Effect of spinning on chemical shifts in magic-angle-spinning nuclear magnetic resonance

Abstract

We have found that the isotropic peak position changes depending on the spinning rate of the sample in 63Cu, 65Cu, 79Br, and 127I magic-angle-spinning nuclear magnetic resonance (NMR) of cuprous halides, CuCl, CuBr, and CuI. With the increase in the spinning rate, the 63Cu and 65Cu signals shift towards lower frequencies, while the 79Br and 127I signals shift towards higher frequencies. The magnitudes of the frequency shift are in the following order: CuCl<CuBr<CuI, and the shifts in the halogen signals are of comparable magnitudes to those in the copper signals except for the shift direction. The magnitude of the spinning-induced shift is found to be proportional to the square of the spinning rate and to the strength of the magnetic field. We have proposed that the spinning-rate-dependent shift has its origin in the Lorentz force working on Cu+ ions being forced to move fast in a strong magnetic field, and have derived formulas to explain the above phenomena. We have also checked other possibilities, and have concluded that no other mechanisms can explain the above phenomena.