Chapter 6: A Qualitative
Theory of Molecular Organic
Photochemistry
December 12, 2002
Larisa Mikelsons
Simplified schematic of the 2 lowest singlet surfaces for a concerted pericyclic reaction:
4N e- concerted disrotatory
pericyclic reactions are generally
photochemically allowed
4N + 2 e- concerted disrotatory
photoreactions are generally
photochemically forbidden
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Concerted pericyclic reactions
which are g.s. forbidden are
generally e.s. allowed in S1 due to
a miminum which corresponds to a
diradicaloid
Pericyclic reactions which are g.s.
allowed are generally e.s. forbidden
in S1 because of a barrier to
conversion to product structure and
the lack of a suitable surface
crossing from S1 to S0
4N or 4N + 2 = # of e-s involved in bond making or bond breaking
6.11 Typical State Correlation Diagrams
for Nonconcerted Photoreactions
Orbital correlation diagram for
H-abstraction by formaldehyde:
Most photochemical reactions are not
concerted and involve reactive
intermediates (D, Z) along *R  I
Use H-abstraction reaction of the n,*
state of ketones as an exemplar for
*R  I reactions
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Assumption: the strictly planar approach
represents the reaction coordinate
(gives the best frontier orbital interaction
between nO and XH; orbitals can
be readily classified)
XH  pX and nO  OH correlations are
assumed to be avoided
State symmetries can be deduced from
the orbital correlation diagram
First Order state correlation diagram for coplanar H-abstraction:
S1 and T1 correlate with the lowest
states of the product and coplanar
H-abstraction is symmetry-allowed.
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S2 and T2 correlate with zwitterionic
forms of the product (which have
very high energies) and coplanar
H-abstraction is symmetry-forbidden.
Destruction of the perfect coplanar
geometry results in a weakly
avoided crossing between the
S11D and S0Z1 surfaces.
The T13D and S0Z1 remains.
6.13 State Correlation Diagrams
for -Cleavage of Ketones
Orbital symmetries for the -cleavage of acetone:
Two diradicaloid geometries possible:
a)
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a bent acyl which is s
b) a linear acyl fragment with s and
a orbitals. The (s,p) state is S
and the (a,p) state is A
Orbital correlation diagrams for -cleavage of acetone:
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Zero Order state correlation diagram for -cleavage of acetone:
For symmetry-allowed -cleavage
of n, * states to yield a linear
acyl fragment, S1(n,*) and
T1(n,*) correlate with 1D(a,pc)
and 3D(a,pc).
For symmetry-forbidden
-cleavage of n, * states to yield
a bent acyl fragment, S1(n,*)
and T1(n,*) correlate with
D*(sp2,*CO).
First Order correlation diagram for -cleavage of acetone:
Situation for cleavage to the
linear fragment essentially
the same
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Occurrence of weakly avoided
crossings for cleavage to the
bent acyl fragment
6.14 A Standard Set of Plausible Primary
Photoreactions for , * and n, * States
Plausible primary photochemical reactions that are initiated in
S1(, *)
T1(, *)
1. Concerted pericyclic reactions
2. Reactions characteristic of carbonium
ions and of carbonanions
3. Cis-trans isomerization
1. Hydrogen atom or e- abstractions
2. Addition to unsaturated bonds
3. Homolytic fragmentations
4. Rearrangement to a more stable
carbon centered radical
Zwitterionic and/or concerted
Diradicaloid and non-concerted
The photochemistry of n,* states is completely diradicaloid to a good approximation.
The plausible primary processes are:
n-Orbital Initiated
*-Orbital Initiated
Atom abstraction
Radical addition
Electron abstraction
-Cleavage
Atom abstraction
Radical addition
Electron donation
-Cleavage
Electrophilic characteristics
Like an alkoxy radical (RO•)
Sensitive to steric factors influencing
substrate’s approach in molecular
plane and near “edges” of carbonyl O
Nucleophilic characteristics
Like a ketyl radical (R2COH•)
Sensitive to steric factors influencing
substrate’s approach above and
below the “faces” of CO
6.15 Intersystem Crossing in
Radical Pairs and Diradicals
S11I processes:
No spin prohibition to 1IP so 1I = 1RP, 1D
1I undergo either recombination or disproportionation which are both extremely rapid
Reactions may be stereospecific
T13I processes:
Spin prohibition to 3IP due to 3I1I
If ISC is slow relative to diffusional separation of the radical pair then free radical
formation occurs
If ISC is slow relative to rotation about C-C bonds then loss of stereochemistry results
in any intramolecular reactions of the diradical
Rate of ISC in RP and D determined by spin-orbit interactions and possibly by very
weak magnetic interactions with nuclear spins and laboratory magnetic fields
6.16 Magnetic Energy Diagrams Including
the Electron Exchange Interaction
Important situations of Zeeman
splitting and exchange splitting:
gB0
gB0
T
T+
T+
S decreases
in energy as J
increases
S
T0
T0
S
J
T-
TS
Condition I (low field, J = 0)
Condition II (low field, J = 0)
Condition I  Solvent separated
spin correlated geminate pairs
and extended biradicals
Condition II  Molecular triplets,
spin correlated pairs in a solvent
cage and small biradicals
S
Condition III (high field, Condition IV (high field,
J ~ gB0 )
J >> gB0)
6.17 Magnetic Interactions and
Magnetic Couplings
Magnetic couplings that are important for ISC: spin-orbit coupling, electron-nuclear
hyperfine coupling and Zeeman coupling.
Dipolar interaction (Eq. 6.12)
Nucleus
with
spin
Contact interaction (Eq. 6/13)
Electron
spin
outside
nucleus
Due to electric or to magnetic dipoles interacting
Interaction  [(12)/r3](3cos2 - 1) (overlap integral)
Rate  (strength of the interaction)2  1 / r6
Electron
spin
inside
nucleus
Due to overlap of wavefunctions
Interaction  ep|(0)|2
Interaction distance independent
Magnetic Coupling Mechanisms
Spin Hamiltonian operator H  used to classify the magnetic coupling mechanism
of an electron spin, S1, with other magnetic moments
Zeeman Coupling: external coupling of electron spin to the magnetic moment of an
applied laboratory field. HZ = geH0S1
Dipole-dipole Coupling: internal coupling of electron spin to the magnetic moment of
another electron spin, S2. HDP = DeS1S2
Hyperfine Coupling: internal coupling of electron spin to the magnetic moment of a
nuclear spin, I. HHF = aS1I
Spin-orbit Coupling: internal coupling of electron spin to the magnetic moment due to
orbital motion of the electron, L. HSO = S1L
Spin-lattice Coupling: coupling of electron spin to the oscillating magnetic fields resulting
from molecular motions of the environment. HSL  S1L
Spin-photon Coupling: coupling of electron spin to the oscillating magnetic field
associated with an electromagnetic field. Hh  S1h
6.18 Coupling Involving Two
Correlated Spins
z
S1 
z
S2

