c nuclear magnetic resonance - chemistrychemistry.syr.edu/totah/che575/support/3a1/5-1.cnmr.pdf ·...
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13C Nuclear Magnetic Resonance
Inherent Difficulties
• Low abundance of 13C (1.1% vs, 99.9% for 1H) • Lower gyromagnetic ratio (1/4 that of 1H) - 13C: 67.28 vs. 1H 267.53 • Slow T1 relaxation times (10-100 sec vs. 0.1-1 sec for 1H) • 13C-1H coupling complicates spectrum; decreases peak intensity
ΔE
magnetic field (B0)
-1/2
+1/2
1,000,000 nuclei at 60 MHz
1,000,009 nuclei at 60 MHz
E
13C Nuclear Magnetic Resonance
Chemical Shift
see also Pavia Table 6.1; Appendices 7 & 8
13C Nuclear Magnetic Resonance
Chemical Shift
Factors the Influence Chemical Shift
1. hybridization 2. electronegativity of element bonded to carbon 3. anisotropy
OH O
**
*
**
δ 22.6 δ 139.0 δ 38.2
δ 62.3 δ 202.8
13C Nuclear Magnetic Resonance
Differences Between 1H and 13C NMR
1. range of 13C chemical shifts is very large compared to 1H 2. 13C chemical shifts are more sensitive to small changes in chemical environment 3. Influence of electronegative atoms on chemical shift greater in 13C NMR than 1H NMR 4. Homonuclear (13C / 13C) splitting is not observed
See Pavia, Appendix 8 for calculation of 13C chemical shifts
CH3 CH2 CH2 CH2 CH2 CH2 OH
0.90
1.32
1.32
1.32
1.56
3.62
CH3 CH2 CH2 CH2 CH2 CH2 OH
14.2
22.8
32.0
25.8
32.8
61.9
1H shifts 13C shifts
NMR Spectra of n-Butylbenzene
13C Nuclear Magnetic Resonance
Heteronuclear Spin-Spin Splitting (1H-13C)
CH2 CH3 CH2
3J = 0-3 Hz
13C Nuclear Magnetic Resonance
Heteronuclear Spin-Spin Splitting
1H coupled spectrum
1H decoupled spectrum
HOH H
HH
cholesterol
13C Nuclear Magnetic Resonance
Proton Decoupled 13C NMR
13C Nuclear Magnetic Resonance
Proton Decoupled 13C NMR 1H coupled spectrum
1H broadband decoupled spectrum
1H off resonance decoupled spectrum
CH3
CH3
13C Nuclear Magnetic Resonance
Nuclear Overhauser Effect & Cross Polarization of Spin
300 MHz
Signal Enhancement: CH3 > CH2 > CH >> C
B0
1H
decoupler
B0
13C
crosspolarization
Boltzmandistribution
NupperNlower
= 1,000,0001,000,048
1,000,0001,000,096
=
NupperNlower
= 1,000,0001,000,048
1,000,0001,000,000
=Boltzmandistribution
11
=
13C Nuclear Magnetic Resonance 13C Pulse Sequence
FID:
13C Nuclear Magnetic Resonance
Relaxation Times (T1)
CH3 C CH2 CH CH3
CH3
CH3
CH3
9.3 s
68 s
13 s23 s
9.8 s
T1 : C > CH > CH2 > CH3
in general
tumbling
HOH H
HH
T1 = 37 s T1 = 20 sT1 = 1-2 s for CH, CH2, CH3T1 = 4-6 s for quaternary C
13C Nuclear Magnetic Resonance
Molecular Symmetry
How Many Lines?
