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

摘要

NH4(C6H5)(4)B represents a prototypical system for understanding aromatic H bonds. In NH4(C6H5)(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 NH4(C6H5)(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 degrees +/- 2 degrees 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. (c) 2006 American Institute of Physics.