Resonance stabilization and gas phase protonation in phosphorus ylides: probing through MO and DFT computations

Protonation energy (PE) calculations at the MP2/6-31G* level of theory on a set of resonance stabilized model ylides reveal that, resonance effect make these ylides multi-site basic. In some cases the ylidic carbon is not the most favored site of protonation and points out to the stability of P+-C− bond. B3LYP/6-31G* calculations yield closely related PE values, indicating its validity. The protonation energy of these ylides are intermediate to those of proton sponges and super bases. PM3 calculations were used to calculate proton affinity (PA) values for a set of the most common triphenylphosphine family of ylides. The PA values of the model ylides at PM3 level bear close resemblances to those calculated at the MP2/6-31G* and B3LYP/6-31G* levels of theory, indicating the inconsiderable contribution of zero point vibration energy (ZPVE) to the PA of these ylides. The proton affinity of the metallated ylide, Ph3PCHLi is computed to be 287.6 kcal/mol and becomes the metal-ylide upon protonation. Introduction of nine NMe2 groups on the three P-phenyl groups of Ph3PCH2 increases the PA to 301.6 kcal/mol.