3. Potentials of Aromatics
Huckle HOMO/LUMO splitting
Hammet parameters
Lowry and Richardson, Mechanism and Theory in Organic Chemistry, 2nd Ed.
Review of MO theory: Benzene
 Gbenzene  4     2  2 
 Gbenzene  6  8
  2

Reference
State is the
p orbital,


  2
bonds lie lower
http://www.ch.ic.ac.uk/vchemlib/course/mo_theory/main.html
Review of MO theory: Benzene
 Gbenzene  6  8
 GB   3     2  2 
 GB   5  7
  2
Reference
State is the
p orbital, 


B

 e B
G    
  2
Note the notation here:
We still write the oxidation
Reaction as a reduction,
But we signal that it is an
Oxidation reaction by writing
Eoxo
 nFE oxo ~   G      HOMO
 nFEoxo  HOMO  Csolvation,ox  Csolvation,red  Dreference electrode
IF
Csolvation,ox ~ Csolvation,red
 nFEoxo  HOMO  Csolvation  Dreference electrode
E
o
ox
 a HOMO  b
A similar analysis can take place for the reduction potentials
E
o
red
 a'  LUMO  b'
You do it
Prove that the free energy change for the reaction of
o
Ered
 Benzene 
Benzene  e 

o
o
 Grxo   G Benzene


G

Benzene
Is related to the energy (in terms of  and ) of the LUMO
 Eredo
LUMO
p
m

HOMO
 Eoxo
More conjugation
 Is referenced to benzene

Prediction of one-electron electrode potentials of some quinones in
dimethylsulfoxide, J. Electroanal. Chem., 2004, 573, 49-53; M.
Namazian, P. Norouzi
Peover, M. E., in Electroanalytical Chem. Vol. 2, 1967
Where would decacylene and biphenyl fit (relatively speaking) on this graph?
0
decacylene
biphenyl
-0.5
coronene
acenapapthylene
E (V vs SCE)
-1
pyrene
-1.5
quaterphenyl
-2
tetracene
-2.5
naphthalene
y = -2.7431x - 0.7306
R2 = 0.949
-3
fluoranthene
stilbene
-3.5
0
0.1
0.2
0.3
0.4
0.5
0.6
Orbital Energy (units of Beta) by Huckel Theory
0.7
0.8
Peover, M. E., in Electroanalytical Chem. Vol. 2, 1967
Where would decacylene and biphenyl fit (relatively speaking) on this graph?
0
decacylene
-0.5
coronene
acenapapthylene
E (V vs SCE)
-1
pyrene
-1.5
quaterphenyl
-2
tetracene
-2.5
naphthalene
y = -2.7431x - 0.7306
R2 = 0.949
-3
fluoranthene
stilbene
biphenyl
-3.5
0
0.1
0.2
0.3
0.4
0.5
0.6
Orbital Energy (units of Beta) by Huckel Theory
0.7
0.8
Streitwieser, 1962
2
Oxidation ~ HOMO
1.5
1
0.5
E vs SCE
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-0.5
-1
-1.5
-2
-2.5
-3
absolute value m
What sets the intercept scale? – reference electrode is SCE
Reduction ~ LUMO
Reduction
absolute value m
-3
-2.5
-2
E vs SCE
-1.5
-1
-0.5
0
0.5
1
1.5
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Reference Electrodes
The absolute reference is

2 H  2e  H 2 , g

aq
V 0
The standard conditions for this reaction are
PH2
RT
o
V V 
ln 
nF  H 
PH2  1atm

H
   1M
An electrode configured this way = Normal Hydrogen Electrode = NHE
Common Reference Systems
NHE
Ag/AgCl
SCE
Fc/Fc+
0V
Re action

