Chemical shift referencing in MAS solid state NMR

Solid state ¹³C magic angle spinning (MAS) NMR spectra are typically referenced externally using a probe which does not incorporate a field frequency lock. Solution NMR shifts on the other hand are more often determined with respect to an internal reference and using a deuterium based field frequency lock. Further differences arise in solution NMR of proteins and nucleic acids where both ¹³C and ¹H shifts are referenced by recording the frequency of the ¹H resonance of DSS (sodium salt of 2,2-dimethyl-2-silapentane-5-sulphonic acid) instead of TMS (tetramethylsilane). In this note we investigate the difficulties in relating shifts measured relative to TMS and DSS by these various approaches in solution and solids NMR, and calibrate adamantane as an external ¹³C standard for solids NMR. We find that external chemical shift referencing of magic angle spinning spectra is typically quite reproducible and accurate, with better than ±0.03 ppm accuracy being straight forward to achieve. Solid state and liquid phase NMR shifts obtained by magic angle spinning with external referencing agree with those measured using typical solution NMR hardware with the sample tube aligned with the applied field as long as magnetic susceptibility corrections and solvent shifts are taken into account. The DSS and TMS reference scales for ¹³C and ¹H are related accurately using MAS NMR. Large solvent shifts for the ¹³C resonance in TMS in either deuterochloroform or methanol are observed, being +0.71 ppm and -0.74 ppm from external TMS, respectively. The ratio of the ¹³C resonance frequencies for the two carbons in solid adamantane to the ¹H resonance of TMS is reported.