What type of radiation flips nuclei in NMR?
NMR Zeeman splitting produces nuclear spin level separations corresponding to photon energies in megahertz range, matching radiofrequency region of electromagnetic spectrum. Applying radiofrequency pulse with matched Larmor frequency drives transitions flipping spin orientation, detected as free induction decay. Ultraviolet photons excite electronic orbitals, infrared excites vibrational modes, microwaves excite electron spin in ESR, each orders of magnitude higher energy than nuclear transitions. Using radiofrequency ensures selective excitation of nuclei without perturbing electronic structure, allowing safe investigation of proteins, nucleic acids, and whole organisms in structural biology and clinical MRI.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.