Download Ground state reactants Ground state products Ground state

Ground state
reactants
Ground state
reactants
hν
Excited state
reactants
Ground state
products
Reaction
Intermediates
Ground state
products
9.1
Criteria to establish reasonable
mechanistic options
In determining a plausible from a possible
mechanistic set, we may first apply criteria
relating to:
1. Energetics
2. Dynamics
3. Structure
4. Electronics
9.2
Additional criteria to select from among plausible
mechanisms the most probable one
• 1. Reactant and Product structure
• 2. Structure of intermediates on the pathway
from reactant to product
• 3. Kinetic rate law
• 4. Labeling experiments
• 5. Structure-reactivity relationships
9.3
Calculated rate constants at 300 K for various preexponential factors and variable activation energies.
16
At 300 K
14
16
10
14
8
12
6
10
4
log (A/s–1) = 8
–1
log k (s
)
12
2
0
0
2
4
6
8
Ea (kcal/mol)
10
12
9.4
Norrish Type I reaction
O
O
CH3
CH3
hν
CH3
CH3
CH3
CH3
3n,π*
~ 100 ns
O
C•
+
The triplet lifetime is around
100 ns at room temperature.
We assume that for this
chemical reaction to
contribute significantly to
triplet decay, its rate constant
will have to be at least 10 5 s -1
for the Norrish Type I
cleavage. We calculate that the
maximum activation energy is
~ 11 kcal/mol.
Experimentally the measured
values are A = 2 x 1012 s -1 and
Ea
= 7.3 kcal/mol and the
(CH 3)3 C •
cleavage occurs with a high
quantum yield.
9.5
The Norrish Type II reaction
O
OH
Ea (kcal/mol)
hν
R1
X
H
R2
Type II H-abstraction
from the triplet state
X
R1
primary
7
R2
secondary
5
tertiary
4.5
Type II 1,4-biradical
typical lifetimes are
around 30-100 ns
OH
R1
+
R2
(a)
OH
R1
(b)
R1
X
O
X
R2
X
R2
X
HO
(c)
O
R1
X
H
R2
9.6
Norrish Type II reaction
Pre-exponential factors
H
O
A = 1013 s–1
H
O
A = 1012 s–1
Entropic factors are important in determining A factors
9.7
Quantum yield and state efficiency
Quantum yield:
Φ=
moles of a given species formed or destroyed
moles of photons absorbed by the system
A mole of photons is
also called an Einstein
State efficiency :
φ =
moles of a given species formed or destroyed
moles of a given state formed by absorption of an Einstein
R
hν
φR*
R*
φI
I
Reactants
φP
P
Products
Φp = ΦR* x φI x φp
9.8
Some mechanistic tools and criteria
1. Does the overall reaction correspond to a standard reaction type
or to a sequence of standard reaction types?
2. How does the product atomic composition and connectivity
relate to the reactant atomic composition and connectivity?
3. How does the product stereochemistry relate to the reactant
stereochemistry?
Reactions can be classified in terms of mechanistic types, and for
any given class of reactions only a small set of mechanistic types
is likely to be required for a detailed analysis
Hammond postulate
If a transition state and an intermediate possess comparable energies and
occur consecutively along a reaction coordinate, the chemical
composition, the chemical constitution (structures) and chemical
properties of the transition state will be similar to those of the intermediate
9.9
Excitation-decay of R*
pulsed excitation
100
[R]
Concentration (% of total)
80
60
40
20
hv

→
R
Reactants
[R * ]
τR

→
Products
R
= Reactants
[R*]
0
0
20
40
60
Time (arbitrary units)
80
100
Pulsed excitation at t = 10 excites 40% of the R molecules,
Reading to an excited state (R*) that decays with τR = 20. Excited
decay is accompanied by the concurrent regeneration of R.
