Note: This is part 6/7 of Marc's NMR lecture notes. 12C is not NMR-active. I = 0
however…. 13C does have spin, I = 1/2 (odd mass). Natural abundance of 13C is
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BACKGROUND ON 13C NMR Note: This is part 6/7 of Marc’s NMR lecture notes
12C
is not NMR-active
however…. 13C
CARBON-13 NMR
13C
I= 0
does have spin, I = 1/2 (odd mass)
signals are 6000 times weaker than 1H because: Natural abundance of
13C
is small (1.08% of all C)
PULSED FT-NMR IS REQUIRED (why??) The chemical shift range is larger than for protons 0 - 200 ppm
BACKGROUND ON 13C NMR For a given field strength 13C has its resonance at a different (lower) frequency than 1H.
1H 1.41 T 2.35 T 7.05 T
60 MHz 100 MHz 300 MHz
Divide the hydrogen frequency by ~4 for 13C
13C 1.41 T 2.35 T 7.05 T
15.1 MHz 25.0 MHz 75.0 MHz
MORE BACKGROUND ON 13C NMR Because of low natural abundance there is a low probability of finding two 13C atoms next to each other in a single molecule. 13C
- 13C coupling
NO!
However, 13C does couple to hydrogen atoms (I = 1/2) 13C
- 1H coupling
YES!
1
COUPLING TO ATTACHED PROTONS 3 protons
2 protons
H 13
C
1 proton
0 protons
H H
H n+1 = 4
13
C
H
n+1 = 3
13
C
H
n+1 = 2
13
C
n+1 = 1
COUPLING TO ATTACHED PROTONS
Methyl carbon
Methylene carbon
Methine carbon
Quaternary carbon
The effect of attached protons on 13C resonances ( n+1 rule applies ) (J’s are large ~ 100 - 200 Hz)
ETHYL PHENYLACETATE
13C
coupled to the hydrogens
DECOUPLED 13C NMR What could that possibly mean?
2
PROTON-DECOUPLED 13C-NMR How? Irradiate the 1H-nuclei while measuring the 13C-nuclei.
RF source 2 “Saturates” the 1H frequencies
1H-13C
13C
Warning! overly-simplified explanation! The hydrogen nuclei are “saturated”: rapid cycling of spin states: +1/2 and -1/2
RF source 1 Pulse tuned to 13C frequencies
signal (FID) measured
When the 13C frequencies are detected, the 13C nuclei see an average coupling (i.e., zero) to the 1H nuclei. 1H nuclei are said to be decoupled from the 13C nuclei Multiplets no longer seen in 13C NMR!
ETHYL PHENYLACETATE Ugly! peaks often overlap!
coupled 13C-NMR
CHEMICAL SHIFTS OF 13C ATOMS
decoupled 13C-NMR
Much easier to interpret
Don’t worry about memorizing, but know the general regions for different types of carbons….
3
APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm (ppm)) R-CH3
Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)
4
2,2-DIMETHYLBUTANE
CYCLOHEXANOL
BROMOCYCLOHEXANE
CYCLOHEXENE
5
CYCLOHEXANONE DEPT 90 and 135: Quick Overview (“distortionless transfer by polarization transfer”)
Normal 13C: shows all C atoms upward! DEPT 90:
shows only CH peaks upward!
DEPT 135: shows CH/CH3 up and CH2 downward!
(note: DEPT eliminates “quaternary” signals which are carbons that lack hydrogens!)
Normal 13C: All carbon upward
Example: ethyl benzene
Example: strychnine
x (TMS)
DEPT 90: only CH upward
Normal 13C: All C upward
DEPT 135: CH and CH3 upward; CH2 downward! x (TMS)
Questions: [1] which peaks are CHs? [2] which peaks are CH2s?
[3] which peaks are CH3s (methyl)? [4] which peaks are quaternaries? (lacking hydrogens)
6
Information derived from a 13C NMR spectrum! 1. Each different type of carbon gives a peak!
OVERVIEW OF 13C NMR
2. The chemical shift (in ppm) gives a clue as to the type of carbon generating the peak (alkane, alkene, benzene, carbonyl, etc.) 3. Integrals are not used in 13C NMR. (But sometimes the size of the peak tells how many carbons present) 4. Spin-spin splitting is suppressed by decoupling to give easy-to-interpret spectra. 5. However, DEPT-90 and DEPT-135 can allow assignment of different carbon types.
