Carbon-13 NMR

Carbon-13 NMR

1. Carbon-13 NMR depends on nuclear spin

   • In the absence of a magnetic field, the spinning nuclei are randomly orientated, but when a magnetic field is applied the nuclei line up so that they either have their spin aligned parallel with the applied field (lower energy state) or with the spin opposed (higher energy state)

2. Electromagnetic radiation in the radio-wave region of the electromagnetic spectrum can provide the energy needed to 'flip' the alignment of the nuclei - hence the term nuclear magnetic resonance

3. The actual energy at which the carbon-13 nucleus flips depends on the atoms to which the carbon is attached

   • Bonded electrons shield the nucleus and thus there is an increase in the energy required to bring about resonance; however, the more electronegative the atom to which the carbon is attached, the less shielding there is
     • For example, the C of a carbonyl, C=O, group has little shielding and is on the downfield (left side) of the spectrum, while the C of a methyl, CH3, group is more strongly shielded and is on the upfield (right) side of the spectrum
     • The difference between the position of absorbance of the nuclei compared with that of a reference (usually tetramethylsilane, Si(CH3)4, referred to as TMS) is referred to as the chemical shift [symbol lower case delta], and is given in units of ppm
       • The shift of TMS is 0 ppm

4. Characteristic values for different carbon environments are given above

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