Electronic Absorption Spectroscopy
Spectroscopy in Inorganic Chemistry
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Diatomic molecule
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C∞v and D∞h
HCN H-H
Electronic Absorption Spectroscopy
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Electronic Absorption Spectroscopy
contribution orbital electron
Σ 0 σ 1
Π 1 π 1
Δ 2 δ 1
Φ 3 δ 1
Σ+ Σ-
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Linear molecule NO
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Electronic Absorption Spectroscopy
A1=Σ+ 0
A2=Σ- 0
E1=Π 1
E2=Δ 2
E3=Φ 3
2Π
2s+1
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Electronic Absorption Spectroscopy
E 2C∞ ... ∞
&sigmav
linear,
rotations quadratic
A1=Σ+ 1 1 ... 1 z x2+y2, z2
A2=Σ- 1 1 ... -1 Rz
E1=Π 2 2cos(Φ) ... 0 (x, y)
(Rx, Ry) (xz, yz)
E2=Δ 2 2cos(2φ) ... 0 (x2-y2,
xy)
E3=Φ 2 2cos(3φ) ... 0
... ... ... ... ...
Character table for C∞v point group
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E 2C∞ ... ∞σv i 2S∞ ... ∞C'2
linear
functions
,
rotations
quadratic
A1g=Σ+
g 1 1 ... 1 1 1 ... 1 x2+y2, z2
A2g=Σ-g 1 1 ... -1 1 1 ... -1 Rz
E1g=Πg 2 2cos(φ) ... 0 2 -2cos(φ) ... 0 (Rx, Ry) (xz, yz)
E2g=Δg 2 2cos(2φ) ... 0 2 2cos(2φ) ... 0 (x2-y2,
xy)
E3g=Φg 2 2cos(3φ) ... 0 2 -2cos(3φ) ... 0
... ... ... ... ... ... ... ... ...
A1u=Σ+
u 1 1 ... 1 -1 -1 ... -1 z
A2u=Σ-u 1 1 ... -1 -1 -1 ... 1
E1u=Πu 2 2cos(φ) ... 0 -2 2cos(φ) ... 0 (x, y)
E2u=Δu 2 2cos(2φ) ... 0 -2 -2cos(2φ) ... 0
E3u=Φu 2 2cos(3φ) ... 0 -2 2cos(3φ) ... 0
... ... ... ... ... ... ... ... ...
Electronic Absorption Spectroscopy
Character table for D∞h point group
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Tetraphenylcyclopentadienone
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Electronic Absorption Spectroscopy
σ*
π*
na
π
σ
nb
ground state excited state excited state
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Electronic Absorption Spectroscopy energy
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Spin-Orbit coupling
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Electronic Absorption Spectroscopy
J
s1
l2
l1
s2
L
S
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Configuration interaction
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t2g
eg
? ?
E E'
E>E'
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Excited states configurations
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Electronic Absorption Spectroscopy
Singly excited conf. Doubly excited conf. Ground state
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Electronic Absorption Spectroscopy
1 N1(A B ); bonding MO
2 N2(A B); antibonding MO
H 1s H 1s
H H
H H
2 N2(A B); antibonding MO
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Configuration interaction
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Electronic Absorption Spectroscopy
B1b
B1a
'
'
ΨB1b
ΨB1a
'
'
ΨB1b
ΨB1a
'
'
B1b
B1a
'
'
C.I. C.I.
Initial state Final state
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E10 – E H12
H12 E20 - E
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Electronic Absorption Spectroscopy
electron-electron repulsions Initial state energy
Initial state energy
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Electronic Absorption Spectroscopy
E1 = ½[E10 + E2
0 + ((E10)2 + (E2
0)2 - 2E10 E2
0 + 4H122)½
E2 = ½[E10 + E2
0 - ((E10)2 + (E2
0)2 - 2E10 E2
0 + 4H122)½
B1b
B1a
'
'
ΨB1b
ΨB1a
'
'
C.I.
Initial state Final state
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Electronic Absorption Spectroscopy
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Criteria to aid in band assignment
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n→π*
- molar absorptivity < 2000
- Blue shift in high dielectric or hydrogen-bonding solvents
Electronic Absorption Spectroscopy
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Dielectric Constant
Pentane 1.84
Hexane 1.88 (25°C) Pyridine 12.4
Heptane 1.92 Methyl Isobutyl Ketone 13.11 (25°C)
Iso-Octane 1.94 Methyl n-Propyl Ketone 15.45
Cyclopentane 1.97 Isobutyl Alcohol 16.68
2-Methoxyethanol 16.93
Cyclohexane 2.02
1,2,4-Trichlorobenzene 2.24 (25°C) n-Butyl Alcohol 17.51 (25°C)
1,4-Dioxane 2.25 Methyl Ethyl Ketone 18.51
Toluene 2.38 (25°C) Isopropyl Alcohol 19.92 (25°C)
n-Propyl Alcohol 20.33 (25°C)
1,1,2-Trichlorotrifluoroethane 2.41 (25°C) Acetone 20.7 (25°C)
Triethylamine 2.42 (25°C)
o-Xylene 2.57 Ethyl Alcohol 24.55 (25°C)
Ethyl Ether 4.33 N-Methylpyrrolidone 32.2 (25°C)
Chloroform 4.81 Methanol 32.70 (25°C)
N,N-Dimethylformamide 36.71 (25°C)
n-Butyl Acetate 5.01 Acetonitrile 37.5
Chlorobenzene 5.62 (25°C)
Ethyl Acetate 6.02 (25°C) Dimethyl Acetamide 37.78 (25°C)
Glyme 7.20 (25°C) Dimethyl Sulfoxide 46.68
n-Butyl Chloride 7.39 Propylene Carbonate 64.9
Water 80.1
Tetrahydrofuran 7.58 (25°C)
Trifluoroacetic Acid 8.55
Dichloromethane 8.93 (25°C)
o-Dichlorobenzene 9.93 (25°C)
Ethylene Dichloride 10.36 (25°C) Electronic Absorption Spectroscopy
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Blue shift
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Electronic Absorption Spectroscopy
e
g
e e
g g
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With increasing polarity n → π* transitions are shifted to lower
wavelengths (blue shift).
