Chapter 1
Ultraviolet/Visible
Spectroscopy
I t d ti off Spectroscopy
Introduction
S
t
• The structure of new compounds that are
isolated from natural sources or p
prepared
p
in the
lab must be determined (and/or verified).
– Chemical analysis
y
((Classical methods))
– Spectroscopy (Modern techniques)
• Spectroscopic techniques are non-destructive
and generally require small amounts of sample
sample.
Types
yp of Energy
gy in Each Region
g
of the
Electromagnetic Spectrum
Region of Spectrum
Energy Transitions
X-rays
Bond Breaking
Ultraviolet/Visible
Electronic
Infrared
Vibrational
Microwave
Rotational
Radiofrequencies
Nuclear Spin (Nuclear Magnetic Resonance)
Electron Spin (Electron Spin Resonance)
3
μ
μ
部分电磁波的波段分布示意图
Four common spectroscopic techniques used
to determine structure:
– Ultraviolet
Ult i l t S
Spectroscopy
t
– Infrared Spectroscopy (IR)
– Nuclear Magnetic Resonance Spectroscopy (NMR)
– Mass Spectrometry (MS or Mass Spec)
• Ultraviolet
Ult i l t S
Spectroscopy
t
– Observes electronic transitions
• Provides information on the electronic bonding in a
molecule
• Infrared
spectroscopy:
– Used to determine the
functional groups
present in a molecule
• Mass spectrometry
–B
Breaks
k molecule
l
l iinto
t
fragments
• Analysis of the masses
off the
th fragments
f
t gives
i
MW and clues to the
structure of the
molecule
• Nuclear
N l
M
Magnetic
ti R
Resonance S
Spectroscopy
t
– Observes the chemical environment of the
hydrogen (or carbon) atoms in the molecule
• Helps provide evidence for the structure of the carbon
skeleton
k l t and/or
d/ the
th alkyl
lk l groups presentt
1H
NMR of CH3CH2OH
13C
NMR of CH3CH2OH
1 1 Principle of UV Light Absorption
1.1
Ultraviolet light: wavelengths between 190 and 400 nm
Visible light: wavelengths between 400 and 800 nm
Ultraviolet/visible spectroscopy involves the
absorption of ultraviolet light by a molecule causing the
promotion of an electron from a ground electronic state
to an excited electronic state.
9
Principle
p of UV
10
There are several types of electronic transitions
available to a molecule including:
σ to σ∗ (alkenes)
π to π∗ (alkenes,
( lk
carbonyl
b
l compounds,
d alkynes,
lk
azo
compounds)
n to σ∗ (oxygen,
(
nitrogen,
it
sulfur,
lf andd halogen
h l
compounds)
n to π∗ (carbonyl
( b
l compounds)
d)
11
Instrument frame
12
Spectrometer
Multichannel photodiode array
Ultraviolet/Visible Spectroscopy
Hewlett-Packard 8452a
Diode Array Spectrophotometer
1.2 Electronic Excitations
15
Vibrational levels
Electronic excited state
Vibrational levels
Electronic ground state
16
1 3 The Origin of UV Band Structure
1.3
A UV Absorption Band at Room Temperature (A)
and at Lowered Temperature (B)
17
1.4 the absorption law—the Beer-Lambert
Law
I
log = ε .l.c
I
0
10
Io = the intensities of the incident light
I = the intensities of the transmitted light
l = the path length of the absorbing in centimetres
c = the concentration in moles per litre
log ( I / I ) is called the absorbance or optical density;
ε is known as the molar extinction cofficient
10
0
18
1.5 Presentation of Spectra
λ = 230nm;
max
log ε = 4.2
272
282
3.1
2.9
Ult i l t S
Ultraviolet
Spectrum
t
off Benzoic
B
i A
Acid
id iin C
Cyclohexane
l h
19
1.6 Choice
C
off S
Solvents
Somee solvents
So
so ve s used in ultraviolet
u v o e spectroscopy
spec oscopy
Solvent
Minimum wavelength for
1 cm cell,
ll nm
Acetonitrile
Water
Cyclohexane
Hexane
Methanol
Ethanol
Ether
Methylene dichloride
Chloroform
Carbon tetrachloride
190
191
195
201
203
204
215
220
237
257
20
The first criterion for solvent
¾ A good solvent should not absorb ultraviolet
radiation in the same region as the substance whose
spectrum is being determined.
determined
¾ Usually solvents which do not contain conjugated
systems
t
are mostt suitable
it bl for
f this
thi purpose, although
lth h
they vary as to the shortest wavelength at which
th remain
they
i transparent
t
t to
t ultraviolet
lt i l t radiation.
di ti
¾ The solvents most commonly used are water, 95%
ethanol, and n-hexane.
