtained by the same methods.