S1
z

S2

S1
S2


Hi
Hi
S1 and S2 coupled and precess
about each other, Hi uncoupled
Hi
S2 and Hi precess about
resultant
S2 uncoupled from S2 and
becomes coupled to Hi
z
z
S1

S2

Hi
S1
HX or HY
S2
Hi
Zero field
T
S
T+
T
High Field
T0
T-
S
T+
X
S

T+  S and T-  S Transitions
Initially S1 and S2 are correlated in T+
Hi (electron or nuclear spin) couples
with S2 and causes T+  S ISC
Zero field: three T sublevels strongly
mixed and radiationless T+  S ISC
plausible (assuming J = 0)
High field: radiationless T+  S ISC not
plausible but radiative transition is
plausible if J is very small
X
T0
T-
S
Vector diagram for T-  S transition
similar to that of T+  S transition
T0  S Transitions
z
z
S1
Hi
S2

z
S1
Hi

S2

HZ

S2 
S2 becomes
uncoupled from S2 and
becomes coupled to Hi
S1 and S2
coupled, Hi
uncoupled
Zero field
T
S
T0
T-
T
S
Hi (electron or nuclear spin) couples
with S1 and causes T0  S ISC
Zero field: three T sublevels strongly
mixed and radiationless T0  S ISC
plausible (assuming J = 0)
S
High field: radiationless T0  S ISC
plausible if J= 0
T+
T+
High Field
S1
Initially S1 and S2 are correlated in T0
Hi

T0
T-
S
6.19 ISC in Radical Pairs and
Diradicals. Exemplar Systems
Simplified paradigm of a photochemical process proceeding
through an n,* triplet electronically excited state, T1:
h
R
1
I
R
1
S0
S1
1
3
3*
*R
2
T1
3
RP
3
1
P
I
4
RP
P
Paradigm for the -cleavage reaction of ketones:
1
1
O
A
B
1R
3
O
A
2
B
3R
O
A
..B
3RP
3
O
A
O
..B
1RP
4
A
+
B
Other Products
Surface energy diagram displaying the spin and
molecular dynamic features of a dynamic radical pair:
Energy surface description uses the
exemplar of the stretching and
breaking of a C-C single bond.
ISC step S1T1 occurs “vertically”
when the exchange interaction is
very large
When J is large it controls the
correlated precessional motion of the
two electron spins so only a strong
interaction can induce ISC
When the RP is not in contact J
decreases and torques can cause the
electron spin to be rephased or
flipped
Visualization of Spin Dynamics.
ISC in Geminate RPs in Zero Field
Distance dependence of spin correlated radical pairs:
ISC not plausible during bond
breaking since breaking takes 10-13 s
while spin precession is of the order
of 10-10 s.
In region 2 J is large so the spins
are strongly correlated and ISC is
implausible in the contact pair.
In region 3 J is comparable to
available magnetic interactions for
the pair so the spins are weakly
correlated and ISC is plausible.
In region 4 J = 0 and neither the
phase nor the orientation of the spin
on one center influences the phase
or orientation of the spin at the other
center.
6.20 Energy Surfaces as Reaction
Maps or Graphs
Orbital interactions and state correlation diagrams supply the basic elements
of a qualitative theory of photoreactions.
The possible products may be deduced from state correlation diagram maps.
The probable products may be deduced from consideration of
a) symmetry-imposed barriers
b) minima which facilitate pathways from an excited surface to the
ground state