1
344 Organic Chemistry Laboratory
Introduction to 1H-NMR Spectroscopy Part 1
Main topics
- Equivalent/non-equivalent protons
- Interpreting simple 1H-NMR spectra
- Spin-spin coupling, n+1 rule
How do we know the structure of these molecules?
Molecular Modeling
E = hν E = energy (kJ/mol)
ν = frequency (Hz), h = Planck constant
Background & theory: pp. 536-538 “Organic Chemistry” 5th. Ed. Loudon.
Spin ½ nuclei 1H, 13C, 19F, 29Si, 31P
NMR active, easy to do, widely used
Spin 1 nuclei 2H (D), 14N
NMR active but not widely used
Spin 0 nuclei 12C, 16O, 32S
NMR silent
Background & theory: pp. 578-636 “Organic Chemistry” 5th. Ed. Loudon.
Nuclear Magnetic Resonance (NMR)
Nuclei behave like a bar magnet
4
6
7.04
2.29
parts per million (ppm)
1,4-Dimethylbenzene
(para-Xylene)
The 1H-NMR spectrum
C8H10
Analyzing para-Xylene
Hb
Hb
Hb
Hb
HbHb
HbHb
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
C8H10
para-Xylene
4
6
7.04
2.29
parts per million (ppm)
1,4-Dimethylbenzene
(para-Xylene)
The 1H-NMR spectrum
C8H10
Ha
Hb
4
6
6.82
3.75
1,4-Dimethoxybenzene
Ha
Ha
Ha
HbHb
Electronegativity O = 3.44, C = 2.55, H = 2.20
O
O
Oxygen is a highly electronegative atom
Oxygen exerts a strong electron-withdrawing effect
in the sigma-framework (i.e. along the C-O bond)
This electron-withdrawing effect reduces the degree
of electron density around the Me-group C-atom
1,4-Dimethoxybenzene1,4-Dimethylbenzene
Electronegativity O = 3.44, C = 2.55, H = 2.20
3.75 ppm2.29 ppm
6.82 ppm7.04 ppm
Shielding/deshielding are relative terms
The Me-group of 1,4-dimethoxybenzene is deshielded relative to the Me-
group of 1,4-dimethylbenzene
1
9
8.00
1.50
tert-Butylformate
Hb
Ha
Ha
Ha
Ha
Hb
SiMe4 = TMS = Tetramethylsilane
0.00 ppm reference compound
Why use TMS?
Si
Don’t assign the TMS signal as part of your molecule!
- chemically inert (won’t react with most molecules)
- signal is strong and unsplit
- volatile (easily removed)
- inexpensive
- protons are strongly shielded
Ranges +/- ~0.5 ppm
2.45
4.56
9
6
Deuterated chloroform (CDCl3) used as solvent
2,4,6-Tris(bromomethyl)mesitylene
Electronegativity Br = 2.96
C = 2.55
2
3
5.80
2.14Standard notation for reporting NMR data
[chemical shift, (multiplicity, integration)]
Bromomethyl acetate
1H-NMR δ: 2.14, (s, 3H); 5.80 (s, 2H).
Reporting 1H-NMR data
Spin-spin coupling
4.42
2
1.58
3
N
O
O
Electronegativity O = 3.44, N = 3.04, C = 2.55
Nitroethane
HaHa
Ha
Hb
Hb
N
O
OHa
Ha
Ha
Hb
Hb
How do neighboring non-equivalent protons interact?
Consider how Hb protons interact with Ha protons
Hb protons align with or against direction of Bo
3 different combinations of Hb spin states
Bo
N
O
OHa
Ha
Ha
Hb
Hb
How do neighboring non-equivalent protons interact?
Consider how Hb protons interact with Ha protons
Hb protons align with or against direction of Bo
1 2 1
Signal from Ha split into a
1:2:1 triplet by Hb
3 different combinations of Hb spin states
Both parallel
ap p
Both anti-parallelp ap
21 1
HaHa
Ha
Hb
Hb
How do neighboring non-equivalent protons interact?
Consider how Ha protons interact with Hb protons
4 different combinations of Ha spin statesN
O
O
Bo
HaHa
Ha
Hb
Hb
How do neighboring non-equivalent protons interact?
Consider how Ha protons interact with Hb protons
4 different combinations of Ha spin states
1 33 1 1 3 3 1
Signal from Hb split into a
1:3:3:1 quartet by Ha
N
O
O
Spin-spin splitting
4.42
2
1.58
3
N
O
O
Nitroethane
HaHaHa
Ha
Hb
Hb
Hb
n + 1 rule
n = # non-equivalent neighboring protons
3.64
12
2
1.12n + 1 rule
Ha
Ha
Ha
Ha
Ha
Ha
Hb
Diisopropylether
O
1
6
15
20
15
6
1
Coupling constant J (Hz) – indicates strength of coupling
J ~ 7 Hz for alkyl (sp3) systems
Ha
Ha
Ha
Ha
Ha
Ha
Hb
O
2
1
Doublet
Pascal’s Triangle
1 1
1
1
1
1
1
1
1
1
1
1
2
3 3
4 46
6 6
5 510 10
2015 15
Singlet
Triplet
Quartet
Quintet
Sextet
Septet
n = 1
n = 2
n = 3
n = 4
1.18
2.793.51
7.02 6.63
4-isopropylaniline
1.18
6
12
2.793.51
4-isopropylaniline
Hb Ha
Hc
Hd
He
1:6:15:20:15:6:1
1
15
20
6
Coupling constant J (Hz) – indicates strength of coupling
J ~ 7 Hz for alkyl (sp3) systems
15
6
1
Part 2
Ha
HbHc
Hd + He Hb Ha
Hc
Hd
He
4-isopropylaniline
1.18
2.793.51
6.637.02
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