Chapter 13: NMR Spectroscopy
Learning Objectives:
1. Know how nuclear spins are affected by a magnetic field, and be able to explain
what happens when radiofrequency radiation is absorbed.
2. Be able to predict the number of proton and carbon NMR signals expected from a
compound given its structure.
3. Be able to predict the splitting pattern in the proton NMR spectrum of a compound
given its structure.
4. With the aid of a chart of chemical shifts from 1H and 13C NMR, be able to assign
peaks in an NMR spectrum to specific protons in a compound.
5. Be able to interpret integration of NMR spectra.
6. Be able to use NMR spectra to determine the structures of compounds, given other
information such as a molecular formula.
7. Be able to calculate coupling constants from 1H NMR spectra, and utilize the
coupling constants for determining compound structure.*
8. Be able to determine the compound structure based on information generated from
mass spectrometry, IR, NMR, and elemental analysis.*
* Supplemental material, not included in the textbook
Sections:
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
13.9
13.10
13.11
13.12
13.13
13.14
13.15
13.16
13.17
13.18
13.19
13.20
13.21
13.22
An Introduction to NMR Spectroscopy
Fourier Transform NMR
Shielding Causes Different Hydrogens to Show Signals at Different
Frequencies*
The Number of Signals in an 1H NMR Spectrum*
The Chemical Shift Tells How Far the Signals Is from the Reference Signal*
The Relative Position of 1H NMR Signals*
Characteristic Values of Chemical Shifts*
Diamagnetic Anisotropiy
Integration of NMR Signals Reveals the Relative Number of Protons Causing
the Signal*
Splitting of the Signals Is Described by the N+1 Rule*
More Examples of 1H NMR Spectra*
Coupling Constants Identify Coupled Protons*
Splitting Diagrams Explain the Multiplicity of a Signal*
Diastereotopic Hydrogens Are Not Chemically Equivalent
The Time Dependence of NMR Spectroscopy
Protons Bonded to Oxygen and Nitrogen*
The Use of Deuterium in 1H NMR Spectroscopy#
Resolution of 1H NMR Spectra
13
C NMR Spectroscopy*
DEPT 13C NMR Spectra#
Two-dimensional NMR Spectroscopy#
NMR Used in Medicines Is Called Magnetic Resonance Imaging
1
* Sections that will be focused
#
Sections that will be skipped
Recommended additional problems
43 – 63, 65 – 72
Class Note
13.1
An Introduction to NMR Spectroscopy and 13.2
Fourier Transform NMR
Applied
magneti
c field
cap
NMR tube
sample
13.3
Shielding Causes Different Hydrogens to Show Signals at Different
Frequencies*
Deshielded
(low electron
density)
intensity
H
Shielded (high
electron
density)
H
H
downfield
upfield
2
The Number of Signals in an 1H NMR Spectrum*
13.4
*Judge the chemically equivalent of H by the symmetry of molecule
H
H
H
H
H
H H
H
H
H H
H
H
Cl
H H
H
Cl
H
H
H H
H H
H H
H
H
H
H
Cl
H
Cl
H
H
CH3
H
H
Br
H
H
H
H
H
H
H
Cl
Cl
NO2
H
H
H
H
H
H
H
Cl
H
H
H
Br
H
H
H
Cl
Cl
H
H
H
NO2
H
H H
H
H
H
3
H
H
H H
H
H
13.5
13.6
The Chemical Shift Tells How Far the Signals Is from the Reference Signal*,
The Relative Position of 1H NMR Signals* and 13.8 Diamagnetic Anisotropiy
Internal reference compound: CHCl3 (from CDCl3) and (CH3)4Si (TMS)
