Short N+--H...Ph hydrogen bonds in ammonium tetraphenylborate characterized by neutron diffraction

The crystal structures of ammonium tetraphenylborate, NH4+.BPh4-, are determined by neutron diffraction at 20 and 293 K. At both temperatures, all four N-H vectors of the ammonium ion are time-average-oriented at the midpoints of the phenyl rings of neighboring anions. The N-H...Ph distances, H...M 2.067 and N...M 3.023 A, are exceptionally short (M = aromatic midpoint). Even at 20 K the ammonium ion performs large amplitude motions which allow the N-H vectors to sample the entire face of the aromatic system.