C-CH2-CH3CH3
CH3CH3
O H3C
H3C
CH3
OH
CH3H3C
A B C D
13C Nuclear Magnetic Resonance
Integration
A
B
C
D
2 peaks
1
2
3
4
13C Nuclear Magnetic Resonance
Distortionless Enhancement by Polarization Transfer (DEPT)
DEPT-45 DEPT-90 DEPT-135 C 0 0 0
CH + + +
CH2 + 0 -
CH3 + 0 +
13C Nuclear Magnetic Resonance
Distortionless Enhancement by Polarization Transfer (DEPT)
CCHCH2CH3
DEPT-450+++
DEPT-900+00
DEPT-1350+-+
DEPT-135
DEPT-90
DEPT-45
13C NMR 1 2 46 78 9 935
CH3
OH
H3C CH3
menthol
3
5
12
4 6
7
89 9
13C Nuclear Magnetic Resonance
DEPT-135 O
NH
CH3O H
OH
CH3O2C OH
OH
OH
O CH3
13C Nuclear Magnetic Resonance
Alkanes
2-methylpentane
CH-CH2-CH2-CH3CH3
CH3a
b dc e
a
e b
c
a
d
R CH3
R2 CH2
CR4
15-55 ppm
30-40 ppm
8-30 ppm
chemical shifts
CHR3 20-60 ppm
13C Nuclear Magnetic Resonance
Alkenes
cis-2-hexene
C C 110-150 ppm
chemical shifts
C CH
H3C CH2CH2CH3
H
ea
b
d
c
f
e b c
a
d 123.9 130.7
29.1 f
13C Nuclear Magnetic Resonance
Alkenes
1-hexene
C C 110-150 ppm
chemical shifts
b
a
114.2
139.2
e abd cf
CH3-CH2-CH2-CH2-CH CH2
13C Nuclear Magnetic Resonance
Alkenes
dihydropyran
C C 110-150 ppm
chemical shifts
b
a
100.7
144.3 O a
b
13C Nuclear Magnetic Resonance
Alkenes
4-methyl-2-penten-1-one
C C 110-150 ppm
chemical shifts
b
a 154.8 124.3 H3C
H3C
O
CH3
a
b
13C Nuclear Magnetic Resonance
Aromatics
nitrobenzene
C110-175 ppm
chemical shifts
b c
a
d
NO2ab
c
d
13C Nuclear Magnetic Resonance
Aromatics
p-cymene
CH3
C110-175 ppm
chemical shifts
13C Nuclear Magnetic Resonance
Aromatics
dichlorobenzenes
Cl
Cl
Cl
Cl
Cl
Cl
C110-175 ppm
chemical shifts
13C Nuclear Magnetic Resonance
Alkynes
2-heptyne
b
a
65-90 ppm
chemical shifts
C≡C
C C CH3CH3CH2CH2CH2ab
79.4
75.3
13C Nuclear Magnetic Resonance
Alkyl Halides
1-chloropentane
C I
25-65 ppm
35-80 ppm
-5-40 ppm
chemical shifts
C Br
C Cl
aCH3 CH2-CH2-CH2-CH2-Cl a
45.1
13C Nuclear Magnetic Resonance
Alcohols, Ethers & Amines
2-hexanol 40-60 ppm
50-80 ppm
chemical shifts
C-OR
C-NR2
CH3CH2CH2CH2 CHCH3OH
aa
68.0
13C Nuclear Magnetic Resonance
Alcohols, Ethers & Amines
1,3-dioxane 40-60 ppm
50-80 ppm
chemical shifts
C-OR
C-NR2
bc
O Oa
a
b
c
13C Nuclear Magnetic Resonance
Alcohols, Ethers & Amines
N-methylisobutylamine 40-60 ppm
50-80 ppm
chemical shifts
C-OR
C-NR2
d
b c
NH
a b d
c
a
13C Nuclear Magnetic Resonance
Carbonyl Compounds
aldehydes & ketones 185-220 ppm carboxylic acids, esters, amides, etc. 155-185 ppm nitriles 110-140 ppm
13C Nuclear Magnetic Resonance
Aldehydes & Ketones
2-hexanone 205-220 ppm
185-200 ppm
chemical shifts
R CO
H
R CO
R'
CH3 CO
CH2-CH2-CH2-CH3a
a
209.0
13C Nuclear Magnetic Resonance
Carboxylic Acid Derivatives
methacrylic acid
175-185 ppm
165-175 ppm
chemical shifts
R CO
OR'
R CO
OH
165-175 ppmR CO
NR'2
CH2CCH3
CO
HOa
a
173.5
13C Nuclear Magnetic Resonance
Carboxylic Acid Derivatives
