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13C NMR Spectroscopy
13C NMR
12C is the most abundant natural isotope of carbon,
but has a nuclear spin I = 0, rendering it
unobservable by NMR.
Limited to the observation of the 13C nucleus which
constitutes only 1.1% of naturally occurring carbon.
40.01.00251.719F
75.07.05
50.04.70
10.71.0067.2813C
6.51.0041.12H
300.7.05
200.4.70
42.61.00267.531H
Frequency νννν
(MHz)
Field
strength B0
(Tesla)
γγγγ
(106 rad/Tesla ×××× sec)
Nucleus
13C Transition Energy
The magnetogyric ratio, γγγγ, for the 13C is 67.3 compared to
267.5 for 1H.
Remember the resonance condition for a nucleus is given by:
νννν = (γγγγ/2ππππ)B0
If the gyromagnetic ratio is lowered, the ∆∆∆∆E is also lowered.
Where a 1H spectrum using a 1.41 T magnet is observed at 60
MHz, a 13C spectrum is observed at 15 MHz – roughly 4 times
less energetic.
Boltzmann: Nupper/Nlower = e-∆∆∆∆E/kT = e-hνννν/kT
@ 298 K the ratio is 1,000,000 / 1,000,002
2
13C NMR
The combined effects of smaller excess populations in the
lower energy state, low natural abundance, and slow
relaxation rates result in a 13C signal that is typically 6000
times weaker than that observed for 1H.
With FT instruments, this is not a problem – simply take
more scans! (recall S/N increases as the square root of the
number of scans).
16 scans on a 5-10 mg sample will give a good 1H spectrum,
512 scans on a 50 mg sample will give a good 13C spectrum.
Fourier Transform NMR
•Radio-frequency pulse given.
•Nuclei absorb energy and precess (spin) like little tops.
•A complex signal is produced, then decays as the nuclei lose energy.
•Free induction decay is converted to spectrum.
13C NMR
• low 13C abundance
• a single molecule will have at most only one 13C
atom
• however, we are sampling a very large number of
molecules, even in a 50 mg sample!
• thus our sampling will ‘see’ a 13C at every C
position in the molecule!
Chem 325
TUTORIAL
TONIGHT @ 7PM
3
13C Shielding
13C spectra are typically recorded from 0 – 220 ppm;
with the zero being the methyl carbon in TMS
(much wider range than 1H spectra!)
13C nuclei are shielded or deshielded (CHEMICAL
SHIFT) due to the same factors as for 1H NMR.
1. Electron withdrawing ability (by inductance or
resonance) of nearby groups.
2. Hybridization.
3. Electron current effects.
13C NMR Chemical Shifts
Several functionalities appear directly on 13C NMR which are
not ‘visible’ in 1H NMR:
- Quaternary carbons
- ipso carbons
- Carbonyl carbonsSi
CH3
H3C CH3CH3
downfield δ (ppm) upfielddeshielded shieldedhigher ∆E lower ∆E
0.020406080100120140160180200220
carbonyl carbons
aromatic carbons
alkene carbons
alkyne carbons
13C-EWG
sp3 carbon
Carbonyl Carbon Chemical Shifts
110120130140150160170180190200210220
ketones
conj. ketones
aldehydes
carboxylic acids
anhydrides nitriles
esters
acid chlorides
amides
Spin-Spin Coupling in 13C NMR
Homonuclear coupling of 13C-13C is possible in theory.
However, due to the low natural abundance of 13C, it is rare to
find two 13C’s in the same molecule, let alone adjacent to one
another.
No need to consider 13C-13C coupling except for enrichment
studies!
Heteronuclear coupling between 13C and the 1H atoms attached
to them is observed (1H abundance ~99%).
Because the 1H atoms are directly attached, the coupling
constants (1J)are large, typically 100-250 Hz.
When such spectra are observed, they are referred to as proton
coupled spectra (or non-decoupled spectra).
