- What is NMR:
NMR stands for "nucleic magnetic resonance". NMR is commonly used to figure out the chemical structures of the unknown compounds by scientists, simply by placing them in NMR machine and interpreting the spectrum. The following guide, shows you what happens and how we get the spectrum.
- Magnetic Field:
All moving charges have:
1) Magnetic Field
2) Electric Field
To start with, we know that electrons move and also have a negative charge, therefore they have magnetic field !
But what about the atoms?
Well they satisfy the first requirement which is the charge as they have protons (+ charge) in the nuclei. However some nuclei with odd atomic number spin, as they have got a Spin Quantum Number ≠ 0. [I ≠ 0]
As a result these special atoms have magnetic field which interacts with other magnetic fields,the concept which allow us to find the chemical structures! :)
It is important to note that atoms with "EVEN" atomic number have I = 0, which results in no spinning --> no magnetic field. [e.g 12C, 16O, 32S]
- Quantum Spining:
When an atom with I ≠ 0 is placed in a strong magnetic field the nuclei will adopt to "2I + 1" orientations.
You might ask how do we know the value of "I" ?!
You can find the values in references as they are constant for each isotope, but the important isotopes for us are 1H and 13C with quantum number "I = 1/2". As a result "(2 x 1/2) + 1 = 2" orientations.
Two alignments means Two possibilities:
1) Aligning WITH the field.
2) Aligning AGAINST the field.
More atoms are aligning with the field as it is energetically favourable
- NMR Effect:
The spinning against the magnetic field has a higher energy state than the spin with the field, for the obvious reason of parallel spin being more stable than anti-parallel.
By applying an external electromagnetic field we can cause a transition from parallel to anti-parallel state and record the transition. The higher the applied magnetic field, the higher the transition, and therefore the higher the quality of the spectrum produced.
- Chemical Shifts:
So how a NMR spectrum can give us insight to the chemical structures?
Atoms are made up of protons and electrons, and it is the electron density around the atom, that leads to a specific characteristics of each atom. So lets see how all these work!
Electrons also have magnetic field as discussed above, however their field opposes the applied external magnetic field. Consequently if the atom is shielded by electrons, the Beff will decrease, and if the atom is deshielded the Beff will increase.
For an atom to be shielded, it needs to be surrounded by an electron donating atoms, and in order to be deshielded it needs to be surrounded by electronegative atom or electron withdrawing groups of atoms as shown in the example below:
- NMR Spectrum:
Two commonly encountered spectrums are 1H and 13C, therefore for simplicity we are just going to talk about the "proton" NMR to explain the features of the spectrum. The spectrum has four main properties that tell us something about the chemical structure:
1) Number of Peaks: Which represents number of differnet environments.
2) Location of Peak: Tells us about shielding and deshielding.
3) Peak Intensity: Shows number of atoms with same environment.
4) Peak Splittings: Indicates the number of protons on adjacent atoms.
The Spectrum looks like the diagram below, note how the position is related to shielding:
- 1H NMR Spectroscopy:
More details and references are available in the interpretation section. So feel free to click on the icon in the right hand side toolbar for a complete guide.
1NMR usually starts from 0 ppm and gives signals up to 13 ppm. 0 ppm shows the most highly shielded proton possible, and as a matter of fact, to determine the 0 ppm, highly shielded compounds such as Tetramethyl Silane is used. There are usually splittings present in the spectrum which gives us an insight to the neighbouring protons.
- 13C NMR Spectroscopy:
More details and references are available in the interpretation section. So feel free to click on the icon in the right hand side toolbar for a complete guide.
The 13C NMR does not show any splittings, and it usually starts from 0 - 200 ppm. It is an important spectrum however the interpretation of it is much more simpler than 1H spectrum as you will see in the in the "interpretation page".
