1. p. 235

2. Electronic transitions
p. 235
Electronic transitions
IR transitions

3. In organic molecules, electrons are in s, p or n orbitals
so transitions are n→p* p→p* n→s*
p. 236
usually, in conjugated molecules
n→p* < p→p* < n→s* 3

4. n → p* in carbonyl, nitro,... X: + C=C or C=O or N=O
p → p* in alkenes, aromatics, carbonyl compounds
n → s* in O: (ethers), N: (amines),
S: (thiols/thioethers),
Hal: (halo-organics)
} uv
s → p* carbonyl, ....
} high energy, far uv
s → s* alkanes

5. because many vibrational transitions, absorptions broad
A UV spectrum of Benzoic acid e
10,000
p → p*
1000
100
because many vibrational transitions, absorptions broad

6. I0 = incident light intensity I = transmitted light intensity
p. 237
Beer’s Law:
A = log(I0/I) = ecl
A is linearly related to c so a calibration plot of A vs. c can be used to determine unknown c from measured A
A = absorbance
I0 = incident light intensity
I = transmitted light intensity
= molar extinction coefficient (or molar absorptivity)
c = concentration in mol L-1
l = pathlength in cm
is in units of L mol-1 cm-1
constant at a given wavelength for any compound
i.e. it is a molecular property

7. p → p* are allowed (orbitals in same plane)
e > 100, commonly 103 to 105
n → p* are forbidden (orbitals orthogonal)
e < 50
Benzoic acid, p → p* e = 10,000
acetone, n → p* e = 18
l = 230 nm
l = 274 nm

8. Simple chromophores absorb in the far uv
p → p*
C=C nm e = 1000
C=O nm e = 1000
n → s*
C-O: nm e = 1000
except
n → p*
C=O nm e = 10-30
} forbidden (weak intensity)
N=O 270 nm e = 5-10

9. BUT conjugation of a chromophore
produces a RED (long l) or BATHOCHROMIC shift
and a HYPERCHROMIC shift

10. conjugation: lMAX increases
p. 238
conjugation: lMAX increases
~ 35 nm per C=C

11. p. 239
C=C
C=C—C=C
C=C

12. Longest l transition is HOMO → LUMO transition
C=C
C=C—C=C
C=C—C=C—C=C
p. 239
p1*
Higher energy transitions
p1*
p2* p*
p2*
p3*
LUMO p3
HOMO
0.45 p2
0.62 p
1.00 p2
1.25 p1
1.62 p1
1.80
Longest l transition is HOMO → LUMO transition

13. }UV Longest l transition is HOMO → LUMO transition C=C 175 nm 15000
p. 239
Longest l transition is HOMO → LUMO transition
C=C nm
C=C—C=C nm 21000
C=C—C=C—C=C 258nm 35000
b-carotene (C=C) nm 125,000
}UV
}VIS

14. ALL transitions are bathochromically shifted on conjugation
p. 240
ALL transitions are bathochromically shifted
on conjugation
190nm
280nm
213nm
320nm
245nm
lost (in under benzene transitions)
p → p*
n → p*

15. p. 240
BENZENE
p1*
p2* p2 p1
184nm 68,000
204nm 8,800
254nm

16. Substituents change the HOMO-LUMO gap, and therefore change lMAX
Ph-H
Ph-OH
Ph-O
Ph-NH
Ph-NH ..
} pH dependant .. ..
} pH dependant }
Auxochromes:
have conjugated lone pairs – smaller effect

17. so a green laser is emitting light ~ 500 nm and a red laser
p. 241
V I B G Y O R
Colors of light:
400nm Violet
500nm Green
600nm Orange
670nm Red
Na D line
so a green laser is emitting
light ~ 500 nm and a red laser
at about 650 nm

18. However, if you shine WHITE light on an object
p. 241
However, if you shine WHITE light on an object
and it appears green, it is because it is reflecting
green and absorbing other wavelengths
approximately
l of light
objects are absorbing
400nm
objects are absorbing
450nm
objects are absorbing
500nm
objects are absorbing
550nm
objects are absorbing
600nm
objects are absorbing
670nm

19. Carrots absorb at 452 nm so appear orange
Tomatoes absorb at 474nm so see
red
Chlorophyll absorbs at 670nm so see
green
Plums absorb 550nm see
violet
Intense colours, LARGE e >1000
Pale colours, SMALL e >50

20. Transition Metal Compounds
INORGANIC COMPOUNDS
Main group compounds usually absorb in far UV
so appear WHITE
NaCl CaCO3 MgSO4 Al2O3
salt chalk Epsom salts Alumina
Transition Metal Compounds
CuSO4 KMnO4 K2Cr2O7 HgI2 Cr2O3
Intense colors involve CHARGE-TRANSFER bands
Pale colors involve d → d transitions

21. KMnO4 K2Cr2O7 HgI2 Cr2O3 Benzene-Cr(CO)3
p. 243
KMnO4 K2Cr2O7 HgI2 Cr2O3 Benzene-Cr(CO)3
CHARGE-TRANSFER COMPLEXES
e > 1000
L → M ligand to metal: :
electron from ligand orbital
transferred to empty metal orbital
{formally L+-M-}
CT*
CT transition ALLOWED
Ligand
orbital
[Donor]
Metal orbital [acceptor]
New CT bonding orbital

22. CH3CH2OH + K2Cr2O7 → CH3CHO + 2Cr3+
p. 244
Breathalyser Test
CH3CH2OH K2Cr2O7 → CH3CHO + 2Cr3+
e ~15
e > 1000
Measure change in A (absorbance) of dichromate solution
not affected by weak Cr3+ absorption
Then change is proportional to conc of alcohol in breath
Some roadside screening devices have a column
of dichromate on silica and measure how much of column
changes

23. Summary UV-Vis Flowchart
p. 244
Summary UV-Vis Flowchart

24. ASSIGNMENT 10