A 13C field-cycling NMR relaxometry investigation of proton tunnelling in the hydrogen bond: Dynamic isotope effects, the influence of heteronuclear interactions and coupled relaxation

Measurement of Proton Tunneling in Short Hydrogen Bonds in Single Crystals of 3,5 Pyridinedicarboxylic Acid Using Nuclear Magnetic Resonance Spectroscopy

“…In order to determine the field dependence, T ðHÞ 1 ðB 0 Þ T , a field-cycling procedure was incorporated into this sequence [48,49]. In general, the proximity of the 14 N nucleus to the migrating hydrogen renders the 1 H spin-lattice relaxation biexponential although, in practice, the 1 H polarization-recovery curves displayed only small deviations from single exponential behavior.…”

1H–19F spin–lattice relaxation spectroscopy: Proton tunnelling in the hydrogen bond studied by field-cycling NMR

“…The relaxation is dominated by the modulation of the homonuclear and heteronuclear dipolar interactions and the elements of the relaxation matrix R ¼ q I r r q S are defined by correlation functions and spectral density functions given elsewhere [8,21]. Evaluating these for double proton transfer in the hydrogen bond we obtain for powder samples [7,13,23,24], …”

“…Since the dipole-dipole interactions dominate, the spin-lattice relaxation is dependent on the orientation of the B-field with respect to the unit cell axes. The expressions for single crystal samples have been given in [13], however, it is well established that an average over all possible orientations of the B-field [7,8] describes well the spin-lattice relaxation time for a powder sample; explicit powder average expressions for the dipolar constants in terms of the molecular geometry and the dipolar contact distances have been given in [23].…”

Proton tunneling in a hydrogen bond measured by cross-relaxation field-cycling NMR

“…(2) the two spectral density components appear with opposite sign; this situation arises because double-quantum and zero-quantum transitions compete within the manifold of dipolar coupled spins, driving the cross-relaxation process in opposite directions. The cross-relaxation processes are driven by modulation of the heteronuclear dipolar interactions; C IS is a lattice sum of such dipolar interactions [9] and the Boltzmann factor 4a ð1þaÞ 2 , with a = exp (A/k B T), arises from the energy asymmetry, A, of the two conformations of the hydrogen bonds that are interchanged by the proton transfer process.…”

“…Experiments have been conducted in the solid state at low temperature on tetrafluoroterephthalic acid (TFTA: C 6 F 4 (COOH) 2 ) where we assign spins I to be 1 H and spins S to be 19 F; Fig. 2 Measurements were made using a custom-built fieldcycling NMR spectrometer [9,12]. To facilitate fast field-switching, the main B-field was provided by a superconducting solenoid with low inductance.…”