Question 1

What does TMS stand for in NMR and why is it used?

Accepted Answer

TMS stands for tetramethylsilane, Si(CH₃)₄. It is used as a reference compound because its 12 equivalent protons give a single sharp signal at δ = 0 ppm. It is chemically inert, volatile (easy to remove), and its signal does not interfere with most organic samples.

Question 2

How do I determine the number of hydrogen environments from an NMR spectrum?

Accepted Answer

Each distinct signal (peak or group of peaks) corresponds to a set of equivalent protons. Count the number of separate signals, ignoring splitting. For example, ethanol (CH₃CH₂OH) has three signals: one for CH₃, one for CH₂, and one for OH. Remember that protons in the same chemical environment (e.g., three CH₃ protons in a methyl group) give one signal.

Question 3

Why does the OH proton in alcohols often appear as a singlet without splitting?

Accepted Answer

Protons on oxygen (or nitrogen) undergo rapid exchange with other labile protons (e.g., from water or other alcohol molecules). This exchange happens faster than the NMR timescale, so the proton does not 'see' neighbouring protons and thus does not split. The signal is typically a broad singlet.

Question 4

What is the difference between ¹H NMR and ¹³C NMR?

Accepted Answer

¹H NMR detects hydrogen nuclei, while ¹³C NMR detects carbon-13 nuclei (only about 1% of carbon atoms). ¹H NMR gives information about the number and environment of hydrogen atoms, including integration and splitting. ¹³C NMR gives information about carbon environments, but integration is not reliable and splitting is usually removed by decoupling, so each carbon gives a single peak.

Question 5

How do I use the n+1 rule to predict splitting patterns?

Accepted Answer

The n+1 rule states that a proton with n equivalent neighbouring protons will be split into (n+1) peaks. For example, a CH₃ group next to a CH₂ group (n=2) gives a triplet. The CH₂ group next to a CH₃ (n=3) gives a quartet. The splitting pattern reveals the number of adjacent protons. Remember that only non-equivalent neighbours cause splitting.

Question 6

Can NMR distinguish between structural isomers?

Accepted Answer

Yes, NMR is excellent for distinguishing isomers because different structures have different numbers of proton/carbon environments, different chemical shifts, and different splitting patterns. For example, propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃) have very different ¹H NMR spectra: propanal has three signals (including an aldehyde proton at ~9-10 ppm), while propanone has only one signal (a singlet at ~2.1 ppm).

Nuclear magnetic resonance spectroscopy (A-level only)

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes