CHEM 2OA3 Lecture Notes - Tetramethylsilane, Chemical Shift, Natural Abundance
This preview shows half of the first page. to view the full 1 pages of the document.
Compare between H-NMR and C13-NMR
Chemical shift, integration, multiplicity
Only chemical shift is important
Singlet, duplet, triplet, etc (n+1 rule)
Have splitting of signals
No splitting b/c of low abundance of C13
Lower frequency (0-13ppm)
C13 atoms resonate a larger frequency than
Not subject to symmetry
Subject to symmetry
Factors affecting the shift:
• Electronegativity of the groups attached
to the C
- Hybridization of C
Other important things to note:
• 13C has only about 1.1% natural abundance (of carbon atoms)
• 12C does not exhibit NMR behaviour (I=0)
• Chemical shift range is normally 0 to 220 ppm
• Chemical shifts are also measured with respect to tetramethylsilane, (CH3)4Si (i.e.
• Similar factors affect the chemical shifts in 13C as seen for H-NMR
• Long relaxation times (excited state to ground state) mean no integrations
• "Normal" 13C spectra are "broadband, proton decoupled" so the peaks show as single
• Number of peaks indicates the number of types of C
The 13C NMR is directly about the carbon skeleton not just the proton attached to it.
• The number of signals tell us how many different carbons or set of equivalent
• The splitting of a signal tells us how many hydrogens are attached to each carbon.
• The chemical shift tells us the hybridization (sp3, sp2, sp) of each carbon.
• Integration: Not useful for 13C NMR
You're Reading a Preview
Unlock to view full version