Monday, February 11, 2008
High Resolution CPMAS NMR of 13C Bonded to 14N
In 13C CPMAS spectra, the resonances of 13C bonded to 14N can give unusual line shapes. They often show up as 1:2 doublets which should not be interpreted as seperate resonances. This effect is observed because the dipolar coupling between the spin I=1/2 13C and the quadrupolar, spin I=1 14N is not fully averaged by the magic angle spinning as is the case for a pair of spin I=1/2 nuclei. The appearance of the spectrum is a function of the quadrupolar coupling constant for the 14N, the orientation of the electric field gradient tensor with respect to the internuclear vector, the distance between the 13C and the 14N, the strength of the magnetic field and the 14N T1 relaxation time. This effect is not specific to 13C and 14N, but a general effect between and spin I=1/2 and quadrupolar neulei. In the figure below, the 50 MHz 13C CPMAS spectrum of D,L-alanine is presented as an example. The resonance shaded and expanded in yellow is the carbon directly bonded to nitrogen.