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71 Generation and Reactions of Σg+ and Δ 1
1
g
Oxygen Molecules in Sensitized Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
Photo-Oxygenations AHSAN
U.
KHAN
and
DAVID
R.
KEARNS
University of California, Riverside, Calif.
In the oxygen-induced quenching of triplet state molecules, M , Σg+ and Δ oxygen molecules can be generated by energy transfer from M ; the Σg+ and Δ generation rates are predicted to be a function of E , the triplet state energy of the donor. Experimental evidence for the formation and reaction of Σg+ and Δ is presented. The energetics of the M . . . O complex indicate that the [ M . . . O ] complex may be stable, whereas the [ M . . . O ] complexes are not. These results allow us to resolve some current controversies. The reaction of O with dienes to form endoperoxides and the decomposition of peroxides were investigated theoretically. A procedure, using state correlation diagrams, can be used to predict the reactivities of Δ and Σg+ oxygen toward organic acceptors. 3
1
1
1
G
3
1
1
1
G
t
1
1
G
3
3
2
1
1
2
1
0
2
2
1
1
G
T t has b e c o m e clear recently that the metastable excited singlet states of m o l e c u l a r o x y g e n 2 / a n d A 1
1
play an unsuspected but important
U
role i n n u m e r o u s p h y s i c a l a n d c h e m i c a l transformations w h i c h i n v o l v e the i n t e r a c t i o n of e l e c t r o n i c a l l y e x c i t e d organic molecules w i t h o x y g e n ( 5 , 11,12, 20, 21, 31, 42, 43, 53, 54).
T h e basis f o r m u c h of the c u r r e n t t h i n k
i n g is a p p a r e n t l y d e r i v e d f r o m suggestions b y K a u t s k y (20) the m e c h a n i s m of d y e sensitized photo-oxygenations. e x c i t e d o x y g e n molecules
regarding
H e p r o p o s e d that
( * 2 / o r A ) , generated b y transfer of elec 1
F 7
t r o n i c e x c i t a t i o n energy f r o m the t r i p l e t state of a sensitizer, are the reactive intermediates i n sensitized p h o t o - o x y g e n a t i o n reactions. A l t h o u g h the K a u t s k y m e c h a n i s m appears to account satisfactorily f o r m u c h of the d a t a r e g a r d i n g sensitized p h o t o - o x y g e n a t i o n reactions, i t raises inter esting questions r e g a r d i n g the m e c h a n i s m b y w h i c h m o l e c u l a r o x y g e n 143 Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
144
OXIDATION
quenches
O F ORGANIC
t r i p l e t state molecules, t h e r e l a t i o n b e t w e e n
COMPOUNDS
HI
the quenching
m e c h a n i s m a n d the o x y g e n a t i o n reactions, a n d p e r h a p s most interestingly, the nature of t h e r e a c t i v i t y of singlet state e x c i t e d o x y g e n m o l e c u l e s . H e r e w e s u m m a r i z e t h e results of o u r p r e v i o u s i n v e s t i g a t i o n of t h e q u e n c h i n g of t r i p l e t state m o l e c u l e s b y o x y g e n (21) n e w results o n t h e energetics of t h e i n t e r a c t i o n of 0 cules.
a n d present
some
w i t h organic mole
W e t h e n s h o w h o w these results p r o v i d e n e w i n s i g h t i n t o t h e
n a t u r e of t h e intermediates reactions. Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
2
reactions
i n v o l v e d i n sensitized p h o t o - o x y g e n a t i o n
F i n a l l y , w e c o n s i d e r t h e p r o b l e m of p r e d i c t i n g t h e course of of g r o u n d a n d singlet e x c i t e d state o x y g e n m o l e c u l e s
with
o r g a n i c acceptors s u c h as dienes.
Quenching of Triplet State Molecules by Oxygen Mechanisms.
M o l e c u l a r o x y g e n is k n o w n to q u e n c h r e v e r s i b l y t h e
e m i s s i o n f r o m t r i p l e t state m o l e c u l e s i n w h a t appears to b e a d i f f u s i o n c o n t r o l l e d process
( J , 42, 43).
D u r i n g t h e past 30 years a n u m b e r of
different m e c h a n i s m s h a v e b e e n suggested to account f o r this q u e n c h i n g . ELECTRON
Since m a n y i n o r g a n i c ions q u e n c h t h e
TRANSFER.
fluores
cence a n d phosphorescence of o r g a n i c m o l e c u l e s i n solutions b y electron transfer, W e i s s p r o p o s e d that the q u e n c h i n g a c t i o n of m o l e c u l a r o x y g e n m i g h t s i m i l a r l y b e c a u s e d b y a n electron transfer o f t h e t y p e ( 5 5 ) : 3
where
3
M
X
+ 0 3
2
-> M + 0 " +
2
M is a t r i p l e t state d o n o r m o l e c u l e , 0 3
X
2
is a g r o u n d state o x y g e n
m o l e c u l e , a n d M a n d 0 " are t h e respective ions. It w o u l d a p p e a r that +
2
this m e c h a n i s m f o r 0 considerations
2
q u e n c h i n g c a n b e e l i m i n a t e d s i m p l y b y energetic
( 5 1 ) , a n d m a n y observations o n d y e sensitized p h o t o -
o x y g e n a t i o n ( 5 , 1 1 , 1 2 , 1 3 , 31, 53, 54).
It is possible that w i t h a v e r y g o o d
e l e c t r o n d o n o r i n a p o l a r solvent, t h e electron transfer m e c h a n i s m m i g h t b e o p e r a t i v e , b u t e v e n t h e n other m e c h a n i s m s s h o u l d b e m o r e i m p o r t a n t (21). INHOMOGENEOUS
MAGNETIC
FIELD
EFFECT.
Since a n i n h o m o g e n e o u s
m a g n e t i c field mixes singlet a n d t r i p l e t states, i t is possible f o r a n i n homogeneous
field,
generated b y m o l e c u l a r o x y g e n , to e n h a n c e inter-
system crossing b e t w e e n t h e t r i p l e t state a n d t h e g r o u n d singlet state of a m o l e c u l e (56).
T h i s m e c h a n i s m is, h o w e v e r , inconsistent w i t h t h e obser
v a t i o n that the efficiency w i t h w h i c h v a r i o u s p a r a m a g n e t i c ions q u e n c h t r i p l e t state m o l e c u l e s is n o t c o r r e l a t e d w i t h t h e m a g n e t i c m o m e n t s of the ions (33, 41).
F u r t h e r , a t h e o r e t i c a l e v a l u a t i o n of t h e m a g n i t u d e of
this effect s h o w e d that i t w a s of n e g l i g i b l e i m p o r t a n c e E N H A N C E D INTERSYSTEM CROSSING.
radiative S
0
-» T
1
0
2
(51).
is k n o w n to e n h a n c e r e v e r s i b l y
transitions i n o r g a n i c molecules (8, 9).