21
A second criterion ( the effect of a solvent on the fine
structure of an absorption band)
Ultraviolet Spectra of Phenol in Ethanol and in Isooctane
22
Large energy
lowering
g caused by
y
solvent interaction
The effect of a polar solvent on a transition
23
A third property of a solvent
The ability of a solvent to influence the wavelength of ultraviolet light
which will be absorbed.
¾ Polar solvents may not form hydrogen bonds as readily with
excited states as with ground states of polar solvents. Transitions of
the n → π * type are shifted to shorter wavelengths by polar
solvents.
¾ On the other hand, in some cases the excited states may form
stronger hydrogen bonds than the corresponding ground state. In
such cases, a polar solvent would shift an absorption to longer
wavelength, since the energy of the electronic transition would be
decreased. Transitions of the π → π * type are shifted to longer
wavelengths by polar solvents.
24
Solvent Shifts on the Transition in Acetone
n →π *
Sl t
Solvent
H2O
λ , nm
264.5
max
CH3OH C2H5OH CHCl3
270
272
277
C6H14
279
25
1.7 the Effect of Conjugation
Ultraviolet Spectra of Dimethylpolyenes, CH3(CH=CH)nCH3 ;
A, n=3; B, n=4; C, n=5
26
Conjugation of two chromophores not only results in a
bathochromic shift, but it also increases the intensity of
th absorption.
the
b
ti
The Effect of Conjugation on Electronic Transitions
Alkenes
Ethylene
1,3-Butadiene
1,3,5-Hexatriene
β-Carotene
β
Carotene (11 double
bonds)
Ketones
Acetone π→π*
n →π*
3-Buten-2-one π→π*
n →π*
π
λmax (nm)
175
217
258
465
ε
15 000
15,000
21,000
35,000
125,000
,
189
280
213
320
900
12
7,100
27
27
28
1.8 the Effect of Conjugation on Alkenes
Formation of the Molecular Orbitals for Ethylene
29
Fig. 1
Fi
1-13
13 A C
Comparison
i
off th
the
Molecular Orbital Energy Levels
and Energy of the π→π*
Transitions in Eth
Ethylene
lene and 1
1,33
Butadiene
Conjugation
j g
makes the energy
gy lower
30
A Comparison of the π→π
π→π* Energy Gap
in a Series of Polyenes of Increasing Chain Length
The longer
g
the conjugated
j g
system,
y
, the longer
g
the
wavelength of the absorption maximum.
31
the Appearance of the Spectra of some Polyenes
32
An extension of the length of the
conjugated system
B = -OH,
OH -OR,
OR -X,
X or –NH
NH2
Δ E1 ＞ Δ E2
Energy Relationships of the New Molecular Orbitals and the
Interacting π System and its Auxochrome
33
Overlap of the electrons
Hyperconjugation
Quasi-resonance structures
The net effect is an extension of the πsystem .
34
1.9 the Effect of conformation and
Geometry on Polyene Spectra
•
•
•
All-trans polyenes show only the Ψ2 → Ψ3* transition
In polyenes with one or more cis double bonds,
bonds the Ψ2 → Ψ4*
transitions may become allowed — called “cis band”.
Any molecule which retains a center of symmetry, even when
possessing
i
severall cis
i double
d bl bonds,
b d will
ill nott show
h
th cis
the
i
band.
35
Spectra of isomeric β-carotenes, showing
the development of the cis band.
36
Empirical rules for caluclating UV/Vis
Absorptions
Woodward-Fieser Rules for Dienes
A. Woodward's Rules for Conjugated Carbonyl
Compounds
B. Mono-Substituted Benzene Derivatives
C. Di-Substituted Benzene Derivatives
D. Benzoyl Derivatives
37
1.10 the Woodward-Fieser Rules for
Dienes
conjugated
j
t d di
dienes
π→π* transition
ε=20,000 to 26,000
λ =217 to 245nm
Butadiene
B
t di
and
d many simple
i l conjugated
j
t d dienes
di
exist
i t in
i a planar
l
strans conformation.
y, alkyl
y substitution p
produces bathochromic shifts and
Generally,
hyperchromic effects.
