Download Chapter 13 Spectroscopy

Chapter 13
Spectroscopy
Nuclear Magnetic Resonance Spectroscopy
Infrared Spectroscopy
Ultraviolet-Visible Spectroscopy
Mass Spectrometry
13.1
Principles of Molecular
Spectroscopy:
Electromagnetic Radiation
Electromagnetic Radiation
Propagated at the speed of light.
Has properties of particles and waves.
The energy of a photon is proportional
to its frequency.
The Electromagnetic Spectrum
Shorter Wavelength ()
400 nm
Longer Wavelength ()
750 nm
Visible Light
Higher Frequency ()
Higher Energy (E)
Lower Frequency ()
Lower Energy (E)
The Electromagnetic Spectrum
Shorter Wavelength ()
Ultraviolet
Higher Frequency ()
Higher Energy (E)
Longer Wavelength ()
Infrared
Lower Frequency ()
Lower Energy (E)
The Electromagnetic Spectrum
Cosmic rays
 Rays
X-rays
Energy
Ultraviolet (UV) light
Visible light
Infrared (IR) radiation
Microwaves
Radio waves
13.2
Principles of Molecular Spectroscopy:
Quantized Energy States
E = h
Electromagnetic radiation is absorbed when the
energy of the photon corresponds to the
difference in energy between two states.
What Kind of States?
Electronic
UV-Visible
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 99.985% 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.
+
+
+
H0
+
+
There is a slight
excess of nuclear
magnetic moments
aligned parallel to
the applied field.
+
+
+
H0
+
+
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
H0
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
H0
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
H0
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
NMR Spectrometers
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
Chloroform
Cl
H
d 7.28 ppm
10.0
9.0
8.0
C
Cl
Cl
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
CH3Cl
CH2Cl2
CHCl3
d 3.1
d 5.3
d 7.3
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
H3C
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
1H
Chemical Shifts of Some Common Groups
Type of proton Chemical shift (d),
ppm
Type of proton
Chemical shift (d),
ppm
H
C
R
0.9-1.8
H
C
C
N
2.1-2.3
H
C
C
C 1.5-2.6
H
C
C
C
2.5
2.0-2.5
H
C
Ar
O
H
C
C
2.3-2.8
1H
Chemical Shifts of Some Common Groups
Type of proton Chemical shift (d),
ppm
H
C
NR
2.2-2.9
Type of proton
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
1H
Chemical Shifts of Some Common Groups
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
13.6
Interpreting Proton NMR
Spectra
Information contained in an NMR
spectrum includes:
1. Number of signals.
2. Their intensity (as measured by area
under peak).
3. Splitting pattern (multiplicity).
Number of Signals
Protons that have different chemical shifts
are chemically nonequivalent.
Exist in different molecular environment.
N
Methoxyacetonitrile
CCH2OCH3
OCH3
NCCH2O
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
Chemically Equivalent Protons
Are in identical environments.
Have same chemical shift.
Replacement test: replacement by some
arbitrary "test group" generates same compound.
H3CCH2CH3
chemically equivalent
Chemically Equivalent Protons
Replacing protons at C-1 and C-3 gives same
compound (1-chloropropane).
C-1 and C-3 protons are chemically
equivalent and have the same chemical shift.
ClCH2CH2CH3
CH3CH2CH2Cl
H3CCH2CH3
Chemically equivalent
Diastereotopic Protons
Replacement by some arbitrary test group
generates diastereomers.
Diastereotopic protons can have different
chemical shifts.
Br
C
H3C
H
d 5.3 ppm
H
d 5.5 ppm
C
Enantiotopic Protons
Are in mirror-image environments.
Replacement by some arbitrary test group
generates enantiomers.
Enantiotopic protons have the same
chemical shift.
Enantiotopic
Protons
H
H
C
CH2OH
H3C
H
Cl
Cl
H
C
H3C
CH2OH
R
C
H3C
CH2OH
S
13.7
Spin-Spin Splitting
in
NMR Spectroscopy
Not all peaks are singlets.