A s this e n
h a n c e m e n t requires some sort of o x y g e n - i n d u c e d m i x i n g of singlet a n d
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
Oxygen
A N D KEARNS
Molecules
145
t r i p l e t states, w e w o n d e r e d w h e t h e r s u c h m i x i n g m i g h t not also enhance the n o n - r a d i a t i v e T i -> S t r a n s i t i o n . If, f o r e x a m p l e , the i n t e r a c t i o n w i t h 0
m o l e c u l a r o x y g e n m i x e d the lowest t r i p l e t state of the m o l e c u l e w i t h the g r o u n d state, intersystem crossing w o u l d d e f i n i t e l y be e n h a n c e d . of the k n o w n 0
2
Because
effect o n the S - » T i a b s o r p t i o n , w e felt this q u e n c h i n g 0
mechanism deserved careful evaluation. ENERGY
TRANSFER.
I n t h e K a u t s k y m e c h a n i s m for q u e n c h i n g t r i p l e t
state molecules, e l e c t r o n i c e x c i t a t i o n energy is t r a n s f e r r e d f r o m Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
the o x y g e n molecules, as f o l l o w s + where 0 x
or
x
2
3
0
-» i M + *0 0
2
M
X
to
2
represents a n o x y g e n m o l e c u l e i n either its A 1
2 / state (37 k c a l . ) .
3
(20):
state (22 k c a l . )
g
T h e studies o n sensitized p h o t o - o x y g e n a t i o n
reactions r e q u i r e that w e g i v e this m e c h a n i s m serious c o n s i d e r a t i o n . O f t h e f o u r m e c h a n i s m s m e n t i o n e d above, o n l y the e n h a n c e d inter system crossing a n d the energy transfer m e c h a n i s m s a p p e a r e d to m e r i t further detailed consideration.
I n o u r s t u d y w e w e r e interested i n the
f o l l o w i n g aspects of the q u e n c h i n g process: ( a ) W h a t is t h e i m p o r t a n c e of q u e n c h i n g b y energy transfer r e l a t i v e to e n h a n c e d intersystem crossing? (b) What mechanisms?
factors
c o n t r o l the
relative importance
of
these
two
( c ) A r e the absolute m a g n i t u d e s of the t h e o r e t i c a l q u e n c h i n g rates sufficient to a c c o u n t f o r the e x p e r i m e n t a l observations? B e l o w , w e o u t l i n e o u r t h e o r e t i c a l treatment of the q u e n c h i n g m e c h a nisms (21),
a n d s u m m a r i z e some of the most i m p o r t a n t results.
n e w results o n c a l c u l a t i n g the energetics of the M . . . 0
2
Some
i n t e r a c t i o n are
g i v e n later i n the text. T h e o r y a n d R e s u l t s . T o discuss the a c t u a l c a l c u l a t i o n of the q u e n c h ing
rate constants,
c o n s i d e r first the v a r i o u s states w h i c h arise w h e n
m o l e c u l a r o x y g e n is w e a k l y c o m p l e x e d w i t h a n organic m o l e c u l e , M . Since the o r g a n i c m o l e c u l e s of interest are p r e s u m e d to h a v e g r o u n d singlet states a n d since the g r o u n d state of o x y g e n is a *% ~ state, the y
g r o u n d state of t h e M - 0
2
c o m p l e x is necessarily a t r i p l e t state. A n u m b e r
of e x c i t e d states of the c o m p l e x c a n be f o r m e d b y e x c i t i n g either M or 0 , 2
a n d these v a r i o u s possibilities are i n d i c a t e d i n F i g u r e 1 ( 5 1 ) .
In
a d d i t i o n to those states of the c o m p l e x w h i c h arise f r o m e x c i t a t i o n of o n l y one of the c o m p o n e n t s , there are also charge transfer states p r o d u c e d b y transfer of a n e l e c t r o n f r o m M to 0 . 2
F o r the b e n z e n e - 0
2
complex,
for e x a m p l e , this charge transfer state is b e l i e v e d to l i e a b o u t 100 k c a l . a b o v e the g r o u n d state
(32).
T h e v a r i o u s states of the c o m p l e x w h i c h w e h a v e just d e s c r i b e d are strictly v a l i d o n l y w h e n M a n d 0
2
are c o m p l e t e l y separated f r o m e a c h
other, a n d it is o n l y u n d e r this c o n d i t i o n that they w o u l d represent "sta-
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
146
OXIDATION
O F ORGANIC
t i o n a r y states." W e c a l l these the zero-order states.
COMPOUNDS
III
When M and 0
2
are b r o u g h t together to f o r m a w e a k c o m p l e x , t h e i n t e r m o l e c u l a r inter a c t i o n w i l l cause these zero-order states to b e c o m e nonstationary w i t h respect
to radiationless transitions to n e a r l y degenerate
same m u l t i p l i c i t y . introducing
states of t h e
I n this sense, c o m p l e x f o r m a t i o n m a y b e v i e w e d as
a time-dependent
zero-order states to b e c o m e
perturbation, w h i c h
causes t h e i n i t i a l ,
nonstationary.
I n terms of t h e states i n d i c a t e d i n F i g u r e 1, e n h a n c e d intersystem Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
crossing a c t u a l l y i n v o l v e s a radiationless t r a n s i t i o n b e t w e e n t h e i n i t i a l state r 3
a n d some n e a r l y degenerate, v i b r a t i o n a l l y excited state of
3
E n e r g y transfer, o n t h e other h a n d , i n v o l v e s a t r a n s i t i o n f r o m r 1
either r x
2
or r , 1
1
3
3
r. 0
to
a g a i n w i t h t h e c o n v e r s i o n of t h e r e q u i s i t e a m o u n t of
electronic energy, Ae, i n t o v i b r a t i o n a l e x c i t a t i o n of t h e c o m p l e x . If t h e i n i t i a l state of t h e c o m p l e x is d e s c r i b e d b y a w a v e f u n c t i o n ^ a n d t h e final state of interest b y
t
t h e n t h e rate at w h i c h a radiationless
t r a n s i t i o n f r o m ^ to ^7 occurs is g i v e n b y (45, 46): *=||ffi||s
(i)
w h e r e $1 is t h e c o m p l e t e H a m i l t o n i a n f o r t h e system, p is t h e d e n s i t y of final states w h i c h are n e a r l y degenerate w i t h *$r a n d N is t h e n u m b e r of h
molecules per cc. I n the B o r n - O p p e n h e i m e r approximation, ^ c a n be expressed as a p r o d u c t of t w o f u n c t i o n s \p a n d E s u c h that ^ = ^ S , w h e r e ^ is a f u n c t i o n of a l l electronic coordinates a n d d e p e n d s o n l y p a r a m e t r i c a l l y o n t h e n u c l e a r coordinates, a n d E is t h e v i b r a t i o n a l w a v e -
*r
[M]
4
[M...OJ
Figure 1. Low-lying electronic states of 1:1 complex (M . . . 0 ) between the electronic states of the free molecules and those of the complex. [This figure is similar to one given by Tsubomura and Mulliken ( 5 1 ) ] 2
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
Oxygen
A N D KEARNS
147
Molecules
f u n c t i o n a n d d e p e n d s o n the electronic state of the c o m p l e x a n d the nuclear
coordinates.