With certain patterns of alkyl substitution, the wavelength
increases but the intensity decreases.
increases,
decreases
38
Cyclic dienes
The central bond is a part of the ring system, the diene chromophore
is usually held rigidly in either the s-trans
s trans or the s-cis
s cis conformation
Homoannular Diene
(s--cis)
(s
less intense,
intense ε= 5,000
5 000 –15,000
15 000
λ longer (273nm)
Heteroannular Diene
(s--trans)
(s
more intense, ε= 12,000 – 28,000
λ shorter (234nm)
The actual rules for predicting the absorption of open chain and sixmembered ring diene were first made by Woodward in 1941.
Since that time they have been modified by Fieser and Scott as a
result of experience with a very large number of dienes and trienes.
39
Rules for diene and triene absorption
Value
V
l assigned
i d to
t parentt heteroannular
h t
l or open diene
di
Value assigned to parent homoannular diene
Increment for
(a) each alkyl substituent or ring residue
(b) the exocyclic double bond
(c) a double-bond extension
(d) auxochrome ―OCOCH3
―OR
―SR
―Cl, ―Br
―NR2
λcalc
Total
214 nm
253 nm
H 3C
H
H
H
H
C
C
C
H
transoid:
observed:
b
d
H 3C
C
C
C
H
H
H 3C
214 nm
217 nm
5 nm
5 nm
30 nm
0 nm
6 nm
30 nm
5 nm
60 nm
C
transoid:
alkyl groups:
observed:
C
H
H
214 nm
3 × 5 = 15
229nm
228 nm
40
CH3
a
A
a
CH3
B
a
Exocyclic Double Bond
transoid:
214 nm
ring residues: 3 × 5 = 15
exocyclic double bond: 5
234 nm
observed:
235 nm
C 3CH
CH
C 2O
observed:
CH3
CH
CH3
CH3
cisoid:
253 nm
ring residues: 3 × 5 = 15
exocyclic double bond: 5
273 nm
observed:
275 nm
240 nm
241 nm
observed:
CH3
278 nm
275 nm 41
CH3
CH3
CH3COO
cisoid:
253 nm
ring residues:
5× 5=
25
double bonds extending
g
conjugation: 2 × 30 =
60
exocyclic double bonds: 3 × 5 = 15
CH3COO
COO–::
0
353 nm
observed:
355 nm
The Woodward-Fieser rules work well for polyenes with from
one to four conjugated
j g
double bonds.
42
1 11 the Fieser-Kuhn Rules for
1.11
polyenes
β-Carotene,
β
Carotene 11 Double Bonds
λmax= 452 nm(hexane), ε=15.2 × 104
Lycopene,
y p , 13 Double Bonds ((11 Conjugated)
j g
)
λmax= 474 nm(hexane), ε=18.6 × 104
43
E ii lR
Empirical
Rules
l ffor P
Polyens
l
λ (hexane) = 114 + 5M + n(48.0 − 1.7 n) − 16.5R − 10 R
ε (hexane) = 1.74 × 10 n
max
endo
d
exo
4
max
where:
M
n
Rendo
Rexo
=
=
=
=
the number of alkyl substituents
the number of conjugated double bonds
the number of rings with endocyclic double bonds
the number of rings
g with exocyclic
y
double bonds
For example, β-Carotene:
M
= 10
Rendo = 2
n
= 11
Rexo = 0
Therefore:
λmax= 453.3 εmax= 19.1 ×104
44
1 12 carbonyl
1.12
b
l compounds;
d enones
T principal
Two
i i l uv transition:
ii
π*
forbidden
280 290 nm
280-290
allowed
190 nm
n
π
Substitution by an auxochrome with a lone pair of
electrons, such as –NR2, – OH, –OR, –NH2, or –X, as in
amides, acids, esters, or acid chlorides, gives a
pronounced hypsochromic effect on the n→π* transition
and lesser bathochromic effect on the π→π* transition.
(resonance interaction)
45
the Hypsocromic Effect of Lone Pair Auxochromes on
the n→π* Transition of a Carbonyl Group
O
CH3
CH 3
CH 3
CH3
H
C
O
CH 3
C
Cl
O
C
O
CH 3
C
O
C
NH2
OCH2CH3
λ max
ε max
S l
Solvent
t
293 nm
12
Hexane
279
15
Hexane
235
53
Hexane
214
-
Water
204
60
Water
This shift is due primarily to the inductive effect of the oxygen, nitrogen,
or halogen
g atoms. They
y withdraw electrons from the carbonyl
y carbon,
causing the lone pair of electrons on oxygen to be held more firmly than
46
would be the case in the absence of an inductive effect.
the Spectra of a Series of Polyene Aldehydes
If the carbonyl group is a part of a conjugated system of double bonds,
b h the
both
h n→π** and
d the
h π→π*
* bands
b d are shifted
hif d to longer
l
wavelengths.
l
h
The energy of the n→π* transition does not decreased as rapidly as that
of the π→π* band, which is more intense.
