RELATIONSHIP BETWEEN PHOSPHINE PROTON AFFINITIES AND LONE-PAIR DENSITY PROPERTIES

Ab initio calculations on the protonation of substituted phosphines ar e used to study the relationship between proton affinities and lone pa ir density properties. It is assumed that these properties are conveni ently represented at the point of maximal concentration of non-bonded charge in the phosphorus valence shell of charge concentration, that i s, at the (3,-3) critical point in del(2) rho. These lone pair propert ies appear to be an excellent indicator for the proton affinity of alk yl phosphines. However, the same properties fail to recover even the q ualitative trend in proton affinities of halo-phosphines. In addition, it is shown that the (3,-3) critical point properties can predict the incorrect site of preferential protonation in halo-substituted phosph ines containing distinct trivalent phosphorus atoms. These results are rationalized in terms of Bader's Atoms-In-Molecules theory. It appear s that integrated properties (atomic charges and energies) may give mo re insight into the mechanism of substituent effects on basicity than del(2) rho or the LP properties.