Six methods, six answers to different questions
NMR (nuclear magnetic resonance) measures how nuclei (1H, 13C) respond to a magnetic field. It reveals molecular connectivity and environment: the number of hydrogens on each carbon, how many bonds separate them, and whether nearby atoms shift the resonance frequency. NMR requires milligrams and works in solution, but it gives unambiguous structure information.
IR spectroscopy identifies functional groups (O-H, C=O, N-H) by their absorption bands. It is fast and requires small samples but gives functional group type, not detailed structure. UV-Vis measures electronic transitions and is sensitive to conjugated systems (how many double bonds in a row). Sample needs and interpretation differ by method.
Matching method to the goal
Mass spectrometry measures molecular weight and fragmentation pattern, revealing both the mass of the intact molecule and how it breaks apart under ionization. Raman spectroscopy, like IR, measures vibrations but with a different selection rule, making it complementary for symmetric molecules.
In practice, structure elucidation combines methods: IR identifies functional groups, MS gives the molecular weight, NMR gives detailed connectivity, UV-Vis refines the extent of conjugation. For purity, a single method (HPLC) often suffices. For concentration, Beer-Lambert law and standard curves are fastest. The cost, sample requirements, speed, and specificity of each method drive which you choose.