Transcript 13_01_05rw

Chapter 13
Spectroscopy
Infrared spectroscopy
Ultraviolet-visible spectroscopy
Nuclear magnetic resonance spectroscopy
Mass spectrometry
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
13.1
Principles of Molecular
Spectroscopy:
Electromagnetic Radiation
Electromagnetic Radiation
Is propagated at the speed of light
Has properties of particles and waves
The energy of a photon is proportional
to its frequency
Figure 13.1: The Electromagnetic Spectrum
Shorter Wavelength ()
400 nm
Longer Wavelength ()
750 nm
Visible Light
Higher Frequency ()
Higher Energy (E)
Lower Frequency ()
Lower Energy (E)
Figure 13.1: The Electromagnetic Spectrum
Shorter Wavelength ()
Ultraviolet
Higher Frequency ()
Higher Energy (E)
Longer Wavelength ()
Infrared
Lower Frequency ()
Lower Energy (E)
Figure 13.1: The Electromagnetic Spectrum
Cosmic rays
 Rays
X-rays
Energy
Ultraviolet light
Visible light
Infrared radiation
Microwaves
Radio waves
13.2
Principles of Molecular Spectroscopy:
Quantized Energy States
E = h 
Electromagnetic radiation is absorbed when the
energy of photon corresponds to difference in
energy between two states.
What Kind of States?
electronic
UV-Vis
vibrational
infrared
rotational
microwave
nuclear spin
radiofrequency
13.3
Introduction to
1H NMR Spectroscopy
The Nuclei that are Most Useful to
Organic Chemists are:
1H
and 13C
both have spin = ±1/2
1H
is 99% at natural abundance
13C
is 1.1% at natural abundance
Nuclear Spin
+
+
A spinning charge, such as the nucleus of 1H
or 13C, generates a magnetic field. The
magnetic field generated by a nucleus of spin
+1/2 is opposite in direction from that
generated by a nucleus of spin –1/2.
The distribution of
nuclear spins is
random in the
absence of an
external magnetic
field.
+
+
+
+
+
An external magnetic
field causes nuclear
magnetic moments to
align parallel and
antiparallel to applied
field.
+
+
+
B0
+
+
There is a slight
excess of nuclear
magnetic moments
aligned parallel to
the applied field.
+
+
+
B0
+
+
Energy Differences Between Nuclear Spin States
+
E
E '
+
increasing field strength
No difference in absence of magnetic field
Proportional to strength of external magnetic field
Some Important Relationships in NMR
Units
The frequency of absorbed
electromagnetic radiation
is proportional to
the energy difference between
two nuclear spin states
which is proportional to
the applied magnetic field.
Hz
kJ/mol
(kcal/mol)
tesla (T)
Some Important Relationships in NMR
The frequency of absorbed electromagnetic
radiation is different for different elements,
and for different isotopes of the same element.
For a field strength of 4.7 T:
1H absorbs radiation having a frequency
of 200 MHz (200 x 106 s-1)
13C absorbs radiation having a frequency
of 50.4 MHz (50.4 x 106 s-1)
Some Important Relationships in NMR
The frequency of absorbed electromagnetic
radiation for a particular nucleus (such as 1H)
depends on its molecular environment.
This is why NMR is such a useful tool
for structure determination.
13.4
Nuclear Shielding
and
1H Chemical Shifts
What do we mean by "shielding"?
What do we mean by "chemical shift"?
Shielding
An external magnetic field
affects the motion of the
electrons in a molecule,
inducing a magnetic field
within the molecule.
The direction of the
induced magnetic field is
opposite to that of the
applied field.
C
H
B0
Shielding
The induced field shields
the nuclei (in this case, C
and H) from the applied
field.
A stronger external field is
needed in order for energy
difference between spin
states to match energy of
rf radiation.
C
H
B0
Chemical Shift
Chemical shift is a
measure of the degree to
which a nucleus in a
molecule is shielded.
Protons in different
environments are shielded
to greater or lesser
degrees; they have
different chemical shifts.
C
H
B0
Chemical Shift
Chemical shifts (d) are
measured relative to the
protons in
tetramethylsilane (TMS)
as a standard.
d =
CH3
H3C
Si
CH3
position of signal - position of TMS peak
spectrometer frequency
CH3
x 106
Downfield
Decreased shielding
Upfield
Increased shielding
(CH3)4Si (TMS)
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
Chemical shift (d, ppm)
measured relative to TMS
1.0
0
Chemical Shift
Example: The signal for the proton in chloroform
(HCCl3) appears 1456 Hz downfield from TMS at
a spectrometer frequency of 200 MHz.
d =
d =
position of signal - position of TMS peak
spectrometer frequency
1456 Hz - 0 Hz
200 x 106 Hz
d = 7.28
x 106
x 106
Cl
d 7.28 ppm
H
C
Cl
Cl
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
13.5
Effects of Molecular Structure
on
1H Chemical Shifts
Protons in different environments experience
different degrees of shielding and have
different chemical shifts.
Electronegative Substituents Decrease
the Shielding of Methyl Groups
least shielded H
CH3F
CH3OCH3
d 4.3
d 3.2
most shielded H
(CH3)3N
d 2.2
CH3CH3
(CH3)4Si
d 0.9
d 0.0
Electronegative Substituents Decrease Shielding
d 0.9
d 1.3
d 0.9
H3C—CH2—CH3
d 4.3
d 2.0
d 1.0
O2N—CH2—CH2—CH3
Effect is Cumulative
CHCl3
CH2Cl2
CH3Cl
d 7.3
d 5.3
d 3.1
Methyl, Methylene, and Methine
CH3 more shielded than CH2 ;
CH2 more shielded than CH
d 0.9
CH3
H3C
C
CH3
H d 1.6
d 0.9
CH3
H3 C
C
CH3
d 1.2
CH2
d 0.8
CH3
Protons Attached to sp2 Hybridized Carbon
are Less Shielded than those Attached
to sp3 Hybridized Carbon
H
H
H
H
H
C
H
H
CH3CH3
C
H
H
H
d 7.3
d 5.3
d 0.9
But Protons Attached to sp Hybridized Carbon
are More Shielded than those Attached
to sp2 Hybridized Carbon
d 5.3
H
H
C
H
C
H
d 2.4
H
C
C
CH2OCH3
Protons Attached to Benzylic and Allylic
Carbons are Somewhat Less Shielded than Usual
H3C
CH3
d 0.8
d 1.5
d 0.9
d 1.3
d 0.9
H3C—CH2—CH3
d 1.2
H3C
d 2.6
CH2
Proton Attached to C=O of Aldehyde
is Most Deshielded C—H
d 2.4
H
H3C
C
O
C
CH3
d 1.1
H d 9.7
Table 13.1
Type of proton
H
H
C
C
R
C
Chemical shift (d), Type of proton
ppm
0.9-1.8
C
C
H
C
C
H
C
Ar
N
2.1-2.3
C 1.5-2.6
O
H
Chemical shift (d),
ppm
2.0-2.5
2.3-2.8
Table 13.1
Type of proton
H
C
NR
Chemical shift (d), Type of proton
ppm
2.2-2.9
H
C
H
H
C
C
Cl
Br
Chemical shift (d),
ppm
C
4.5-6.5
3.1-4.1
2.7-4.1
H
Ar
6.5-8.5
O
H
C
O
3.3-3.7
H
C
9-10
Table 13.1
Type of proton
Chemical shift (d),
ppm
H
NR
1-3
H
OR
0.5-5
H
OAr
6-8
O
HO
C
10-13