A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Nuclear Magnetic Resonance (NMR) spectroscopy exploits the quantum spin properties of atomic nuclei (especially ¹H and ¹³C) in an external magnetic field to provide detailed structural information. In ¹H NMR, the chemical shift (in ppm, referenced to TMS at 0 ppm) encodes the electronic environment of each proton — deshielded protons (near electronegative groups or in aromatic rings) resonate at higher ppm values. Integration gives the relative count of equivalent protons in each environment, and the splitting pattern (multiplet structure following the n+1 rule) reveals the number of adjacent non-equivalent protons. Together, these three features allow unambiguous structural assignment.
Work through ¹H NMR spectra of simple known molecules (ethanol, acetone, diethyl ether) before tackling unknowns. For each spectrum: first count signals (distinct environments), then use integration for H counts, then decode splitting. Sketch expected shift ranges: CH₃ (~1 ppm), vinyl (~5–6 ppm), aromatic (~7–8 ppm), aldehyde (~9–10 ppm), carboxylic acid (~11–12 ppm).
NMR spectroscopy works because certain atomic nuclei — particularly ¹H and ¹³C — behave like tiny bar magnets. When placed in a strong external magnetic field, these nuclei can align with or against the field, and they absorb radiofrequency energy to flip between those states. The exact frequency at which a nucleus absorbs depends on its electronic environment: electrons surrounding a nucleus partially shield it from the external field, so electronegative neighbors that pull electrons away cause the nucleus to resonate at a higher frequency (higher ppm on the chemical shift axis). This is why an aldehyde proton (~10 ppm) appears far downfield compared to a simple alkyl CH (~1 ppm).
The three pieces of information you read from a ¹H NMR spectrum work together like three independent clues. The chemical shift tells you what type of environment a proton is in (alkyl, next to oxygen, aromatic, etc.). The integration tells you the relative number of protons producing each signal — if one signal is twice as tall as another, it represents twice as many equivalent protons. The splitting pattern (multiplicity) tells you how many non-equivalent protons are on adjacent carbons: the n+1 rule states that n neighboring protons split a signal into n+1 lines, creating doublets, triplets, quartets, and so on.
Consider ethanol (CH₃CH₂OH). You expect three signals: the CH₃ group, the CH₂ group, and the OH proton. The CH₃ is adjacent to two CH₂ protons, so it appears as a triplet (2+1=3). The CH₂ is adjacent to three CH₃ protons, so it appears as a quartet (3+1=4). The OH proton is often a broad singlet because fast proton exchange averages out coupling. Integration confirms the 3:2:1 ratio of protons.
¹³C NMR is complementary but interpreted differently. It tells you how many distinct carbon environments exist, but — crucially — peak heights are not proportional to the number of carbons (unlike ¹H integration). This is because different carbons relax at different rates during the experiment. Broad-band decoupling also removes the C-H splitting, so each carbon environment appears as a single line regardless of attached protons.
The power of NMR for structural determination comes from combining all these signals. Unknown compound? Count the ¹H signals to count distinct proton environments, use integration to tally protons in each, decode splitting to map connectivity, and match chemical shifts to functional group tables. Cross-checking with ¹³C NMR and other spectroscopic methods (IR, mass spec) allows complete structure assignment — often without ever synthesizing a reference compound.