Chapter 10: NMR Microcoils for On-line Reaction Monitoring

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most important and powerful analytical tools available to the scientific community, and to synthetic chemists in particular. Standard, commercially available, high-field NMR spectrometers (running from 4.7 to 23.5 T, corresponding to 200, respectively 1000 MHz ¹H Larmor frequency) have their radiofrequency antennas incorporated in probe heads that allow measuring samples in 5 mm tubes. Commercial probe heads that allow on-flow monitoring of reactions are based on, typically 5 mm, saddle coil designs, but these require relatively large amounts of material and/or have poor filling factors and correspondingly poor mass sensitivity. In 1994 Sweedler and co-workers launched the field of microcoil NMR spectroscopy, and the past two decades have seen several groups starting to fabricate their own small-volume probe-heads. Here we provide an overview of the different types of NMR microcoils that haven been developed to measure volumes in the lower microliter and (sub-)nanoliter scale, and then focus on the main geometries of microcoils exploited for use in reaction monitoring as solenoids, planar spiral, and stripline coils. Several examples are presented of on-flow and stationary reaction monitoring with such microcoils. The rapid progress in the field promises that many more groups will enter the field of NMR microcoil reaction monitoring in the coming years.