Signals can be split by coupling of
nuclear spins.
Cl2CHCH3
1,1-Dicholoroethane
4 lines;
quartet
2 lines;
doublet
CH3
CH
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
Two-Bond and Three-Bond Coupling
H
C
H
C
C
C
H
H
Protons separated by
two bonds
(geminal relationship).
Protons separated by
three bonds
(vicinal relationship).
Two-Bond and Three-Bond Coupling
H
C
H
C
C
C
H
H
In order to observe splitting, protons cannot
have same chemical shift.
Coupling constant (2J or 3J) is independent
of field strength and are measured in Hz.
1,1-Dicholoroethane
4 lines;
quartet
Cl2CHCH3
2 lines;
doublet
CH
CH3
Coupled protons are vicinal (three-bond coupling).
10.0 9.0
8.0
7.0
6.0
5.0
4.0
3.0 2.0
1.0
CH splits CH3 into a doublet.
CH3 splits CH into aChemical
quartet. shift (d, ppm)
0
Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
Cl
H
H
C
C
Cl
H
Signal for methyl
H protons is split into
a doublet.
To explain the splitting of the protons at C-2,
we first focus on the two possible spin
orientations of the proton at C-1.
Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
Cl
H
H
C
C
Cl
H
Signal for methyl
H protons is split into
a doublet.
There are two orientations of the nuclear spin
for the proton at C-1. One orientation shields
the protons at C-2; the other deshields the C2 protons.
Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
Cl
H
H
C
C
Cl
H
Signal for methyl
H protons is split into
a doublet.
The protons at C-2 “feel” the effect of both the
applied magnetic field and the local field
resulting from the spin of the C-1 proton.
Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
H
Cl
H
C
C
Cl
H
H
“True” chemical
shift of methyl
protons (no coupling).
This line corresponds
to molecules in which
This line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
the nuclear spin of
the proton at C-1
reinforces
the applied field.
opposes
the applied field.
Why does the methine proton of
1,1-dichloroethane appear as a quartet?
Signal for methine
proton is split into
a quartet.
H
Cl
H
C
C
Cl
H
H
The proton at C-1 “feels” the effect of the
applied magnetic field and the local fields
resulting from the spin states of the three
methyl protons. The possible combinations
are shown on the next slide.
Why does the methine proton of
1,1-dichloroethane appear as a quartet?
H
Cl
H
C
C
Cl
H
There are eight combinations of
nuclear spins for the three methyl
protons.
H
These 8 combinations split the
signal into a 1:3:3:1 quartet.
The Splitting Rule for 1H NMR
For simple cases, the multiplicity of a signal
for a particular proton is equal to the number
of equivalent vicinal protons + 1.
13.8
Splitting Patterns:
The Ethyl Group
CH3CH2X is characterized by a triplet-quartet
pattern (quartet at lower field than the triplet).
BrCH2CH3
Ethyl bromide
4 lines;
quartet
3 lines;
triplet
CH3
CH2
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
Splitting Patterns of Common Multiplets
Splitting Patterns of Common Multiplets
Number of equivalent
protons to which H
is coupled
Appearance Intensities of lines
of multiplet in multiplet
1
Doublet
1:1
2
3
4
5
6
Triplet
Quartet
Pentet
Sextet
Septet
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
13.9
Splitting Patterns:
The Isopropyl Group
(CH3)2CHX is characterized by a doubletseptet pattern (septet at lower field than the
doublet).
ClCH(CH3)2
Isopropyl chloride
2 lines;
doublet
7 lines;
septet
CH3
CH
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
13.10
Splitting Patterns:
Pairs of Doublets
Splitting patterns are not always symmetrical,
but lean in one direction or the other.
Pairs of Doublets
H
C
C
H
Consider coupling between two vicinal
protons.
If the protons have different chemical shifts,
each will split the signal of the other into a
doublet.
Pairs of Doublets
H
C
C
H
Let  be the difference in chemical shift in Hz
between the two protons.
Let J be the coupling constant between peaks
for each proton in Hz.
