The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering

Abstract

NH 4 ͑C 6 H 5 ͒ 4 B represents a prototypical system for understanding aromatic H bonds. In NH 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 NH 4 ͑C 6 H 5 ͒ 4 B were studied by quasielastic incoherent neutron scattering in a broad temperature range ͑20ഛ T ഛ 350 K͒. 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.

Dynamics of Methyl Iodide Clathrate Hydrate, Investigated by MD Simulations and QENS Experiments

“…This model yields an excellent agreement, as shown in Figure , with a rotation radius of 2.28 Å and the free parameters p = 0.19 ± 0.02 and σ D = 24.9° ± 0.8°. It should be noted that several additional models have been considered and disproved (tetrahedral jump model, octahedral jump model with equiprobable and nonequiprobable sites; Supporting Information for details).…”