Infrared spectroscopy identifies functional groups
Infrared (IR) spectroscopy measures the frequency at which molecular bonds vibrate. Different functional groups absorb at characteristic frequencies. A C-H stretch appears around 3000 cm-1; a C=O stretch (carbonyl) appears around 1700 cm-1; an O-H stretch (alcohol or carboxylic acid) appears around 3300-3500 cm-1 as a broad band; an N-H stretch (amine or amide) appears around 3200-3500 cm-1. By scanning the full spectrum and looking for these peaks, you can identify which functional groups are present. IR tells you what is in the molecule, not the structure or connectivity.
NMR reveals connectivity and environment
Nuclear magnetic resonance (NMR) spectroscopy measures how nuclei (usually H or C) respond to a magnetic field. The number of signals tells you how many different H or C environments exist in the molecule. The position of each signal (chemical shift) depends on the electron density around that nucleus, which is influenced by nearby atoms. Integration of each peak tells you how many equivalent nuclei give that signal. Coupling between neighboring nuclei creates splitting patterns (singlets, doublets, triplets) that reveal how many neighbors each nucleus has. Together, chemical shift, integration, and coupling patterns allow you to deduce the connectivity and structure of the molecule.
Mass spectrometry and UV-Vis provide weight and chromophore info
Mass spectrometry (MS) fragments the molecule under electron bombardment, producing ions of various m/z ratios. The molecular ion peak (M+) gives the molecular weight; fragmentation patterns reveal how the molecule breaks apart and can suggest functional groups or structural features. UV-Visible spectroscopy measures absorption in the ultraviolet and visible range. Conjugated double bonds and aromatic rings absorb at characteristic wavelengths; the wavelength and intensity can hint at the degree of conjugation and aromaticity. For a complete structural determination, all four techniques together (IR, NMR, MS, UV-Vis) are more powerful than any one alone.