Determination of Intermolecular Interactions Using Polarization Compensated Heteronuclear Overhauser Effect of Hyperpolarized Spins

The nuclear Overhauser effect (NOE) has long been used as a selective indicator for intermolecular interactions. Due to relatively small changes of signal intensity, often on the order of several percent, quantitative NOE measurements can be challenging. Hyperpolarization of nuclear spins can dramatically increase the NOE intensity by increasing population differences, but poses its own challenge in quantifying the original polarization level. Here, we demonstrate a method for the accurate measurement of intermolecular heteronuclear cross-relaxation rates by simultaneous acquisition of signals from both nuclei. Using this method, we measure cross-relaxation rates between water protons and ¹⁹F of trifluoroacetic acid at concentrations ranging from 23 to 72 mM. A concentration-independent value of 2.46 × 10^-4 ± 1.02 × 10^-5 s^-1 M^-1 is obtained at a temperature of 301 K and validated using a nonhyperpolarized measurement. In a broader context, accurate measurement of heteronuclear cross-relaxation rates may enable the study of intermolecular interactions including those involving macromolecules where ¹⁹F atoms can be introduced as site-selective labels.