NMR spectroscopy
Summary
NMR (nuclear magnetic resonance) spectroscopy detects ¹H or ¹³C nuclei absorbing radio-frequency energy in a magnetic field, revealing the kinds of hydrogens or carbons in a molecule. Most testable: chemical shift gives the environment, integration gives relative H count, and splitting gives neighbors (n + 1).
Detail
Meaning: Nuclei with spin, such as ¹H and ¹³C, occupy different energy levels in a strong magnetic field. Radio-frequency photons of the matching energy flip them (resonance). Surrounding electrons shield each nucleus, so hydrogens in different environments absorb at slightly different frequencies, reported as chemical shift δ in ppm relative to TMS (0 ppm). Electronegative atoms and carbonyls deshield, moving signals downfield (larger δ). Equivalent hydrogens give one signal.
Equation: Number of peaks = n + 1, where n is the number of equivalent hydrogens on adjacent carbons (a unitless count).
On the MCAT: You are given a molecular formula and a spectrum and asked to build or choose the structure. Use shift (alkyl near 1 ppm, alkene about 5–6, aromatic about 6.5–8, aldehyde about 9–10), integration (peak area is proportional to the number of H), and splitting. Questions on how many signals a molecule gives are really symmetry questions.
Common confusion: Splitting comes from nonequivalent hydrogens on neighboring carbons, not from hydrogens on the same carbon or from equivalent ones. A CH₃ next to a CH₂ is a triplet because it has two neighbors, not because it has three H. Also, IR mainly identifies functional groups, whereas NMR maps the carbon–hydrogen framework.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.