AX
H
C
C
J
H
J

When  is much larger than J the signal for
each proton is a doublet, the doublet is
symmetrical, and the spin system is called
AX.
AM
H
C
C
J
H
J

As /J decreases the signal for each proton
remains a doublet, but becomes skewed. The outer
lines decrease while the inner lines increase,
causing the doublets to "lean" toward each other.
AB
H
C
H
C
J
J

When  and J are similar, the spin system is
called AB. Skewing is quite pronounced. It is
easy to mistake an AB system of two doublets
for a quartet.
A2
H
C
C
H
When  = 0, the two protons have the same
chemical shift and don't split each other. A
single line is observed. The two doublets have
collapsed to a singlet.
2,3,4-Trichloroanisole
(1,2,3-Trichloro-4-methoxybenzene) H
Skewed doublets
H
OCH3
Cl
Cl
Cl
OCH3
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
13.11
Complex Splitting Patterns
Multiplets of multiplets
m-Nitrostyrene
H
H
O2N
H
Consider the proton shown in red.
It is unequally coupled to the protons shown
in blue and white.
Jcis = 12 Hz; Jtrans = 16 Hz
H
m-Nitrostyrene
H
O2N
The signal for
the proton
shown in red
appears as a
doublet of
doublets.
16 Hz
12 Hz
H
12 Hz
m-Nitrostyrene
H
H
O2N
H
Doublet of doublets
13.12
1H NMR Spectra of Alcohols
What about H bonded to O?
O—H
H
C
O
H
The chemical shift for O—H is variable and
depends on temperature and concentration.
Splitting of the O—H proton is sometimes
observed but usually is not. It usually appears
as a broad singlet peak.
Adding D2O converts O—H to O—D.
The O—H peak disappears.
13.13
NMR and Conformations
NMR is “Slow”
Most conformational changes occur faster
than NMR can detect them.
An NMR spectrum is the weighted average of
the conformations.
For example, cyclohexane gives a single peak
for its H atoms in NMR. Half of the time a
single proton is axial and half of the time it is
equatorial. The observed chemical shift is half
way between the axial chemical shift and the
equatorial chemical shift.
13.14
13C
NMR Spectroscopy
1H
and 13C NMR Compared
Both give us information about the number of
chemically nonequivalent nuclei
(nonequivalent hydrogens or nonequivalent
carbons).
Both give us information about the
environment of the nuclei (hybridization state,
attached atoms, etc.).
It is convenient to use FT-NMR techniques for
1H; it is standard practice for 13C NMR.
1H
and 13C NMR Compared
13C
NMR requires FT-NMR because the
signal for a carbon atom is 10-4 times weaker
than the signal for a hydrogen atom,
because of differences in the magnetic
properties of the two nuclei and,
at the “natural abundance” level, only 1.1% of
all the C atoms in a sample are 13C (most are
12C).
1H
and 13C NMR Compared
13C
signals are spread over a much wider
range than 1H signals making it easier to
identify and count individual nuclei
For 1-chloropentane, it is much easier to
identify the compound by its 13C spectrum
than by its 1H spectrum.
1H
1-Chloropentane
ClCH2CH2CH2CH2CH3
10.0
9.0
8.0
7.0
6.0
CH3
ClCH2
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
13C
1-Chloropentane
ClCH2CH2CH2CH2CH3
a separate, distinct
peak appears for
each of the 5
carbons
200
180
160
140
120
CDCl3
100
80
60
Chemical shift (d, ppm)
40
20
0
13.15
13C
Chemical Shifts
Measured in ppm (d)
from the carbons of TMS.
Factors Affecting 13C Chemical Shifts
• Electronegativity of groups attached to
carbon.
• Hybridization state of carbon.
Electronegativity Effects
Electronegativity has an even greater effect
on 13C chemical shifts than it does on 1H
chemical shifts.
