An NMR relaxometry study of heteronuclear effects upon proton transfer in hydrogen bonds

The inherent quantum-mechanical nature of the proton transfer process in hydrogen bonds has been investigated through its effects on the nuclear spin-lattice relaxation rate. The fast magnetic field-cycling techniques employed allowed a direct measure of the rate characterising this dynamic process, which is closely related to the potential energy environment experienced by the mobile proton. Various heteronuclear effects from magnetic and non-magnetic nuclei outside the hydrogen bond were characterised. The contribution to proton tunnelling from the displacement of heavy atoms in the molecule is an important consideration within a complete description of the process. This interdependence was accurately measured for the carboxyl-group oxygen atoms in benzoic acid dimers through the isotope effect. Careful comparison of $^{16}$O and $^{18}$O-enriched benzoic acid relaxation allowed this relationship to be measured from the difference in low-temperature tunnelling rates. Fluctuating dipolar interactions caused by proton transfer motion couples the Zeeman states of different nuclear species. The cross-relaxation occurring through this natural coupling was explored as a function of field in 2,4,6-trifluorobenzoic acid and $^{13}$C-enriched pure benzoic acid. Characterising the strength of this interaction endeavoured to broaden the comprehension of heteronuclear coupling and served as confirmation of the model used. Beyond the carboxylic acid dimer, this investigation also showed dynamic disorder in intermolecular short, strong hydrogen bonds of pyridine-3,5-dicarboxylic acid. This proton transfer mechanism was found to be strongly dependent on the molecular vibrational modes creating a pathway between two potential minima. A finite change in entropy between the proton sites ensured that greatest proton mobility occurred at intermediate temperature, between relatively stable configurations at the extremes of temperature. A study of different sources of molecular dynamics within one compound showed the efficiency of field-cycling NMR at separating their contributions to relaxation. Dynamic rates from the proton transfer and methyl group rotation in 4-methylbenzoic acid were reliably extracted to the extent of identifying separate contributions from a small percentage of molecules around impurity centres