Sizable concentration-dependent frequency shifts in solution NMR using sensitive probes

With the growing use of high fields and ultrasensitive probes, radiation damping emerges as a significant feedback interaction in modern solution NMR. Motivated by recent observations of mysterious concentration-dependent frequency shifts, experiments carried out on a cryoprobe at 600 MHz have revealed a time-averaged frequency shift of up to +83/-81 Hz. The sizable frequency shifts arise from deviations in the phase of the radiation damping field from perfect orthogonality relative to the net transverse magnetization. The frequency shift is shown to depend on the longitudinal magnetization and probe tuning conditions through experiments and numerical simulations. Such unexpected shifts in the solvent precession frequency provide a physical explanation for the empirical practice of adjusting the irradiation frequency of the saturating B1 field in solvent presaturation to achieve optimal suppression. Additional applications of the radiation damping induced frequency shift to solvent suppression and NMR methodology are discussed.