13C NMR - Predicting signal position
The position at which a particular carbon atom absorbs in the 13C NMR spectrum of a molecule depends on the electronic environment of the carbon.
The signal position is measured in parts per million of the magnetic field and is quoted relative to the position of the signal for a standard substance.
The signals due to carbon in most organic molecules appear between 0 δ (delta) and 200 δ.
sp3 carbons appear between 0 and 90 δ.
sp2 carbons appear at higher than 100 δ.
This is because the electrons in the molecule shield the absorbing nucleus from the magnetic field to different extents.
The signal position is measured in parts per million of the magnetic field and is quoted relative to the position of the signal for a standard substance.
The standard is (CH3)4Si. All of the carbons in this molecule are equivalent, and the position at which they absorb is defined as 0 δ (delta).
The signals due to carbon in most organic molecules appear between 0 δ (delta) and 200 δ.
Carbon atoms that are shielded to a greater extent from the external magnetic field appear at lower δ.
The spectrum can be divided into two regions.sp3 carbons appear between 0 and 90 δ.
sp2 carbons appear at higher than 100 δ.