4
1H – 13C Splitting
The splitting follows the simple N+1 rule:
The multiplet analysis gives useful information, but there are
two major limitations:
1) If the 13C signal is weak (common) the outer peaks of the
multiplet may be lost in the noise of the spectrum.
2) Due to the large J-constants, the multiplets quickly begin to
overlap and become congested.
C13
C
H
13C
H
H13
C
H
H
H13
quaternary
singlet
methine
doublet
methylene
triplet
methyl
quartet
13C NMR Spectrum
Proton-Coupled
Effect of Coupling
Coupling can cause 13C NMR spectra to become very complicated (convoluted) quite easily.
1H Coupled
Three equal intensity lines
at 77 ppm
CDCl3 solvent
13C- 2D coupling
1H Decoupling
To simplify the 13C spectrum, and to increase the intensity of
the observed signals, a decoupler is used to remove the spin
effects of the 1H nucleus.
A second RF generator irradiates at the 1H resonance
frequency causing the saturation – effectively averaging all
their spin states to zero.
1H channel-
13C channel
13C νννν pulse
13C FID
5
13C Proton Decoupled Spectrum
13C{1H}
Effect of Decoupling
1H Coupled
1H Decoupled
13C NMR Spectra
Due to signal enhancement and spectral simplification, 13C
spectra are usually reported as 1H decoupled.
Each chemically unique carbon in the molecule gives rise to a
single peak.
Of course chemically equivalent carbons contribute to the
same peak!
The number of different signals (peaks) indicates the number
of different kinds of carbon.
The location (chemical shift) indicates the type of functional
group.
13C NMR Intensities
Peak areas (~heights) are NOT proportional to number of carbons.
Carbon atoms with more hydrogens give strongersignals, due to more efficient relaxation (transfer of spin to the hydrogens).
However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)
6
Example: Ethanol
CH2 CH3
OH
Example: 1-bromohexane
CH2
CH2
CH2
CH2
CH2
CH3Br
Example: cyclohexane Example: cyclohexene
7
Example: 1,3-cyclohexadiene Example: 1,4-cyclohexadiene
Example: m-nitrotoluene
CH3O2N1
23
46
7
5
3
1
2
65
7
4
28
13C Chemical Shift Predictions
Examining a large set of chemical shift data has allowed the
development of ‘empirical’ rules or substituent parameters to
allow chemical shift predictions for most commonly
encountered situations.
Example: the carbon atoms of a substituted benzene ring.
Benzene itself → single peak at 128.7 ppm
Add to this value substituent increments which depend on the
chemical nature of the substituent and where it is on the ring
relative to the carbon whose shift is being predicted.
8
29
13C Aromatic Substituent Parameters
CH3O2N 12
3
46
7
5
C1 = 128.7 + (CH3)ipso + (NO2)meta = 128.7 + 8.9 + 0.8 = 138.9 ppm
C2 = 128.7 + (CH3)ortho + (NO2)ortho = 128.7 + 0.7 + (-5.3) = 124.1 ppm
31
Example: m-nitrotolueneCH3O2N
12
3
46
7
5
3
1
2
65
7
4
135.4135.46
129.2129.45
120.6120.54
148.4148.23
123.8124.12
139.9138.41
Obs’dCalc’dC
32
Example: p-Hydroxyacetophenone
CO CH3
OH
1
6
5
4
3
2
131.4130.26
115.8115.85
162.1158.54
115.8115.83
131.4130.22
129.2128.71
Obs’dCalc’dC
4
3
5
2
61
9
33
13C Shift Predictions – Alkyls
Can also make predictions for alkyl groups
Base value: use
unsubstituted
hydrocarbon 34
Example: bromocyclopentane
Br
23.3233
37.9362
53.5511
Obs’dCalc’dC 1
2
3
1
2
3
35
13C NMR Intensities
Peak areas (~heights) are NOT proportional to number of carbons.
Carbon atoms with more hydrogens give strongersignals, due to more efficient relaxation (transfer of spin to the hydrogens).
