Part 1: Carbon-13 NMR and chemical environments

Use symmetry to count environments, interpret chemical shifts and explain the role of TMS and deuterated solvents.

A signal represents an environment, not one atom

NMR uses the response of suitable nuclei in a magnetic field. Their surrounding electrons affect the local magnetic environment, so chemically different positions can absorb at different frequencies. Chemical shift, δ, reports position relative to a reference on a parts-per-million scale.

For carbon-13 NMR, count chemically distinct carbon environments. Equivalent carbons contribute to the same signal, so a molecule with six carbons need not show six peaks. Symmetry can make carbons equivalent even when they are far apart in a drawing.

OCR carbon-13 spectra here are proton-decoupled: do not apply the proton n + 1 splitting rule to these carbon peaks. Peak intensities in ordinary carbon-13 spectra are not used as a simple carbon-count integration scale in this course.

Worked environment count: compare both ends

Pentan-3-one, CH₃CH₂COCH₂CH₃, has a plane of symmetry through its carbonyl group. The two terminal CH₃ carbons are equivalent, the two CH₂ carbons are equivalent and the carbonyl carbon is unique, so there are three carbon environments.

Pentan-2-one, CH₃COCH₂CH₂CH₃, has no corresponding equivalence between its ends. Its five carbons are in different environments, so five signals are expected in the simplified interpretation. Both molecules have the same formula; signal count helps distinguish their connectivity.

Label environments with letters directly on the structure. Compare the whole route of bonds from each candidate carbon, not just whether it is labelled CH₃ or CH₂. An ethoxy CH₂ and an alkyl-chain CH₂ are not equivalent merely because both carry two hydrogens.

Use the data sheet to narrow structural possibilities

Carbonyl carbons appear much further downfield than ordinary saturated alkyl carbons. Carbons attached to electronegative atoms and unsaturated carbons occupy characteristic regions. Use the supplied OCR chemical-shift data rather than treating a single memorised value as an exact fingerprint.

Chemical shifts vary with molecular surroundings. A peak in a characteristic range supports a type of carbon environment, but nearby functional classes can overlap. Combine the shift with the formula, number of signals and other evidence.

For an illustrative ethyl ethanoate spectrum, four signals near 14, 21, 60 and 171 ppm fit two different methyl environments, an O–CH₂ carbon and an ester carbonyl. These are teaching values, not an experimentally measured spectrum attached to this page. The structure must explain all four signals.

Why TMS and deuterated solvent are used

Tetramethylsilane, TMS, provides the 0 ppm reference. Its equivalent methyl groups give a single reference signal in each relevant spectrum, and its signal is well separated from many ordinary organic signals. It is chemically suitable for many routine organic measurements and can be removed readily because it is volatile.

A deuterated solvent reduces interference from solvent hydrogen in a proton NMR spectrum because deuterium is not observed as ordinary ¹H in that experiment. This does not mean every real spectrum is completely free of residual solvent or water peaks. Use any labelled solvent peaks appropriately.

Distinguish dissolving the sample in a deuterated solvent from adding D₂O as an exchange test. The former supports acquisition; the latter deliberately tests exchangeable OH/NH protons by changing their isotopic identity.

A repeatable carbon-spectrum method

Start with the molecular formula and count the total carbons. Then count observed sample signals, excluding identified reference/solvent signals. Fewer signals than carbons suggests equivalence or symmetry, not lost atoms.

Assign distinctive regions, especially carbonyl and unsaturation, using the data sheet. Propose structures that fit both the environment count and shifts. Check them against proton NMR and IR before making a final identification.

An absent expected signal can challenge a proposal, but real experimental limitations or supplied simplifications matter. For examination spectra, use the stated data and do not invent missing peaks simply to rescue a preferred structure.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources