How do you use combined spectroscopic data to determine a compound's structure?

Combined spectroscopic data is used to determine a compound's structure by analysing its infrared, nuclear magnetic resonance, and mass spectrometry data.

Infrared (IR) spectroscopy is used to identify functional groups in a molecule. It works by measuring the vibrations of different bonds in response to infrared radiation. Each type of bond absorbs infrared radiation at a specific frequency, producing a unique peak on the IR spectrum. By comparing these peaks to known values, you can identify the functional groups present in the molecule.

Nuclear Magnetic Resonance (NMR) spectroscopy provides information about the carbon and hydrogen atoms in a molecule. In proton NMR (1H NMR), the spectrum shows the different environments of hydrogen atoms in the molecule. Each unique environment corresponds to a peak in the spectrum, and the area under each peak indicates the number of hydrogens in that environment. Carbon-13 NMR (13C NMR) works similarly, but for carbon atoms. By analysing the NMR spectra, you can determine the arrangement of atoms in the molecule.

Mass spectrometry (MS) gives information about the molecular weight of the compound and its fragmentation pattern. The molecular ion peak (M+ peak) indicates the molecular weight of the compound. The other peaks in the spectrum represent fragments of the molecule, which can provide clues about its structure. For example, the presence of a peak at m/z = 15 suggests a methyl group (CH3) in the molecule.

By combining the data from these three types of spectroscopy, you can piece together the structure of the compound. The IR spectrum identifies the functional groups, the NMR spectra show the arrangement of atoms, and the MS data provides the molecular weight and possible fragments. This process requires careful analysis and interpretation of the spectroscopic data, but with practice, it becomes a powerful tool for determining the structure of unknown compounds.