Relaxation during the INEPT experiment

For small molecules in isotropic liquid relaxation typically has little impact on the efficiency of coherent polarization transfer. However, in the presence of sources that induce rapid nuclear relaxation, it can become significant, leading to nuclear enhancements far from theoretical maximum. Examples of such sources include paramagnetic metals and organic radicals used in dynamic nuclear polarization (DNP) experiments. Here, we examined the role of relaxation in the classical refocused insensitive nuclei enhanced by polarization transfer (INEPT) experiment, in the presence of stable organic radicals. Despite the plethora of literature on relaxation, disagreement has arisen in recent literature regarding the mechanism responsible for the loss of magnetization during the INEPT experiment, prompting us to revisit this topic. In the present work, it is reestablished that for heteronuclear scalar-coupled spin-½ systems, transverse magnetization decays at an average rate of the in-phase and antiphase coherences - analogous to the homonuclear case - and in line with literary precedent. A variety of spin-locking experiments are used to validate the derived theory, and which are found to be in close agreement. Details and practical considerations are provided on the use of spin-locking experiments in the particular context of liquid-state Overhauser DNP (ODNP), as well as the use of sapphire NMR tubes in this.