With
this
a p p r o x i m a t i o n , the
rate
expression
becomes:
k
where p
el
=
W
=
^
1
^
'
, F
if
• S
2
=
n
2
= ^ ^ F
2
a n d w h e r e n runs over a l l
Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
v i b r a t i o n a l states of the c o m p l e x w h i c h p e r m i t the t o t a l energy +
(2)
v
(electronic
v i b r a t i o n a l ) of the final state to be n e a r l y degenerate i n energy w i t h
the i n i t i a l electronic
state.
is the
F
if
F r a n c k - C o n d o n factor
for
the
complex. T h e above expression is a p p r o p r i a t e for the case w h e r e there is d i r e c t m i x i n g of a p a r t i c u l a r p a i r of i n i t i a l a n d final states ( z e r o o r d e r ) of the c o m p l e x ( 1 8 ) . It m a y h a p p e n , h o w e v e r , that direct m i x i n g of ^ a n d 1/7 is q u i t e s m a l l , i n w h i c h case i n d i r e c t m i x i n g r e s u l t i n g f r o m strong m i x i n g of these t w o states w i t h a c o m m o n t h i r d state, \j/ , m a y be m o r e i m p o r t a n t k
(35, 51).
W h e n i n d i r e c t m i x i n g is i m p o r t a n t , the expression for q u e n c h
i n g rate constant is f o r m a l l y as that g i v e n i n E q u a t i o n 2, except that /3 i e
is r e p l a c e d b y f$'
where
where (E* — E)
is the difference b e t w e e n the electronic energies
eh
k
of
t h e i n i t i a l state ^ a n d the i n t e r m e d i a t e state \j/ . k
W i t h this f o r m u l a t i o n of the p r o b l e m the c a l c u l a t i o n of q u e n c h i n g rate constants is r e d u c e d to e v a l u a t i n g v a r i o u s m a t r i x elements of the t y p e , of
F r a n c k - C o n d o n factors
for the
complex,
a n d of
the
T h e p r o c e d u r e u s e d to evaluate
the
d e n s i t y of states factor p. EVALUATION
OF
p
et
A N D
fi'
eh
electronic m a t r i x elements has b e e n d e s c r i b e d (21).
Before presenting
the results, w e note the v a r i o u s c o m p l e x geometries c o n s i d e r e d , a n d these are i n d i c a t e d i n F i g u r e 2. T h e results are s u m m a r i z e d i n T a b l e I. F r o m these results, several i m p o r t a n t conclusions c a n b e d r a w n . F i r s t , d i r e c t m i x i n g m a t r i x elements are s m a l l c o m p a r e d w i t h those f o r i n d i r e c t m i x i n g , i n v o l v i n g a n i n t e r m e d i a t e charge transfer state. S e c o n d l y , a n d p e r h a p s m o r e i m p o r t a n t l y , the m a t r i x elements for i n d i r e c t m i x i n g are the same for a l l three q u e n c h i n g m e c h a n i s m s .
Since w e expect
d e n s i t y of final states to be the same for a l l three cases (45, 46), p r i m a r i l y the F r a n c k - C o n d o n factor w h i c h
u l t i m a t e l y determines
relative i m p o r t a n c e of the three q u e n c h i n g m e c h a n i s m s .
the
It is this factor
w h i c h w e n o w consider.
American Chemical Society. Library 1155 16th St.,
the it is
N.W.
Mayo; Oxidation of Organic Washington O XCompounds . 20036 Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
148
OXIDATION
O F ORGANIC
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a Figure
2.
COMPOUNDS
m
b
Schematic
representation M ...0
2
of two possible
geometries for
the
complex
(a) : The atomic orbitals of O and the two adjacent orbitals of carbon atoms in M are shown approaching each other face-to-face. R is the distance between the planes containing the molecular axis of M and O (b) : Orbitals of O and M are shown for an end-on approach of the molecules. Here R is distance between carbon and oxygen atoms t
t
t
FRANCK-CONDON
0
2
I n t h e l i m i t of w e a k c o u p l i n g o f M a n d
FACTORS.
t h e v i b r a t i o n a l w a v e f u n c t i o n f o r t h e c o m p l e x m a y b e w r i t t e n as a
p r o d u c t of t h e v i b r a t i o n a l w a v e f u n c t i o n s f o r the t w o c o m p o n e n t s : B * = M(*,n)xo,(M) fc
w h e r e XM.(k,n)
(4)
X
is the v i b r a t i o n a l w a v e f u n c t i o n f o r M w h e n the M . . . 0
c o m p l e x is i n some v i b r o n i c state s p e c i f i e d b y k a n d n a n d x o ( ^ ) * 2
corresponding vibrational wavefunction for 0 .
n
s
t
n
2
e
W i t h this a p p r o x i m a t i o n ,
2
the F r a n c k - C o n d o n f a c t o r f o r t h e c o m p l e x b e c o m e s |xM(/,n)>2<
Fif = %< u(i,o) n X
In
X02
( M > ) |xo (/>")>
the various quenching mechanisms
2
2
(5)
w h i c h w e are c o n s i d e r i n g ,
the t o t a l energy of t h e system is c o n s e r v e d b y c o n v e r t i n g t h e p r o p e r a m o u n t of electronic e x c i t a t i o n energy i n t o v i b r a t i o n a l e x c i t a t i o n of M
Table I. Direct
Electronic Matrix Elements for Direct and Indirect Mixing"
Mixing -
P(*T
3
3
2 cm."
r) 0
^Ors - r ) /^rs x
Indirect Mixing
2
v i a a Charge Transfer 0'( r 3
3
-
3
r ) 0
PVTn ~ ^2) PVT, ~ ^ x )
1
o
2
State 20 c m . - i
20 20
Values calculated for complex geometry a with R = 4A or with geometry b but with R = 3A (see Figure 2 ) . a
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
and
0 .
Oxygen
A N D KEARNS
A c t u a l l y to d e t e r m i n e F ,
2
149
Molecules
w e m u s t d e c i d e just h o w a g i v e n
if
a m o u n t of v i b r a t i o n a l energy Ac w i l l be p a r t i t i o n e d b e t w e e n M a n d 0 . 2
F r o m studies of e m i s s i o n f r o m e l e c t r o n i c a l l y e x c i t e d o x y g e n m o l e cules, w e k n o w that the o x y g e n F r a n c k - C o n d o n factor r a p i d l y d i m i n i s h e s ( f a c t o r of >
100) w h e n there is a change of 1 or 2 q u a n t a i n the v i b r a
t i o n a l state of 0
2
(26,
36).