Types of Carbons
Classification
CH4
Chemical shift, d
1H
13C
0.2
-2
CH3CH3
primary
0.9
8
CH3CH2CH3
secondary
1.3
16
(CH3)3CH
tertiary
1.7
25
(CH3)4C
quaternary
28
Replacing H by C (more electronegative) deshields
C to which it is attached.
Electronegativity Effects on CH3
Chemical shift, d
1H
13C
CH4
0.2
-2
CH3NH2
2.5
27
CH3OH
3.4
50
CH3F
4.3
75
Electronegativity Effects and Chain Length
Cl
Chemical
shift, d
CH2
CH2
CH2
CH2
CH3
45
33
29
22
14
Deshielding effect of Cl decreases as
number of bonds between Cl and C increases.
Factors Affecting 13C Chemical Shifts
• Electronegativity of groups attached to
carbon.
• Hybridization state of carbon.
Hybridization Effects
sp3-Hybridized
carbon is more
shielded than sp2.
36
114
138 36 126-142
sp-Hybridized
carbon is more
shielded than
sp2, but less
shielded than
sp3.
H
C
C
CH2
68
84
22
CH2
20
CH3
13
Carbonyl Carbons Are Especially Deshielded
O
127-134
CH2
C
41
171
O
CH2
CH3
61
14
13C
Chemical Shifts for Some Common Groups
Type of carbon Chemical shift (d),
ppm
Type of carbon
Chemical shift (d),
ppm
RCH3
0-35
RC
CR
65-90
R2CH2
15-40
R2C
CR2
100-150
R3CH
25-50
110-175
R4C
30-40
13C
Chemical Shifts for Some Common Groups
Type of carbon Chemical shift (d),
ppm
RCH2Br
RCH2Cl
20-40
25-50
Type of carbon
RC
Chemical shift (d),
ppm
N
110-125
RCOR
160-185
O
RCH2NH2
35-50
RCH2OH
50-65
O
RCH2OR
50-65
RCR
190-220
13.16
13C
NMR and Peak Intensities
Pulse FT-NMR distorts intensities of signals.
Therefore, peak heights and areas can be
deceptive.
m-Cresol
CH3
7 carbons give 7
signals, but
intensities are not
equal
OH
200
180
160
140
120
100
80
60
Chemical shift (d, ppm)
40
20
0
13.20
Infrared Spectroscopy
Gives information about the functional groups
in a molecule.
Infrared Spectroscopy
Characteristic functional groups usually found
between 4000-1600 cm-1.
From 1300-625 cm-1 called “fingerprint region.”
Depends on transitions between vibrational
energy states:
Stretching.
Bending.
Stretching Vibrations of a CH2 Group
Symmetric
Antisymmetric
Bending Vibrations of a CH2 Group
In plane
“scissoring”
In plane
“rocking”
Bending Vibrations of a CH2 Group
Out of plane
“wagging”
Out of plane
“twisting”
Infrared Absorption Frequencies
Structural unit
Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H
3310-3320
sp2 C—H
3000-3100
sp3 C—H
2850-2950
sp2 C—O
1200
sp3 C—O
1025-1200
Infrared Spectrum of Hexane
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Benzene
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Dihexyl Ether
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Absorption Frequencies
Structural unit
Frequency, cm-1
Stretching vibrations (multiple bonds)
C
1620-1680
—C
C—
2100-2200
—C
N
2240-2280
C
Infrared Spectrum of 1-Hexene
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Hexanenitrile
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Absorption Frequencies
Structural unit
Frequency, cm-1
Stretching vibrations (carbonyl groups)
C
O
Aldehydes and ketones 1710-1750
Carboxylic acids
1700-1725
Acid anhydrides
1800-1850 and 1740-1790
Esters
1730-1750
Amides
1680-1700
Infrared Spectrum of 2-Hexanone
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Hexanoic Acid
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Methyl Hexanoate
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Absorption Frequencies
Frequency, cm-1
Structural unit
Bending vibrations of alkenes
RCH
CH2
910-990
R2C
CH2
890
cis-RCH
CHR'
trans-RCH
R2C
CHR'
CHR'
665-730
960-980
790-840
Infrared Spectrum of 1-Hexene
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Absorption Frequencies
Structural unit
Frequency, cm-1
Bending vibrations of derivatives of benzene
Monosubstituted
730-770 and 690-710
ortho-Disubstituted
735-770
meta-Disubstituted
750-810 and 680-730
para-Disubstituted
790-840
Infrared Spectrum of Hexylbenzene
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Absorption Frequencies
Structural unit
Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols)
3200-3600
O—H (carboxylic acids)
3000-3100
N—H
3350-3500
Infrared Spectrum of 1-Hexanol
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Hexylamine
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectrum of Hexanamide
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.
