Topic 11: Measurement and data processing - 11.3 Spectroscopic identification of organic compounds

Nature of science:

Improvements in instrumentation-mass spectrometry, proton nuclear magnetic resonance and infrared spectroscopy have made identification and structural determination of compounds routine. 1.8

Models are developed to explain certain phenomena that may not be observable - for example, spectra are based on the bond vibration model. 1.10


Essential idea: Analytical techniques can be used to determine the structure of a compound, analyse the composition of a substance or determine the purity of a compound. Spectroscopic techniques are used in the structural identification of organic and inorganic compounds.

The degree of unsaturation or index of hydrogen deficiency (IHD) can be used to determine from a molecular formula the number of rings or multiple bonds in a molecule

Mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy (1H NMR) and infrared spectroscopy (IR) are techniques that can be used to help identify compounds and to determine their structure.

Applications and skills

Determination of the IHD from a molecular formula.

Deduction of information about the structural features of a compound from percentage composition data, MS, 1H NMR or IR.


The electromagnetic spectrum (EMS) is given in the data booklet in section 3. The regions employed for each technique should be understood.

The operating principles are not required for any of these methods.

The data booklet contains characteristic ranges for IR absorptions (section 26), 1H NMR data (section 27) and specific MS fragments (section 28). For 1H NMR, only the ability to deduce the number of different hydrogen (proton) environments and the relative numbers of hydrogen atoms in each environment is required. Integration traces should be covered but splitting patterns are not required.