Ag aq
 e
 Ag s


AgCls 
Ag

Cl

aq
aq
Cons tan t
Free Energy
V   0.7996
K sp  18
. x10 10
 nF V 
 
 RT ln K sp
Controlled by the presence of KCl solid, saturated
4 M solution of KCl
You Finish This Problem………………….
Why might this make a good reference system…..
Hint think of what you would like a reference electrode to be…..
V Answer
 0197
.
Common Reference Systems
NHE
Ag/AgCl
SCE
Fc/Fc+
0V
Re action

Ag aq
 e
 Ag s
Cons tan t
Free Energy
V   0.7996


AgCls 
Ag

Cl

aq
aq
K sp  18
. x10 10
 nF V 
 
 RT ln K sp
You Finish This Problem………………….
Why might this make a good reference system…..
Hint think of what you would like a reference electrode to be…..
V Answer
 0197
.
Common Reference Systems
NHE
Ag/AgCl
SCE
Fc/Fc+
0V
0.2412
Saturated Calomel Electrode
Hg2 Cl2 ,s  2e 
 2 Hg   2Cl 
V  0.2412
Write out the Nernst equation for this reaction
Looking at the Nernst equation – explain how this reaction
is one particularly suited for a reference system
Common Reference Systems
NHE
Ag/AgCl
SCE
Fc/Fc+
0V
0.2412
0.40 (water)
Fe
Fc =Ferrocence = Fe(Cp)2
Fc   e 
 Fc
Used as a reference in nonaqueous solvents because
The potential is relatively solvent independent
Fc Eo vs NHE
Water
0.40
Acetonitrile
0.69
DMF
0.72
Dimethylsulfoxide 0.68
Electrochemical Window********
Streitwieser, 1962
2
Oxidation ~ HOMO
1
0.5
E vs SCE
0
0
-0.5
-1
-1.5
Pt in MeCN, 0.1 M TBABF
Hg, Water
Pt, Water
1.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-2
-2.5
-3
absolute value m
Reduction ~ LUMO
Can we make measurements of all of these easily? – related to solvents and electrodes

Electron affinity
Ionization Potential
Expect (for a homologous series) correlations
between
a) Ered and A, Electron affinity
b) Eox and Ionization Potentials
 Eredo
LUMO
p
m

HOMO
 Eoxo
9.5
S
9
CH3
S
8.5
H3C
CH3
H3C
H3C
IP (eV)
8
7.5
7
6.5
H3C
CH3
H3C
6
CH3
H3C
CH3
5.5
5
0.5
1
1.5
2
Eox (V vs NHE)
Eberson, L. Electron-Transfer Reactions in Organic Chemistry,
1982, 79-185
2.5
3
3.5
Doesn’t fit the series – can only
Do this for compounds which have
Similar solvation effects
N
3
N
2.5
2
EA
1.5
1
0.5
0
-0.5
-1
-3
-2.8
-2.6
-2.4
-2.2
-2
Ered (V)
-1.8
-1.6
-1.4
-1.2
-1
1. Accurate Calculation of Absolute One-Electron Redox Potentials of Some para-Quinone Derivatives in
Acetonitrile, JPCA, 2007, 111, 7227, Mansoor Namzian and Michelle L. Coote.
2. Quantum Chemical Modeling of the Reduction of Quinones, J. R. T. Johnsson Waas, E. Ahlberg, Itai
Panas, D. J. Schiffrin, JPC A, 2006, 110, 2005
3. Substituent Effect ona Family of Quinones in Aprotic Solvents: An Experimental and Theoretical
Approach; JPC A, 2006, 110, 9411; C. Frontana, A. Vazquez-Mayagoitia, J. Garza, R. Vargas, I. Gonzalez
4. Quantum-Chemical Predictions of Absolute Standard Redox Potentials of Diverse Organic Molecules and
Free Radicals in Acetonitrile, JACS, 2005, 127, 7227, Yao Fu, Lei Liu, Hai-Zhu Yu, Yi-Min Wang and QingZiang Guo
5. DFT calculation of electrode potentials for substituted quinones in aqueous solution. Chem Phys Letters,
396, 2004, 424; M Namzian, Hora A. Almodarresieh, M. R. Noorbala, H. R. Zare
6. Prediction of one-electron electrode potentials of some quinones in dimethylsulfoxide, J. Electroanal.
Chem., 2004, 573, 49-53; M. Namazian, P. Norouzi
7. Using Cyclic Voltammetry and Molecular Modeling to Determine Substituent Effects in the One-Electron
Reduction of Benzoquinones, J. E. Heffner, J. C. Raber, O. A. Moe, C. T. Wigal, J. Chem Ed., 1998, 75, 3, 365
8. Calculated One-Electron Reduction Potentials and Solvation Structures for Selected p-Benzoquinones in
Water; JPCB, 1997, 101,. 623, K. Raymond, A. K. Grafton, R. A. Wheeler
9. A Method for Computing One-Electron Reduction Potentials and Its Application to pBenzoquinone in Water at 300 K, R. A. Wheeler, JACS, 1994, 116, 11048
The main advance in calculations has been in the solvation effects
Difference in HOMO/LUMO electron
4. Quantum-Chemical Predictions of Absolute Standard Redox Potentials of Diverse Organic Molecules and
Free Radicals in Acetonitrile, JACS, 2005, 127, 7227, Yao Fu, Lei Liu, Hai-Zhu Yu, Yi-Min Wang and QingZiang Guo
Computation methods for the solvation effects now allows
Relatively good predictions of formal potentials even accounting for
Changes in the series structure
1. Accurate Calculation of Absolute One-Electron Redox Potentials of Some para-Quinone Derivatives in
Acetonitrile, JPCA, 2007, 111, 7227, Mansoor Namzian and Michelle L. Coote.
0.6
Cl
Cl
O
O
0.4
Cl
0.2
NC
Cl
O
CN
O
Cl
Cl
0
O
Cl
O
CH3
O
Computed E
O
Cl
-0.2
O
O
CH3
-0.4
O
O
O
O
Cl
Expensive calc.
Cl
O
O
H3C
-0.6
O
O
-0.8
CH3
O
O
O
CH3
-1
O
NH2
O
O
-1.2
O
-1.4
-1.4
Notice that the amine groups
No longer fall out of line!
NH2
NH2
O
-1.2
-1
-0.8
-0.6
-0.4
-0.2
Experimental E
0
0.2
0.4
0.6
The POINT?
Can now calculate
Despite major solvation
effects!
4. Quantum-Chemical
Predictions of Absolute
Standard Redox Potentials of
Diverse Organic Molecules
and Free Radicals in
Acetonitrile, JACS, 2005,
127, 7227, Yao Fu, Lei Liu,
Hai-Zhu Yu, Yi-Min Wang
and Qing-Ziang Guo

BQ  e 
Q

What accounts for this difference?
More difficult to reduce
H3C
Implies that
Cl has ability
To stabilize
The charge introduced
By reduction but CH3
Does not!
CH3
duroquinone
O
O
H3C
quinone
chloranil
O
-0.26- -0.23
4.52-4.54
0.08-0.1
Cl
O
O
Cl
4.18-4.21
CH3
O
Cl
VvsNHE
eV
4.78
0.34
Cl
 1J 
 
 C
 16
. x10 19 C 
1eV  1electron
 16
. x10 19 J
 1V 
V
 electron 
8. Calculated One-Electron Reduction Potentials and Solvation Structures for Selected p-Benzoquinones in
Water; JPCB, 1997, 101,. 623, K. Raymond, A. K. Grafton, R. A. Wheeler
Hammet Parameters and Formal Potentials
OH
H
OH
O
+H+
O
RCOO  H 



