Dynamics of molecules on excited state
surfaces
Chapter 6
Principles of Molecular Photochemistry: An Introduction
NJT, VR and JCS
• Transition state connects a single reactant to a single product and it is a saddle point
along the reaction course.
• Collisions are a reservoir of continuous energy (~ 0.6 kcal/mol per impact).
• Collisions can add or remove energy from a system.
• Concerned with a single surface.
Visualization of Thermal Reactions
Visualization of Thermal Reactions
Radiative transition vs Reaction
How do molecules move on surfaces?
How molecules navigate between surfaces?
Radiationless Transition Photoreaction
A Model for Photochemistry and Photophysics of Organic Molecules
Classification of Photoreactions
R
R*
P
P*
PR + hν *R
PhotochemicalPhotophysical
?
R*
P*
R P
Adiabatic
Diabatic
Hot ground state
R P
R*
P#
A fancier way of drawing the actions on excited surfaces
Molecular Photochemistry: Recent Developments in Theory, S. Mai and L. Gonzalez,
Angew. Chem. Int. Ed. 2020, 59, 16832 – 16846
Models used to understand
reactions on excited surfaces
• Adiabatic and diabatic surfaces
• Crossing and avoided crossing
• Funnel
• Conical intersection
• Energy gap law / Fermi’s golden rule
• Wave packets vs particles (marbles)
O*
H
H
H
H
H
H O
H
H
H
H
H
H
O
H
H
H
H
H
H
O
H
H
H
H
H
H
Photodissociation of s bond
H2
H H H H
• E. Schrödinger developed the famous F equation in 1926
• Based on E.Sch. eq. Heitler and London developed the concept
of bonding for H2 molecule in 1927.
• L. Pauling extended this to the concept of valence-bond theory.
W. Heitler F. London
These formed the basis of energy diagrams for photodissociation.
Linus Pauling, 1938
Potential energy curves for dissociation of NaCl
Covalent
Ionic
Electronic correlation diagram for a given reaction co-ordinate
F1. c1. S1
Y1R
F2. c2. S2
Y2R*
Y3
Y4P*
P
RC
Y1R
Y2R*
Y1P
Y2P*
RC
discon
discon
The curves are based on simple coulombic
attraction between charged spheres (of
which there is zero for the covalent
model), and repulsion of nuclear charge
and physical interaction between hard
spheres.
To obtain the best wavefunctions and
energies at any particular point, we must
mix the ionic and covalent wavefunctions.
The resulting energies repel each other.
Therefore, at every internuclear separation,
the energies of the original wavefunctions
are “split apart.” At the crossing point, the
new energies (which will be closer to the
true energies than the original ionic and
covalent wavefunction energies) split apart
and the curves no longer cross. This point
of nearest approach is called an avoided
crossing point
Crossing and avoided crossing
Probability of jump from lower to upper or the reverse
• Energy gap (DE)
• Velocity (v)
• Slope difference (Ds)
Landau-Zener-Stuecktberg equation
P = exp(–DE2/vDs)
DEDE
Large gapSmall gap
A
A’
B
B’
Conical Intersection
Minimum at Avoided Crossing & CI may not be at the same place
CI
AC
Avoided crossing, true crossing and conical
intersection
Avoided crossing
in two dimensions
Product appears in ns
Product appears in ps-fs
Diatomic
Allowed crossing in higher
dimensions (polyatomic
molecules); Conical
intersection
"for his studies of the transition
states of chemical reactions using
femtosecond spectroscopy."
Ahmed H. Zewail
The Nobel Prize in Chemistry 1999
Chem. Eng. News., 1988, Nov
7, 24
Physics Today, May 1990, 24
J. Phys. Chem. C., 1996, 100,
12701
Angew. Chem. Int. Ed., 2000,
39, 2586
J. Chem. Edu., 2001, 78, 737
10-15 sec time resolution (1980s)
The Nobel Prize in
Chemistry 1967 was
divided, one half
awarded to M. Eigen,
the other half jointly to
R. G. W. Norrish and G.
Porter "for their studies
of extremely fast
chemical reactions,
effected by disturbing
the equilibrium by
means of very short
pulses of energy."
