Proton ¹H-NMR Spectroscopy

Specification Reference Organic Chemistry, Analytical techniques 37.4

Quick Notes

1H NMR shows the environments of hydrogen atoms in a molecule and gives information about neighbouring groups.
Each peak = a different proton (¹H) environment.
Chemical shift (δ, ppm) tells us the type of environment.
Integration (peak area) = relative number of protons in each environment.
Splitting pattern (multiplets) = number of adjacent non-equivalent protons using the n + 1 rule.
TMS is used as a reference (δ = 0 ppm).
Deuterated solvents (e.g. CDCl₃) are used to avoid interfering peaks.
O–H and N–H protons can be identified by proton exchange with D₂O.

Full Notes

The background theory of carbon and hydrogen NMR has been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

¹H NMR (proton nuclear magnetic resonance) is a powerful technique that provides detailed information about the hydrogen atoms (protons) in an organic molecule. Graphs (spectra) are produced that are analysed

The spectra can tells us:

How many different hydrogen (proton) environments there are.
The ratio of hydrogens in each environment.
How the hydrogens are arranged relative to one another.

Key Features of a ¹H NMR Spectrum

Chemical Shift (δ, ppm)

Each type of proton appears at a specific chemical shift depending on its chemical environment.

Generally, more electronegative groups shift peaks downfield (higher δ values).

Data book values are used to compare peaks to identify possible bonding groups within a sample.

CIE A-Level Chemistry data booklet table of typical chemical shift ranges for hydrogen NMR.

Relative Peak Area (Integration)

Integration patterns (peak area) show the ratio of protons in each unique environment.

CIE A-Level Chemistry hydrogen NMR spectrum of propane showing integration ratio of 1:3.

Matt’s exam tip

Be really careful, integration ratios aren’t necessarily the actual number of protons in each environment — just the ratio. For example, propane has 2Hs in one environment and 6Hs in another. The integration ratio is 1:3. With a molecular formula of C₃H₈, that ratio corresponds to 2H and 6H.

Spin-Spin Coupling and the n+1 Rule

Hydrogens bonded to adjacent, non-equivalent carbon atoms can cause a peak to be split.

The number of hydrogens bonded to adjacent, non-equivalent carbons determines how many times the peak is split. This is summarised by the n + 1 rule.

Where n is the number of protons bonded to adjacent, non-equivalent carbon atoms and n+1 is the number of times a peak will be split.

CIE A-Level Chemistry diagram showing peak splitting in proton NMR according to the n+1 rule.

This is useful to know when analysing spectra as it means the number of hydrogen atoms bonded to adjacent, non-equivalent carbon atoms can be determined from peak splitting.

Example Ethanol (CH₃CH₂OH)¹H NMR Spectra

CIE A-Level Chemistry proton NMR spectrum of ethanol showing triplet, quartet, and singlet peaks.

CH₃ group shows a triplet (next to CH₂).
CH₂ group shows a quartet (next to CH₃).
OH appears as a singlet (no splitting).

Predicting and Interpreting Spectra

You’ll often be asked to match a molecule to its ¹H NMR spectrum.

Some steps to follow:

Count and identify unique H environments.
Predict δ values for each.
Use integration to match peak areas.
Use splitting patterns to identify neighbouring Hs.
Match or deduce the structure.

The Role of TMS (Tetramethylsilane)

Tetramethylsilane, TMS, (CH₃)₄Si, is used as an internal standard (δ = 0 ppm). This allows peaks to be compared to reference values.

CIE A-Level Chemistry proton NMR spectrum showing the TMS reference peak at 0 ppm.

Why is TMS used?

Produces a single peak (all protons equivalent).
Non-reactive and volatile (easily removed).
Low chemical shift (does not interfere).

Deuterated Solvents

For H-NMR, samples must be dissolved in deuterated solvents (contain only isotopes 2H that don’t have an overall spin) or solvents that have no hydrogens in, such as tetrachloromethane, CCl₄.

Why not just use normal solvents like H₂O or CHCl₃?

Protonated solvents interfere with the ¹H NMR signal.
Deuterated solvents contain deuterium (²H), which does not show up in ¹H NMR.

Identifying O–H and N–H Protons with D₂O Exchange

D₂O (deuterated water) contains ²H isotopes of hydrogen, which do not appear in ¹H NMR spectra.

If a sample contains O-H or N-H bonds and is dissolved in a D₂O solvent, the H in the O-H bond will be exchanged with deuterium atoms from the D₂O. Because the deuterium doesn’t show on a NMR spectra, no peak will appear for the O-H or N-H environment now.

CIE A-Level Chemistry diagram showing how O–H protons are exchanged with deuterium using D2O in NMR.

To confirm O-H environments in a compound, we can run an NMR without D₂O and then repeat with D₂O as the solvent. If any peaks in the spectra disappear, we know they must be O-H groups.

CIE A-Level Chemistry spectra comparison of propanol with and without D2O showing disappearance of O–H peak.

The O–H or N–H protons are replaced with deuterium, and the corresponding peak disappears from the spectrum.

Summary

¹H NMR spectra show hydrogen environments in organic molecules.
Chemical shift (δ) identifies environments.
Integration shows relative numbers of protons.
Splitting (n+1 rule) shows neighbouring protons.
TMS is the reference at 0 ppm.
Deuterated solvents avoid solvent peaks.
D₂O exchange helps confirm O–H and N–H protons.

Proton 1H-NMR Spectroscopy