SHIELDING AND CHARACTERISTIC CHEMICAL SHIFTS IN

In i C NMR spectroscopy the i C signal due to the carbon in CDCI3 appears as a triplet centred at 5 77.3 with peaks intensities in the ratio 1 1 1 (due to spin-spin coupling between i C and H). This resonance serves as a convenient reference for the chemical shifts of i C NMR spectra recorded in this solvent. 

This section deals briefly with a number of more advanced topics in NMR spectroscopy, which indicate of the power of NMR spectroscopy in solving complex structural problems. 

The averaging effects of exchange apply to any dynamic process that takes place in a molecule (or between molecules). However, many processes occur at rates that are too fast or too slow to give rise to visible broadening of NMR spectra. The NMR time-scale happens to coincide with the rates of a number of common chemical processes that give rise to variation of the appearance of NMR spectra with temperature and these include  

For example, cyclohexane gives a single averaged resonance in the H NMR at room temperature, but separate signals are seen for the axial and equatorial hydrogens when spectra are acquired at very low temperature. 

Exchange broadening is frequently observed in amides due to restricted rotation about the N-C bond of the amide group. 

---