10-6 sec time resolution (1960s)
R. G. W. Norrish
M. Eigen
G. Porter
Can a horse fly
The birth of high-speed camera
Photo series by Eadweard Muybridge, 1870
Can a horse fly
Photo series by Eadweard Muybridge, 1870
Millions of molecules are reacting
Although same reaction – they don’t run
in lock-step
How to synchronize ?
As if a whole herd of horses in lock-step
in one camera
• Millions of molecules could march in lockstep
from a common starting point
• A series of pictures can be taken at different
times
• Equivalent to a movie of a chemical reaction
Coherence
Coherence
Coordinated fashion or acting together.
How to achieve this?
Molecular beam
Waves can be added.
constructive interference
amplitude doubledin phase
(0°)
+
− +
−
destructive interference
amplitude zero
+
−
+
−
out of phase
(180°)
Waves oscillate positive and negative and travel
+
−
Waves of different
wavelengths can be
added. Add 5 waves.
λ = 1.2, 1.1, 1.0, 0.9, 0.8
Superposition
Sum of the 5 waves
Classical – can know momentum p and position x exactly at the same time.
Quantum – know p exactly, x completely uncertain. Equal probability of
finding particle anywhere.
Waves of different
wavelengths can be
added. Add 5 waves.
λ = 1.2, 1.1, 1.0, 0.9, 0.8
Superposition
Sum of the 5 waves
-20 -10 10 20
-4
-2
4
2
Superimposing 5 waves concentrates probability in a region of space, but now there
are 5 values of the momentum.
Wave packets
The wavepacket initially created in the FC region of the excited state of the reactant rapidly evolves
along the reaction pathway towards the conical intersection, and the stimulated emission (SE)
progressively shifts to the red as the band gap between the excited and ground states narrows. Near the
conical intersection region, which is reached in ∼80 fs according to both experiments and simulations,
the SE signal vanishes as the two surfaces approach each other. Following the “jump” to the hot ground
state of the photoproduct, a symmetric photo-induced absorption (PA) signal is formed. This PA band
rapidly shifts to the blue as the surfaces move away from each other energetically and the wavepacket
relaxes to the bottom of the photoproduct well.
Photochem. Photobiol. Sci., 2015, 14, 213
Ultrafast time resolution (fs) has changed the way we visualize reactions
on excited surfaces
Classification of photoreactions
• Bond breaking----cleavage reactions
• Bond twisting-----geometric isomerization
• Addition at one end----abstraction, substitution
• Addition at two ends-----cycloaddition
• Concerted pericyclic reactions---electrocyclization
• Rearrangement
p-bonds break upon twisting
s-bond break upon stretching
How do bonds break?: Stretching and twisting
H2
H H H H
• E. Schrödinger developed the famous F equation in 1926
• Based on Sch. Eq. Heitler and London developed the concept of
bonding for H2 molecule in 1927.
• L. Pauling extended this to the concept of valence-bond theory.
W. Heitler F. London
These formed the basis of energy diagrams for photodissociation.
A model for breaking s bond by stretching
Homolytic photodissociation of I2, ICN, IBr, CH3I etc.
