I previously used data mining of crystal structures to explore the directing influence of substituents on aromatic and heteroaromatic rings. Here I explore, quite literally, a different angle to the hydrogen bonding interactions between a benzene ring and OH or NH groups. I start by defining a benzene ring with a centroid.
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 Å, 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.
Related Article: T.Steiner, S.A.Mason|2000|Acta Crystallogr.,Sect.B:Struct.Sci.|56|254|doi:10.1107/S0108768199012318
Related Article: O.Danylyuk, B.Lesniewska, K.Suwinska, N.Matoussi, A.W.Coleman|2010|Cryst.Growth Des.|10|4542|doi:10.1021/cg100831c