*Signal of TMS = 0 ppm (CHCl3 = 7.27 ppm)
*Chemical shift ()
A. Effect from electronegativity (inductive effect)
H
Cl
H
H
H
Cl
H H
H
H
H H
H H
4
H H
B. Effect from resonance
O
O
6.15
O
O
7.63
4.92
OCH3
6.88
H
OCH3
H
H
OCH3
H
H
H
H
OCH3
H
H
H
H
H
H
7.26
H
H
6.92
H
H
H
H
H
C. Effect from structure
4.73
O
O
3.75
O
O
3.52
2.54
2.72
1.85
5
1.51
D. Diamagnetic Anisotropiy (anisotropic effect)
applied magnetic field (Bo)
induced magnetic field (Bi)
H 7.3
actual magnetic field
(Bo + Bi)
H
13.7
Characteristic Values of Chemical Shifts*
Table 13.1
13.9
Integration of NMR Signals Reveals the Relative Number of Protons Causing
the Signal*
* Diagnostic for 1H NMR but less accurate for 13C NMR
* Ratio rather than exact number
Hb
H
H
H
Ha
Cl
H H
H H
H
H
H
Ha
H
H H
H H
Hc
I
Hb
Hb
Cl
Hb
H
6
H
Ha
H
H H
O
III
II
H
H
H H
Ha
13.10
Splitting of the Signals Is Described by the N+1 Rule*
A. Multiplicity of Signal and Relative Intensities
Ratio
1:1
1:2:1
1:3:3:1
1:4:6:4:1
1 : 5 : 10 : 10 : 5 : 1
1 : 6 : 15 : 20 : 15 : 6 : 1
Multiplicity
doublet
triplet
quartet
quintet
sextet
septet
Original signal
1st splitting
2nd splitting
3rd splitting
4th splitting
5th splitting
6th splitting
3
5
6
2
4
1
1
1
1
1
1
1
1
3
6
10
15
1
4
10
20
Two important criteria:
* For I = 1/2
* For chemically equivalent nuclei
7
1
5
15
1
6
1
B. Examples
Hb
H
Cl
H
H
Ha
Cl
H H
H H
H
H
H
Ha
H
H H
H H
Hc
I
Hb
Hb
Hb
H
8
H
Ha
H
H H
O
III
II
H
H
H H
Ha
More Examples of 1H NMR Spectra*
13.11
A. More examples
Ha
H
Hd
H
Br
Br
H H
H
H3C
H H
O
H H
Hb
Hb
H
Ha
Hc
IV
9
O
V
H
H3C CH3
Hb
He
B. Difference between quartet (q) and doublet of doublet (dd)
Hb
H
Hb
OCH3
Ha
Cl
Br
Cl
H
Cl
H
Br H
O
H H
Hc
Ha
H
VI
O
VII
10
H
H H Hc
13.12 Coupling Constants Identify Coupled Protons* and 13.13 Splitting Diagrams
Explain the Multiplicity of a Signal*
A. Table 14.3 and handout
B. Calculation of coupling constant (J value)
pattern A
pattern B
2.5 mm
2.5 mm
5 mm
5 mm
400 MHz 1H NMR
400 MHz 1H NMR
integral ratio of peaks:
1:3:3:1
integral ratio of peaks:
1:1:1:1
15 mm
3.3 ppm
15 mm
3.2 ppm
3.3 ppm
11
3.2 ppm
C. Splitting diagrams and J values
(1)
Ha
Hb
Hc
(1) Jab = Jac
(2) long range coupling (4 bonds)
12
(2) Jab > Jac
D. Structure determination and J values
(1) Example 1
Ha
COCH3
Hb
Hc
Jab = 2 Hz
Jac = 15 Hz
Jbc = 7 Hz
(2) Example 2: determination of cis and trans isomers
Ha
Ha
Hb
COCH3
H3C
H3C
Hb
trans
Jab = 15 Hz or 7 Hz
COCH3
cis
13
(3) Example 3: determination of the regioisomers of di-substituted benzene
derivatives
NH2
NH2
NH2
H
Br
H
H
H
H
H
H
H
Br
H
H
H
1,2-di-substituted
(ortho)
H
Br
1,3-di-substituted
(meta)
1,4-di-substituted
(para)
14
13.14
Diastereotopic Hydrogens Are Not Chemically Equivalent
H
H
H
H
H
H
Br
H
13.15
The Time Dependence of NMR Spectroscopy
Figure 13.29
H
H
H
H
15
13.16 Protons Bonded to Oxygen and Nitrogen* and 13.17 The Use of Deuterium in
1
H NMR Spectroscopy#
13.18
Resolution of 1H NMR Spectra
1
60 MHz
90 MHz
H NMR
300 MHz
16
400 MHz
600 MHz
13.19
13
C NMR Spectroscopy*
A. Table 13.4
Chemical shift and height (intensity)
B. Proton-coupled and proton-decoupled 13C spectra
17
13.22
NMR Used in Medicines Is Called Magnetic Resonance Imaging
18