Exemplars for s bond cleavage in the excited state
• Photodissociation of CH3 I
o Zewail et. al., J. Chem. Phys., 83, 1996, 1985
o Zewail et. al., Chem. Phys. Lett, 142, 426, 1987
• Photodissociation of ICN
o Zewail et. al., J. Chem. Phys., 87, 2395, 1987
o Zewail et. al., Science, 241, 4870, 1988
o Zewail et. al., J. Chem. Phys., 89, 6128, 1988
o Zewail et. al., J. Chem. Phys., 90, 829, 1989
• Photodissociation of NaI
o Zewail et. al., Chem. Phys. Lett., 146, 175, 1988
o Zewail et. al., J. Chem. Phys., 88, 6672, 1988
o Zewail et. al., J. Chem. Phys., 91, 7415, 1989
o Zewail et. al,. Nature, 348, 225, 1990
• Photodissociation of I2
o Zewail et. al., Chem. Phys. Lett, 161, 297, 1989
Pump-Probe-Detect in
fs time scale
J. C. Polanyi
D. Hersbach
Y. T. Lee
R. Bershon
R. Zare
James L. Kinsey
Potential energy curves for dissociation of CH3I
Dissociates in
400 fs
The first study on photodissociation by Zewail (1985)
Y. Amatatsu, S. Yabushita and K. Morokuma, J. Chem. Phys. 100, 4894 1994
J. Qian, D. J. Tannor, Y. Amatatsu and K. Morokuma, J. Chem. Phys. 101, 9597 1994
Potential energy curves for dissociation of ICN
Photodissociation of ICN (1987)
Rise and decay of laser induced
fluorescence signal of CN*
388.9 nm
389.8 nm
390.4 nm
391.4 nm
• Excited state surface of ICN is dissociative, not bound (no oscillation)
• ICN dissociates within 205 ± 30 fs (~0.2 ps)
Goal (map the surface)
Experiment (pump & probe)
ICN I + CN I + CN*
LIF
Photodissociation of ICN
Tracing the unbound surface (1987)
Potential energy curves for dissociation of NaCl
• If there is no mixing, the two
curves would remain separate and
the excited NaI will retain covalent
character and dissociate like CH3I
and ICN.
• If the two first order curves mix
the crossing will become avoided.
• The avoided crossing will lead to a
‘well’ where the excited NaI will
get trapped and establish a
resonance (oscillation) between
covalent and ionic character.
• The molecule resonates between
two electronic configuration
Detection of crossing and avoided crossing
Resonance
Allowed crossing
Avoided crossing
Experimental demonstration for curve crossing (1988)
~10%
pump
probe
LIF
pump
probe LIF
~10%
Na*
~10%
LIF
Na*
Ultrafast (fs) photodissociation of NaI: Consequences of surface crossing
pump
probe
Pump: 310-390 nm
Probe: 590-700 nm
LIF: Na emission
Free Na atom resulting
from dissociation is
detected
Every time the wave packets hit the outer potential wall, some tunneling
occurs and a little puff of products, Na + I, comes out.
Photodissociation of NaI (1988)
Zewail et. al.
Chem. Phys. Lett., 146, 175, 1988
J. Chem. Phys., 88, 6672, 1988
J. Chem. Phys., 91, 7415, 1989
Nature, 348, 225, 1990
Can we get experimental support
for avoided crossing?
I2
Photodissociation of Iodine (1989)
R. S. Mulliken, J. Chem. Phys, 55, 288, 1971
l1
l2
lem
Pump
Probe
Detect
Photodissociation of I2: A model for cleavage reactions
R. S. Mulliken, J. Chem. Phys, 55, 288, 1971
Photodissociation of Iodine (1989)
Tracing the unbound and the bound surfaces (a, b & c)
700/310/ (a state)
620/310/ (b state)
505/310/ (c state)
Time of oscillation
500 fs
• Considering molecules as particles (classical mechanics) or wave
packets (quantum mechanics) lead to similar results. Wave packets can
leak through a barrier (tunneling) while particles cannot.
• Surfaces generated based on electronic correlation diagrams help predict
reaction dynamics
• Crossing of surfaces are common and these could lead to ‘real’,
‘avoided’ and conical intersections
• On excited state surfaces oscillation (resonance) of electronic structures
occurs in fs time scale.
