: Carbon (13C) NMR Flashcards
13C has only about 1.1% …
natural abundance (of carbon atoms)
12C does not exhibit …..
NMR behavior (I=0)
The 13C nucleus has a spin …
1/2 nucleus (as 1H)
The 13C nucleus is about 400 times …..
less sensitive than H nucleus to the NMR phenomena
Due to the low abundance
we do not usually see 13C-13C coupling (labeling experiments)
Chemical shift range is normally…….
0 to 220 PPM
Chemical shifts are also measured with respect ..
to tetramethylsilane, (CH3)4Si (i.e. TMS
The resonances due to 13C nuclei are split by neighboring 1H atoms. These splittings would complicate the appearance of the spectra making them harder to interpret
Therefore, in a “normal” 13C spectra, these couplings are “removed” by applying a continuous second radio frequency signal of a broad frequency range that excites all the 1H nuclei and cancels out the coupling patterns due to the interaction of the 1H with the 13C. This means that each carbon is seen as a single line.
13C chemical shifts
-The most significant factors affecting the chemical shifts are:
Electronegativity of the groups attached to the carbon atom
Hybridisation of carbon
DEPT
distortionless enhancement of polarization transfer) which can be used to give similar information to an off-resonance decoupled spectra i.e. the number of attached protons to each carbon.
In a DEPT experiments, a sequence of pulses with various delay times are used to create the DEPT spectra where -CH3, -CH2-, CH and Cq peaks respond in different fashions, based upon the number of protons attached.
All signals will be around the same height unless originating from two chemically and structurally equivalent protons ……….
with the exceptions of quaternary carbons (and occasionally CH carbons) which normally appear shorter than the other signals
DEPT-135
CH & CH3 yields positive peaks; CH2 gives negative peaks and Cq are invisible.
DEPT-90
CH yields peaks; Cq, CH2, and CH3 are invisible
