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Nuclear
Magnetic
Resonance
Chapter 15
15-1
Electromagnetic Radiation

Electromagnetic radiation: light and other
forms of radiant energy  = c & E = h

Wavelength (): the distance between
consecutive identical points on a wave
 Frequency
(n): the number of full cycles of a
wave that pass a point in a second

Hertz (Hz): the unit in which radiation
frequency is reported; s-1 (read “per
second”)
15-2
Electromagnetic Radiation

 Wavelength
15-3
Molecular Spectroscopy
 We
study three types of molecular
spectroscopy
Region of the
Electromagnetic
Spectrum
Absorption of Electromagnetic
Radiation Results
in Transition Between
radio frequency
nuclear spin energy levels
infrared
vibrational energy levels
ultraviolet-visible
electronic energy levels
15-4
A pictorial view of UV/Vis
UV/Vis radiation is measured in nm
(wavelength)
15-5
IR Spectroscopy
radiation is measured in cm-1
 This is actually a frequency. Remember that
frequency and wavelength are inversely
proportional.
 IR
15-6
NMR Spectroscopy
 NMR
uses radiowaves, measured in MHz
15-7
Nuclear Magnetic Resonance Spectroscopy
Introduction to NMR
• When a charged particle such as a proton spins on its axis, it
creates a magnetic field. Thus, the nucleus can be considered
to be a tiny bar magnet.
• Normally, these tiny bar magnets are randomly oriented in
space. However, in the presence of a magnetic field B0, they
are oriented with or against this applied field.
• The energy difference between these two states is very small
(<0.1 cal).
15-8
Nuclear Spins in B0
and 13C, only two orientations are
allowed.
higher
Energy
 For 1H
energy state
spin -1/2
(aligned against
the applied field
lower
energy state
spin +1/2
(aligned with
the applied field
15-9
Nuclear Spins in B0
 In
an applied field strength of 7.05T, which is
readily available with present-day
superconducting electromagnets, the
difference in energy between nuclear spin
states for
• 1H is approximately 0.0286 cal/mol, which
corresponds to electromagnetic radiation of 300
MHz (300,000,000 Hz)(300MHz)
• 13C is approximately 0.00715 cal/mol, which
corresponds to electromagnetic radiation of
75MHz (75,000,000 Hz)(75 MHz)
15-10
Population in high vs low
 E=
0.0286 cal/mol RT=582cal/mol
 If pop in high E state is 1,000,000 then pop in
low energy state is 1,000,049
nuclei in high E state
 E / RT
e
nuclei in low E state
15-11
NMR Spectroscopy
 NMR
uses radiowaves, measured in MHz
 The energy transitions depend on the
strength of the magnetic field which is
different from machine to machine
 We define the machine independent ppm as
n
6

 10
Oscillator frequency
15-12
Nuclear Magnetic Resonance
we were dealing with 1H nuclei isolated
from all other atoms and electrons, any
combination of applied field and radiation
that produces a signal for one 1H would
produce a signal for all 1H. The same is true
of 13C nuclei
 But hydrogens in organic molecules are not
isolated from all other atoms; they are
surrounded by electrons, which are caused
to circulate by the presence of the applied
field
 If
15-13
Electrons
Shield
What causes differences?
Electrons shield. Remove electrons they de-shield.
15-14
Electron Withdrawing groups deshield
by removing electron density
“I suck”
15-15
Electron density can be added or
removed through the p or s systems
15-16
Field currents in benzene
H0
15-17
Ring currents usually deshield
15-18
Alkenes
15-19
Nuclear Magnetic Resonance
 It
is customary to measure the resonance
frequency (signal) of individual nuclei
relative to the resonance frequency (signal)
of a reference compound
 The reference compound now universally
accepted is tetramethylsilane (TMS)
CH 3
H3 C
Si
CH 3
CH 3
Tetramethylsilane (TMS)
15-20
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—The Spectrum
• An NMR spectrum is a plot of the intensity of a peak against its
chemical shift, measured in parts per million (ppm).
15-21
Nuclear Magnetic Resonance
For a 1H-NMR spectrum, signals are
reported by their shift from the 12 H signal
in TMS
 For a 13C-NMR spectrum, signals are
reported by their shift from the 4 C signal in
TMS


Chemical shift (): the shift in ppm of an
NMR signal from the signal of TMS
=
Shift in frequency from TMS (Hz)
Frequency of s pectrometer (Hz)
15-22
Equivalent Hydrogens
Equivalent hydrogens: have the same
chemical environment (Section 2.3C)
 Molecules with

