In what two aspects does IR and NMR spectroscopy differ?

Answer 1

IR allows identification of absorption bands due to particular bonds within a molecule, but often doesn't allow the full molecular structure to be determined. however the equipment is relatively cheap and the method is not very time consuming.

NMR works differently, the signal is due to atomic nuclei, and is modified by chemical environment. so rather often it allows determination of the full molecular structure (especially if the more sophisticated methods are used). however, the equipment is expensive (requires a powerful magnet) and methods can be time consuming.

in practice, often both methods are used in a complementary way. in synthetic work for example, it is common to use IR to get an idea if a reaction worked as expected, and NMR to definitively characterise the final product of a series of reactions.