Difficulty: Intermediate | Prerequisites: General Chemistry, Lewis structures, electronegativity, understanding of functional groups, basic IR spectroscopy.
Proton (¹H) NMR spectroscopy tells you about the hydrogen environments in a molecule: how many distinct types of hydrogen are present, how many of each type there are, and what electronic environment each occupies. Together with IR and ¹³C NMR, it is the primary tool for structure determination in Organic Chemistry II. The chemical shift (δ, in ppm) of a proton signal reveals what the hydrogen is attached to, and this correlation table is the reference you will use to interpret spectra. If you are not comfortable with hybridisation (sp³, sp², sp) and how electronegativity affects electron density, review those concepts first.
¹H NMR chemical shifts tell you the electronic environment of each hydrogen in a molecule. Hydrogens on sp³ carbons far from electronegative groups appear upfield (low ppm), while those near electronegative atoms, on double bonds, on aromatic rings, or on aldehyde and carboxylic acid groups appear progressively further downfield (higher ppm). The correlation table is your roadmap for matching a peak's position to a structural feature.
Chemical shift (δ)
The position of an NMR signal on the spectrum, measured in parts per million (ppm) relative to a reference compound (TMS). In simple terms, it tells you where a peak sits on the x-axis, and that position reflects the electronic environment around the hydrogen.
Tetramethylsilane (TMS)
The standard reference compound in NMR, defined as δ = 0.0 ppm. All other chemical shifts are reported relative to TMS. Silicon is less electronegative than carbon, so TMS protons are highly shielded and resonate at the far right of the spectrum.
Shielding and deshielding
A shielded proton is surrounded by relatively high electron density, which partially blocks the external magnetic field, causing it to resonate at a lower ppm (upfield). A deshielded proton has lower electron density around it, so it feels a stronger effective magnetic field and resonates at higher ppm (downfield). Electronegative atoms, pi systems, and ring currents all cause deshielding.
Upfield vs. downfield
Upfield means toward lower ppm values (right on the spectrum, closer to TMS). Downfield means toward higher ppm values (left on the spectrum). Think of downfield as "more deshielded."
Integration
The area under each NMR peak, which is proportional to the number of equivalent hydrogens producing that signal. If one peak integrates for 3 and another for 2, the ratio of those hydrogen types is 3:2.
Splitting (multiplicity)
The pattern of sub-peaks within a signal, caused by neighbouring non-equivalent hydrogens (spin-spin coupling). A proton with n equivalent neighbours splits into n + 1 peaks. This is not a chemical shift concept, but it works alongside chemical shift data to assign structure.
R-CH₃ (methyl on sp³ carbon): δ 0.8 – 1.0 ppm. The most shielded common protons after TMS.
R-CH₂-R (methylene): δ 1.2 – 1.4 ppm.
R₃C-H (methine on sp³ carbon): δ 1.4 – 1.7 ppm.
R-C≡C-H (terminal alkyne): δ 2.5 ppm. Despite being on an sp carbon, alkyne protons are not as downfield as you might expect because the cylindrical electron cloud of the triple bond shields them (anisotropic effect).
Allylic protons (C=C-CH): δ 1.6 – 1.9 ppm. One carbon removed from a double bond, mildly deshielded.
Protons alpha to a carbonyl (O=C-CH): δ 2.1 – 2.6 ppm. The electron-withdrawing carbonyl pulls electron density away.
Benzylic protons (Ar-CH): δ 2.2 – 2.5 ppm.
R-CH₂-Cl: δ 3.6 – 3.8 ppm.
R-CH₂-Br: δ 3.4 – 3.6 ppm.
R-CH₂-I: δ 3.1 – 3.3 ppm.
R-CH₂-OR (protons alpha to oxygen): δ 3.3 – 3.9 ppm.
R-CH₂-OH: δ 3.3 – 4.0 ppm.
R-NH₂: δ 1.0 – variable (amine N-H protons are often broad and their position varies with concentration, temperature, and solvent; they often do not follow the n+1 rule cleanly).
H on C=C (vinyl protons): δ 4.6 – 6.0 ppm, depending on substitution. The sp² carbon is more electronegative than sp³ (more s-character), and the pi electrons create an anisotropic deshielding effect.
Aromatic ring protons (Ar-H): δ 6.0 – 8.5 ppm, most commonly 7.0 – 7.8 ppm for simple benzene derivatives. The ring current of the aromatic pi system creates a strong deshielding cone above and below the plane.
R-CHO (aldehyde proton): δ 9.5 – 10.0 ppm. Highly deshielded by the adjacent C=O. This signal is distinctive and usually integrates for exactly one hydrogen.
R-COOH: δ 10 – 12 ppm, broad. The most deshielded common proton. Often broad or may not appear if exchangeable with the solvent.
O-H (alcohols): δ 0.5 – 5.0 ppm, highly variable, often broad. Can be confirmed by a D₂O shake: the peak disappears when deuterium replaces the proton.
O-H (phenols): δ 4.5 – 7.7 ppm, variable.
N-H (amines): δ 1.0 – 5.0 ppm, variable and broad.
O-H (carboxylic acids): see above, δ 10 – 12 ppm.
"Higher ppm means more shielded." The opposite is true. Higher δ (further left, downfield) means more deshielded. Lower δ (further right, upfield) means more shielded.
"Alkyne protons should appear very far downfield because sp carbons are very electronegative." Terminal alkyne protons actually appear around δ 2.5 ppm, which is upfield of vinyl protons. The cylindrical pi electron cloud around the triple bond creates an anisotropic shielding effect that partially offsets the electronegativity of sp carbon.
