Effect of substituents on ionization potentials of phosphorus compounds. Conjugation in radical cations

The first vertical ionization potentials (I) of phosphorus compounds P(X-i) (3), OP(X-i)(3), SP(X-i)(3), (4-XC6H4)(3)P, and P=CX are related to the ind uctive, resonance, and polarizability parameters of inorganic, organic, and organometallic substituents X by dependences of the type I = I-H + a Sigma sigma (1) + b Sigma sigma (+)(R) + c Sigma sigma (alpha), where I-H is the I value for X = H. The I values are also affected by hyperconjugation. The ratio of the contributions of the resonance (b Sigma sigma (+)(R)) and pol arizability (c Sigma sigma (alpha)) effects to the I value is determined by the degree of delocalization of the unpaired electron and the positive cha rge in the radical cations formed upon ionization of neutral molecules. The sigma (+)(R) resonance parameters of organosilicon, organogermanium, and o rganotin substituents bound to the P.+ radical cation center were calculate d for the first time.