Light-Assisted NMR

Nuclear Magnetic Resonance (NMR) has long been a major technique for both characterization of compounds and mechanistic studies. While ex-situ applications may provide considerable information on reaction mechanisms, they are often associated with loss of resolution in reaction time. Consequently, transcient intermediates might not be seen. In-situ investigations, on the other hand, have the advantage that timing of acquired NMR spectra can be tailored to the reaction conditions. New methods that combine NMR with various light sources and offer access to a wide range of light-enabled in-situ experiments have been developed in our labs.

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is a powerful approach for sensitivity enhancement in NMR spectroscopy. In liquids, intermolecular photo-CIDNP depends on the transient bimolecular reaction between a photoexcited dye and the sample of interest. The large enhancements arising from 13C photochemically induced dynamic nuclear polarization (13C photo-CIDNP) in solution, as developed by the Cavagnero group, dramatically improve biomolecular NMR sensitivity in the context of heteronuclear correlation spectroscopy.[1]

A simple, cost-effective setup using ultra high power (UHP)-LED systems enables support for photochemical research, as well photochromic investigations. LED systems may be preferred over lasers when lower power is sufficient, and/or continuous irradiation are needed. Our setup is portable and can easily be moved between spectrometers. Fiber patch cords are attached to the LED via a coupler adapter, and are capable of delivering variable fiber optic output power of up to 500 mW at the tip of the fiber. Dedicated pulse sequences allow for precise LED control using spectrometer TTL ports. This experimental setup has been utilized by the Cavagnero group as a cost-effective approach to increase NMR sensitivity at low protein concentrations.[2]

The LED setup was also used in continuous mode by the Yoon group to study several types of fast photochemical reactions.[3] For example, mechanistic studies of an iridium catalyzed [4+2] cycloaddition were enabled by NMR-based rapid kinetic monitoring.

1H spectra of reaction between trans-anethole and isoprene with different LED irradiation time

For a good introduction to the LED setup please check out W. Swords Flash Talk on LED-NMR.



[1] Okuno, Y., Cavagnero, S. “Fluorescein: a Photo-CIDNP Sensitizer Enabling Hyper-Sensitive NMR DataCollection in Liquids at Low Micromolar Concentration” J. Phys. Chem. B 120, 715-723 (2016).

[2] Yang, HM, Hofstetter, H, Cavagnero, S. Fast-pulsing LED-enhanced NMR: A convenient and inexpensive approach to increase NMR sensitivity. J. Chem. Phys. 151, 245102-245112 (2019).

[3] Skubi, K. L.; Swords, W. B.; Hofstetter, H.; Yoon, T. P. LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition, ChemPhotoChem 4, 685–690 (2020).