Ko
OH
X
OX
O
 K
  log 
 Ko 

 RCOOH 
+H+
O
XRCOO  H 



K

 XRCOOH 
As  increases
greater acid dissociation,
Implies negative charge can be more easily stabilized
Implies electron withdrawing group
A general trend: greater electron withdrawing = easier reduction
http://www.wiredchemist.com/chemistry/data/hammett_sigma_constants.html
A large list of hammett sigma constants
Group
σpara
σ
N
(
C
H
N
H
O
O
3
)
2
2
H
C
H
3
OCH2CH3
t-C4H9
CH3
n-C4H9
C2H5
i-C3H7
NHCOCH3
n-C3H7
Si(CH3)3
H2C=CH**
C6H5**
H
m
e
t
-
0
.
8
3
-
0
.
1
5
-
0
.
6
6
-
0
.
1
6
-
0
.
3
7
0
.
1
2
-
0
.
2
7
0
.
1
2
-0.24
-0.2
-0.17
-0.16
-0.15
-0.15
-0.15
-0.13
-0.07
-0.02
-0.01
0
a
0.1
-0.1
-0.07
-0.08
-0.07
-0.07
0.07
-0.07
-0.04
0.05
0.06
0
G
r
S
N
o
C
H
u
H
C
σ
p
H
H
2
a
3
O
F
C
p
C
l
SH
I
Cl
Br
CHO
CO2H**
COCH3
CF3
CN
NO2**
N(CH3)3+
NO
r
a
σ
m
e
t
0
0
.
1
5
0
0
.
1
9
0
.
0
6
0
.
3
4
0
.
1
2
0
.
1
1
0.15
0.18
0.23
0.23
0.42
0.45
0.5
0.54
0.66
0.78
0.82
0.91
a
0.25
0.35
0.37
0.39
0.35
0.37
0.38
0.43
0.56
0.71
0.88
0.62
A good description of the effect of electron withdrawing and donating groups on an arene:
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch12/ch12-8d.html
VvsNHE
eV
H3C
CH3
duroquinone
O
O
H3C
Electron withdrawing
quinone
chloranil
O
4.52-4.54
0.08-0.1
Cl
O
O
Cl
-0.26- -0.23
CH3
O
Cl
4.18-4.21
4.78
0.34
Cl
Explains the trend:
Easier to reduce (less negative potential)
with greater Hammet value
more electrons withdrawn
7. Using Cyclic Voltammetry and Molecular Modeling to Determine Substituent Effects in the One-Electron
Reduction of Benzoquinones, J. E. Heffner, J. C. Raber, O. A. Moe, C. T. Wigal, J. Chem Ed., 1998, 75, 3, 365
0.8
0.6
Cl
Cl
O
Ered, V vs Ag/AgCl
0.4
O
NC
0.2
Cl
Cl
O
0
O
Cl
O
O
Cl
Cl
O
-0.2
CN
O
H3C
O
O
-0.4
H3C
O
H3C
O
-0.6
H3C
O
H3C
-1
O
CH3
O
-0.8
O
CH3
-0.5
0
0.5
1
1.5
Hammet parameter, sigma
2
2.5
3
3. Substituent Effect ona Family of Quinones in Aprotic Solvents: An Experimental and Theoretical
Approach; JPC A, 2006, 110, 9411; C. Frontana, A. Vazquez-Mayagoitia, J. Garza, R. Vargas, I. Gonzalez
1
Second publication gives exact same trend, but uses different reference
Electrode so the value are displaced
Ered, V vs Ag/AgCl or Fc
0.5
0
Cl
-0.5
O
F
Same sigma
But different
Locations on
The ring
O
Cl
F
Cl
O
-1
O
O
F
O
F
Cl
-1.5
-1
-0.5
0
0.5
1
1.5
Hammet parameter, sigma
2
2.5
3
OH
para
O-
X
X
O
O
OH
OH
O
X
ortho
+H+
O
OH
X
O
X
meta
para
Electron Withdrawing = Easier Reduction
The withdrawing effect occurs for a variety of reasons:
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch12/ch12-8d.html
Electron Withdrawing Groups
“1. Substituents with pi bonds to electronegative atoms (e.g. -C=O, -NO2) adjacent to the pi
system are electron withdrawing groups (EWG) - they deactivate the aromatic ring by
decreasing the electron density on the ring through a resonance withdrawing effect. The
resonance decreases the electron density at the ortho- and para- positions. Hence these sites are
less nucleophilic, and so the system tends to react with electrophiles at the meta sites.
2. Substituents with several bonds to electronegative atoms (e.g. -CF3) adjacent to the pi system
are electron withdrawing groups (EWG) - they deactivate the aromatic ring by decreasing the