This shift is due to unpaired electrons (orbital energy decreases n)
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Electronic Absorption Spectroscopy
n → π*
Increase solvent polarity
e
g
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Electronic Absorption Spectroscopy
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Formic acid H-C(=O)OH 101 °C 58 1.21 g/ml 1.41 D
n-Butanol CH3-CH2-CH2-CH2-OH 118 °C 18 0.810 g/ml 1.63 D
Isopropanol CH3-CH(-OH)-CH3 82 °C 18 0.785 g/ml 1.66 D
n-Propanol CH3-CH2-CH2-OH 97 °C 20 0.803 g/ml 1.68 D
Ethanol CH3-CH2-OH 79 °C 24.55 0.789 g/ml 1.69 D
Methanol CH3-OH 65 °C 33 0.791 g/ml 1.70 D
Acetic acid CH3-C(=O)OH 118 °C 6.2 1.049 g/ml 1.74 D
Nitromethane CH3-NO2 100–103 °C 35.87 1.1371 g/ml 3.56 D
Water H-O-H 100 °C 80 1.000 g/ml 1.85 D
Electronic Absorption Spectroscopy
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Hydrogen bonding solvents
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Electronic Absorption Spectroscopy
n → π*
π*
n
π*
n
e
g
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Electronic Absorption Spectroscopy
M. Homocianu et al. / Journal of Advanced Research in Physics 2(1), 011105 (2011)
Solvent
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Acidic media
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Electronic Absorption Spectroscopy
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Electron-donating group
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Electronic Absorption Spectroscopy
< < <
chromophore
Electron-donating group
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200 270
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π→π*
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his transition is available in compounds with
unsaturated centres, e.g., simple alkenes, aromatics,
carbonyl compounds, etc. this transition requires lesser
energy then transition in a simple alkene, although
several transitions are available , the lowest energy
transition is the π→π* transition and a absorption band
around 170-190 nm in unconjugated alkenes is due to
this transition in the case of , e.g., saturated ketones,
the most intense band around 150 nm is due to
π→π* transition
Electronic Absorption Spectroscopy
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Red shift
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Electronic Absorption Spectroscopy
e
g
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Solvent effect (polarity)
Electronic Absorption Spectroscopy
e
g
e
g
π → π*
Increase solvent polarity
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K- Band
One may designate the UV absorption bands by using electronic
transitions or the letter designation. The band due to π→ π* transitions
in a compound with conjugated π system is usually intense
(εmax.>10000) and is frequently referred to as the k-band (german-
konjugierte). The examples of the compounds in which k-band appears
are butadiene, Mesityl oxide. Benzene itself displays three absorption
bands at 184,204 and 256nm and of these the band at 204nm is often
designated as k-band, and this used in other benzenes as well.
Eg. Conjugated diene, triene, polyene, enones and aromatic rings
Electronic Absorption Spectroscopy
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R- Band
The n→π* transition (R-band german radikalartig) in compounds with
single chromatographic groups i.e., carbonyl or nitro are forbidden with
ε value less than 100.
In conjugated systems the energy separation between the ground and
excited states is reduced and the system then absorbs at longer
wavelengths and with a greatly increased intensity (k-band is intense
and at longer wavelength). Moreover, due to the lessening of the energy
gap, the n→ π* transition due to the presence of the heteroatom and
lone pair i.e. the r-band also undergoes a red shift with little change in
intensity.
Eg. Acetone, acroline, methyl vinyl ketone, acet aldehyde, acetophenone, croton
aldehyde
Electronic Absorption Spectroscopy
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B-Band
These bands are observed in aromatic compounds and hetero aromatic
compounds. Here B refers to Benzenoid bands
Eg. Benzene, tolune, acetophenone, benzoic acid, napthelene, styrene
E- Bands
Such band originate due to electronic transition in the benzenoid system
of the ethylinic part enclosed in cyclic conjugation. Here E refers to
Ethylinic. These are further classified as E1 and E2
Eg. Benzene, nepthelene, anthracene, quinolene
Electronic Absorption Spectroscopy
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The intensity electronic transition
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Oscillator strengths
Electronic Absorption Spectroscopy
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Electronic Absorption Spectroscopy
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Transition moment integral
ƒ = ∫ ΨeMΨe dv =D
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Electronic Absorption Spectroscopy
+x
-x
ex 2 ^
Ground state electronic wave function Excited state electronic wave function
Electric dipole moment operator
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Electronic Absorption Spectroscopy
moment integral
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Electric dipole moment
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M=Σer
M = e1r1 + e2r2 + e3r3 + …
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moment integral
∫ ΨgΣerΨg dτ
∫ ΨgMΨg dτ
∫ ΨgΣerΨ ex dτ
Electronic Absorption Spectroscopy
^
M=Σer
Then
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Transition moment integral
ƒ = ∫ ΨeMΨe dv =D
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Electronic Absorption Spectroscopy
+x
-x
ex 2 ^
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∫ ΨeMxΨe dv
∫ ΨeMyΨe dv
∫ ΨeMzΨe dv
Electronic Absorption Spectroscopy
^
^
^
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