9.10
Decay of triplet xanthone following pulsed
laser excitation
0.025
O
0.020
Absorbance signal
Decay of triplet xanthone
monitored at 600 nm following
355 nm laser excitation in
acetonitrile. The faster decay
process is in the presence of 9
mM pyridine. Quenching is due
to charge-transfer interactions
O
0.015
no quencher
0.010
Lifetime of R*, no quencher
1 =k
1
τ1
with
quencher
0.005
0.000
0
5
10 (nn.20)
15
20
25
Time, µs
30
35
40
Lifetime of R*, quencher present
1 = k + k [Q] = 1 + k [Q]
1
q
q
τ1
τ2
9.11
Stern-Volmer plot based on fluorescence
2.5
o
Φ F/ Φ F
2
slope = k s τ
1.5
q
F
1
0
0.05
0.1
0.15
0.2
0.25
0.3
[trans-1,2-dicyanoethylene], M
Experimental plot of Φ0F/f ΦF vs.
[trans-1,2-dicyanoethylene] for
the cycloaddition of acetone
singlets to the ethylene.
The intercept is 1.0 and the
slope is 7 M-1 = kqτ.
CN
O*
+
kq
CN
S 1 (n,π*)
CN
CN
1
9.12
Stern-Volmer plots in laser photolysis
CH3
O*
CH3
Butyrophenone
triplet
+
1-Methylnaphthalene
ground state
[ MeN *] ∞ =
1
∆OD∞
CH3
Butyrophenone
ground state
k q [ MeN ]
τ -1 + k [MeN ]
º
= a +
CH3
O
Energy
transfer
q
+
*
1-Methylnaphthalene
triplet
(~420 nm absorption)
[ BTP * ] º
a
kq τ0 [ MeN ]
9.13
Detecting the excited quencher
S1
91
T1
80
ener
gy
trans
fer
triplet state
with a characteriatic
signature
T1
60
transient absorption
87
S1
solvent: methanol
hν
320
S0
S0
360
400
440
Wavelength, nm
480
CH3
O
H 3C
R
9.14
Kinetics of Reactions Involving More than One
Excited State
O
hν
CH3
H3C
S1
T1
CH3 COCH3 + CH2 =CHCH3
Relative
efficiencies
Relative
rates
S
φq
φqT
=
kSq τ
S
Norrish Type II products
formed from both S 1 and T 1
kq τT
T
rate of S quenching
1
rate of T quenching
1
=
kqS [diene]
k qT [diene]
=
k Sq
k qT
9.15
Possible forms of Stern-Volmer plots
Plot type
A) Linear
Cause
1) Only one excited state reacts
2) Two rapidly equilibrating excited states with either or both
quenched
3) Two non-interconverting states coincidentally quenched with the
same value of kq Z
4) The first of two consecutively formed states is quenched, with
either one reacting
B) Downward
curvature
1) Reaches a zero slope. Two reactive excited states, but only one
quenched
2) Reaches a positive asymptotic slope. (a) Two non-interconverting
excited states quenched with different kq values, or (b) Two
interconverting excited states with the shorter lived or both reactin
the quencher disrupts the interconversion equilibrium. Final slop
corresponds to initial shorter state
C) Upward
curvature
1) Two consecutive excited states are quenched, but only the second
one is reactive
2) Two non-interconverting excited states are both reactive and
quenched with different kq2 values
3) Two interconverting excited states, with only the lower one reacti
and with the quencher disrupting the interconversion equilibrium
4) Same as B2
5) A sensitized reaction with both sensitizers and substrate states
quenched
6) Static or time dependent quenching.
9.16
Two different approaches to the study of short
lived reaction intermediates
•
By making the technique sufficiently fast that it can
access the time scale in which the intermediate
naturally lives.
•
By isolating the intermediate under conditions where
its lifetime is long enough for the technique of choice,
such as in matrix isolation.
9.17
Sensitization: why and how
The idea of triplet sensitization is to test if a given triplet reaction can be
induced while completely bypassing the singlet manifold of that
compound. Thus, for efficient and selective sensitization, the
sensitizer should have the following characteristics:
(1) Triplet energy higher than that of the molecule to be sensitized.
(2) An absorption in a region where the molecule under study is
transparent, so that selective excitation can be readily achieved.
(3) Either a very short singlet lifetime, or a sufficiently large S-T energy
gap to place the sensitizer S1 level significantly above the S1 level of
the sensitized molecule.
