Chemical Shift

Chemically different hydrogens in an organic molecule do not experience the same magnetic field. Electrons shield the nucleus thereby reducing the effective magnetic field and requiring energy of a lower frequency to cause resonance. On the other hand, when electrons are withdrawn from a nucleus, the nucleus is deshielded and feels a stronger magnetic field requiring more energy (higher frequency) to cause resonance. Thus, NMR can provide information about a hydrogen's electronic environment.

Generally, hydrogens bound to carbons attached to electron withdrawing groups tend to resonate at higher frequencies (more downfield (to the left) from TMS, tetramethylsilane, a common NMR standard). The position of where a particular hydrogen atom resonates relative to TMS is called its chemical shift. Learning where typical hydrogens resonate requires experience and study, but learning some common chemical shifts will provide you with a tremendous advantage at solving structural problems using NMR.

Click here to see a list of chemical shifts of some typical hydrogen atoms.

Review Questions

1.	A shielded proton is one with a relatively ? greater lesser electron density and absorbs ? downfield upfield (to the ? left right ) in the 1H NMR spectrum.
2.	For ethyl acetate Would you expect the hydrogens labeled C to be ? downfield upfield relative to TMS? Would you expect the hydrogens labeled B to be ? downfield upfield relative to C? Would you expect the hydrogens labeled A to be ? downfield upfield relative to B? Which hydrogens will be the most downfield? A B C Which hydrogens will be the most upfield? A B C