Proton nuclear magnetic resonance Click on the link on the icon below to view a video introducing NMR spectroscopy.
How are spectra created? Nuclei behave like tiny magnets – similar to a compass needle. The compass needle aligns with the Earths magnetic field and points north. When energy is put in, the needle can be made to align in a direction opposite to the Earths magnetic field. The needle will swing back as soon as the energy is removed.
When an external magnetic field is applied, hydrogen nuclei can align with the external field or against it. E radio waves External magnetic field Nucleus aligned with magnetic field – low-energy state. Nucleus aligned opposed to magnetic field – high-energy state. As nuclei relax back to the low-energy alignment, energy in the radio wave frequency is released. This energy is detected and recorded as peaks on a spectrum.
Proton environments In a compound, the protons are bonded to other atoms and so there are more electrons in the region of the protons. These electrons affect the external magnetic field experienced by the proton. The energy gap between high- and low-energy states will be slightly different. The frequency of radiation emitted as the nuclei relax back to the low-energy state will also be different. Protons emitting radiation of the same frequency are said to be in the same proton environment.
Determining proton environments Example: How many proton environments are there in ethanol? The three CH 3 protons are in one environment. The two CH 2 protons are in a second environment. The OH proton is in a third environment.
Preparing a sample To obtain the 1 H NMR spectrum of a sample it is usually necessary to dissolve the sample in a solvent. Solvents must not contain protons that will interfere with the sample being measured. A solvent must: – contain no hydrogen atoms, eg tetrachloromethane, CCl 4 or – have the hydrogen atoms replaced with deuterium ( 2 H), eg CDCl 3 or CD 3 OD.
Explaining spectra The scale runs from right to left and is called the chemical shift. It is measured in parts per million (ppm). Peaks to the left of TMS peak are said to be downfield of TMS. TMS (tetramethylsilane) is added to the solvent and provides a reference peak. The protons in TMS are assigned the value 0 ppm and the rest of the spectrum is calibrated relative to this.
Interpreting low-resolution 1 H NMR spectra Points to note: Each peak in a low-resolution spectrum represents one proton environment. The type of proton environment can be identified by looking up the chemical shift in a correlation table (data book). The area under the peak relates to the number of protons in the environment.
By comparing the heights of the integration curves we can determine the ratio of protons in each environment. In this spectrum the height ratio is 1:1 so there is an equal number of protons in each environment. The area is given as an integration curve and the height of the curve can be measured. The area under the peak gives information about how many protons are in each environment.
Solving problems It is helpful to lay out your interpretations in a table. Peak shift (ppm)Integration height (mm)Ratio 2.1301 3.8301
High resolution High-resolution spectra are run using a higher radio frequency and the peaks have more detail. Compare the spectra below for methyl propanoate. Low-resolution NMR for methyl propanoate. High-resolution NMR for methyl propanoate.
This spectrum is for pentan-3-one. The peaks show that there are two proton environments. Can you assign these peaks to the structure? CH 3 CH 2
When the spectrum is expanded it can be seen that each peak is made up of a number of peaks. These are called multiplets. Triplet: three peaks in the group. Quartet: four peaks in the group. Other multiplets include singlets and doublets.
n + 1 rule The number of peaks in a multiplet can give additional information about the structure. The splitting of peaks is caused by the neighbouring carbons hydrogen atoms. Protons in the same environment are said to be equivalent and as such behave as one proton. This follows the n + 1 rule. – n is the number of hydrogen atoms attached to the next-door carbon – n + 1 is how many peaks will be seen in the cluster.
CH 2 CH 3 CH 3 CH 2 COCH 2 CH 3 Split by 3 protons on next- door carbon so n = 3, n + 1 = 4 peaks. Split by 2 protons on next- door carbon, so n = 2, n + 1 = 3 peaks.
Example 2 – Assign the peaks and suggest a structure Molecular formula C 3 H 7 Br CH 3 A triplet due to CH 2 group adjacent. CH 2 This is not a simple quartet. There are extra splittings due to CH 3 and CH 2 neighbouring groups. CH 2 A triplet due to CH 2 group adjacent.