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CHEM 2OA3 Lecture Notes - Tetramethylsilane, Chemical Shift, Natural Abundance

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Jeff Landry

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Compare between H-NMR and C13-NMR
Chemical shift, integration, multiplicity
Only chemical shift is important
Singlet, duplet, triplet, etc (n+1 rule)
Only singlet
Have splitting of signals
No splitting b/c of low abundance of C13
in nature
Lower frequency (0-13ppm)
C13 atoms resonate a larger frequency than
H. (0-220ppm)
Not subject to symmetry
Subject to symmetry
Factors affecting the shift:
Electronegativity of the groups attached
to the C
- Hybridization of C
Requires FT-NMR
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.
(N+1 rule)
The chemical shift tells us the hybridization (sp3, sp2, sp) of each carbon.
Integration: Not useful for 13C NMR
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