Variable hydrogen bond lengths in hydrated complexes of formate and methylammonium ions

Abstract

The importance of the full hydrogen bond connectivity pattern on a specific hydrogen bond length has been examined by carrying out theoretical ab initio calculations for various hydrated complexes of formate (HCO₂^–) and methylammonium (MeNH₃⁺) ions, as well as for related complexes in which the water molecules were replaced by HF or NH₃. For the HCO₂^–⋯ HOH interaction substantial cooperative and competitive effects were attained that caused the principal hydrogen bond length to vary between 1.46 and 2.05 Å at the HF/6-311 ++ G(2d,p) level of theory without significant changes to other geometry parameters. A similar range was obtained for the MeNH₃⁺⋯ OH₂ hydrogen bond. Increasing H-bond lengths are generally paralleled by decreasing interaction energies. The observed trends, which are relevant for crystallographic studies of H-bond lengths, are essentially independent of basis set size, inclusion of electron correlation effects or correction for basis set superposition error. The effect of hydrogen bonding on the energy potential for HOO₂^–⋯⁺HNH₂Me interactions has been investigated.