"O-H and N-H peaks always appear in the same place." These exchangeable protons are notoriously variable. Their position depends on concentration, solvent, temperature, and hydrogen bonding. A D₂O shake experiment is the reliable way to identify them.
"Integration tells you the absolute number of hydrogens." Integration gives ratios, not absolute counts. A 3:2 integration ratio could mean 3H and 2H, or 6H and 4H. You need the molecular formula to pin down absolute numbers.
⚠️ Memorise the approximate ranges: alkyl 0 – 2, near O/N/X 2.5 – 4.5, vinyl 4.5 – 6.5, aromatic 6.5 – 8.5, aldehyde ~9.5, carboxylic acid ~11. This is the minimum for exam speed.
⚠️ Be ready to identify aldehyde vs. carboxylic acid by chemical shift and by the shape of the peak (aldehyde is a sharp singlet or doublet; acid O-H is very broad).
⚠️ The D₂O shake is a common exam scenario: "after adding D₂O, a peak at δ 3.5 disappears." You should recognise this as an exchangeable O-H or N-H.
⚠️ Expect questions that combine chemical shift data with integration and splitting to identify an unknown structure. The chemical shift narrows down which functional group; integration tells you how many; splitting tells you about neighbours.
True or false: A proton at δ 7.3 ppm is most likely attached to an aromatic ring.
True. Aromatic protons typically fall in the 6.5 – 8.5 ppm range.
Fill in the blank: Aldehyde protons appear near δ ______ ppm.
9.5 – 10.
True or false: A D₂O shake will cause a methyl singlet at δ 2.1 ppm to disappear.
False. Only exchangeable protons (O-H, N-H) disappear with D₂O. A methyl group on a C-H bond is not exchangeable.
Fill in the blank: Vinyl (alkene) protons typically appear between δ ______ and ______ ppm.
4.5 and 6.5.
True or false: Integration in ¹H NMR gives you the absolute number of hydrogens.
False. It gives ratios of hydrogens. The molecular formula is needed to determine absolutes.
Q: A ¹H NMR spectrum shows signals at δ 1.2 (triplet, 3H), δ 4.1 (quartet, 2H), and δ 8.1 (singlet, 1H). The molecular formula is C₃H₆O₂. What is the structure?
A: Methyl formate (HCOOCH₃) does not fit the splitting. The triplet-quartet pattern with 3H and 2H suggests an ethyl group (CH₃CH₂-). The singlet at δ 8.1 (1H) is consistent with a formate proton. The compound is ethyl formate (HCOOCH₂CH₃). The CH₂ at δ 4.1 is deshielded by the adjacent oxygen.
Q: You see a broad peak near δ 11.5 that disappears upon addition of D₂O. What type of proton is this?
A: A carboxylic acid O-H proton. The high chemical shift (~11 ppm) and exchangeability with D₂O are characteristic of -COOH.
Q: A signal at δ 9.7 ppm integrates for one hydrogen and appears as a singlet (or very small doublet). What is it?
A: An aldehyde proton (R-CHO). Aldehyde protons appear near δ 9.5 – 10 ppm and integrate for 1H.
Q: Two compounds, A and B, each have the molecular formula C₃H₆O. Compound A shows a singlet at δ 2.1 (3H) and a singlet at δ 9.8 (1H). Compound B shows a doublet at δ 1.1 (6H) and a septet at δ 2.5 (1H) with no peaks above δ 3. Identify both.
A: Compound A is propanal (CH₃CH₂CHO): the singlet at 9.8 is the aldehyde H, and the 3H at 2.1 is a methyl adjacent to the carbonyl (this simplified description assumes the spectrum; the actual splitting of propanal is more complex, but the aldehyde peak at ~9.8 is the key). Compound B is acetone ((CH₃)₂CO): the 6H doublet is the two equivalent methyls, and the septet at 2.5 fits protons split by six equivalent neighbours, though in practice acetone shows a sharp singlet at ~2.1. Re-examining: Compound A with δ 9.8 (1H, singlet) and δ 2.1 (3H, singlet) could be propanal, and Compound B with a 6H doublet and 1H septet is more consistent with an isopropyl group. The isomer question is testing whether you recognise the aldehyde signal.
Q: Why do aromatic protons appear further downfield than vinyl protons, even though both are on sp² carbons?
A: The aromatic ring current creates an additional deshielding effect. The circulating pi electrons generate a local magnetic field that reinforces the external field at the plane of the ring where the protons sit. Isolated vinyl protons experience deshielding from electronegativity and the pi bond but lack this ring-current effect.
The ¹H NMR chemical shift table works hand in hand with ¹³C NMR shifts. A carbonyl carbon appears at δ 190 – 220 in ¹³C NMR, while the aldehyde proton on that same carbon appears at δ 9.5 – 10 in ¹H NMR. Correlating data from both experiments gives you a much more complete structural picture.
IR spectroscopy complements NMR by confirming functional groups: if ¹H NMR shows a peak near δ 9.8 and IR shows a strong absorption near 1720 cm⁻¹ with the aldehyde C-H doublet at 2720 cm⁻¹, you can be confident you have an aldehyde.
Splitting patterns and coupling constants (covered separately) build on the chemical shift framework. Once you know what region a peak falls in, splitting and integration narrow down the exact connectivity.
¹H NMR, proton NMR, chemical shift table, delta ppm, NMR correlation table, shielding, deshielding, upfield, downfield, TMS, aromatic ring current, D₂O shake, exchangeable protons, organic chemistry II NMR, structure determination NMR