¹H NMR Spectroscopy

What information does ¹H NMR spectroscopy provide?

• It provides information about the number of different proton environments, the relative number of protons in each environment, and the number of non-equivalent protons on adjacent carbon atoms.

What is used as the standard for ¹H NMR and why?

TMS (tetramethylsilane, (CH₃)₄Si.

Reasons:

1. All 12 protons are in the same environment, producing one strong, sharp signal.

2. It is inert and non-toxic.

3. It has a low boiling point (easily removed).

4. Its signal is at  δ = 0 (by definition) and is away from most other organic signals.

What characteristics of CCl₄ make it suitable as a solvent?

• Non polar due to tetrahedral shape, so allows it to function as a solvent for non-polar organic molecules

• No protons, so they give of no peaks in the ¹H spectrum

Why are deuterated solvents (e.g., CDCl₃) or CCl₄ used in ¹H NMR?

• These solvents contain no protons (¹H atoms). This ensures that a large solvent peak does not appear on the spectrum and obscure the peaks from the sample.

What is chemical shift (δ) in ¹H NMR?

• The position of a signal on the spectrum (in ppm) relative to the TMS standard. It indicates the chemical environment of the proton(s).

What does it mean if protons are deshielded?

• The protons are near an electronegative atom or group (like O, Cl, or C=O). This pulls electron density away from them, so they feel the external magnetic field more strongly and absorb at a higher chemical shift (further left/downfield)

What information does the peak integration provide?

• The relative ratio of the number of protons in each chemical environment. This is shown by the area under the peaks.

What is spin-spin splitting and what is the n+1 rule?

• Spin-spin splitting is when a ¹H NMR signal is split into a cluster of peaks by non-equivalent protons on adjacent carbon atoms.

• The n+1 rule predicts the number of peaks in the cluster, where n = the number of non-equivalent protons on adjacent carbon(s).

What splitting pattern is caused by 0, 1, 2, and 3 adjacent, non-equivalent protons?

• 0 adjacent protons → Singlet (1 peak)

• 1 adjacent proton → Doublet (2 peaks)

• 2 adjacent protons → Triplet (3 peaks)

• 3 adjacent protons → Quartet (4 peaks)

What are equivalent protons?

• Protons that are in the exact same chemical environment. They will all absorb at the same chemical shift and do not split each other's signals. (e.g., the 3 protons in a CH₃ group).

What splitting pattern would you expect for the –OH proton in an alcohol (like ethanol)?

• Usually, a singlet. The proton exchange in the –OH group is often fast and it does not typically couple with (split) adjacent protons.

Question 1 (1 mark) State why CDCl₃ is a suitable solvent for use in ¹H NMR spectroscopy.

• Answer: It contains no protons / ¹H atoms (1).