Spectroscopic in Inorganic Chemistry

Electronic Absorption Spectroscopy

Spectroscopy in Inorganic Chemistry

Ferdo

wsi Univ

ersity

of Mash

had

Diatomic molecule

Ferdowsi University of Mashhad 2

C∞v and D∞h

HCN H-H


Ferdo

wsi Univ

ersity

of Mash

had



contribution orbital electron

Σ 0 σ 1

Π 1 π 1

Δ 2 δ 1

Φ 3 δ 1

Σ+ Σ-

Ferdo

wsi Univ

ersity

of Mash

had

Linear molecule NO



A1=Σ+ 0

A2=Σ- 0

E1=Π 1

E2=Δ 2

E3=Φ 3

2Π

2s+1

Ferdo

wsi Univ

ersity

of Mash

had



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

Ferdo

wsi Univ

ersity

of Mash

had


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

... ... ... ... ... ... ... ... ...


Character table for D∞h point group

Ferdo

wsi Univ

ersity

of Mash

had

Tetraphenylcyclopentadienone



σ*

π*

na

π

σ

nb

ground state excited state excited state

Ferdo

wsi Univ

ersity

of Mash

had


Electronic Absorption Spectroscopy energy

Ferdo

wsi Univ

ersity

of Mash

had

Spin-Orbit coupling



J

s1

l2

l1

s2

L

S

Ferdo

wsi Univ

ersity

of Mash

had

Configuration interaction


t2g

eg

? ?

E E'

E>E'

Ferdo

wsi Univ

ersity

of Mash

had

Excited states configurations



Singly excited conf. Doubly excited conf. Ground state

Ferdo

wsi Univ

ersity

of Mash

had



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

Ferdo

wsi Univ

ersity

of Mash

had

Configuration interaction



B1b

B1a

'

'

ΨB1b

ΨB1a

'

'

ΨB1b

ΨB1a

'

'

B1b

B1a

'

'

C.I. C.I.

Initial state Final state

Ferdo

wsi Univ

ersity

of Mash

had


Ferdo

wsi Univ

ersity

of Mash

had

E10 – E H12

H12 E20 - E



electron-electron repulsions Initial state energy

Initial state energy

Ferdo

wsi Univ

ersity

of Mash

had



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

Ferdo

wsi Univ

ersity

of Mash

had


Ferdo

wsi Univ

ersity

of Mash

had



Ferdo

wsi Univ

ersity

of Mash

had

Criteria to aid in band assignment


n→π*

- molar absorptivity < 2000

- Blue shift in high dielectric or hydrogen-bonding solvents


Ferdo

wsi Univ

ersity

of Mash

had


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

Ferdo

wsi Univ

ersity

of Mash

had

Blue shift



e

g

e e

g g

Ferdo

wsi Univ

ersity

of Mash

had

With increasing polarity n → π* transitions are shifted to lower

wavelengths (blue shift).

This shift is due to unpaired electrons (orbital energy decreases n)



n → π*

Increase solvent polarity

e

g

Ferdo

wsi Univ

ersity

of Mash

had



Ferdo

wsi Univ

ersity

of Mash

had


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


Ferdo

wsi Univ

ersity

of Mash

had

Hydrogen bonding solvents



n → π*

π*

n

π*

n

e

g

Ferdo

wsi Univ

ersity

of Mash

had



M. Homocianu et al. / Journal of Advanced Research in Physics 2(1), 011105 (2011)

Solvent

Ferdo

wsi Univ

ersity

of Mash

had

Acidic media



Ferdo

wsi Univ

ersity

of Mash

had

Electron-donating group



< < <

chromophore

Electron-donating group

Ferdo

wsi Univ

ersity

of Mash

had


200 270

Ferdo

wsi Univ

ersity

of Mash

had

Ferdowsi University of Mashhad 30 Ferdo

wsi Univ

ersity

of Mash

had

Ferdowsi University of Mashhad 31 Ferdo

wsi Univ

ersity

of Mash

had

π→π*


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


Ferdo

wsi Univ

ersity

of Mash

had

Red shift



e

g

Ferdo

wsi Univ

ersity

of Mash

had


Solvent effect (polarity)


e

g

e

g

π → π*

Increase solvent polarity

Ferdo

wsi Univ

ersity

of Mash

had


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


Ferdo

wsi Univ

ersity

of Mash

had


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


Ferdo

wsi Univ

ersity

of Mash

had


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


Ferdo

wsi Univ

ersity

of Mash

had

The intensity electronic transition


Oscillator strengths


Ferdo

wsi Univ

ersity

of Mash

had



Ferdo

wsi Univ

ersity

of Mash

had

Transition moment integral

ƒ = ∫ ΨeMΨe dv =D



+x

-x

ex 2 ^

Ground state electronic wave function Excited state electronic wave function

Electric dipole moment operator

Ferdo

wsi Univ

ersity

of Mash

had



moment integral

Ferdo

wsi Univ

ersity

of Mash

had

Electric dipole moment


M=Σer

M = e1r1 + e2r2 + e3r3 + …

Ferdo

wsi Univ

ersity

of Mash

had


moment integral

∫ ΨgΣerΨg dτ

∫ ΨgMΨg dτ

∫ ΨgΣerΨ ex dτ


^

M=Σer

Then

Ferdo

wsi Univ

ersity

of Mash

had

Transition moment integral

ƒ = ∫ ΨeMΨe dv =D



+x

-x

ex 2 ^

Ferdo

wsi Univ

ersity

of Mash

had


∫ ΨeMxΨe dv

∫ ΨeMyΨe dv

∫ ΨeMzΨe dv


^

^

^

Ferdo

wsi Univ

ersity

of Mash

had

Documents

Spectroscopic in Inorganic Chemistry