W e , therefore, a n t i c i p a t e that most of the v i b r a t i o n a l e x c i t a t i o n w i l l be d e p o s i t e d i n M rather t h a n i n 0 Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
plex might somewhat v a r i a t i o n of F
if
2
( f o r m a t i o n of a strong M - 0
relax this "selection
rule").
com
2
Consequently,
the
w i t h Ac w i l l b e almost e n t i r e l y c o n d i t i o n e d b y v a r i a t i o n s
i n / , the F r a n c k - C o n d o n factor for the m o l e c u l e .
Fortunately, semi-
m
e m p i r i c a l f o r m u l a s for e v a l u a t i n g f
m
h a v e b e e n d e v e l o p e d (19,
24,
48),
a n d one s u c h expression is g i v e n i n E q u a t i o n 6. / = 0.15 e x p [ - (Ac - 4 0 0 0 ) / l l , 5 0 0 ]
(6)
m
w h e r e Ac is n o w the a m o u n t of electronic energy
( i n c m . " ) w h i c h is 1
c o n v e r t e d i n t o v i b r a t i o n a l e x c i t a t i o n of M . Regardless of exact m a g n i tudes of f
and f ,
M
it is e v i d e n t f r o m the a b o v e d i s c u s s i o n that the process
02
w h i c h i n v o l v e s the c o n v e r s i o n of the least a m o u n t of electronic into v i b r a t i o n a l energy w i l l b e the
d o m i n a n t one.
Since
energy
intersystem
crossing a l w a y s i n v o l v e s the c o n v e r s i o n of the largest a m o u n t of elec t r o n i c e x c i t a t i o n energy i n t o v i b r a t i o n a l energy, w e c o n c l u d e that energy transfer to o x y g e n is the major q u e n c h i n g process.
T h e s e conclusions d o
not d e p e n d u p o n exact n u m e r i c a l e v a l u a t i o n of m a t r i x elements a n d other factors, a n d , therefore, m a y b e c o n s i d e r e d to b e strong p r e d i c t i o n s . MAGNITUDES
OF
T H E RATE
CONSTANTS.
T h e electronic factors
and
the F r a n c k - C o n d o n factors h a v e a l r e a d y b e e n e v a l u a t e d above, a n d a l l that r e m a i n s is to o b t a i n a v a l u e f o r the d e n s i t y of final states. A s i n o u r earlier p a p e r , w e use R o b i n s o n ' s v a l u e of p/N
1 c m . (45).
=
q u e n c h i n g of a t r i p l e t state m o l e c u l e w i t h a n energy E
t
=
F o r the
60 k c a l . , w e
o b t a i n the f o l l o w i n g u n i m o l e c u l a r r a t e constants: ( ^ a " ^ ^ ) 10 /sec. (energy transfer to !$/) 1
12
(^3—^ri)
1 0 / s e c . (energy transfer to
(r
10 /sec. (enhanced intrasystem crossing).
3
3
—> r ) 3
0
n
A)
x
ff
9
W h i l e the exact m a g n i t u d e s of these rate constants are not expected to b e p a r t i c u l a r l y accurate, the relative m a g n i t u d e s s h o u l d be r e l i a b l e . Since the rate constants f o r energy transfer are o n the o r d e r of
10 1 1
1 0 / s e c , o u r calculations easily a c c o u n t for the e x p e r i m e n t a l o b s e r v a t i o n 12
that the 0 0
2
2
q u e n c h i n g of t r i p l e t states is d i f f u s i o n c o n t r o l l e d .
(If M and
o n l y f o r m c o l l i s i o n complexes, the rate constant for d i s s o c i a t i o n w o u l d
b e o n the o r d e r of 1 0 / s e c , m u c h smaller t h a n 1 0 - 1 0 / s e c . )
The
a b o v e results answer the three questions w e p o s e d earlier.
now
lo
11
12
We
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
150
OXIDATION
OF
ORGANIC
COMPOUNDS
III
c o n s i d e r other aspects of o u r results w h i c h seem to b e r e l e v a n t to inter preting sensitized photo-oxygenations. Variation i n the X /A 1
Sensitizer.
+ 1
g
Ratio w i t h Triplet
g
State E n e r g y o f t h e
T h e c a l c u l a t i o n s i n d i c a t e that q u e n c h i n g of h i g h e n e r g y t r i p l e t
state m o l e c u l e s (E
> 50 k c a l . ) generates m o r e 2 / t h a n A 1
t
a f a c t o r of 10.
T h i s large l / A 1
+ 1
tg
1
g
by roughly
r a t i o arises s i m p l y f r o m the f a c t that
g
less electronic e n e r g y is c o n v e r t e d i n t o v i b r a t i o n a l e n e r g y w h e n the state is p r o d u c e d t h a n w h e n the A 1
/ 7
state is generated.
1
2
+
the expression u s e d to e v a l u a t e f of the b e h a v i o r of the % / A 1
+ 1
g
g
M
generated. >
E
t
37 k c a l . )
breaks d o w n , a n d to o b t a i n some i d e a
r a t i o w i t h sensitizers i n this r a n g e w e h a v e
u s e d e x p e r i m e n t a l observations o n t r i p l e t - t r i p l e t e n e r g y transfer.
Porter
a n d W i l k i n s o n f o u n d that as the t r i p l e t state e n e r g y of a sensitizer a p proaches
(say, w i t h i n 5 k c a l . ) the e n e r g y of the acceptor, the
p r o b a b i l i t y is s i g n i f i c a n t l y r e d u c e d (40).
transfer
If a s i m i l a r result w e r e o b t a i n e d
w i t h m o l e c u l a r o x y g e n as the acceptor, the rate of 2 / g e n e r a t i o n w o u l d x
a c t u a l l y decrease as E
t
a p p r o a c h e s 37 k c a l .
o n the other h a n d , is e x p e c t e d to increase. effects is that the X / A 1
+ 1
g
g
T h e rate of g e n e r a t i n g
A,
x
g
T h e net result of these t w o
r a t i o w o u l d decrease s m o o t h l y to zero as the
sensitizer t r i p l e t state energy is decreased f r o m 45 to 37 k c a l . T h e c u r v e i n F i g u r e 3a shows the e x p e c t e d v a r i a t i o n i n the %// A 1
1
ratio w i t h
g
E. t
Energetics of Interaction of Organic Molecules with 0
2
I n the t h e o r y o u t l i n e d a b o v e w e a s s u m e d a w e a k c o l l i s i o n c o m p l e x b e t w e e n the t r i p l e t state of the o r g a n i c m o l e c u l e a n d the t r i p l e t state of o x y g e n , a n d w e w e r e a b l e to a c c o u n t f o r the rate of q u e n c h i n g of t r i p l e t states b y 0
2
o n this basis.
F u r t h e r i n v e s t i g a t i o n i n t o the n a t u r e of this
i n t e r a c t i o n i n d i c a t e s that it is not necessarily w e a k
(7).
