Chapter 13: Structure Determination: Nuclear Magnetic Resonance Spectroscopy
Spinning nuclei absorb radiofrequency energy in a magnetic field
A hydrogen or carbon-13 nucleus possesses a property called nuclear spin that makes it behave as a minute bar magnet. Not all nuclei qualify: a nucleus with an even number of protons and an even number of neutrons, such as ¹²C or ¹⁶O, has zero spin and is invisible to NMR. Organic chemistry is fortunate in its two workhorses — ¹H, the dominant hydrogen isotope, and ¹³C, the 1.1% minor isotope of carbon — both of which are spin-active. Outside a magnetic field, the nuclear magnets point randomly. Inside a strong applied field, quantum mechanics allows a spin-½ nucleus only two orientations:…
Shielding sets the chemical shift, reported on the delta scale
The applied magnetic field acts on every electron in a molecule, forcing the electron clouds into a circulation that produces small magnetic fields of their own. At most nuclei this induced field opposes the applied one, so the nucleus experiences an effective field slightly weaker than the magnet delivers. The phenomenon is called shielding, and its magnitude depends on the electron density around the nucleus. A proton on a carbon bearing only other carbons and hydrogens is electron-rich and well shielded; a proton near an electronegative atom such as oxygen or a halogen has electron density…
Equivalent protons share one signal
A ¹H NMR spectrum does not show one peak per hydrogen atom; it shows one peak per set of hydrogens that share the same chemical environment. Two protons share an environment — they are chemically equivalent — when the molecule's own dynamics or symmetry makes them indistinguishable. Rotation about carbon–carbon single bonds is far faster than the NMR measurement, so the three hydrogens of a methyl group are averaged into a single environment, and the same is true of the two hydrogens of a typical CH₂ group. Symmetry operations of the whole molecule then extend equivalence across larger…
Shift regions locate environments; integration counts their protons
The chemical shift of a proton is set almost entirely by its immediate structural environment, so a compact table of regions covers most of organic chemistry. Protons on saturated carbons with no nearby substituent absorb between δ 0.9 and 1.8. Electron-withdrawing neighbors move signals downfield in proportion to their pull: protons on a carbon adjacent to a carbonyl group or a carbon–carbon double bond appear at δ 2.0–2.6, and protons on a carbon directly bonded to a halogen or an oxygen appear at δ 2.5–4.5 — the methoxy singlet of an ester near δ 3.7 and the OCH₂ quartet of ethyl acetate…
Neighboring protons split signals: the n + 1 rule
Each spin-active nucleus is a small magnet, and its two orientations slightly raise or lower the effective field at nearby nuclei. An observed proton with a single neighboring proton therefore resonates at two slightly different frequencies, depending on the neighbor's orientation, and its signal appears as two lines of equal intensity — a doublet. With two equivalent neighbors the possible orientation combinations produce three effective fields in a 1:2:1 population ratio, giving a triplet; three neighbors give a 1:3:3:1 quartet. In general, n equivalent neighboring protons split a signal…
Coupling constants measure the splitting and identify the relationship
The n + 1 rule counts the lines in a multiplet; the coupling constant measures the distance between them, and that number carries structural information the line count alone does not. J is the spacing between adjacent lines of a multiplet, quoted in hertz rather than ppm. Spectra are plotted on the δ scale, so reading a coupling off a spectrum means converting: J = Δδ(ppm) × spectrometer frequency (MHz). Adjacent lines 0.024 ppm apart on a 300 MHz spectrometer are 0.0243 × 300 = 7.3 Hz apart. Record the same compound on a 60 MHz instrument and those lines move to 0.122 ppm apart — five times…
¹³C NMR maps the carbon skeleton; DEPT sorts CH₃, CH₂, CH, and C
Carbon NMR faces a sensitivity obstacle that proton NMR does not: the spin-active isotope ¹³C makes up only 1.1% of natural carbon, so the signal available from a given sample is roughly a hundredfold weaker, and spectra are accumulated over many scans. The low abundance carries a compensating simplification — the chance of two ¹³C atoms sitting on adjacent carbons is negligible, so carbon–carbon splitting does not complicate the spectrum. Coupling to the attached protons would still split each carbon signal, so routine spectra are recorded with broadband proton decoupling, which averages the…
A four-question inventory converts spectra into structures
With the individual tools in hand — shifts, integrals, multiplicities, and the carbon spectrum — structure determination becomes a disciplined inventory rather than inspired guessing. The degrees of unsaturation implied by the molecular formula are computed first — for a compound CcHh containing no other elements, that is (2c + 2 − h) / 2, each unit standing for one ring or one pi bond. C3H6O gives (2·3 + 2 − 6)/2 = 1, so the spectrum must account for exactly one ring or double bond; C4H8O2 likewise gives 1, since a C=O or a ring changes what the spectrum must contain. The proton spectrum is…
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