spectroscopy
Summary
Spectroscopy is the use of how molecules absorb electromagnetic radiation at specific energies to work out their structure. The MCAT's main types are IR (functional groups), ¹H NMR (hydrogen environments) and UV-vis (conjugation). Photon energy rises with frequency: E = hf.
Detail
Meaning: Spectroscopy measures which photon energies a molecule absorbs. Each technique matches a different transition: IR excites bond vibrations, NMR flips nuclear spins in a magnetic field, and UV-vis excites electrons. Mass spectrometry is usually grouped with these but measures the mass-to-charge ratio of ions, not absorbed light.
Equation: E = hf = hc/λ, where E is photon energy (J), h is Planck's constant (6.63 × 10⁻³⁴ J·s), f is frequency (Hz), c is the speed of light (3.00 × 10⁸ m/s) and λ is wavelength (m).
On the MCAT: A passage gives a spectrum or peak list and asks you to pick the structure. In IR, a broad O–H band near 3300 cm⁻¹ suggests an alcohol, and a strong C=O peak near 1700 cm⁻¹ suggests a carbonyl. In ¹H NMR, the number of signals equals the number of distinct hydrogen environments, integration gives relative hydrogen counts, and splitting follows n + 1 for n neighboring hydrogens. In UV-vis, more extended conjugation shifts λmax to longer wavelength.
Common confusion: IR is plotted in wavenumber (cm⁻¹), which is proportional to energy, so a larger wavenumber means higher energy and shorter wavelength. Also, IR mainly reveals functional groups, while NMR shows how the carbon-hydrogen framework is connected.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Spectrometric Identification of Organic Compounds (Silverstein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.