A model proton-transfer system in the condensed phase: NH4+OOH-, a crystal with short intermolecular H-bonds

The crystal structure of NH4+OOH- is determined from single-crystal x-ray data obtained at 150 K. The crystal belongs to the space group P2(1)/c and has four molecules in a unit cell. The structure consists of discrete NH4+ and OOH- ions. The OOH- ions are linked by short hydrogen bonds (2.533 angstrom) to form parallel infinite chains. The ammonium ions form links between these chains (the N center dot center dot center dot O distances vary from 2.714 to 2.855 angstrom) giving a three-dimensional network. The harmonic IR spectrum and H-bond energies are computed at the Perdew-Burke-Ernzerhof (PBE)/6-31G** 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 (similar to 2 kJ/mol) and is described as P2(1)/c <-> 2Cc. 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 NH4+OOH- crystal as a promising model for the experimental and DFT based molecular dynamics simulation studies of proton transfer along the chain are discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3493688]