T h e c a l c u l a t i o n of the rate of q u e n c h i n g of t r i p l e t states b y 0
2
was
essentially a n e v a l u a t i o n of the m a g n i t u d e of i n t e r a c t i o n b e t w e e n
two
different states of the c o m p l e x . D e t e r m i n a t i o n of p o t e n t i a l e n e r g y curves f o r the M - 0
2
i n t e r a c t i o n , r e q u i r e s a n e v a l u a t i o n of the t o t a l e n e r g y of
the c o m p l e x as a f u n c t i o n of R , the i n t e r m o l e c u l a r distance. A c o n s i d e r a t i o n of the v a r i o u s states of the c o m p l e x at r e l a t i v e l y large i n t e r m o l e c u l a r distances (R
> 4 A ) , as g i v e n above, is the starting
p o i n t of the present i n v e s t i g a t i o n . T h e energies of these states are w e l l k n o w n a n d are i n d i c a t e d , i n o r d e r of i n c r e a s i n g energy, o n the r i g h t side of F i g u r e 4
(21).
S i n c e w e h a v e e x p e r i m e n t a l i n f o r m a t i o n r e g a r d i n g the g r o u n d state of the c o m p l e x ( 3 , 6, 7 ) , a n d since it w o u l d be i m p o s s i b l e to c a l c u l a t e
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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71.
KHAN AND KEARNS
Figure
3.
Oxygen
151
Molecules
(a) Predicted variation in the 'SgV'Ag ratio with sensitizer triplet state energy
(b) Observed variation in the product distribution from the dye sensitized photo-oxygenation of cholest-4-en-3/3-ol with the triplet state energy of the sensitizer (37, 38). 1, Triphenylene; 2, Fluorescein; 3, Eosin Y; 4, Acridine Orange; 5, Sulfurhodamine B; 6, Erythrosin B; 7, Rose Bengal; 8, Hematoporphyrin; 9, Chlorine ; 10, Methylene blue (i
r e l i a b l y t h e steeply r i s i n g r e p u l s i v e p o r t i o n of the p o t e n t i a l energy c u r v e , w e h a v e e v a l u a t e d t h e p o t e n t i a l energy curves f o r the excited states w i t h reference to t h e g r o u n d states.
I n d o i n g so w e have assumed that t h e
strongly r e p u l s i v e i n t e r m o l e c u l a r interactions at s m a l l R are the same for a l l states of the c o m p l e x . W e n o w i n d i c a t e i n m o r e d e t a i l the p r o c e d u r e f o r c o n s t r u c t i n g t h e p o t e n t i a l energy curves f o r t h e states of the c o m p l e x s h o w n i n F i g u r e 4. G r o u n d State. T h e heat of f o r m a t i o n of a c o m p l e x b e t w e e n a n o n i o n i c , g r o u n d state o r g a n i c m o l e c u l e a n d a g r o u n d state o x y g e n is v e r y s m a l l , a n d these complexes are dissociative at r o o m t e m p e r a t u r e ( 3 , T h i s i m p l i e s that as M a n d 0
2
6,7).
a p p r o a c h each other, or R [ M . . . 0 ] d e 2
creases, the p o t e n t i a l energy c u r v e goes t h r o u g h a s m a l l or n e g l i g i b l e m i n i m u m before it starts to rise, a n d the g r o u n d state c u r v e i n F i g u r e 4 is d r a w n a c c o r d i n g l y .
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
152
OXIDATION
OF
ORGANIC
COMPOUNDS
HI
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E
R [M
—0 ] 2
Figure 4. Potential energy curves for the different states of the (M . . . 0 ) complex. Dashed curves represent the energy of the respective states of the complex in the absence of configuration interaction. Continuous curves indicate the result of including configuration interaction 2
E x c i t e d States. If w e neglect c o n f i g u r a t i o n i n t e r a c t i o n , the e x c i t e d states T i , r , a n d r 2
3
behave
essentially l i k e the g r o u n d state a n d are
s h i f t e d r e l a t i v e to one another o n l y b y a r e p u l s i v e i n t e r m o l e c u l a r
ex
change integral.
are
Since t y p i c a l values f o r these exchange integrals
of the o r d e r of 1 - 1 0
cm.
1
for i n t e r m o l e c u l a r separations d o w n to 3 A
t h e i r effect o n the p o t e n t i a l energy curves is n e g l i g i b l e . W e also
(21),
neglect the l i f t i n g of the degeneracy of the t w o A 1
a n d r'i 1
states of o x y g e n ,
g
w h i c h occurs as R becomes smaller since w e expect the
s p l i t t i n g to be s m a l l
A — A'
1
u
1
g
(16).
T h e charge transfer ( C T ) states, r , CT
behave quite differently f r o m
the n o n p o l a r states. B e c a u s e of the strong c o u l o m b i c i n t e r a c t i o n b e t w e e n the n e g a t i v e l y c h a r g e d o x y g e n a n d the p o s i t i v e M i o n , the e n e r g y C T state varies a p p r o x i m a t e l y as 1/R. b y d o t t e d curves. figuration
A s the r
state closely approaches t h e r
ct
3
state, c o n
i n t e r a c t i o n b e t w e e n the singlet components, a n d b e t w e e n
t r i p l e t components of r
c t
and r
3
1
3
the
c a n no longer b e i g n o r e d .
A s a result of this c o n f i g u r a t i o n i n t e r a c t i o n , r the singlet c o m p o n e n t of r
of
T h i s is i n d i c a t e d i n the F i g u r e 4
ct
is d e s t a b i l i z e d , b u t
is s t a b i l i z e d b y at least 1 k c a l , a n d p r o b a b l y
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
more.
Oxygen
A N D KEARNS
Molecules
153
A s i m i l a r i n t e r a c t i o n occurs b e t w e e n the respective t r i p l e t c o m
ponents of r
c t
and r
a s t a b i l i z a t i o n of r 3
a n d results i n a d e s t a b i l i z a t i o n of the r 3
3
( s o l i d lines i n F i g u r e 4 ) .
3
is also present b e t w e e n
and
r
1
and between
2
c t
state a n d
Configuration interaction 1
r
a n d ^Ti b u t w i t h
c t
decreased effectiveness because of the increased energy differences tween
the
interacting
states.
The
s t a b i l i z a t i o n of
'r
1 8
is,
3
expected to be larger t h a n the s t a b i l i z a t i o n of either * r or r 1
2
1
(compare
the difference b e t w e e n the s o l i d a n d d o t t e d curves i n F i g u r e 4 ) . of the absence of a n y n e a r b y q u i n t e t state, the r Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
5
3
be
therefore, Because
state is u n p e r t u r b e d b y
c o n f i g u r a t i o n i n t e r a c t i o n a n d w i l l p r o b a b l y b e h a v e l i k e the g r o u n d state. T o s u m m a r i z e , the r x
2
t i a l l y dissociative, whereas T h e complex
1 , 3
and ^ 1 , 3
r
states a n d the g r o u n d state are essen e x h i b i t significant p o t e n t i a l m i n i m a .