If the conjugated chain becomes long enough, the n→π* band becomes
47
“buried” under the more intense π→π* band.
the Orbitals of an Enone System Compared to those of the Noninteracting Chromophores
48
1.13 Woodward’s Rules for Enones
¾ Conjugation of a double bond with a
carbonyl group leads to absorption (ε = 8,000
8 000
to 20,000), π→π*, at 220 ~ 250 nm,
predictable
¾ n→π
n→π*, at 310 ~ 330 nm
nm, much less intense (ε
= 50 to 100), not predictable
49
O
O
H 3C
C
H 3C
C
C
CH3
OCOCH3
CH 3
CH 3
acyclic enone:
α －CH3:
β －CH
CH3:
observed:
215nm
10
24
249nm
249nm
6 membered enone:
6-membered
double bond extending
conjugation:
homocyclic diene
δ-ring residue:
observed:
b
d
CH3
CH3
215nm
30
39
18
302nm
300
300nm
O
O
Br
5-membered enone:
β-ring residue: 2× 12=
exocyclic double bond:
observed:
202nm
24
5
231nm
226nm
5-membered enone:
α-Br:
B
β-ring residue: 2× 12=
exocyclic double bond:
observed:
202nm
25
24
5
256nm
251nm
1 14 Solvent Shifts – a more detailed
1.14
examination
1. In most π→π* transitions
((bathochromic shift, lower energy)
gy)
π*
E
N
E
R
G
Y
π*
π
Non-Polar
Solvent
}
Large energy lowering
caused by solvent
interaction
π
Polar
Solvent
the Effect of a Polar Solvent on a Transition
51
The ground state of the molecule is relatively non-polar, and the excited
state is often more polar than the ground state. As a result, when a polar
solvent is used, it interacts more strongly with the excited state than with
the g
ground state, and the transition is shifted to longer
g wavelength.
g
th groundd state
the
t t
the π electron density equally distributed, the nuclei of C are shielded
the π* excited state
the electron is promoted, the C becomes electron deficient
The excited state interacts more strongly with polar
or hydrogen bonding solvents
52
2 In most n→π
2.
n→π* transitions
(hypsochromic shift, higher energy)
π*
E
N
E
R
G
Y
becomes electron
deficient
π * C* O
n
}
n
π
π
Non-Polar
Solvent
Large solvent stabilization
due to hydrogen bonding
C O
H
O
H
Polar
Solvent
the
h Eff
Effect off a Polar
P l Solvent
S l
on an n→π** T
Transition
ii
•The ground state is more polar than the excited state.
•Hydrogen bonding solvents interact more strongly with unshared
electron pairs in the ground state molecule
53
1 15 Aromatic Compounds
1.15
M l
Molecular
l Orbitals
O bit l and
dE
Energy St
States
t ffor Benzene
B
184 and 202 nm
255nm
primary bands
secondary bands (fine-structure)
54
¾The second primary band (184 nm,
ε=47,000)
in vacuum ultraviolet
region
¾The 202 nm band is much less
intense (ε=7,400), it corresponds to a
forbidden transition
¾The secondary band is the least
intense of the benzene bands (ε
(ε=230)
230).
It corresponds to a symmetry
forbidden electronic transition.
Ultraviolet Spectrum of Benzene
55
A Substitutents with Unshared Electrons
A.
Y
Y
Y
Y
• The non-bonding
g electrons can increase the length
g of
the π system though resonance.
• The more available these n-electrons are for
interaction with the π system of the aromatic ring, the
greater the shifts will be. (-NH2, -OH, -OCH3, -X)
• Interactions of this type between the n and π
electrons usually cause shifts in the primary and
secondary
y benzene absorption
p
bands to longer
g
wavelength.
• In addition, the presence of n-electrons in these
compounds gives the possibility of n→π* transition.
56
If an n-electron is excited into the extended π* chromophore,
p
,
the atom from which it was removed becomes electrondeficient, while the π system of the aromatic ring acquires an
extra electron. This causes a separation of charge in the
molecule.
