1 Chapter 14: NMR Spectroscopy Learning Objectives: 1. Know how ...

Chapter 14: 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 1 H and 13 C 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 1 H 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:
14.1 Introduction to NMR Spectroscopy
14.2 Fourier Transform NMR
14.3 Shielding*
14.4 The Number of Signals in the 1 H NMR Spectrum*
14.5 The Chemical Shift*
14.6 The Relative Position of 1 H NMR Signals*
14.7 Characteristic Values of Chemical Shifts*
14.8 Integration of NMR Signals*
14.9 Diamagnetic Anisotropiy
14.10 Splitting of the Signals*
14.11 More Examples of 1 H NMR Spectra*
14.12 Coupling Constants*
14.13 Splitting Diagrams*
14.14 Time Dependence of NMR Spectroscopy
14.15 Protons Bonded to Oxygen and Nitrogen*
14.16 Use of Deuterium in 1 H NMR Spectroscopy #
14.17 Resolution of 1 H NMR Spectra
14.18 13 C NMR Spectroscopy*
14.19 DEPT 13 C NMR Spectra #
14.20 Two-dimensional NMR Spectroscopy #
* Sections that will be focused
# Sections that will be skipped
1

Recommended additional problems
41 – 61, 63 – 71
Class Note
14.1 Introduction to NMR Spectroscopy and 14.2 Fourier Transform NMR
Applied
magneti
c field
cap
NMR tube
sample
14.3 Shielding
Deshielded
(low electron
density)
Shielded (high
electron
density)
intensity
H
H
H
downfield
upfield
2

14.4 The Number of Signals in the 1 H NMR Spectrum
*Judge the chemically equivalent of H by the symmetry of molecule
H
H
H
H
H
H
H
Cl
H
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
H
H
H
H
H
H
H
Cl
H
H
H
H
H
H
H
CH 3
H
H
Br
Cl
H
NO 2
Cl
H
Cl
Cl
NO 2
H
H
H
H
H
H
H
H
H
H
H
Br
H
H
H
H
H
H
H
H
H
Cl
H
H
H
H
H
H
H
H
3

14.5 The Chemical Shift, 14.6 The Relative Position of 1 H NMR Signals, and 14.9
Diamagnetic Anisotropiy
Internal reference compound: CHCl 3 (from CDCl 3 ) and (CH 3 ) 4 Si (TMS)
*Signal of TMS = 0 ppm (CHCl3 = 7.27 ppm)
*Chemical shift (δ)
A. Effect from electronegativity (inductive effect)
H
H
H
Cl
H
Cl
H
H
H
H
H
H
H
H
H
H
4

B. Effect from resonance
O
O
O
6.15
O
4.92
7.63
6.88
H
OCH 3
H
H
OCH 3
H
H
OCH 3
H
H
OCH 3
H
7.26
H
H
6.92
H
H
H
H
H
H
H
H
H
H
C. Effect from structure
O
4.73
O
3.75
O O
3.52
2.54 2.72
1.85
1.51
5

D. Diamagnetic Anisotropiy (anisotropic effect)
applied magnetic field (B o )
induced magnetic field (B i )
H 7.3
H
actual magnetic field
(B o + B i )
14.7 Characteristic Values of Chemical Shifts
Table 14.1
14.8 Integration of NMR Signals
* Diagnostic for 1 H NMR but less accurate for 13 C NMR
* Ratio rather than exact number
H b
Hb
H b
H b
H
Cl
H
H
H
Cl
H
H
H
H a
H
H H H H
a H H H H
H a
H c
H
H
H
H
O
H
H
H H
H
H a
I
II
III
6

14.10 Splitting of the Signals
A. Multiplicity of Signal and Relative Intensities
Ratio
Multiplicity
1 : 1 doublet
1 : 2 : 1 triplet
1 : 3 : 3 : 1 quartet
1 : 4 : 6 : 4 : 1 quintet
1 : 5 : 10 : 10 : 5 : 1 sextet
1 : 6 : 15 : 20 : 15 : 6 : 1 septet
Original signal
1 st splitting
1 1
2 nd splitting
1
2
1
3 rd splitting
1
3 3
1
4 th splitting
1
4 6 4
1
5 th splitting
1
5 10 10 5
1
6 th splitting
1
6
15
20
15
6
1
Two important criteria:
* For I = 1/2
* For chemically equivalent nuclei
7

B. Examples
H b
Hb
H b
H b
H
Cl
H
H
H
Cl
H
H
H
H a
H
H H H H
a H H H H
H a
H c
H
H
H
H
O
H
H
H H
H
H a
I
II
III
8

14.11 More Examples of 1 H NMR Spectra
A. More examples
H d
Br
H H
Br
H a
H a
H 3 C
H H H H
H b
H b
IV
H
H H
H c
H
O
V
O H
H 3 C CH 3
H b
H e
9

B. Difference between quartet (q) and doublet of doublet (dd)
Cl
H
Br
H a
H
H
H b
Cl
O
H
Cl
H
H a
H b
H c H c
OCH 3
H
Br
H
H
O
H
H
VI
VII
10

14.12 Coupling Constants and 14.13 Splitting Diagrams
A. Table 14.3 and handout
B. Calculation of coupling constant (J value)
2.5 mm
pattern A
5 mm
2.5 mm
pattern B
5 mm
400 MHz 1 H NMR
400 MHz 1 H NMR
integral ratio of peaks:
1:3:3:1
integral ratio of peaks:
1:1:1:1
3.3 ppm
15 mm
3.2 ppm
3.3 ppm
15 mm
3.2 ppm
11

C. Splitting diagrams and J values
(1)
(1) J
H a H b
H ab = J ac (2) J ab > J ac
c
(2) long range coupling (4 bonds)
12

D. Structure determination and J values
(1) Example 1
H b
H a
H c
COCH 3
J ab = 2 Hz
J ac = 15 Hz
J bc = 7 Hz
(2) Example 2: determination of cis and trans isomers
H a
H a
COCH 3
H 3 C H 3 C
J ab = 15 Hz or 7 Hz
H b
trans
H b
cis
COCH 3
13

(3) Example 3: determination of the regioisomers of di-substituted benzene
derivatives
NH 2
NH 2
NH 2
H
Br
H
H
H
H
H
H
H
Br
H
H
H
H
Br
1,2-di-substituted
(ortho)
1,3-di-substituted
(meta)
1,4-di-substituted
(para)
14

14.15 Protons Bonded to Oxygen and Nitrogen and 14.16 Use of Deuterium in 1 H NMR
Spectroscopy
14.17 Resolution of 1 H NMR Spectra
1 H NMR
60 MHz 90 MHz 300 MHz 400 MHz 600 MHz
15

14.18 13 C NMR Spectroscopy
A. Table 14.4
Chemical shift and height (intensity)
B. Proton-coupled and proton-decoupled 13 C spectra
16