9.18
The Norrish Type II reaction
An example of intramolecular hydrogen abstraction
Ph
OH
Me
Ph
O*
• OH
2 ns
H
Me
Me
Ph
Ph
Me
100 ns
OH
•
Me
Me
Ph
n,π*
triplet
O
H
Me
γMeVLP
Me
hν
Acetophenone (ACP)
9.19
Experimental tests for the involvement of
biradicals in the Norrish Type II reaction
Hydrogen abstraction
CH3
Ph
+
CH3
Ph
RS–H
OH
OH
CH3
CH3
H
+
RS •
Electron transfer
CH3
+
H 3C N
N CH3
Ph
+
CH3
MV+
•
O
MV2+
Initiation of polymerization
H 3C
+
CO2CH 3
polymerization
CH2
Addition to reactive double bonds
+
H 3C
But
C Se
Ph
But
OH
But
Se C
But
CH3
9.20
The Norrish Type I reaction of cyclic ketones
O
Ph
O*
Ph
Ph
O
Ph
hν
fast
↑ Ph
↑
– CO
Ph
Ph ↑
↑ Ph
Ph ↑
↓
k > 10 8 s–1
Ph
Ph
+
Ph
Ph
Ph
The Norrish Type I reaction of 2,6-diphenylcyclohexane involves the
fragmentation of a triplet biradical to another triplet biradical
following rapid decarbonylation.
9.21
The paradigm for biradical reactions
RULES
Rule # 1
Rule # 2
Biradical paradigm concepts
1
3
BR will only yield singlet products
BR will only yield triplet products. These reactions are uncommon, unless another
biradical is produced (see the example of Scheme nn.11)
Rule # 3
Biradicals undergo monoradical-like reactions with essentially the same rate constants a
typical monoradicals. Whether or not these processes dominate will largely depend on
the dynamics of biradical-specific reactions (see rule # 4)
Rule # 4
3
BR will undergo biradical-specific reactions when they interact with paramagnetic
species, such as oxygen, nitroxides and certain transition metal ions
Rule # 5
3
1
Intersystem crossing ( BR ↔ BR) plays an important role in biradical reactions.
Equilibration is rarely achieved. For triplet biradicals intersystem crossing is frequently
irreversible and determines their lifetime
Rule # 6
1
The decay of BR to molecular products can be fast enough to compete with bond
rotations. As a result the partition anong different biradical products may depend on th
conformation at which intersystem crossing occurs
Rule # 7
3
The lifetime of BR will depend on the factors that control spin-spin interactions, such
as distance, S-T energy gap, and spin-orbit coupleing interactions (these effects will be
covered in detail in Chapter zz)
Rule # 8
Structures in which odd electrons are of the same spin are more stable the fusther apart
are the location of the unpaired electrons
9.22
Oxygen in Organic Photochemistry
a common statement:
The reaction is quenched by oxygen,
thus,
it must be a triplet reaction
true or false?
10.1
Electronic structure
The basic electronic structure of the oxygen molecule in the ground
state can be written as:
O2 (1σg)2 (1σµ)2 (2σg)2 (2σµ)2 (3σg)2 (1π µ)4 (1π g)2
or
O2
(1σ)2 (1σ∗)2 (2σ)2 (2σ∗)2 (3σ)2 (1π) 4 (1π*) 2
10.2
Energy levels for molecular oxygen
1
1
∆ g, ∆ g 44.8 kcal/mol, 15764 cm
1
Energy
Σg 37.5 kcal/mol, 13121 cm
1∆
g
-1
-1
22.4 kcal/mol, 7882 cm-1
3
Σg (ground state)
Energy levels for molecular oxygen. Excited triplet states have not been included because they
are much higher in energy. The 1∆g state is the one normally refereed to as singlet oxygen.
