📝 NMR spectroscopy

Task

🎧️ Listen to the recording and mind pronunciation of words.

Different chemical environments (bonds and atoms surrounding a nucleus) affect the strength of magnetic field that must be applied to a nucleus in order for it to enter the resonance state.
By measuring the strength of magnetic field that must be applied, NMR spectroscopy gives us information about the local environment of specific atoms in a molecule. This can be used to deduce information about a chemical shift. The environments of C-13 and H-1 atoms are most commonly studied in NMR spectroscopy. The horizontal scale on an NMR spectrum represents chemical shift (δ). It is measured in parts per million (ppm) of the magnetic field strength needed for resonance in a reference chemical called TMS.
Tetramethylsilane is universally used as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum. The signal from the carbon atoms in TMS is defined as having a chemical shift of zero.
The chemical shift values of peaks on the C-13 NMR spectrum can help us identify the types of carbon atom in a compound. Carbon-13 NMR spectroscopy investigates the absorption of radiation by nuclei of carbon-13 atoms (13C) and is thus a technique for obtaining information about the number and arrangement of carbon atoms in a molecule. Identical carbon atoms all absorb radiation at the same frequency and so contribute to the same peak. The number of different peaks, therefore, tells you the number of different environments of carbon atoms in the molecule. The frequency of the radiation absorbed depends on the environment of the hydrogen atom and gives information about the position of the carbon in the molecule relative to other carbon atoms and functional groups.
Proton NMR spectroscopy investigates the absorption of radiation by nuclei of hydrogen atoms and is thus a technique for obtaining information about the number and arrangement of hydrogen atoms in a molecule. The area under the peaks on a H-1 NMR spectrum is proportional to the number of hydrogen atoms causing the signal. The ratio of the areas under the peaks tells you the ratio of H-1 atoms in each environment. The chemical shift values of peaks on an H-1 NMR spectrum give information about the likely types of proton environment in a compound.
The peaks are often not single peaks; they are split into a number of peaks very close together. The way in which each peak is split depends on the total number of hydrogen atoms on adjacent. The level of splitting of the peaks is given by (n+1), where n = number of hydrogen atoms on adjacent atoms. A hydrogen atom with no hydrogen atoms on adjacent carbon atoms will have no splitting and will give a single peak known as a singlet. If n = 1 the peak will be split once and will thus appear as a doublet. Proton NMR spectra are very useful for working out the structure of organic molecules, particularly if the molecular formula is known.

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