13.21
Ultraviolet-Visible (UV-VIS)
Spectroscopy
Gives information about conjugated  electron
systems
Transitions between Electron Energy States
E = h
Gaps between electron energy
levels are greater than those
between vibrational levels.
Gap corresponds to wavelengths
between 200 and 800 nm.
Conventions in UV-VIS
X-axis is wavelength in nm (high energy at left,
low energy at right).
max is the wavelength of maximum absorption
and is related to electronic makeup of molecule—
especially  electron system.
Y axis is a measure of absorption of electromagnetic
radiation expressed as molar absorptivity ().
UV Spectrum of cis,trans-1,3-Cyclooctadiene
2000
max 230 nm
Molar
absorptivity ()
max 2630
1000
200
220
240
260
Wavelength, nm
280
* Transition in cis,trans-1,3-Cyclooctadiene
LUMO




HOMO 

Most stable
-electron
configuration
E = h


-Electron
configuration of
excited state
* Transition in Alkenes
HOMO-LUMO energy gap is affected by
substituents on double bond.
As HOMO-LUMO energy difference
decreases (smaller E), max shifts to longer
wavelengths.
Effect of Substitution
Methyl groups on double bond cause max
to shift to longer wavelengths
H
H
C
H
H
CH3
C
C
H
max 170 nm
H
C
CH3
max 188 nm
Effect of Conjugation
Extending conjugation has a larger effect
on max; shift is again to longer
wavelengths.
H
H
C
H
H
H
C
C
H
C
H
H
C
C
H
max 170 nm
max 217 nm
H
H
Effect of Conjugation
H
C
C
H
H
C
C
H
H
H3C
max 217 nm
for conjugated diene
H
C
C
H
H
C
H
C
H
C
H
C
CH3
max 263 nm
for conjugated triene
plus two methyl
groups
Lycopene
Orange-red pigment in tomatoes.
max 505 nm
13.22
Mass Spectrometry
Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron.
e–
Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron.
e–
Electron is deflected but transfers much of
its energy to the molecule.
Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron.
e–
Electron is deflected but transfers much of
its energy to the molecule.
Principles of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron.
Principles of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron.
+
•
e–
Forming a positively charged, odd-electron
species called the molecular ion.
Principles of Electron-Impact Mass Spectrometry
Molecular ion passes between poles of a
magnet and is deflected by magnetic field.
Amount of
deflection depends
on mass-to-charge
ratio (m/z).
Highest m/z
deflected least.
Lowest m/z
deflected most.
+
•
Principles of Electron-Impact Mass Spectrometry
If the only ion that is present is the molecular
ion, mass spectrometry provides a way to
measure the molecular weight of a compound
and is often used for this purpose.
However, the molecular ion often fragments to
a mixture of species of lower m/z.
Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation
and a radical.
+
•
Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation
and a radical.
+
•
Usually several fragmentation pathways are
available and a mixture of ions is produced.
Principles of Electron-Impact Mass Spectrometry
Mixture of ions of
different mass
gives separate peak
for each m/z.
Intensity of peak
proportional to
percentage of each
ion of different
mass in mixture.
Separation of peaks
depends on relative
mass.
+
+
+
+
+
+
Principles of Electron-Impact Mass Spectrometry
Mixture of ions of
different mass
gives separate peak
for each m/z.
Intensity of peak
proportional to
percentage of each
atom of different
mass in mixture.
