Flashcards · 9620 · A Level Topic 33

Nuclear magnetic resonance spectroscopy (A Level) flashcards

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Why is TMS, Si(CH₃)₄, used as the standard in NMR?: It gives one intense signal (12 equivalent H, 4 equivalent C) at the low-δ end, clear of nearly all organic signals. It is inert and volatile, so it can be removed and the sample recovered.
Why are ¹H NMR samples dissolved in CDCl₃ or CCl₄, not CHCl₃?: CDCl₃ (deuterium, ²H) and CCl₄ (no H) give no ¹H signal. The ¹H in CHCl₃ would give a huge signal that swamps the sample’s.
What does an integrated ¹H NMR spectrum show?: The relative areas of the signals: the ratio of H atoms in each environment. Scale the simplest ratio to the molecular formula.
The n+1 rule: A ¹H signal is split into n + 1 peaks by n non-equivalent H on the adjacent carbon(s): 0 → singlet, 1 → doublet, 2 → triplet, 3 → quartet.
2H quartet + 3H triplet (¹H NMR): An ethyl group, CH₃CH₂–.
9H singlet (¹H NMR): (CH₃)₃C–.
¹³C vs ¹H NMR: ¹³C shows carbon environments with no splitting; ¹H gives integration and splitting.
What affects chemical shift (δ)?: The molecular environment: an electronegative atom (O, Cl, Br) or a C=O group close to the nucleus gives a larger δ. Splitting, by contrast, counts H on the neighbouring carbons.
How many ¹³C lines and ¹H signals does butanedione, CH₃COCOCH₃, give?: 2 ¹³C lines (C=O and CH₃, by symmetry) and 1 ¹H signal (a singlet: all 6 H equivalent).