Structure and UV spectra of aliphatic-aromatic phosphines

The basicities of 10 tertiary phosphines were determined. Equations were derived to relate pK_a(H₂O) for these phosphines to the sum of the Kabachnik _ph-constants and pK_a(H₂O) for aryldiethylphosphines to the Hammett _n-constants. Analysis of these equations leads to the conclusion that in the ground state the unshared pair of electrons of the trivalent phosphorus is not conjugated with the -electron system of the aryl group.The bands in the UV spectra of the aryldiethylphosphines were assigned on the basis of this fact. The most intense absorption in the spectra of the phosphines (K-band) is due to the 'A_1g 'B_1u transition of the -electrons. The B-band, which is produced by the 'A_1g 'B_2u transition, only appears in the spectra of phosphine oxides; in the case of the phosphines it is obscured by the stronger R-band, which arises from the n -transition of the unshared electrons of phosphorus.