3
( M . . . 0 ) 3
3
m a y a c t u a l l y b e m o r e stable t h a n w e
2
o r i g i n a l l y assumed, i n w h i c h case the F r a n c k - C o n d o n factors
for
c o m p l e x m a y h a v e to be m o d i f i e d to i n c l u d e v i b r a t i o n a l excitation of
0
2
as i n d i c a t e d p r e v i o u s l y .
A l t h o u g h these considerations
nature of t h e [ M . . . 0 ]
c o m p l e x d o not alter a n y of the major c o n c l u
2
regarding
the the
sions r e g a r d i n g the q u e n c h i n g m e c h a n i s m , they are i m p o r t a n t i n inter p r e t i n g sensitized p h o t o - o x i d a t i o n reactions.
Relation of Theoretical Quenching Results to Interpretation of Sensitized Photo-Oxygenations W e n o w discuss sensitized p h o t o - o x y g e n a t i o n reactions how
some of o u r t h e o r e t i c a l results c l a r i f y some c u r r e n t
and show
controversies
a n d g i v e us n e w insight into the r e a c t i o n m e c h a n i s m s . Comparison of Terenin-Schenck Mechanism a n d the Singlet Mechanism.
T e r e n i n (49, 50),
a n d later S c h e n c k (47),
Oxygen
p r o p o s e d the f o l
l o w i n g m e c h a n i s m f o r d y e sensitized p h o t o - o x y g e n a t i o n reactions: h
v
M
1
3
M
1
+ 0 3
X
[M . . . 0 ] 2
2
+ A
W h e r e the m o l o x i d e , [ M . . . 0 ] , 2
->
M
3
X
—> [ M . . . 0 ] 2
—>
X
M
0
+ A0
2
is the reactive i n t e r m e d i a t e
which
transfers o x y g e n to the acceptor, A , to f o r m p r o d u c t s A 0 . T h i s m e c h a 2
n i s m , w h i c h was g i v e n strong s u p p o r t b y the extensive w o r k of S c h e n c k and
co-workers (15),
was u n t i l recently the generally a c c e p t e d
n i s m f o r sensitized p h o t o - o x y g e n a t i o n reactions.
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
mecha
154
OXIDATION
OF
ORGANIC
COMPOUNDS
III
A n alternative scheme, w h i c h w e m e n t i o n e d earlier, is that p r o p o s e d b y K a u t s k y a n d c a n be w r i t t e n as f o l l o w s
3
M
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1
In
+ 0 3
X
0
2
-> %
+ A —> A 0
2
+ ^
(20):
(%
or *A„)
+
2
the K a u t s k y scheme the reactive i n t e r m e d i a t e is a n e x c i t e d singlet
state o x y g e n m o l e c u l e ( 2 / o r A ) 1
1
g
i n s t e a d of the m o l o x i d e .
Although
m u c h of the recent w o r k i n this field has c o n c e n t r a t e d o n d e t e r m i n i n g w h i c h of these, a p p a r e n t l y m u t u a l l y exclusive, m e c h a n i s m s is correct, o u r c a l c u l a t i o n o n the energetics of the [ M . . . 0 ]
i n t e r a c t i o n , suggest that
2
the T e r e n i n - S c h e n c k m e c h a n i s m c a n a c t u a l l y be v i e w e d as a n i n c o m p l e t e v e r s i o n of the singlet o x y g e n m e c h a n i s m .
T h e m o l o x i d e of T e r e n i n -
S c h e n c k c a n b e i d e n t i f i e d w i t h the c o m p l e x
1
( M . . . 0 )
3
3
3
which we
2
discussed p r e v i o u s l y . T h i s e x c i t e d state of the c o m p l e x is expected to be v e r y short l i v e d ( < 1 0 "
10
sec.) because of radiationless transitions to t h e
l o w e r l y i n g dissociative states of the c o m p l e x r 1
or A 1
2
and r , 1
yielding
i
1
2/
oxygerr. It is these species w h i c h go o n to react w i t h acceptors as
g
proposed by Kautsky. Evidence f o r the Participation of Both % 1
g
+
and A 1
Oxygen.
g
of the conclusions of o u r i n v e s t i g a t i o n w a s that b o t h * 2 / a n d A 1
g
One
oxygen
c o u l d be generated i n the q u e n c h i n g of t r i p l e t state molecules a n d that the 2 / / A , ratio d e p e n d s u p o n E . 1
1
i
t
T h e i m p o r t a n c e of this i n i n t e r p r e t i n g
t r i p l e t sensitized p h o t o - o x y g e n a t i o n reactions lies i n the p o s s i b i l i t y that %
and A
+
1
g
m i g h t e x h i b i t different c h e m i c a l reactivities. If this is true,
the d i s t r i b u t i o n of p r o d u c t s f r o m a sensitized p h o t o - o x y g e n a t i o n m i g h t v a r y s i g n i f i c a n t l y w i t h the t r i p l e t state energy of the sensitizer. U n t i l n o w it has u s u a l l y b e e n a s s u m e d that most singlet o x y g e n reactions c o u l d b e a t t r i b u t e d to reactions of A 1
g
(11,
12, 13, 39, 54).
far as w e are aware, the o n l y p u b l i s h e d attempt to
find
e v i d e n c e f o r a difference i n t h e r e a c t i v i t y of 2 / a n d A x
results (10).
x
g
As
experimental
gave n e g a t i v e
W e n o w discuss some d a t a o n the sensitized p h o t o - o x y g e n a
t i o n of cholesterol, w h i c h w e b e l i e v e p r o v i d e s e v i d e n c e for i n v o l v e m e n t of b o t h %
+
and
A.
x
g
T h e d y e sensitized p h o t o - o x y g e n a t i o n of cholest-4-en-3/?-ol only two products (II)
(37,
38),
a n enone
(III)
a n d an epoxy
yields ketone
as i l l u s t r a t e d . T h e anomalous aspect of these results is that
the
enone/epoxy ketone ratio varies b y a factor of 150 d e p e n d i n g u p o n the c h o i c e of sensitizer. I n l i g h t of o u r p r e d i c t i o n r e g a r d i n g the v a r i a t i o n i n the % / A 1
+ 1
g
g
ratio w i t h E, t
w e w e r e interested i n w h e t h e r there w a s a
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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71.
KHAN
Oxygen
A N D KEARNS
Molecules
155
1
HE
s i m i l a r c o r r e l a t i o n b e t w e e n the p r o d u c t d i s t r i b u t i o n a n d E
t
T h e results are presented i n F i g u r e 3b.
(22,
23).
C o m p a r i s o n of C u r v e s a a n d b
shows that the v a r i a t i o n i n the I I I / I I ratio c a n be a c c o u n t e d for i n terms of a v a r i a t i o n i n the 2 / / A 1
T
1
J 7
ratio w i t h E . t
W i t h h i g h e n e r g y sensitizers
2 / is the major species i n i t i a l l y generated, a n d this species a p p a r e n t l y
reacts to p r o d u c e m a i n l y enone ( p e r h a p s b y a h y d r o g e n a t o m abstrac tion).