Conclusions based on ultrafast experiments
“The study of chemical events that occur in the femtosecond time scale is
the ultimate achievement in half a century of development and, although
many future events will be run over the same course, chemists are near
the end of the race against time”
George Porter, 1993
H2
Covalent
Ionic
NaCl
Models for photodissociation of covalent and ionic s bond
Energy surfaces generated by following the electrons as the nuclei move
along the reaction co-ordinate.
discon
discon
R
R*
P
P*
R
R*
P
P*
Concerted (pericyclic) reactions: Cyclization
Assigning symmetry to relevant orbitals
Disrotatory
Conrotatory
Conrotatory Orbital Correlation Diagram
R. B. Woodward, R. Hoffmann, JACS, 87, 396,1965
Disrotatory Orbital Correlation Diagram
R. B. Woodward, R. Hoffmann, JACS, 87, 396,1965
State correlation diagram for an electrocyclic reaction
Thermal---allowed
Photo---forbidden
Thermal---forbidden
Photo---??
H. C. Longuet-Higgins, E. W. Abrahamson, JACS, 87, 2045,1965
W. Th. A. M. van der Lugt and L. J. Oosterhoff, J. Am. Chem. Soc., 91, 6041, 1969
The photochemical reaction is facilitated by crossings of surfaces
in the excited state
UP hill, NO
Down hill, YES
H. C. Longuet-Higgins, E. W. Abrahamson, J. Am. Chem. Soc., 87, 2045, 1965
van der Lugt and Oosterhoff,
J. Am. Chem. Soc., 91, 6041, 1969
19691965
Longuet-Higgins and Abrahamson,
J. Am. Chem. Soc., 87, 2045, 1965
1975
D. Grimbert, G. Segal, and A. Devaquet,
J. Am. Chem. Soc., 97, 6629, 1975
The photochemical reaction is facilitated by crossings of
surfaces
in the excited state
R. Mathies et.al, Acc. Chem. Res. 28, 493, 1995
Ultrafast dynamics studies
Conical Intersection and Multiple Products
Cyclobutene ring opening is not stereospecific?
Adiabatic Photochemistry?
Conical intersection with multiple exits?
C
H
C
H
hν
C
H
C
H
C
H
C
H
hν
C
H
C
H
C
H
C
H
hν
C
H
C
H
C
H
C
H
hν
C
H
C
H
C
H
C
H
hν
C
H
C
H
Geometric Isomerization
Ethylene - HOMO
Ethylene - LUMO
hυ
φ1
φ2
φ2φ1
Geometric Isomerization: Twisting of a C=C p Bond
Ethylene
p
p*
p p* p p**
So
S1
S2
So
S1
S2
φ1φ1
φ1φ2
φ2φ2 φ2
φ2
φ1φ2
φ1φ1
Correlation Diagram for Geometric Isomerization
φ1
φ2
φ2φ1
Note the similarity between this diagram and WH diagram
for electrocyclization
R. Hochstrasser, Pure & Appl.
Chem., 1980, 52, 2683
Energy surface for the isomerization of stilbene
T. Tahara et. al., Phys. Chem. Chem.
Phys., 2012, 4, 6225
J. Saltiel, et. al., J. Photochem.
Phobiol A, Chem. 1992, 65, 29
Geometric Isomerization of Ethylene
Conical Intersection is a Possibility
Conical intersection at a twisted, mono-pyramidalized geometry
suggested through computation.
Ethylene decays with a lifetime of 20±10 fs (2 x 10-14 sec); computed
T. J. Martinez et. al., Annu. Rev. Phys. Chem. 2007, 58, 613.
T. J. Martinez et. al., Chem. Phys., 2000, 259, 237.
Suggested references on excited state surfaces, crossings, avoided
crossings, conical intersections etc.