electron density on the ring through a inductive withdrawing effect. The net overall effect is
similar to that described above for other electron withdrawing groups.
3. Halogen substituents are a little unusual in that they are deactivating but still direct ortho- /
para-. The reason is that they are both inductive electron withdrawing (due to their
electronegativity) but they are also resonance donating (lone pair donation). The inductive
effect lowers the reactivity of the starting material but the resonance effect controls the
regiochemistry due the stability of the intermediate carbocations.”
Electron Donating
1. “Substituents with lone pairs (e.g. -OCH3, -NH2) on the atoms adjacent to the p system
are electron donating groups (EDG) - they activate the aromatic ring by increasing the
electron density on the ring through a resonance donating effect. The resonance effect
only allows electron density to be positioned at the ortho- and para- positions. Hence
these sites are more nucleophilic, and the system tends to react with electrophiles at these
ortho- and para- sites.
2. Alkyl substituents (e.g. -CH3, -CH2CH3) are also electron donating groups - they
activate the aromatic ring by increasing the electron density on the ring through an
inductive donating effect. This is the same effect that allows alkyl groups to stabilise
simple carbocations. They overall effect is similar to that described above.
3. Substituents with C=C (e.g. -vinyl or -aryl) are also electron donating groups - they
activate the aromatic ring by a resonance donating effect. This is a similar effect to that
for type 1 except that the electrons are from a bonded pair not a lone pair.”
Because of these variety of mechanisms
– it is not possible to necessarily assume that there is an equivalent
rise in the HOMO with a decrease in the LUMO
– such that easier to reduce = easier to oxidize
This principle is
Demonstrated in the
Next study
Daminelli, J. Chem. Phys., Vol. 115, No. 10, 8 September 2001
Withdrawing: here
generally shifts both
HOMO/LUMO down
so these should be harder to
oxidize and easier to reduce
Group
N
(
C
H
N
H
O
O
3
)
2
2
H
C
H
3
OCH2CH3
t-C4H9
CH3
n-C4H9
C2H5
i-C3H7
NHCOCH3
n-C3H7
Si(CH3)3
H2C=CH**
C6H5**
H
σpara
σ
m
.
e
t
5
0
.
8
3
-
-
0
.
6
6
-
-
0
.
3
7
0
.
1
2
-
0
.
2
7
0
.
1
2
-0.24
-0.2
-0.17
-0.16
-0.15
-0.15
-0.15
-0.13
-0.07
-0.02
-0.01
0
0
1
-
0
.
1
a
6
0.1
-0.1
-0.07
-0.08
-0.07
-0.07
0.07
-0.07
-0.04
0.05
0.06
0
G
r
S
N
o
C
H
u
C
σ
p
H
H
H
2
a
O
C
r
a
0
0
F
C
p
3
l
SH
I
Cl
Br
CHO
CO2H**
COCH3
CF3
CN
NO2**
N(CH3)3+
NO
σ
m
e
t
0
.
1
5
0
.
1
9
0
.
0
6
0
.
3
4
0
.
1
2
0
.
1
1
0.15
0.18
0.23
0.23
0.42
0.45
0.5
0.54
0.66
0.78
0.82
0.91
Donating: here generally
Shifts both HOMO/LUMO
Up so these should be easier
to oxidize and harder to
reduce
a
0.25
0.35
0.37
0.39
0.35
0.37
0.38
0.43
0.56
0.71
0.88
0.62
P-sigma: 0.66
0.5
?
0.78 -0.83 -0.66 -0.27
0
Thus far……
Eo correlates with
HOMO/LUMO
EA
IP
Hammett parameters
One more important correlation – very significant for photoelectrochemistry:
pi-pi* optical transitions
The optical transition should be correlated to
the difference in HOMO/LUMO….
which is correlated to the difference between Eox and Ered
hv
E E
o
ox
o
red
 a HOMO  b  a'  LUMO  b'
LUMO, Ered
p



