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