The dynamical properties of the aromatic hydrogen bond in N H 4 ( C 6 H 5 ) 4 B from quasielastic neutron scattering

N H 4 ( C 6 H 5 ) 4 B represents a prototypical system for understanding aromatic H bonds. In N H 4 ( C 6 H 5 ) 4 B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in N H 4 ( C 6 H 5 ) 4 B were studied by quasielastic incoherent neutron scattering in a broad temperature range ( 20 ⩽ T ⩽ 350 K ) . We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C 3 axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T = 350 K to 150 ps at T = 67 K . The activation energy was found to be 3.14 kJ ∕ mol , which means only 1.05 kJ ∕ mol per single H bond for reorientations around the C 3 symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 ° ± 2 ° and a correlation time of 22 ± 8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.