10.3
Dimol emissions
Oxygen shows several "dimeric" emissions, the best known of which
is the dimol emission at ~635 nm. No dimer is formed under normal
laboratory conditions. Kasha has pointed out that the simultaneous
transition for a pair of emitting species does not require an actual
complex to exist, although the two molecules must be within contact
distance; i.e. close enough for electron exchange to be possible. The
process has also been described as energy pooling and is reminiscent
of triplet-triplet annihilation processes
( )
( )
( )
( )
O 2 1∆ g + O2 1∆ g → O 2 3 Σg + O2 3 Σg + hν
10.4
Jablonski diagram for the excited states of
molecular oxygen
1Σ
g;
t = 130 ns
(sum of knr from 1 Σg = 7.6 x 106 s-1)
krad = 3.4 x 103 s-1
knr =
Φ em = 4.5 x 10-4
krad = 0.40 s -1
knr =
1
∆g ; t = 87 ms
Φ em = 5.2 x 10-8
CCl4
krad = 1.1 s-1
knr = 10.4 s-1
Φ em = 0.087
3
Σg
10.5
Singlet oxygen spectroscopy
Solvent
Φ em
τo∆ (µs)
C6H6
4.7 x 10 -5
31
CH3CN
7.1 x 10 -5
75
CHCl3
3.6 x 10 -4
207
CS2
0.040
34000
CCl4
0.087
87000
Freon-113
0.15
99000
H 2O
CH3OH
Sources: (Schmidt, 1989),
krad (s-1)
~5
10.4
Singlet oxygen (1∆g) lifetimes, emission quantum yields and radiative decay rate
constants (krad) in various solvents at room temperature
10.6
Effect of deuteration on singlet oxygen
lifetimes
O2 (1∆g) lifetimes in
Pr =
CH3OH
10.4 µs
CH3OD
37 µs
CD3OD
227 µs
k M[ M]
−1
τ
+ kM [ M]
Probability of reaction
10.7
Effect of deuterium on reaction yields
Pr =
k M[ M]
−1
τ
+ kM [ M]
Probability of reaction
There are two extreme situations in which we would NOT
expect an effect:
(i)
if the reaction does not involve singlet oxygen; and
(ii) if the reaction with singlet oxygen is extremely fast
10.8
Simple rules let us anticipate changes in O2 ( 1∆ g)
lifetime as a function of the solvent
•
The longest lifetimes are observed in perhalogenated solvents.
•
τo∆ decreases on increasing the number of H atoms in the solvent
molecule.
•
The shortest τo∆ values are observed with solvents having O–H
groups, notably water.
•
The presence of heavy atoms reduces τo∆ .
•
Solvent deuteration invariably increases τo∆ .
10.9
Bond dissociation energies for selected
oxygen containing species
Molecule
O2
Bond type
O=O
BDE (kcal/mol)
119.0
H2O2
O–O
51
H–O
H2O
H–O
H–O
102.2
119.3
H2O2
H–O
88.1
HO2
H–O
ROH
ROOH
H–O
H–O
105
R2O2
ketone
O–O
C=O
37
methanol
H–O
104.4
10.10
Redox properties
• Oxygen is a good electron acceptor, but a very poor donor.
• Reduction of oxygen can lead to O2• , H O2• , H O2–, H 2O2 and HO•
• It is usually the first electron transfer to O2 that is the rate limiting step.
• The O2/ O2• couple has an immense importance in nature.
•It has an E˚ of -0.15 V in water and -0.60 in dimethylformamide.
• Under many conditions, O 2• is itself a good reductant.
• Superoxide is a poor oxidant, since E˚ (O2• / O22–) < -1.7 V
10.11
Redox properties of singlet oxygen
Singlet oxygen is a better oxidant than ground state oxygen.
When the excitation energy of singlet oxygen is taken into
consideration the values of E˚ (1O2/ O2• ) are 0.34 V in
dimethylformamide and 0.79 V in water.
Singlet oxygen oxidizes molecules such as N,N,N',N'-tetramethyl-pphenylenediamine to its radical cation.