p

HOMO, Eox
This equation shows that the relationship between the observed wavelength
And the formal potentials is complicated by solvation energies:
h   E o   R   R
5
fluorancene
0.618
delta E =3.68
lambda= 277
4.5
4
3.5
log E
3
biphenyl
m=0.704
deltaE=4.18V
lamba=246 nm
anthracene
m=0.414
delta E=2.8
lambda=362 or344
2.5
2
1.5
phenanthene
m=0.605
delta E =3.69
lambda=249
1
0.5
0
220
240
260
280
300
320
340
360
380
400
nm
UV-Vis spectra is from the NIST webbook data base
http://webbook.nist.gov/chemistry/
(Should get a much better correlation
With more pi pi* transitions)
400
350
hV, nm
300
250
y = -89.041x + 602.93
R2 = 0.8921
200
150
2
2.5
3
3.5
Delta E
4
4.5
Streitwieser, 1962
2
Oxidation ~ HOMO
1.5
1
0.5
E vs SCE
0
0
-0.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pi-pi* wavelength
-1
-1.5
-2
-2.5
-3
absolute value m
Reduction ~ LUMO
Metal phthalocyanines (porphyrins) as a good example class
1. Metal centered electron transfer (d orbitals)
2. Ligand centered electron transfer (HOMO/LUMO)
Quinone
On Graphite
electrode
wave
O
O
graphite
Metal phthalocyanines (porphyrins) as a good example class
1. Metal centered electron transfer (d orbitals)
1. Shifted by ligand strength (electron withdrawing donating toward the
Nitrogens
2. Ligand centered electron transfer (HOMO/LUMO)
Cobalt metalloporphyrins
1. Shifted by sigma parameters
Of interest in Vitamin B
3. Related to spectroscopic transitions
calculated
Metal
centered
Ligand
centered
Experimental
By spectroscopy
Metal centered redox chemistry of substituted cobalt phthalocyanines adsorbed on graphite and correlations with MO
calculations and Hammett parameters. Electrocatalytic reduction of a disulfide, Zagal et al Polyhedron 19 2000, 2255.
Vitamin B12 is a cobalt phtalocyanin
Cyanocobalamin
5,6-dimethylbenzimidazolylcanocobamide
CN
Co3+
+e
?
N
http://www.3dchem.com/imagesofmolecules/vitaminb12.gif
What do you think will happen from what we have learned so far?
1. Co2+ less able to hold ligand
2. Following chemical reaction (substitution by water)
CN
CN
+e
+e
Co2+
Co3+
N
CN
Ered,on
N
Co2+
Ered,L
N
H2O
CN
CN
CN
+e
Co3+
Co2+
Ered,off
H2O
H2O
N
N
Ered,on<<Ered,off
+e
Co2+
Ered,L
H2O
N
  H O
Co 2  CN  L N off
Co 2   CN  L N on 


