A model proton-transfer system in the condensed phase: NH 4+ OOH-, a crystal with short intermolecular H-bonds

The crystal structure of NH 4+ OOH- is determined from single-crystal x-ray data obtained at 150 K. The crystal belongs to the space group P 21/c and has four molecules in a unit cell. The structure consists of discrete NH 4+ and OOH- ions. The OOH- ions are linked by short hydrogen bonds (2.533 Å) to form parallel infinite chains. The ammonium ions form links between these chains (the NO distances vary from 2.714 to 2.855 Å) giving a three-dimensional network. The harmonic IR spectrum and H-bond energies are computed at the Perdew-Burke-Ernzerhof (PBE)/6-31 G level with periodic boundary conditions. A detailed analysis of the shared (bridging) protons' dynamics is obtained from the CPMD simulations at different temperatures. PBE functional with plane-wave basis set (110 Ry) is used. At 10 K the shared proton sits near the oxygen atom, only a few proton jumps along the chain are detected at 70 K while at 270 K numerous proton jumps exist in the trajectory. The local-minimum structure of the space group Cc is localized. It appears as a result of proton transfer along a chain. This process is endothermic (∼2 kJ/mol) and is described as P 21/c2Cc. The computed IR spectrum at 10 K is close to the harmonic one, the numerous bands appear at 70 K while at 270 K it shows a very broad absorption band that covers frequencies from about 1000 to 3000 cm-1. The advantages of the NH 4+ OOH- crystal as a promising model for the experimental and DFT based molecular dynamics simulation studies of proton transfer along the chain are discussed.