Me 2 N
NMe 2
10.12
Energy transfer
Singlet oxygen as a donor
There are not many examples of energy transfer from oxygen, largely
because few molecules have such a low excitation energy
1O
2
1∆
g
+
β-C
3O
2
3Σ
g
+
3β-C*
CH3
CH3
CH3
CH3
CH3
CH3
H 3C
CH3
CH3
CH3
β-carotene (β-C)
10.13
Singlet oxygen: chemical quenching
H 3C
H 3C
CH3
H 3C
1O
CH2
2
H 3C
CH3
CH3
OOH
(A)
1
O2
O
(B)
O
CH3O
CH3O
1O
CH3O
2
CH3O
O
O
(C)
10.14
The Schenck or ene reaction
H3C
H3C
H3C
CH3
1
O2
H3C
CH3
(A)
CH3
OOH
H
O
H
CH2
O
O
O
10.15
Singlet oxygen: reversible addition
Addition to a conjugated system can be reversed thermally with
regeneration of singlet oxygen
O
1O
2
O
∆
+ 1O2
endoperoxide
Provides a chemical mechanism
for ‘storing’ singlet oxygen
10.16
Dioxetane formation is not reversible
H3C
CH3
CH3
+
H3C
1
O2
CH3
H3C
O
H3C
O
minor
CH3
dioxetane
CH3
H3C
O
H3C
O
CH3
H3C
∆
Ea ~ 27 kcal/mol
H3C
O*
+
H3C
ΦS ~ 0.25
ΦT ~ 0.35
CH3
H3C
H3C
O
O•
H3C
O•
CH3
?
10.17
Interaction of oxygen with excited singlet
states
S1
T2
T1
Possible but unlikely
( )
1 *
X + 3O 2 → 3X* + O 2 1∆ g
requires large S-T gap
So
Common
Assisted intersystem crossing
1 *
X
+ 3O 2 → 3 X* + 3 O 2
10.18
Energy transfer processes
Quenching by oxygen of excited triplet
states
2 ]*
X +
1O
2
singlet
path
[X····O2 ]*
X +
3
triplet
path
1 [X····O
1/9
3
X* +
3
1/3
O2
3
O2
5/9
5 [X····O
2 ]*
quintet
path
Spin statistics plays a key role indetermining the probabilities of the vatious reaction
paths between an excited triplet state and molecular oxygen
10.19
Detecting singlet oxygen
The emission at 1270 nm provides a
convenient tool to study the chemistry of
singlet oxygen in the 1∆g state. The next
higher electronic state is the 1Σg, 37.5 kcal/mol
or 13121 cm -1 above the ground state. It emits
weakly by decay to both the 1∆g state and the
ground state. Its lifetime of 135 ns in carbon
tetrachloride is surprisingly long for an upper
electronic state (Kasha's Rule)
laser
1Σ
g;
t = 130 ns
(sum of knr from 1 Σg = 7.6 x 106 s-1)
krad = 3.4 x 103 s-1
knr =
Φ em = 4.5 x 10-4
krad = 0.40 s -1
1∆
g
; t = 87 ms
sample
power
monitoring
silicon filter
germanium diode
knr =
-8
Φ em = 5.2 x 10
krad = 1.1 s-1
knr = 10.4 s-1
Φ em = 0.087
3Σ
g
signal
monitoring
10.20
Paradigm for electronic energy transfer from
a triplet sensitizer to molecular oxygen
• It cannot occur if the sensitizer energy is significantly below 22 kcal/mol.
• It can only populate the 1∆g level of molecular oxygen if the sensitizer energy is between 22
and 37 kcal/mol, since population of the 1Σg level would be energetically unfavorable.
• If the sensitizer energy exceeds 38 kcal/mol, excitation of oxygen to either the 1∆g or 1Σg
levels is possible.
• If the energy of the sensitizer is between ~21 and ~25 kcal/mol, it is possible for the process
to be reversible, with 1O2 (1∆g) also transferring energy back to repopulate the triplet state
of the sensitizer.
• If the energy of the sensitizer is in the neighborhood of 37-40 kcal/mol – i.e. matching
reasonably well the energy of 1O2 (1Σg) – the process is not expected to show reversibility.
This is due to the fact that the 1Σg state is too short-lived for reversible transfer to occur
with any significant probability at the concentrations of organic solutes frequently used in
photochemistry. The process is possible but not probable.
10.21
Reaction paths available in the interaction of
sigma singlet oxygen with the substrate RX
krxn
1O 1Σ
2
g
RX
kbypass
Products
3O 3Σ
2
g
(The upper state of singlet oxygen)
kΣ∆
O2 1∆ g
1
10.22
Quenching by oxygen of excited triplet
states. Chemical trapping
In the majority of cases, interaction of oxygen with triplet states involves energy transfer.
