9701 · 37.3
Carbon-13 NMR spectroscopy flashcards
Revision flashcards for Cambridge 9701 Carbon-13 NMR spectroscopy (syllabus 37.3). Flip, recall, then mark a real past-paper question.
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What is a 'carbon environment'?
The specific bonding and neighbouring atoms for a particular carbon atom in a molecule. Carbons in identical environments due to symmetry are 'chemically equivalent' and produce a single signal.
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Why is TMS (Tetramethylsilane) used as the reference standard in NMR?
It is chemically inert, volatile (easily removed), and its four equivalent carbons produce a single, sharp signal defined as 0 ppm, which is away from the signals of most organic compounds.
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What does 'proton-decoupled' mean in ¹³C NMR?
A technique is used to irradiate the sample at proton resonant frequencies. This removes the splitting effect (coupling) of neighbouring ¹H nuclei on ¹³C signals, simplifying the spectrum to single peaks for each carbon environment.
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What information is given by the number of signals in a ¹³C NMR spectrum?
The number of signals directly corresponds to the number of different, non-equivalent carbon environments in the molecule.
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What information is NOT reliably given by a standard ¹³C NMR spectrum?
The relative number of carbons in each environment. Unlike in proton NMR, the integration (area under the peak) is generally not proportional to the number of ¹³C nuclei.
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What causes a carbon nucleus to be 'deshielded' and have a higher chemical shift (δ)?
The presence of nearby electronegative atoms (e.g., O, Cl) or π-bonds (e.g., C=C, C=O). These withdraw electron density, reducing the shielding from the external magnetic field.
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State the approximate chemical shift (δ) range for a carbon in a C=O group of a ketone or aldehyde.
Approximately 190 - 220 ppm. This is one of the most deshielded regions in a ¹³C NMR spectrum.
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State the approximate chemical shift (δ) range for a carbon in a C=O group of a carboxylic acid or ester.
Approximately 160 - 185 ppm. It is less deshielded than a ketone due to resonance with the adjacent single-bonded oxygen atom.
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How many signals would you expect in the ¹³C NMR spectrum of propane, CH₃CH₂CH₃?
Two signals. The two outer CH₃ carbons are in an identical environment due to symmetry. The central CH₂ carbon is in a different environment.
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How many signals would you expect in the ¹³C NMR spectrum of toluene (methylbenzene)?
Five signals. One for the methyl carbon, and four for the benzene ring carbons (the carbon attached to the CH₃, and the ortho, meta, and para carbons are all unique).