However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)
Nuclear Overhauser Enhancement (NOE)
A phenomenon observed with proton-decoupled 13C-NMR is
that the intensity of the signal for a given 13C increases versus
the proton-coupled spectrum roughly proportional to the
number of protons attached.
The degree of this signal enhancement is called the Nuclear
Overhauser Enhancement (NOE).
This effect is general, and appears anytime when one of two
types of atoms is irradiated, while the spectrum of the other is
observed. In this case, while the 1H population is irradiated to
saturation, the 13C is observed. Here: a heteronuclear effect.
10
NOE
The effect can be a positive or negative one, but for the case
of 1H-13C, the effect is positive
The maximum enhancement is given by:
NOEmax = 1 (γγγγ irradiated)
2 (γγγγ observed)
This value is what is added to the observed intensity in the
coupled spectrum to give the intensity observed in the
decoupled spectrum:
total predicted intensity = 1 + NOEmax
NOE
For 1H – 13C, NOE = ½ (267.5/67.28) = 1.988
A maximum enhancement of almost 200% is possible.
NOE operates in both directions – 13C nuclei (if decoupled)
would enhance the signal of 1H – however, this signal would
be weak due to the low abundance of 13C.
Because NOE for 13C – 1H operates in the opposite direction
(a rare nuclei always bound to an abundant one) it is a useful
probe into structural assignments.
The NOE effect is very short-range, falling off as 1/r3 the
distance between the nuclei.
Origin of NOE
An isolated two spin system between
a single carbon and single hydrogen
atom
The effects of coupling are left out for
simplicty
Shown are the four combinations of
spin states of these two nuclei, N1-4
The two energy states where both are
spin up or spin down are the lowest
and highest energy states
The “mixed” states are roughly
degenerate in energy
N1
N3
N2
N4
C H
C H
C HC H
Origins of NOE
Quantum mechanics dictates that
allowed transitions involve only
one change of spin at a time –
single quantum transitions
The allowed transitions are
shown in red
N1
N3
N2
N4
C H
C H
C HC H
11
Origins of NOE
Let the equilibrium population
of the two degenerate states be B
The N1 level would be higher
than B by a small amount, δδδδ
The N4 level would be lower
than B by a the same amount, δδδδ
The signal for a 13C in this case
would be proportional to δδδδ at
equilibrium
The two 13C transitions are N1 –
N2 and N3 – N4
N1
N3
N2
N4
C H
C H
C HC H
Origins of NOE
When a decoupler is used, the 1H
populations are disturbed from
their equilibrium values
Relaxation processes restore these
disturbed populations to their
equilibrium values
One such process is a double-
quantum transition, where both
the C and H nuclei relax
simultaneously (blue line)
This “leak” in the upper state
enhances the population of the
lower energy state for carbon – the
excess population is larger – and
the signal intensifies
N1
N3
N2
N4
C H
C H
C HC H
double
quantum transition
NOE
NOE: an example of cross-polarization, polarization of spin
states of one type of nucleus causes a polarization of the spin
states of another nucleus.
A heteronuclear NOE effect is always observed in ‘normal’ 1H
decoupled 13C spectra.
Total NOE for a given C increases with number of nearby H’s.
Thus intensities of C signals are generally:
CH3 > CH2 > CH > C
NOE effect is quite general. Can also be applied in a
homonuclear sense, i.e. 1H{1H}
NOE
Difference
Difference
12
NOE
Depends on cross-polarization of spin states.
Can tell us what nuclei are close together.
In contrast to J-coupling (spin-spin) which operates through
the bonding electrons, NOE is a through-space effect.
Thus NOE can tell us about the proximity of atoms which are
separated by many bonds, e.g. proteins, RNA, DNA
46
Example: m-nitrotoluene
CH3O2N1
23
46
7
5
3
1
2
65
7
4
47
Example: benzonitrile
C
N
Very weak: no attached H’s
No NOE effect!
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