In a few examples, notably diketones, a chemical reaction occurs between the triplet state and O 2: addition to
a carbonyl carbon (Schenck mechanism) and subsequent C–C bond cleavage to an acylperoxy and an acyl
radical, which itself is scavenged by molecular oxygen to yield a second acylperoxy radical
Schenck mechanism
3*
O
R
3
O
O2
3
Σg
R
•
O•
R
R
O
R
O
O
O
+
R
3
O O•
O•
O2 3 Σg
O
2
R
O O•
10.23
Efficiency of singlet oxygen, O2(1∆ g), generation:
selecting a good singlet oxygen sensitizer
Φ∆ =
ΦISC • S∆ •
kq [O2]
τ–1 + kq [O2]
S∆ is a true indicator of the 'quality' of a triplet sensitizer in
the generation of singlet oxygen
10.24
Values of S∆ for some singlet oxygen sensitizers
•
The π,π* triplet states of polynuclear aromatics are generally highly efficient, frequently with
S∆ ≥ 0.8. Many other π,π* triplet states are also very efficient.
•
The n,π* triplet states of ketones have low values of S∆, for example for benzophenone in the
0.3-0.4 range. There is a modest increase in the value of S∆ with decreasing triplet energy.
Sensitizer
Solvent
S∆
naphthalene
cyclohexane
1.0
anthracene
benzophenone
benzene
benzene
0.8
0.3
fluorenone
benzene
0.8
tetraphenylporphyrin
Ru(bipy) 3Cl2
benzene
0.58
methanol
0.92
α-Terthienyl
benzene
0.8
phenazine
acridine
benzene
acetonitrile
0.83
0.82
10.25
Parameters to take into consideration in
selecting the singlet oxygen sensitizer and
conditions
• High value of S∆.
• Long triplet lifetime in order to maximize triplet quenching.
• High rate constant for triplet quenching by oxygen (true in almost all cases), and low rate constant
for triplet quenching by substrate.
• High sensitizer stability toward singlet oxygen. Some good (i.e. high S∆) sensitizers may also trap
singlet oxygen efficiently, thus reducing their own usefulness.
• Good spectral properties making possible the selective excitation of the sensitizer (as opposed to
the substrate) with a readily available light source.
• A sensitizer with efficient intersystem crossing under the experimental conditions (that could
include oxygen-assisted intersystem crossing).
• A solvent with good solubility for oxygen (e.g. halogenated) and where singlet oxygen has a long
lifetime.
• Easy sensitizer removal. For synthetic applications it may be desirable to eliminate the sensitizer
at the end of the reaction. Some heterogeneous sensitizers have been developed (e.g. on polymer
particles) that can be readily filtered at the end of the oxidation.
10.26
Reaction of oxygen with reaction intermediates:
Mechanisms and kinetics
Free radical scavenging by oxygen
Carbon-centered free radicals frequently react with oxygen with rate
constants exceeding 109 M -1 s-1 in fluid solution, to yield a peroxyl radical
R• + O 2 → R –OO•
ROO• + RH → R• + ROOH
ANTIOXIDANTS
CH 3
OH
Bu t
Bu t
HO
CH 3
H 3C
CH 3
BHT
O
CH 3
CH 3
CH 3
CH 3
CH 3
Vitamin E
10.27
Biradical scavenging by oxygen
A) Assisted intersystem crossing
3
1
3
O2
Products
H
H
Ph
Ph
OH
3O
2
OH
E.g.
+
+ PhCOCH 3
B) Hydroperoxide formation
3
3
OO•
O2
H
OOH
H
C) Peroxide formation
3
3
O
O2
O
H
H
10.28
Reactions of carbenes with oxygen
[( C6 H5) 2 C:]
3
3
+
O2
→
( C 6H5 ) 2COO
Carbonyl oxide or Criegee intermediate
Making carbenes:
Diazo precursor of carbene with triplet ground state
N
N
1
3
hν
fast
–N2
Diazirine precursor of carbene with singlet ground state
N N
Cl
1
hν
–N2
Cl
10.29