E. Teller, THE CROSSIKG OF POTENTIAL SURFACES, J. Phys. Chem , 1937, 41, 109.
E. Teller, INTERNAL CONVERSION IN POLYATOMIC MOLECULES, Isr. J. Chem. 1969, 7, 227
C. Zener, Non-Adiabatic Crossing of Energy Levels, Proc. Roy. Soc. London 1932, 137A,696
T. J. Martinez et. al., Annu. Rev. Phys. Chem. 2007, 58, 613.
T. J. Martinez el. al., J. Phys. Chem. A, 2000, 104, 5161
M. Robb et. al., Rev. Computational Chemistry, 2000, 15, 87-146.
Oliviucci, Photochem. Photobiol. Sci., 2011, 10, 867.--- Rev on CI
Domcke and Yarkony, Annu. Rev. Phys. Chem., 2012, 63, 325–52
Yarkony, Chem. Rev., 2012, 112, 481–498
F. F. Crim, Faraday Discuss., 2012, 157, 9–26
F. F. Crim, J. Phys. Chem., 1996, 100, 12725-12734J.
https://chemistry.as.miami.edu/research-groups/ramamurthy-group/video-
lectures/video-lectures-miami/index.html
Watch two videos by Oliviucci in this site (Murthy group); search, they are at the end
Michl, J. "Photochemical Reactions of Large Molecules. I. A Simple Physical
Model of Photochemical Reactivity", Mol. Photochem. 1972, 4, 243.
Michl, J. "Photochemical Reactions of Large Molecules. II. Application of the
Model to Organic Photochemistry", Mol. Photochem. 1972, 4, 257.
Michl, J. "Photochemical Reactions of Large Molecules. III. Use of Correlation
Diagrams for Prediction of Energy Barriers", Mol. Photochem. 1972, 4, 287.
J. Michl, Topics Curr. Chem., 1974, 46, 1
M. Klessinger and J. Michl, Excited States and Photochemistry of Organic
Molecules, VCH, 1995, pp. 179-241
*R
F
I
(*I or *P)
P
hν
F = funnel from excited to ground state surface
I = ground state reactive intermediate
*I = excited state of a reactive intermediate
*P = excited state of product
R
Mechanistic Possibilities
Hydrogen abstraction is a common photoreaction
H2
Covalent
Ionic
NaCl
Models for photodissociation of covalent and ionic s bond
Energy surfaces generated by following the electrons as the nuclei move
along the reaction co-ordinate.
Types of surfaces
Electronic States of Carbonyl Compounds
p
p*
n
n p*
p
p*
n
p p*
p
n
p*
Frontier orbital view of hydrogen abstraction by carbonyl np* triplet
Geometry of orbital interaction
Less likely
More likely
Original publications on correlation diagrams, avoided crossings
on reactions with intermediates
Salem Correlation Diagram
L. Salem, J. Am. Chem. Soc., 96, 3486, 1974
L. Salem, Israel. J. Chem., 14, 89, 1975.
L. Salem, Science, 191, 822, 1976
W. Dauben, L. Salem and N. J. Turro, Acc. Chem. Res., 8, 41,
1975
Non-concerted Photoreactions
L. Salem
1937Allowed
and Avoided Crossings
B. Bigot, A. Devaquet and N. J. Turro, J. Am. Chem. Soc., 103, 6, 1981
L. Salem, C. Leforestier, G. Segal and L. Salem, J. Am. Chem. Soc., 97, 479, 1975
Electrons in Chemical Reactions First Principles, L. Salem, Wiley, 1982, pp. 124-157
O O
nπ*
O
ππ*GS
Relevant
electronic states
R
H
O
R
H
O
R
H
n-plane π- plane
O
R
H
O O
H
R
O
H
R
Constructing Salem Diagram
Geometries of
approach
Diradical
intermediates
formed
Constructing Salem diagram for n-plane attack
Identify a symmetry
plane to which relevant
orbitals are s or a
Enumerate the relevant
orbitals of the reactant
and the intermediate in
the order of their energy.
Build the orbital
correlation diagram based
on energy and symmetry.
Salem correlation diagrams:
L. Salem, JACS, 1974, 96, 3486; Science, 1976, 191, 822.
Dauben, Salem and Turro, Acc. Chem. Res., 1975, 8, 41.
4S 2A
4S 2A
3S 3A
3S 3A
A3S3
A4S2
A2S4
A3S3
A2S4
A4S2
Hydrogen abstraction along n-plane is likely
from np* state.