2


Co 3  CN  L( N on )  e
2
Co 3  CN  L( N on )  e 
 Co  CN  L N on 
Co 2   CN  L N on   H2 O 
 Co 2   CN  L H2 O( N off )
fast
  H O  e
Co 3  CN  L N off
2
Example using the potentials shifts for a sensor
Notice it is shifted from
Zero because of
Electron withdrawing
Effect of porphyrin
810
370
630
280
Irreversible formation of isoporphyrin
Ring ring interaction
Bucher et al Chem Comm. 2003
Shape reflects the individual
Binding strengths effect on electron
N
pyridine
N
imidazole
N
H
N
H3C
N
H
D-methylimidazole
Effect of non-bonding electrons?
High HOMO or NBMO = easier oxidation
LUMO
NBMO
HOMO
OH
Benzene
phenol
SH
Benzenethiol
NH2
aniline
Montilla Electrochimica Acta, 2002, 47, 4399
Benzene
Phenol
Aniline
V vs NHE=~1.5
An example of this
NHE
0
~1.5
Ag/AgCl
+0.197
~1.05
OH
SCE
+0.2412
~1.259
You calc this
1.0058
~0.92
Do you notice anything
Diagnostic about these
CVs?
Oliviera, Chemosphere 2007, 66, 2152
Boron Doped Diamond Electrode
NH2
V vs Ag/AgCl~ 1.05
Vs SCE~0.92
An examples of this
Direct CV of benzenethiol and aniline are
Difficult to get because the resulting radicals
Polymerize so here are some corresponding IP
Of substituted anthracene which should follow same
rend
Ionization potential 7.4eV
SH
Eberson, L. Electron-Transfer
Reactions in Organic Chemistry,
1982, 79-185
NH2
Kim, JACS 2006
One Last Example of HOMO/LUMO correlations
Oxygen p orbital overlap with Si-C sigma
Bond is at a maximum when Si-C-O-H bond
Angle is 90o
Mixing of the bonds results in
A different HOMO of higher
Energy: Predict – Easier Oxidation
With rotation
Yoshida, J.; Maekawa, T. Murata, T.;
Matsunaga, S.; and Isoe, S. JACS, 1990, 112,
1962-1970
Some Rules of Thumb
? Some Rule of Thumbs?
1. Ease of oxidation related to HOMO. The higher the HOMO, easier to oxidize (less
positive Eox)
2. HOMO energy related to delocalization. Sigma bonds least delocalized, lowest
HOMO, hardest to oxidize (Most positive Eox)
a)
Sigma hardest
i.
affected by hybridization involved.
b)
pi easier
i.
pi electrons within a ring are generally stable unless activated by
electron donors
c)
n (non bonding) easiest of all
3. Molecules with heteroatoms contain Non-Bonding Molecular Orbitals lying above
HOMO – easiest to oxidize
a)
Availability of NBMO electrons related to electronegativity of first row
elements (C>N>O)
4. General relationship – harder to oxidize harder to reduce (lower the HOMO, higher
the LUMO)
5. Dispersing charge across the molecule enhances the stability of the system
Summary Points thus far
Metal complex oxidation/reduction
kinetics fit d/d induced bond length changes
Potential related to attraction between metal and
ligand
1) Dq (driven by crystal field splitting) of Mn+ or
2) Ligand …Metal covalent bond
Ligand oxidation reduction related to
resonance
substituents
END here
SAWYER PROPOSES
Metal solid oxidations are facilitated by
solvent oxidation (electron donation to the metal)
electrolyte anion oxidation
ligand electron donation.
N
N
7.6V
Ligand oxidation is 2.32V vs NHE
Removal of an electron (IP) from d6sp would
Require 7.9eV activation energy
Fe II bpy3  Fe II bpy  3  Fe( III )bpy3
2
3
e
3
+N
N
-e
N
N
Fe(II)
Fe(II)
+
N
-e
Fe(II)
N
N
Fe(III)
N
Measure of the covalent bond between
The metal (III) center and the ligand
250
200
Co
Cu(II)
delta G (kJ/mol)
O
150
O
N
O
N
100
O
Fe
H3C
50
OH
CH3
Mn
N
N
0
-2.5
-2
-1.5
-1
-0.5
delta E (V)
Sawyer shows a nice correlation between the difference in oxidation potential and the
Ligand M(III) binding energy – consistent with their theory
0