11-2 The Nature of NMR Absorptions The absorption frequency is not the same for all 1H and 13C nuclei Nuclei in molecules are surrounded by electrons Electrons set up tiny local magnetic fields that act in opposition to the applied field,shielding the nucleus from the full effect of the external magnetic field The effective field actually felt by the nucleus is the applied field reduced by the local shielding effects Beffective=Bapplied-Blocal The absorption frequency is not the same for all 1H and 13C nuclei ▪ Nuclei in molecules are surrounded by electrons ▪ Electrons set up tiny local magnetic fields that act in opposition to the applied field, shielding the nucleus from the full effect of the external magnetic field ▪ The effective field actually felt by the nucleus is the applied field reduced by the local shielding effects Beffective = Bapplied – Blocal 11-2 The Nature of NMR Absorptions