The extra electron in the ring is actually in a π* orbital.
Y
Y
*
*
*
Y
Y
*
A charge transfer or an electron transfer excited state
With compounds which are acids or bases, pH
changes can have a very significant effect on the
positions of the primary and secondary bands.
57
B Substituents
B.
S b i
capable
bl off π-conjugation
j
i
C O
C O
C O
C O
R
R
R
R
Interaction of the benzene ring electrons and the π
electrons of the substituent can also produce a new
electron transfer band. This new band may be so intense
as to obscure the secondary band of the benzene
system.
Notice that the opposite polarity is induced; the ring
becomes electron deficient.
The effect of acidity or basicity of the solution on such a
chromophoric substituent group
58
C. Disubstituted benzene Derivatives
A.. For
o pa
para-,
a , two
wo poss
possiblities:
b es:
1. Both groups are electron-releasing or electronwithdrawing, similar to monosubstituted benzenes;
2. One is electron-releasing while the other is electronwithdrawing, enhanced shifting (the magnitude of the
shift of the primary band is greater than the sum of
the shifts due to the individual group.)
O
O
H2N
N+
O
H2N
N+
O
resonance interactions
59
Empirical Rules for Benzoyl Derivatives
O
C
R
Parent chromophore:
y or ring
g residue
R = Alkyl
R=H
R = OH or OAlkyl
Increment for each substituent:
Alkyl or ring residue
-Alkyl
-OH,-OCH 3 ,or –OAlkyl
-OO -
-Cl
-Br
-NH 2
-NHCOCH 3
-NHCH
NHCH 3
-N(CH 3 ) 2
246
250
230
o-,mompo-,mpo
ompo-,mpo-,mpo-,m,
po-,mppo-,mp-
3
10
7
25
11
20
78
0
10
2
15
13
58
20
45
73
20
85
60
Br
O
HO
O
OH
HO
parent chromophore: 246nm
o-ring
i residue:
id
3
m-Br:
2
251nm
observed:
C
253nm
OH
parent chromophore: 230nm
m-OH: 2× 7 =
14
p-OH:
25
269nm
observed:
270nm
61
Application of UV
1.To obtain the information of conjugation, ε
2 Qualification analysis
2.Qualification
3.Quantitative analysis: A=εbc
Example spectra
Example spectra
A practical guide: Exercises
What do you look for in an ultraviolet spectrum? A single band of low intensity (10 to 100) in the region 250­
360nm,
360nm with no major absorption at shorter wavelengths
(200­250nm)
n→π*
A simple, or unconjugated, chromophore (contains an O, N,or S): A
i l
j
d h
h
(
i
O N S)
C=O, C=N, N=N, ‐NO2,‐COOR, ‐COOH, or –CONH2
65
Two bands of medium intensity (ε
(ε=1,000
1,000 to 10,000), both with
λmax above 200 nm
an aromatic system
•A
A good
d ddeall off fine
fi structure
t t
in
i the
th longer
l
wavelength
l th band
b d (in
(i non-polar
l
solvents only).
•Substituent on the aromatic rings will increase the molar absorptivity
above 10,000.
10 000
•In polynuclear aromatic substances, a third band will appear near 200 nm.
Bands of high intensity (ε=10,000 to 20,000), above 210 nm
unsaturated ketone or a diene or polyene
an α,β-
•The longer the length of the conjugated system, the longer the observed
wavelength will be.
•For dienes,
dienes using the Woodwood-fieser
Woodwood fieser rules
•More than four double bonds, using Fiese-Kuhn rules
66
Simple
p ketones,, acids,, esters,, amides,, and other compounds
p
containing both π system and unshared electron pairs, will show
two absorptions:
n→π* at longer wavelength (>300nm,
(>300nm low intensity)
π→π* at shorter wavelength (<250nm, high intensity)
•With conjugation (enones), the λmax of the π→π* band moves to linger
wavelengths , Woodward’s rules
ε above 10,000, buryy the weaker n→π* transition
•α,β-unsaturated esters and acids, Nielsen’s rules
Compounds which are highly colored are likely to contain a
long-chain conjugated system (4~5) or a polycyclic aromatic
chromophore.
67
Homework and Course
Requirements
• Comprehend the rationale Ultraviolet and visible
spectra
t
• Master the fundamental type of electronic
transition
• Master the major component parts and the
demand of every part of visible ultraviolet
spectrophotometer
• Finish Homework