Structure and dynamics of intramolecular hydrogen bonds in carboxylic acid dimers: A solid state NMR study

Abstract

Direct evidence for dynamic intramolecular double proton exchange in carboxylic acid dimers in the solid is obtained from NMR measurements. For the p-toluic acid dimer, an asymmetric double minimum potential with an energy difference ΔG = 1.0 kJ/mol and an activation energy of Ea = 4.8 kJ/mol between the two tautomeric forms is found. The equilibrium position of the carboxylic proton pair at low temperature is derived from the corresponding dipolar coupling tensor. The importance of double proton tunneling at low temperature is discussed.

Molecular structure and motion in zero field magnetic resonance

“…These H-bonded dimers exhibit interesting, small amplitude motions. As shown previously in single-crystal studies by X-ray crystallography [42,43] and NMR [31], the carboxylic protons in this crystalline solid are in a state of dynamic disorder at room temperature. This means that one orientation of the internuclear vector is not preferred over the other, and exchange between the two orientations is occuring.…”

“…To answer this question, consider the simplest case of two coupled spin-1/2 nuclei. Previous studies in high field have shown [31] and theoretical calculations of motion in zero field [32,33,34,35] reveal that motion alters the dipole-dipole coupling. To simplify matters, all of the motions discussed below are considered to be fast on a time scale of the dipole-dipole couplings.…”

Dynamic allostery governs cyclophilin A–HIV capsid interplay

“…If motions that are faster than the magnitude of the rigid-limit dipolar coupling constant (microseconds to nanoseconds) are present, the dipolar tensor is dynamically averaged, resulting in a reduced effective dipolar coupling (22). The dipolar coupling constants and the asymmetry parameters of the motionally averaged dipolar tensors are very sensitive probes of motional amplitudes and symmetries in the nanosecond to microsecond timescales, as we and others have demonstrated (22)(23)(24). We used 3D RN-symmetry based experiments (25) for the measurement of 1 H-15 N H and 1 H-13 C α dipolar lineshapes of individual resonances of CA assemblies.…”

Strong Coupling Theory for Tunneling and Vibrational Relaxation in Driven Bistable Systems

“…This has been studied experimentally in detail in Refs. [43]. A specific control scheme ("Hydrogen-Subway") has been proposed [44,45] to steer intramolecular hydrogen transfer reactions in malonaldehyde by ultrashort laser pulses.…”