HCP and H3C-CP as Proton Acceptors in Protonated Complexes Containing Two Phosphorus Bases: Structures, Binding Energies, and Spin-Spin Coupling Constants
Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp2, and sp3phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H−CP and H3C−CP as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the π cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C−CP as the proton acceptor are more stable than those with H−CP as the acceptor, a reflection of the greater basicity of H3C−CP. In most complexes with sp2- or sp3-hybridized P−H donor ions, the P−H bond lengthens and the P−H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the CP bond and a red shift of the CP stretching vibration. The two-bond coupling constants 2πhJ(P−P) and 2πhJ(P−C) are significantly smaller than 2hJ(P−P) and 2hJ(P−C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these π complexes.