Chemically different carbons in an organic molecule do not experiencethe same magnetic field. Just like protons, electrons shield the nucleusthereby reducing the effective magnetic field and requiring energy of alower frequency to cause resonance. On the other hand, when electrons arewithdrawn from a nucleus, the nucleus is deshielded and feels a strongermagnetic field requiring more energy (higher frequency) to cause resonance.Thus, NMR provides information about a carbonís electronic environment.
Generally, carbons attached to electron withdrawing groups tend to resonateat higher frequencies (more downfield (to the left) from TMS, tetramethylsilane,a common NMR standard). The position of where a particular carbon atomresonates relative to TMS is called its chemical shift. Again, since carbonatoms resonate over such a wide range, learning some common chemical shiftswill provide you with a tremendous advantage at solving structural problemsusing NMR.
For example carbonyl carbons resonate furthest downfield (typically160-200 ppm), aromatic carbons (115-145 ppm) and saturated aliphatic carbons(5-40 ppm).
Click here to see a list of chemicalshifts of some typical carbon atoms.