Transcript NMRSEM2
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)
CH3
H3 C
Si
CH3
CH3
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
of H
CH3 F
CH3 OH
CH3 Cl
CH3 Br
CH3 I
(CH 3 ) 4 C
4.0
3.5
3.1
2.8
2.5
2.1
4.26
3.47
3.05
2.68
2.16
0.86
(C H3 ) 4 Si
1.8
0.00 (by definition
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 split into (1 + 1) or
2 peaks ; a doublet
Cl
CH3 -CH-Cl
n = 3. Its signal
is split 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