Introduction
Spectroscopy determines organic structure by observing how a molecule interacts with electromagnetic radiation (Physics Chapter III) of different energies — each technique probes a different type of molecular motion or transition, and together they let chemists deduce a structure without needing to break the molecule apart chemically.
1. UV/Visible spectroscopy
UV/Visible spectroscopy probes electronic transitions — electrons jumping between molecular orbitals upon absorbing a photon of appropriate energy (Physics Chapter II). It is most informative for molecules with extended conjugation (alternating double and single bonds, or aromatic rings, Chapter 13), where increased conjugation lowers the energy gap between orbitals, shifting absorption to longer wavelengths — this is the basis of colour in many organic dyes and pigments.
2. Infrared (IR) spectroscopy
IR spectroscopy probes molecular vibrations — bonds stretching and bending at frequencies corresponding to infrared photon energies. Different bond types (and the functional groups they belong to) absorb at characteristic frequencies (e.g., a broad O–H stretch around 3200–3550 cm⁻¹, a sharp C=O stretch around 1700 cm⁻¹), making IR spectroscopy primarily a tool for identifying which functional groups are present, though it is less useful for determining the overall carbon skeleton.
3. Proton (¹H) NMR spectroscopy
Nuclear magnetic resonance exploits the fact that certain nuclei (including ¹H) have spin (Physics Chapter II) and behave like tiny magnets, absorbing radiofrequency energy at a frequency dependent on the local magnetic environment when placed in a strong external magnetic field. Three key pieces of information are read from a ¹H NMR spectrum: chemical shift (position of a signal, reflecting the electronic environment/shielding around that hydrogen — hydrogens near electronegative atoms are typically shifted downfield), integration (signal area, proportional to the number of equivalent hydrogens producing that signal), and spin–spin splitting (a signal split into multiple peaks by neighbouring, non-equivalent hydrogens, following the n+1 rule, which reveals how many hydrogens are adjacent to the one being observed).
4. Mass spectrometry
Mass spectrometry (introduced in Physics Chapter IV in its nuclear/isotope application) ionises a molecule and measures the mass-to-charge ratio of the resulting molecular ion (giving the molecule’s overall molar mass) and of the smaller fragment ions produced when the molecular ion breaks apart in characteristic, chemically informative ways — the fragmentation pattern often reveals specific structural features, such as the loss of a recognisable small group (like water or a methyl radical).
5. Combined interpretation for structure determination
In practice, no single spectroscopic technique fully determines a structure; the real skill (and the way these are always tested together) is combining evidence from all four: molecular mass and key fragment losses (mass spectrometry) narrow down the molecular formula and possible substructures, characteristic absorptions (IR) confirm which functional groups are present, and the number, position, integration and splitting pattern of signals (¹H NMR) piece together the remaining carbon-hydrogen framework — cross-checking that every piece of evidence is consistent with a single proposed structure.
Key Takeaways
- UV/Visible probes electronic transitions (informative for conjugated systems); IR probes bond vibrations (informative for functional group identification).
- ¹H NMR provides three independent pieces of structural information from a single spectrum: chemical shift (environment), integration (hydrogen count), and splitting pattern (neighbouring hydrogens).
- Mass spectrometry gives molecular mass from the molecular ion and structural clues from characteristic fragmentation.
- Real structure determination combines evidence from all techniques together rather than relying on any single spectrum alone.