Nuclear Magnetic Resonance (NMR) Spectroscopy
Quick Notes
- NMR is an analytical technique used to help determine molecular structures.
- ¹³C NMR and ¹H NMR give information about the number of unique carbon and hydrogen environments in a molecule (number of peaks in a spectra) and positions of carbon and hydrogen atoms in a molecule (chemical shift, ppm, of each peak).
- ¹³C NMR produces simpler spectra compared to ¹H NMR.
- Chemical shift (δ) values depend on the molecular environment of carbon-13 or H-1 nuclei.
- ¹H NMR spectra use integration to show the relative number of protons in each environment and the n+1 rule to show splitting patterns, giving information about the number of hydrogens bonded to adjacent, non-equivalent carbon atoms.
- Tetramethylsilane (TMS) is used as a standard because it produces a single peak (with a chemical shift set as 0 ppm).
- For H-NMR, Deuterated solvents or CCl4 are used to avoid interfering signals.
Full Notes
The background theory of carbon and hydrogen NMR has been outlined in more detail here.
This page is just a quick refresh for what you need to know for AQA A-level Chemistry :)
If anything on this page is confusing or feels too difficult to understand, I recommend checking out the link above first and watching the videos it contains.
Introduction to NMR Spectroscopy
NMR (Nuclear Magnetic Resonance) Spectroscopy detects nuclei in a magnetic field.
¹³C and ¹H NMR are the most commonly used techniques. The hydrogen-1 and carbon-13 atoms have a property called nuclear spin, which means they act like tiny magnets. When placed in a strong magnetic field, these tiny magnets can absorb energy and 'flip' their spin – this is called resonance.
The amount of energy needed to do this depends their chemical environment. Because of this, atoms in different environments absorb slightly different amounts of energy for resonance to occur.
Chemical shift (δ) is used to describe this energy and is measured in parts per million (ppm) relative to TMS (δ = 0 - see below for more detail).
Spectra are produced that show peaks at certain ppm. Each peak refers to a unique carbon or hydrogen environment in the sample.
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. This ensures that the solvent doesn’t interfere with the spectra and there are no signals from the solvent.
The most important thing to be able to do with NMR is to recognise ‘unique’ carbon and hydrogen environments in a molecule, before worrying about more advanced areas such as peak splitting and the n+1 rule. Remember it isn’t just the immediate atoms bonded to a carbon or hydrogen you need to look at. It is also what those atoms are themselves bonded to.
¹³C NMR vs ¹H NMR
| Feature | ¹³C NMR | ¹H NMR |
|---|---|---|
| Detects | Carbon-13 nuclei | Hydrogen nuclei |
| Complexity | Simpler (1 peak per unique C environment) |
More complex (splitting patterns) |
| Chemical Shift Range | 0 – 200 ppm | 0 – 12 ppm |
| Peak Integration | Not relevant | Shows relative proton numbers |
| Spin-Spin Splitting | Not observed | n+1 rule |
Chemical Shift (δ) and Molecular Environment
Chemical shift (δ) depends on electron density around a nucleus responsible for the peak in the spectra.
Generally, more electronegative groups shift peaks downfield (higher δ values).
Data book values are used to compare the peaks on a spectra to identify possible bonding groups within a sample.
Tetramethylsilane (TMS) as a Standard
Tetramethylsilane (TMS) is used as an internal standard (δ = 0 ppm). This enables us to compare absorbances in NMR spectra and link them to data book values.
This means that all absorbance values are relative to the absorbances for C and H in TMS.
Why TMS?
- Produces a single peak (all carbon and hydrogen atoms in the same environment).
- Non-reactive and volatile (easily removed).
- Low chemical shift (does not interfere with peaks).
¹H NMR Integration
Integration (peak area) shows the ratio of protons in each, unique environment.
For example:The NMR spectra of propane (C3H8) shows two peaks, with a ratio of 1:3, matching that of the ratio of hydrogens in each environment.
Be really careful, integration ratios aren’t necessarily the actual number of protons in each environment - just the ratio. For example the propane spectra above has 2Hs in one environment and 6Hs in another. However, the integration ratios on the spectra are 1:3. You then have to look at the molecular formula (C2H8) and link the ratio of Hs to the actual number in each environment. 8Hs in total with a ratio of 1:3 means 2 Hs in one environment and 6Hs in the other :)
¹H NMR Spin-Spin coupling (peak splitting) and n+1 rule
Hydrogens bonded to adjacent, non-equivalent carbon atoms can cause a peak in a H-NMR spectra to be ‘split’.
The number of hydrogens bonded to adjacent, non-equivalent carbon atoms 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.
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) in ¹H NMR:
- CH3 group shows a triplet (next to CH2).
- CH2 group shows a quartet (next to CH3).
- OH appears as a singlet (no splitting).
Summary
- NMR reveals how many unique carbon and hydrogen environments are present and where they are (chemical shift, δ).
- ¹³C NMR spectra are simpler (no splitting, no integration), while ¹H NMR shows integration ratios and splitting (n+1).
- Electronegative groups shift peaks downfield; use data tables to match δ ranges to likely environments.
- TMS gives a single peak at δ = 0 ppm and is used as the standard reference.
- Deuterated solvents or CCl4 are used so the solvent doesn’t contribute proton signals.
- Integration ratios reflect relative numbers of protons; use the molecular formula to convert ratios into actual counts.
- Peak splitting follows the n+1 rule and tells you the number of hydrogens on adjacent, non-equivalent carbons.