Field Dependence of 13C Line Shapes for Nitrogen Bearing Carbons in the Solid State

The frequency separation between the two peaks in the 1:2 doublets typically observed for ¹³C nuclei bonded to ¹⁴N in ¹³C CPMAS spectra is inversely dependant on the strength of the magnetic field. As a result narrower resonances are observed for nitrogen bearing carbons at higher field. The figure below shows the ¹³C CPMAS spectrum of D,L-alanine acquired on both a 200 MHz and a 500 MHz instrument.