Since some A 1
is also generated b y h i g h energy sensitizer, it is
!J
possible that 2 / reacts to p r o d u c e e x c l u s i v e l y enone. 1
tizers generate o n l y A 1
g
L o w energy sensi
oxygen, a n d this species a p p a r e n t l y reacts w i t h I
to f o r m a n i n t e r m e d i a t e of t y p e I V , w h i c h t h e n decomposes
to give
m a i n l y the epoxy ketone. M o s t of the sensitizers w e u s e d are k n o w n to abstract
hydrogen
atoms f r o m g o o d donors l i k e p h e n o l , a n d it was possible that some of the enone p r o d u c t i o n m i g h t arise f r o m h y d r o g e n a t o m a b s t r a c t i o n b y
the
excited d y e . T h i s p o s s i b i l i t y appears to b e e l i m i n a t e d b y the o b s e r v a t i o n that w i t h eosin, for example, there was no change i n the p r o d u c t d i s t r i b u t i o n w h e n the o x y g e n pressure was v a r i e d f r o m 0.2 to 10 a t m . results o b t a i n e d u s i n g t r i p h e n y l e n e (E
t
=
The
67 k c a l . ) as a sensitizer also
s u p p o r t this c o n c l u s i o n . O f a l l the sensitizers used, t r i p h e n y l e n e is least susceptible to p h o t o r e d u c t i o n , a n d yet w h e n u s e d as a sensitizer it still gave
the 3:1
enone/epoxy
ketone
ratio expected
for a h i g h
energy
sensitizer. Since the l i f e t i m e of 2 / i n s o l u t i o n is m u c h shorter t h a n A , one 1
1
/ y
m i g h t expect that a r e d u c t i o n i n the c o n c e n t r a t i o n of the acceptor
(I)
w o u l d f a v o r reactions i n v o l v i n g A . T h e results of a n e x p e r i m e n t a l test 1
/ /
of this p o s s i b i l i t y are presented i n T a b l e II.
Since m e t h y l e n e b l u e is a
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
156
OXIDATION
OF ORGANIC
l o w energy sensitizer c a p a b l e of g e n e r a t i n g o n l y A 1
COMPOUNDS
in
oxygen, no concen
g
t r a t i o n effect o n t h e p r o d u c t d i s t r i b u t i o n is expected, a n d little, i f a n y , is observed fluorescein 10" M 4
experimentally. and
However, with
the h i g h
energy
sensitizers
eosin, a r e d u c t i o n i n t h e c o n c e n t r a t i o n of I f r o m 10" t o 2
causes a decrease i n t h e enone/epoxy ketone ratio f r o m 3/1 to
a b o u t 1/4. I f w e assume a s i m p l e r e a c t i o n m e c h a n i s m i n w h i c h m o l e c u l e s are lost, either b y d e c a y to A 1
1
2/
or b y a d i f f u s i o n - c o n t r o l l e d r e
g
a c t i o n w i t h I, o u r c o n c e n t r a t i o n d a t a w o u l d r e q u i r e a 2 ^ l i f e t i m e i n 1
+
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s o l u t i o n w h i c h is greater t h a n 1 0 ° sec. T h i s appears to b e inconsistent w i t h t h e recent observations of O g r y z l o o n t h e q u e n c h i n g of 2 , i n the 1
gas phase
(39).
i
+
W i t h o u t f u r t h e r i n v e s t i g a t i o n of t h e kinetics of t h e
reactions of I w e c a n o n l y suggest that t h e a c t u a l r e a c t i o n m e c h a n i s m is m o r e c o m p l i c a t e d t h a n t h e s i m p l e o n e suggested above. Table II. P r o d u c t V a r i a t i o n w i t h Concentration o f Substrate i n the Photosensitized Photo-Oxygenation o f Cholest-4-en-3/?-ol (I) a
Sensitizer
Concentration 1.3 X l O " 1.0 X 10"
Methylene Fluorescein
1.3 X 10" 1.0 X 10"
Eosin Y
Lumichrome Riboflavin
of P
1.3 8.2 4.0 2.5 1.6 8.3 f
2 4
X 10" X 10r4 2
X X X X
10" 10" 10" 10"> 4
4
4
r
1.0 X 10" 1.3 5.0 1.0 7.5
2 4
X X X X
10" 10" 10" 10"
2
2 4 4 5
Ratio of
III/II
e
1/3* 1/4-1/5 3/1 1/3 3/1
e
d
1/1 1/4 1/4 1/4 1/5 6/1 30/1 * 6/1 1/2 1/5
° Photo-oxygenation conditions were essentially the same as those used by Nickon, except that a 500-watt tungsten lamp was used, and typical runs were 24 hours long. Concentration in moles/liter. Ratios determined by chromatography and infrared spectroscopy. * This value also reported by Nickon (37, 38). Data of Nickon (37,38). Three-day irradiation. b 0
e
1
Prediction of the Reactivity of Ground State and Singlet Excited Molecular Oxygen with Organic Molecules T h e v a l i d i t y o f t h e K a u t s k y singlet o x y g e n m e c h a n i s m seems t o b e c o n f i r m e d b o t h t h e o r e t i c a l l y ( 2 1 ) a n d e x p e r i m e n t a l l y ( 5 , 11, 12, 13, 31,
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
53, 54),
Oxygen
A N D KEARNS
157
Molecules
a n d w e h a v e n o w b e e n a b l e to i n c o r p o r a t e c e r t a i n aspects of
the " T e r e n i n - S c h e n c k " m e c h a n i s m w i t h i n the f r a m e w o r k of the singlet o x y g e n m e c h a n i s m (27).
I n v i e w of these interesting results w e t h o u g h t
it w o r t h w h i l e to explore t h e o r e t i c a l l y t h e reactions
between
organic
m o l e c u l e s a n d g r o u n d a n d e x c i t e d state o x y g e n m o l e c u l e s a n d the reverse process, d e c o m p o s i t i o n of the o x y g e n a t i o n p r o d u c t . F o r this i n v e s t i g a t i o n w e u t i l i z e the i n f o r m a t i o n i n h e r e n t i n the s y m m e t r y properties of
the
orbitals a n d states of the " r e a c t a n t " a n d " p r o d u c t " m o l e c u l e s to construct Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 15, 2017 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch071
c o r r e l a t i o n d i a g r a m s f o r the system. B y u s i n g s u c h d i a g r a m s w e are able to m a k e p r e d i c t i o n s r e g a r d i n g n a t u r e a n d course of reactions
(28,
29).