Reaction from T1(np*) is likely to be more
efficient than from S1 (np*).
If T1 is pp* the reaction from n-face is
unlikely.
How do reactive intermediates in the triplet state
transform to singlet products?
What is the mechanisms of intersystem crossing in
diradicals?
Why excited singlet and triplet give different
products?
Mechanism of photoreactions involving intermediates
R + hn R*
Photophysical
I P
Photochemical
Thermal
Molecular Structure and Dynamics
R P
R *R I P
R
1
*R 3*
R
3
I
1
I P
hν
General Photochemical Paradigm
hν
hν
S0 S1 T1
3
RP 1
RP P
ISC ISC
O
BA
1*
O
BA
3* O
A B
3
O
A B
1
O
BA
1
hn
P
Importance of intersystem crossing
I3
I1
What is a diradical and a biradical?
The Electronic Properties of Diradicals, L. Salem and C. Rowland., Angew. Chem.
Internat. Edit. Eng.,1972, 11, 92.
L. Salem, Pure & Appl. Chem., 1973, 33, 317.
Concerted Reactions That Produce Diradicals and Zwitterions: Electronic, Steric,
Conformational, and Kinetic Control of Cycloaromatization Processes, R. K.
Mohamed, P. W. Peterson, and Igor V. Alabugin, Chem. Rev., 2013, 113, 7089.
Do Diradicals Behave Like Radicals? T. Stuyver, B Chen, T. Zeng, P. Geerlings, F. De
Proft, and Roald Hoffmann, Chem. Rev., 2019, 119, 11291.
Diradicals, M. Abe, Chem. Rev., 2019, 113, 7011.
What controls the singlet-triplet energy gap in
molecules and diradicals?
DEST = J - B in intermediates where HOMO-LUMO gap is very small
B: bonding interaction, interaction between nuclei due to the presence
of two electrons in a bonding orbital
• Molecules
• Intermediates (Diradicals)
DEST = 2J in molecules where HOMO-LUMO gap is large
J: exchange integral, depends on the overlap of orbitals with one
electron each
*R[S1(n,π∗)] 3I(D) 1I(D)
hν
3RP
1RP
R P
S0 S1 T1 P
*R[T1(n,π∗)]
Electronic Energy Difference between Singlet and Triplet States in
Diradical Reactive Intermediates, I(D)
S1 > T1
S1 > T1 S1 < T1
HOMO–LUMO gap large
Only J, no B
HOMO–LUMO gap small or equal energy
Only J Both J and B
J will be proportional to the electron exchange integral for the (NBL)1(NBU)1 configuration
B will be proportional to the contribution of the (NBL)2 configuration. The latter
corresponds to the bonding contribution.
{(NBL)1(NBU)1}1 {(NBL)1(NBU)1}3 (NBL)2 (NBL)2
(NBL)2
NBL NBU
DEST = J - B
Singlet-triplet energy gap
Energy Ordering in Diradicals Depends on Values of J and B
DEST = J - B
• Bonding
• Two states
• No longer diradical
Degenerate
• No bonding
• Triplet lower than singlet
Close in energy
• Slight bonding
• Singlet lower than triplet
Close in energy
Salem’s rules for ISC in diradicals-1
(1) The value of the exchange interaction, J,
between the two radical centers is less than the
strongest available magnetic coupling mechanism.
In order to mix the S and T states of I(D) effectively, the two states must have
essentially the same energy, i.e., they must be very close to degenerate.
Since J causes the energy of the S and T state to “split,” the value of J must
approach zero if the energies of the S and the T state of I(D) are to become
degenerate and mix effectively.
Salem’s rules for ISC in diradicals-2
(2) The non-bonding orbitals of the diradical are in
an orbital orientation that can interact to some
extent and can create orbital angular momentum
that couples with the spin angular momentum
during the ISC step.
