Flashcards · 7405 · A Level Topic 33

Nuclear magnetic resonance spectroscopy (A Level) flashcards

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Four reasons TMS is used as the standard: 12 equivalent H atoms give one strong single peak; its peak (δ = 0) is at lower δ than almost all other protons; it is inert; it is volatile, so it is easily removed.
Why are ¹H NMR samples dissolved in CDCl₃ or CCl₄?: Deuterium gives no signal in a ¹H spectrum, and CCl₄ has no H atoms, so the solvent does not swamp the sample’s peaks.
The n+1 rule: A ¹H signal is split into n + 1 lines by n H atoms on adjacent carbon atoms.
Splitting pattern of an ethyl group, CH₃CH₂–: A quartet (CH₂, 2H) and a triplet (CH₃, 3H).
What does integration of a ¹H spectrum give?: The relative number of H atoms in each environment, as a ratio. You need the molecular formula to get actual numbers.
How many peaks in the ¹³C NMR spectrum of methyl ethanoate, CH₃COOCH₃?: Three: the C=O carbon, the O–CH₃ carbon and the CH₃ on the C=O.
¹³C vs ¹H: ¹³C counts carbon environments and shows no splitting. ¹H counts hydrogen environments and gives integration and splitting.
Aldehyde vs ketone: Both show C=O near 190–220 in ¹³C. Only an aldehyde has a ¹H peak at δ 9–10.