Strong, positive-ion hydrogen bonds: The binary complexes formed from NH3, OH2, FH, PH3, SH2, and ClH

Abstract

A systematic, ab initio electronic structure analysis of the strong, positive‐ion hydrogen bond is reported. Energies and wave functions have been obtained at the 4‐31G level for the twenty‐one complexes, (B⋅⋅⋅H–A)⁺, where A,B=NH₃, OH₂, FH, PH₃, SH₂, ClH. The A–H bond length (r₁), the B⋅⋅⋅A separation (R(, and the angle (ϑ) measured relative to the symmetry axis of B have been optimized. Calculated dimerization energies E_D are found to be in reasonable agreement with experiment. Charge density difference plots of these complexes exhibit a remarkable similarity to the pattern of alternating charge gain and loss known for the neutral H‐bonded dimers. The proton donor is characterized by a charge gain region between A and the proton and charge loss on the proton; the electron donor by a charge loss between B and the proton. Four of the complexes (HF⋅⋅⋅H⋅⋅⋅FH)⁺, (H₂O⋅⋅⋅H⋅⋅⋅OH₂)⁺, (HCl⋅⋅⋅H⋅⋅⋅ClH)⁺, (HCl⋅⋅⋅H⋅⋅⋅FH)⁺, have unusually short internuclear separations and show large charge gain around the protons. This is the first theoretical evidence of a transition from predominantly electrostatic to predominantly covalent binding in hydrogen bonding and it corroborates a recent experimental X–N study. An estimate of the amount of charge lost from the proton, Δq_H, has been obtained from the difference plots and is found to bear a linear relation with the dimerization energy E_D for a series of complexes with a single proton donor. The inverse relation between E_D and the difference in monomer proton affinities, ΔPA, reported in the literature for substituted pyridinium ions, is shown to hold as well for all A and B. Our calculated results also give a quantitative demonstration of the recently proposed inverse relationship between proton position and ΔPA. Several useful new organizing principles have been found: (a) R varies linearly with r₂, the B⋅⋅⋅H distance. Covalent bonding in the four complexes noted above is indicated by deviation from this line. Crystal structures taken from the literature also obey this relationship and the satisfactory agreement between experiment and calculations show that the 4‐31G basis is adequate for predicting strong H‐bond geometries. (b) R_N/R?1.2, where R_N is the A⋅⋅⋅B separation in neutral hydrogen bonds. (c) E_D displays a smooth inverse relation to r₂ for a sequence of complexes with a single electron donor.