In order to generate angular momentum an orbital jump from a “pZ ® pX” type
is required. The best orbital orientation for spin-orbit mixing is when the
two non-bonded orbitals of the diradical are at a 900 orientation with
respect to one another.
Salem’s rules for ISC in diradicals-3
(3) The degree of electron pairing character in the
singlet can become significant during the ISC
step.
In order to effectively generate orbital angular momentum, the electron must
jump from one orbital to the other half occupied orbital which is at a 900
orientation, as the singlet is created. This produces a situation for which
there are two electrons in a non-bonding orbital to a "zwitterionic"
structure (1(Z)). Thus, for the most effective creation of angular momentum,
the singlet must possess a certain amount of spin paired character.
Conformation dependent product formation
Conformation dependent ISC and its effect on product distribution
kisckisckisc
3 DR
1 DR
Product
A
B
C
Fast
Slow
P-1
Not all conformations would have the same J, and orientation
of the two p-orbitals
Conformation dependent ISC and its effect on product distribution
Slow
P-2
hν
Sens.
hν
Direct
+
CH2
hν
Direct
hν
Sens.
C6H5 hν
C6H5
OCH3
CH3OH
C6H5
CH2OH
hν
Sens.
O
O
O
hν
sensitized
hν
Excited Singlet and Triplet May Undergo
Different Reactions
Loose and Tight Biradicals
*1
*3
stereorandom
stereospecifc
*1
*3
Why excited singlet and triplet give different products?
Photochemistry of dibenzyl ketone as an exemplar of cage effect
Definition of cage effect
C12 C16
SDS CTABCore (2-3 nm)
Stern Layer
(up to a few A)
Gouy-Chapman Layer
(up to several hundred A)
Water molecule
SO3
-
Na+
N
+
BrSurfactant
monomers
Schematic representation of a
guest@micelle complex
G
G
107
GC traces of product
distributions upon irradiation in
solution and in HDTCl micelle
O
Me
hν
BA
O
Me
+ MeMe
+
AA AB BB
A comparison between solution and micellar irradiations
In micellar solutions the % cage depends on the surfactant
concentration
Cage effect dramatically increases at a certain concentration of surfactant
Sulfate surfactants
CH3(CH2)nOSO3Na
Bigger micelles,
more hydrophobic cage,
slower exit to water
In micellar solutions the % cage depends on the the cage size
Reactive radicals escape
from smaller cages more
easily.
Intersystem Crossing in Radical Pairs
Spin Dynamics - pictorial view
The system, in its T1 state, slides down along the T1 surface energy. This correspond to an increased
distance between the two nuclei: the bond breaking step occurs.
Cage effect and nuclear isotope effect
(a) Initial sample of
13C=O enriched DBK
(b) The degree of 13C=O
in DBK increases with
photolysis (90%
conversion)
PhCH2CCH2Ph
O
hν
T1S1 PhCH2C
O
CH2Ph
3
12
C
escape13
C CAGE
PhCH2CCH2Ph
O
DBK
PhCH2C
O
CH2Ph
1
b
PhCH2CH2
O
CH3 PhCH2CH2
CH2
PMAP
ENRICHED IN 13
C
FREE RADICALS
PhCH2CH2Ph + CO
IMPOVISHED IN 13
C
O
PhCH2C
O
CH2Ph
Cage effect can be utilized for isotope enrichment
The competion is between cage escape and hyperfine induced ISC
Effect of an applied magnetic field on the T splitting
T+ T0 T- S
ββ
αβ + βα
αα
Tαβ
− βα
T0
T+
S
T levels split apart, T0 has the same energy as S
Hz
Only T0 ® S ISC allowed
Magnetic field effect will
decrease cage reactions
Nuclear isotope effect will
increase cage reactions
The effect of external magnetic field on the cage effect
The cage effect decreases. More exit from host cage.
Initial 90% photolysis
at 0 applied mag. field
90% photolysis
at 15,000 G applied
mag. field
Isotope enrichment decreases in presence of applied magnetic field
48% 13C 55% 13C62% 13C