Comparison of proton transfers in heterodimers and homodimers of NH3 and OH2

Abstract

Ab initio molecular orbital methods are used to study proton transfers in the cationic heterodimer (H₃NHOH₂)⁺ as well as the symmetric homodimers (H₂OHOH₂)⁺ and (H₃NHNH₃)⁺. All calculations are carried out at the Hartree–Fock level with a 4‐31G basis set to ensure consistency. For proton transfers along a linear hydrogen bond in the heterodimer, asymmetric single‐well potentials with a minimum corresponding to (NH₄)⁺(OH₂) are obtained for R(NO) distances of less than 2.85 Å. Longer intermolecular separations lead to appearance of a second minimum in the potential (NH₃)(OH₃)⁺. The energy barrier between these two minima is much greater for transfer from N to O than for the reverse O to N. Transfer barriers in the two homodimers lie between these two extremes, with interoxygen transfer barriers somewhat higher than for the internitrogen process. Barriers for all systems are found to be sensitive to angular deformations as well as stretches of the H bond. Electronic redistributions occurring at various stages of proton transfer are monitored by means of density difference maps and population analyses. Greater amounts of charge transferred from the proton‐accepting molecule to the donor are associated with more facile proton transfers. The calculated energies of the lone pair orbitals of the proton‐accepting atom provide another accurate indicator of the height of the barrier to proton transfer. These observations are explained in terms of fundamental principles of electronegativity and orbital interactions.