Multiple spin order generated by photo-CIDNP hyperpolarisation NMR

Hetero-nuclear multiple quantum coherences build a basis of multi-dimensional nuclear magnetic resonance (NMR) and are typically established through scalar coupling-based polarisation transfers in liquids, starting from 1H due to the low population of the other spin nuclei. Here, the presence of heteronuclear double and triple quantum spin orders resulting from in-situ photochemically induced dynamic nuclear polarisation (photo-CIDNP) was explored with ¹³C,¹⁵N-labelled tyrosine and Atto Thio 12 acting as photosensitizer. From the longitudinal spin orders, double and triple quantum coherences are obtained by a single pulse. Hence, the ¹H,¹³C double coherence is detected in a [¹H, ¹³C]-heteronuclear single quantum coherence experiment and alike without the initial insensitive nuclei enhanced by polarisation transfer (INEPT) pulse train. The photo-CIDNP signal intensities of double and triple quantum spin orders/coherences were evaluated to be only about one order of magnitude less than for single quantum. This suggests that the photo-CIDNP polarisation enhancements of the double and triple quantum coherences/spin orders are in the order of a factor of 10⁵ for the double and 10⁹ for the triple coherence. These extensive enhancements are attributed to the concerted spin sorting of the nuclei within a single molecule, which originates from the radical pair mechanism of photo-CIDNP hyperpolarisation.