heptyl acetate
175-185 ppm
165-175 ppm
chemical shifts
R CO
OR'
R CO
OH
165-175 ppmR CO
NR'2
CH3 COO-CH2CH2-CH2-CH2-CH2-CH2-CH3
a b
a 171.0
b 64.6
13C Nuclear Magnetic Resonance
Amides
N-methyl-6-hexanelactam
175-185 ppm
165-175 ppm
chemical shifts
R CO
OR'
R CO
OH
165-175 ppmR CO
NR'2
a
175.7 d
36.9
c
51.3 N
OCH3ab
c
d b
35.7
13C Nuclear Magnetic Resonance
Nitriles
hexanenitrile
119.9
110-140 ppm
chemical shifts
R C N
a
C NCH3CH2CH2CH2a
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to Deuterium
multiplicity
relative intensity
multiplicity = 2nI + 1
where: n = # of nuclei I = spin
Cb
Da
Jab = 45 Hz
13C Nuclear Magnetic Resonance
Solvent Peaks
CDCl3 D3C(S=O)CD3
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to 19F
fluorotribromomethane
1J = 370 Hz
C Cb
FCa
F
JFb = ~50 HzJFa = ~350 Hz
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to 19F
2,2,2-trifluoroethanol (proton decoupled)
1J = 278 Hz
2J = 35 Hz
C Cb
FCa
F
JFb = ~50 HzJFa = ~350 Hz
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to 19F
commonly observed systems
C Cb
FCa
F
JFb = ~50 HzJFa = ~350 Hz
F3C SO
OROF3C
C OR
O
trifluoroacetates trifluoromethanesulfonates(triflates)
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to 31P
tetramethylphosphonium chloride (proton decoupled)
1J = 56 Hz
C Cb
PCa
P
JPa, JPb variable
13C Nuclear Magnetic Resonance
Heteronuclear Coupling to 31P
dimethyl methylphosphonate (proton decoupled)
2J = 6 Hz 1J = 144 Hz
C Cb
PCa
P
JPa, JPb variable
1H Nuclear Magnetic Resonance
1. Get an Overview • identify any reference solvents in proton decoupled spectrum 2. Symmetry: • count the number of carbons (remember 13C is not integrated) • verify carbon count based on the molecular formula • is there any symmetry?
- rarely see overlap of peaks in absence of symmetry - ok to see fewer carbons (if reasonable); not okay to have more!
3. Chemical Shift: • determine the chemical shift of each resonance in the spectrum
- what functional groups may be present - consider the presence of heteroatoms 4. If Needed Collect DEPT Data • determine how many Hs are on each carbon 5. Put it all Together: • often helpful to consider 1H NMR data simultaneously
How to Interpret a 13C NMR Spectrum
1H Nuclear Magnetic Resonance
Reporting 13C NMR Data (1H decoupled)
CH3CH2CH2CH2CH2C≡N
13C NMR (25 MHz, CDCl3) δ 119.9, 30.9, 25.2, 22.0. 17.1, 13.8.
119.9030.8725.2121.9617.1113.78
2619701000920814950
ppm Int
1H Nuclear Magnetic Resonance
Reporting 13C NMR Data (1H decoupled)
13C NMR (25 MHz, CDCl3) δ 119.9, 30.9, 25.2, 22.0. 17.1, 13.8.
frequency (1/4 1H frequency)
list chemical shift of each peak to 1 decimal place*
solvent
*if two peaks round to same value, can add second decimal place (e.g. 69.87, 69.89);
13C Nuclear Magnetic Resonance
Chemical Shift