course notes nmr 3 13cnmr and (advanced 1d...
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13C NMR
• much lower signal to noise (6000 x less sensitive than 1H NMR)
Noise
-more sample (higher concentration)-more scans (shorter time between scans)-increase line broadening (LB = 2 - 10)
-due to a combination of lower inherent sensitivity and also lower abundance of 13C isotope (~1%).
• improve signal to noise by:
Line Broadening (LB)
!"#$!%&#'()*+,-&.,/&!"#&01'./2.,/&
345'()*+,-
1H!C coupling not observed;
abundance of 13C too low.
13C!H coupling is observed,
and complicates spectrum.
Solution: Decoupling pulse.
13C NMR, 1H (broadband) decoupled
13C NMR
O
-CH3
-CH3-CH3
-CH2-
-CH2-
bridgehead
bridge
13C NMR
no decoupling
!"#$!%&#'()*+,-&.,/&!"#&01'./2.,/&
345'()*+,-
1H!C coupling not observed;
abundance of 13C too low.
13C!H coupling is observed,
and complicates spectrum.
Solution: Decoupling pulse.
13C NMR, 1H (broadband) decoupled
13C NMR
O
-CH3
-CH3-CH3
-CH2-
-CH2-
bridgehead
bridge
• usually broad band decoupled (C-H coupling 100-250 Hz)-each carbon is a singlet
• cannot integrate normal broadband decoupled spectra!-due to NOE, decoupling, and incomplete FID
!"#$!%&#'()*+,-&.,/&!"#&01'./2.,/&
345'()*+,-
1H!C coupling not observed;
abundance of 13C too low.
13C!H coupling is observed,
and complicates spectrum.
Solution: Decoupling pulse.
13C NMR, 1H (broadband) decoupled
13C NMR
O
-CH3
-CH3-CH3
-CH2-
-CH2-
bridgehead
bridge1H decoupled
• much wider frequency range (200 ppm) versus 1H NMR (12 ppm)
13C NMR
http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__5%3A_Structure_Determination_II/Section_5.6%3A_13C-NMR_spectroscopy
• much more sensitiveto subtle differences in local environment (less overlap of peaks)
75 MHz
150 MHz
usually see one singlet per chemically equivalent Carbon(coincidental overlap is rare compared to 1H NMR)
13C NMR chemical shifts
• chemical shifts similar to 1H NMR (spread out over 200 ppm)- also referenced to TMS
http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__5%3A_Structure_Determination_II/Section_5.6%3A_13C-NMR_spectroscopy
examples
-Much less structural information in 13C NMR • only chemical shift (no integration, no coupling)
•C’s with protons have enhanced signal (due to NOE)-carbons without protons will be shorter-also explains why deuterated solvent peak is small
•Why is CDCl3 peak a “triplet”?
Heights of peaks does provide some information
DEPT (number of attached H’s)
normal13C NMR
DEPT 90°CH only
DEPT 135°CH, CH3 upCH2 down
DEPT (number of attached H’s)
normal13C NMR
DEPT 90°CH only
DEPT 135°CH, CH3 upCH2 down
6
7 (8, 10)4
5
2, (9, 1)
3
What information can we get from NMR?
1) proton environments and spin systems
1I) carbon environment and #’s of attached protons
Other 1D experiments can tell us:
a) confirm that two protons are really coupled to each other
b) 3-D shape (what protons are close to each other in space)
c) presence and chemical environments of other heteroatoms
d) estimate of molecular size
(from 1D studies- that we have discuss so far)
e) measure enantiomeric excess