NMR phenomenon was discovered in 1945:

• in a strong magnetic field (B_o)
• nuclei whose spin quantum number (l)
  is not zero)
• absorb radiofrequency (RF) electromagnetic radiation at a particular resonance frequency:
  
  

NMR active nuclei: ¹H, ¹³C, ¹⁵N, ³¹P, ¹⁴N