HC=P and H3C-C=P 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 sp3 phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C≡P and H 3C-C≡P as proton acceptors. These protonbound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the n cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H 3C-C≡P as the proton acceptor are more stable man those with H-C≡P as the acceptor, a reflection of the greater basicity Of H 3C-C≡P. In most complexes with sp2- or sp 3-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 C=P bond and a red shift of the C≡P stretching vibration. The two-bond coupling constants 2πhV(P-P) and 2πhV(P-C) are significantly smaller than 2hJ(P-P) and 21V(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. © 2007 American Chemical Society.Peer Reviewe