What chemical shift measures
In nuclear magnetic resonance spectroscopy, chemical shift describes where a nucleus resonates relative to a reference, reported in parts per million (ppm) and measured from tetramethylsilane (TMS) at 0 ppm. The shift arises from electron shielding: electrons around a nucleus generate a small opposing magnetic field, so more shielded nuclei resonate at lower ppm (upfield) and deshielded nuclei resonate at higher ppm (downfield).
Electronegative neighbors, ring currents, and hydrogen bonding all reduce shielding and push signals downfield, making chemical shift a direct readout of a nucleus's chemical environment.
Typical 1H and 13C shift ranges
For proton (1H) NMR, common ranges run roughly 0.5 to 2 ppm for alkyl CH, 2 to 3 ppm for protons near carbonyls or alkynes, 3 to 4.5 ppm for protons on carbons bearing oxygen or halogen, 4.5 to 6.5 ppm for vinyl protons, 6.5 to 8 ppm for aromatic protons, 9 to 10 ppm for aldehydes, and 10 to 13 ppm for carboxylic acids.
Carbon (13C) NMR spans a wider window of about 0 to 220 ppm: alkyl carbons near 0 to 50, carbons bonded to oxygen or nitrogen around 50 to 90, aromatic and alkene carbons 110 to 150, and carbonyl carbons 160 to 220 ppm.