Probing P-H+-P hydrogen bonds: Structures, binding energies, and spin-spin coupling constants

Abstract

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of 22 open and 3 cyclic complexes formed from the sp2 [H2C=PH and HP=PH (cis and trans)] and sp3 [PH2(CH3) and PH3] hybridized phosphorus bases and their corresponding protonated ions. EOM-CCSD calculations have been carried out to obtain 31P-31P and 31P-1H coupling constants across P-H+-P hydrogen bonds. Two equilibrium structures with essentially linear hydrogen bonds have been found along the protontransfer coordinate, except for complexes with P(CH3)H3 + as the proton donor to the sp2 bases. Although the isomer having the conjugate acid of the stronger base as the proton donor lies lower on the potential energy surface, it has a smaller binding energy relative to the corresponding isolated monomers than the isomer with the conjugate acid of the weaker base as the donor. The hydrogen bond of the latter has increased proton-shared character. All of the complexes are stabilized by traditional hydrogen bonds, as indicated by positive values of the reduced coupling constants 2hKP-P and 1KP-H, and negative values of 1hK H-P. 2hJP-P correlates with the P-P distance, a correlation determined primarily by the nature of the proton donor. For open complexes, 1JP-H always increases relative to the isolated monomer, while 1hJH-P is relatively small and negative. 2hJP-P values are quite large in open complexes, but are much smaller in cyclic complexes in which the P-H+-P hydrogen bonds are nonlinear. Thus, experimental measurements of 2hJP-P should be able to differentiate between open and cyclic complexes. © 2007 American Chemical Society.