Separation of peaks
depends on relative
mass.
+
+
+
+
+
+
Some Molecules Undergo Very Little Fragmentation
Benzene is an example. The major peak
corresponds to the molecular ion.
Relative
intensity
100
m/z = 78
80
60
40
20
0
20
40
60
80
100
120
m/z
Isotopic Clusters
H
H
79
H
H
93.4%
All H are
1H and all
C are 12C.
H
H H
H
78
H
H
H
H
79
H H
H
H
6.5%
One C is 13C.
H
H
0.1%
One H is 2H.
Isotopic Clusters
in Chlorobenzene
35Cl
37Cl
Visible in peaks
Relative for molecular ion.
intensity
100
112
80
60
40
114
20
0
20
40
60
80
100
120
m/z
Isotopic Clusters
in Chlorobenzene
Relative
intensity
100
80
H
No m/z 77, 79
pair; therefore, ion
responsible for
m/z 77 peak does
not contain Cl.
60
+
H
H
H
77
40
m/z
20
0
H
20
40
60
80
100
120
Alkanes Undergo Extensive Fragmentation
CH3—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH3
Relative
intensity
43
57
100
80
Decane
60
71
40
85
20
0
142
99
20
40
60
80
m/z
100
120
Propylbenzene Fragments Mostly
at the Benzylic Position
Relative
intensity
100
91
80
CH2—CH2CH3
60
40
120
20
0
20
40
60
80
m/z
100
120
13.23
Molecular Formula
as a
Clue to Structure
Molecular Weights
One of the first pieces of information we try to
obtain when determining a molecular
structure is the molecular formula.
However, we can gain some information even
from the molecular weight. Mass spectrometry
makes it relatively easy to determine
molecular weights.
The Nitrogen Rule
A molecule with an
odd number of
nitrogens has an odd
molecular weight.
A molecule that
contains only C, H,
and O or which has
an even number of
nitrogens has an
even molecular
weight.
O2N
NH2
93
NH2
138
NO2
O2N
NH2
183
Exact Molecular Weights
O
CH3(CH2)5CH3
Heptane
CH3CO
Cyclopropyl acetate
Molecular formula
C7H16
C5H8O2
Molecular weight
100
100
Exact mass
100.1253
100.0524
Mass spectrometry can measure exact
masses. Therefore, mass spectrometry can
give molecular formulas.
Molecular Formulas
Knowing that the molecular formula of a
substance is C7H16 tells us immediately that it is
an alkane because it corresponds to CnH2n+2.
But C7H14 lacks two hydrogens of an alkane;
therefore, it contains either a ring or a double
bond.
Index of Hydrogen Deficiency
Relates molecular formulas to multiple bonds
and rings.
Index of hydrogen deficiency =
1
2
(molecular formula of alkane –
molecular formula of compound)
Example 1
C7H14
Index of hydrogen deficiency
= 1 (molecular formula of alkane –
2
molecular formula of compound)
= 1 (C7H16 – C7H14)
2
= 1 (2) = 1
2
Therefore, one ring or one double bond.
Example 2
C7H12
= 1 (C7H16 – C7H12)
2
= 1 (4) = 2
2
Therefore, two rings, one triple bond,
two double bonds or one double bond + one ring.
Oxygen Has no Effect
CH3(CH2)5CH2OH (1-heptanol, C7H16O) has
same number of H atoms as heptane.
Index of hydrogen deficiency =
1
2
(C7H16 – C7H16O) = 0
No rings or double bonds.
Oxygen Has no Effect
O
CH3CO
Cyclopropyl acetate
Index of hydrogen deficiency =
1 (C H – C H O ) = 2
5 12
5 8 2
2
One ring plus one double bond.
If Halogen is Present
Treat a halogen as if it were hydrogen.
H
Cl
C
H
C3H5Cl
C
CH3
Same index of hydrogen
deficiency as for C3H6.
Rings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum of
rings plus multiple bonds.
Catalytic hydrogenation tells us how many
multiple bonds there are.