• 1 set of equivalent hydrogens give 1 NMR signal
• 2 or more sets of equivalent hydrogens give a
different NMR signal for each set
Cl
CH3 CHCl
1,1-Dichloroethane
(2 signals)
Cl
O
Cyclopentanone
(2 signals)
CH3
C
C
H
H
(Z)-1-Chloropropene
(3 signals)
Cyclohexene
(3 signals)
15-23
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Chemical Shift Values
15-24
15-25
Chemical Shift


Depends on (1) electronegativity of nearby atoms,
(2) the hybridization of adjacent atoms, and (3)
magnetic induction within an adjacent pi bond
Electronegativity
CH3 -X
Electronegativity of X
CH3 F
CH3 OH
CH3 Cl
4.0
3.5
3.1
4.26
3.47
3.05
CH3 Br
CH3 I
(CH 3 ) 4 C
2.8
2.5
2.1
2.68
2.16
0.86
(C H3 ) 4 Si
1.8
0.00 (by definition
 of H
15-26
Methyl Acetate
15-27
Signal Splitting (n + 1)
 Peak:
the units into which an NMR signal is
split; doublet, triplet, quartet, etc.
 Signal
splitting: splitting of an NMR signal
into a set of peaks by the influence of
neighboring nonequivalent hydrogens
+ 1) rule: the 1H-NMR signal of a
hydrogen or set of equivalent hydrogens is
split into (n + 1) peaks by a nonequivalent
set of n equivalent neighboring hydrogens15-28
 (n
Signal Splitting (n + 1)
n = 1. Their signal
is s plit into (1 + 1) or
2 peaks ; a doublet
Cl
CH3 -CH-Cl
n = 3. Its s ignal
is s plit into (3 + 1)
or 4 peaks ; a quartet
predict the number of 1H-NMR
signalsOand the splitting pattern O
of each
 Problem:
(a) CH 3 CCH2 CH3
(b) CH3 CH2 CCH2 CH3
O
(c) CH3 CCH(CH 3 ) 2
15-29
Origins of Signal
Splitting
 When
the chemical shift of one nucleus is
influenced by the spin of another, the two
are said to be coupled
 Consider nonequivalent hydrogens Ha and
Hb on adjacent carbons
• the chemical shift of Ha is influenced by whether
the spin of Hb is aligned with or against the
applied field
Ha Hb
C
C
15-30
Origins of Signal
Splitting
B0
Hb
Magnetic field of H b
subtracts from the applied
field; H b signal appears at
a higher applied field
Hb
Magnetic field of H b adds
to the applied field; H a
signal appears at a lower
applied field
Ha
15-31
Origins of Signal
Splitting
 Table
13.8 Observed signal splitting
patterns for an H with 0, 1, 2, and 3
equivalent neighboring hydrogens
Structure
Spin States of H b
Signal of H a
Ha
C
C
Ha Hb
C
1
1
C
15-32
Origins of Signal
Splitting
 Table
13.8 (contd.)
Ha Hb
C
C
1 2
1
Hb
Ha Hb
C
C
Hb
1 3
3
1
Hb
15-33
Coupling Constants

Coupling constant (J): the distance between peaks
in an NMR multiplet, expressed in hertz
• J is a quantitative measure of the magnetic
interaction of nuclei whose spins are coupled
Ha
Ha
HaHb
Hb
-C-CHb
6-8 Hz
8-14 Hz
Ha
Ha
C
C
Ha
Hb
C
C
Hb
11-18 Hz
0-5 Hz
5-10 Hz
C
C
Hb
0-5 Hz
15-34
Ethyl acetate
15-35
Isopropyl alcohol
15-36
13C-NMR
Spectroscopy
Each nonequivalent 13C gives a different
signal
 A 13C is split by the 1H bonded to it
according to the (n + 1) rule
 Coupling constants of 100-250 Hz are
common, which means that there is often
significant overlap between signals, and
splitting patterns can be very difficult to
determine
 The most common mode of operation of a
13C-NMR spectrometer is a hydrogendecoupled mode
15-37

13C-NMR

Spectroscopy
In a hydrogen-decoupled mode, a sample is
irradiated with two different radio
frequencies
• one to excite all 13C nuclei
• a second is a broad spectrum of frequencies that
causes all hydrogens in the molecule to undergo
rapid transitions between their nuclear spin
states

On the time scale of a 13C-NMR spectrum,
each hydrogen is in an average or
effectively constant nuclear spin state, with
the result that 1H-13C spin-spin interactions
are not observed; they are decoupled
15-38
Carbon – 13 shifts
15-39
15-40
C8H10
15-41
C7H12O4
15-42
C7H14O
15-43