NMR spectra

Meaning: In ¹H NMR, each set of chemically equivalent hydrogens gives one signal. Position reflects shielding: electronegative atoms and π systems deshield a proton and move its signal downfield (left, higher δ). Area (integration) is proportional to the number of hydrogens in the signal. Splitting reflects hydrogens on adjacent carbons.

Equation: peaks in a signal = n + 1, where n is the number of equivalent hydrogens on adjacent carbons (a dimensionless count). Chemical shift: δ = (ν − νTMS)/ν0 × 10⁶, where ν is the signal frequency, νTMS is the tetramethylsilane reference frequency, and ν0 is the spectrometer frequency, all in Hz; δ is in ppm.

On the MCAT: You get a formula and a spectrum and must pick the structure. Approximate shifts: alkyl 0.9–1.5; next to C=O 2–2.5; alkene 4.5–6; aromatic 6.5–8; aldehyde 9–10; carboxylic acid 10–12. Expect questions on how many signals a molecule gives, or which signal is farthest downfield.

Common confusion: Equivalent hydrogens do not split each other, so splitting comes only from nonequivalent neighbors on adjacent carbons. A CH₃ next to a CH₂ appears as a triplet, but the three CH₃ hydrogens themselves do not make it a quartet. Also, O–H and N–H signals are often broad and can appear as singlets because of exchange.