R e a c t i o n s o f S i n g l e t O x y g e n w i t h C o n j u g a t e d Dienes. T h e r m o c h e m i c a l a n d spectroscopic d a t a c a n b e c o m b i n e d w i t h o r b i t a l c o r r e l a t i o n d i a g r a m s (17)
to construct state c o r r e l a t i o n d i a g r a m s (34)
of singlet o x y g e n w i t h conjugated dienes.
f o r reactions
W i t h these state c o r r e l a t i o n
d i a g r a m s w e c a n a c c o u n t f o r the e x p e r i m e n t a l observations a n d d e r i v e i n f o r m a t i o n o n the factors w h i c h c o n t r o l the r e l a t i v e reactives of and A x
g
i n other c o n c e r t e d a d d i t i o n reactions.
features of
our approach b y
*2/
W e illustrate the essential
c o n s i d e r i n g the
concerted
addition
of
m o l e c u l a r o x y g e n to a conjugated d i e n e (e.g., c y c l o p e n t a d i e n e , 1,3-cyclohexadiene, f u r a n ) to f o r m a n e n d o p e r o x i d e ( g e o m e t r y of the t r a n s i t i o n state i n d i c a t e d i n F i g u r e s 5 a n d 6)
(15).
T h e state c o r r e l a t i o n d i a g r a m
w h i c h forms the basis for u n d e r s t a n d i n g this r e a c t i o n is s h o w n i n F i g u r e 6 a n d w a s c o n s t r u c t e d as f o l l o w s .
First, thermochemical data
(bond
strengths, resonance, energies, etc.) w e r e u s e d to scale the g r o u n d state energy of the reactants r e l a t i v e to the g r o u n d state of the p r o d u c t . S p e c t r o s c o p i c d a t a w e r e t h e n u s e d to locate the e x c i t e d states of the reactants ( o x y g e n + w i s e f o r the p r o d u c t .
d i e n e ) w i t h respect to t h e i r g r o u n d state, a n d l i k e F i n a l l y , s y m m e t r y a n d s p i n selection rules w e r e
u s e d to correlate states of the reactants w i t h those of the p r o d u c t a n d t h e r e b y construct a " p r i m i t i v e " state c o r r e l a t i o n d i a g r a m ( d o t t e d curves, Figure 6).
A l t h o u g h this shows w h i c h i n i t i a l state of the reactants u l t i
m a t e l y correlate w i t h a p a r t i c u l a r state of the p r o d u c t , i t does not i n d i c a t e exactly how
the states correlate.
B a r r i e r s a r i s i n g f r o m i n t e n d e d crossing
of levels are, for e x a m p l e , not i n d i c a t e d (17).
T o o b t a i n this d e t a i l e d
i n f o r m a t i o n , w e u s e d the o r b i t a l c o r r e l a t i o n d i a g r a m presented s c h e m a t i c a l l y i n F i g u r e 5. T h e m o l e c u l a r orbitals o n the left side of the d i a g r a m are those of the w e a k l y c o m p l e x e d d i e n e a n d 0 . 2
are the m o l e c u l a r orbitals of 0
2
T h e o- , 00
00
fx,
T, y
tt *, x
and
TT^*
a n d ^ i , ^ , i/% a n d ^ are the p i - m o l e c u 2
4
lar orbitals of the d i e n e , a n d the n o d a l p a t t e r n of e a c h o r b i t a l is s c h e m a t i cally indicated i n the
figure.
S, S', a n d A , a n d A ' i n d i c a t e w h e t h e r the
orbitals are s y m m e t r i c or a n t i s y m m e t r i c w i t h respect to the p l a n e of s y m m e t r y .
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
assumed
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158
OXIDATION
O F ORGANIC
COMPOUNDS
H I
Figure 5. Orbital correlation diagram for the concerted addition of molecular oxygen to a diene. The molecular orbitals of 0 (or , TT , ?r , TT *, 7r *, and (T *) and of the diene (d/ ana ^ J are given on the left, and the molecular orbitals of the product peroxide are on the right. The orbitals are arranged vertically in order of increasing energy, and nodal patterns of the orbitals are indicated schematically 2
00
x
y
X
y
00
l9
T h e m o l e c u l a r orbitals of t h e e n d o p e r o x i d e a r e i n d i c a t e d o n t h e r i g h t side of F i g u r e 5, w i t h t h e orbitals a r r a n g e d i n t h e o r d e r of energy a n d a g a i n classified as s y m m e t r i c o r a n t i s y m m e t r i c . B e c a u s e of t h e d e g e n e r a c y o f the 7r* orbitals i n t h e o x y g e n m o l e c u l e , it is s i m p l e r , f o r purposes of d i s c u s s i o n , to c o n s i d e r first t h e r e g e n e r a t i o n of t h e d i e n e a n d m o l e c u l a r o x y g e n f r o m t h e p e r o x i d e r a t h e r t h a n t h e
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
Oxygen
A N D KEARNS
reverse process.
159
Molecules
A c c o r d i n g to the o r b i t a l c o r r e l a t i o n d i a g r a m the g r o u n d
state c o n f i g u r a t i o n ( c r o o ^ ^ , ^ ^ ^ * ) is expected 2
2
2
2
2
2
to increase signifi
c a n t l y i n energy as the C — O b o n d s are b r o k e n because of the increase i n the energy of a
±
orbitals.
It is p r i m a r i l y this same factor w h i c h causes
the states a r i s i n g f r o m the c o n f i g u r a t i o n
(o- o- o-_7r7r7r*(7o*) to 2 00
2 +
2
2 1/
2 c
2/
1
0
1
in
crease i n i t i a l l y i n energy. B e c a u s e of i n t e r a c t i o n w i t h other h i g h e r energy states, these states a n d h i g h e r l y i n g states u l t i m a t e l y pass t h r o u g h a m a x i m u m i n energy a n d finally correlate w i t h the S / a n d A ^ * states, respec 1
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tively.
X
I n c o r p o r a t i n g the o r b i t a l i n f o r m a t i o n i n t o the " p r i m i t i v e " state
c o r r e l a t i o n y i e l d s the i m p r o v e d state c o r r e l a t i o n d i a g r a m s g i v e n b y the s o l i d curves i n F i g u r e 6.
Figure 6. Schematic of the state correlation diagram for the concerted addition of molecular oxygen to a diene. The electronic configurations for states of the (0 + diene) complex are indicated on the left (state of 0 in the complex given in brackets). States of the cyclic peroxides with their appropriate electronic configurations are on the right in order of increasing energy. Dashed curves indicate the "primitive" correlations obtained by straightforward application of symmetry and spin restrictions. Solid curves indicate final correlations obtained by using the additional information contained in the orbital correlation diagram 2
2
A l t h o u g h c o n s t r u c t i o n of this d i a g r a m was discussed i n terms of d e c o m p o s i t i o n of the p e r o x i d e , w e c a n use it to m a k e several i m p o r t a n t
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
160
OXIDATION
O F ORGANIC
COMPOUNDS
H I
p r e d i c t i o n s r e g a r d i n g t h e reverse r e a c t i o n i n w h i c h m o l e c u l a r o x y g e n a d d s t o a g r o u n d state d i e n e . ( a ) U n d e r n o r m a l c o n d i t i o n s % ~ o x y g e n is n o t e x p e c t e d t o b e reactive since this i n i t i a l state o f the reactants ( a t r i p l e t ) correlates e n d o t h e r m i c a l l y w i t h a n excited t r i p l e t state o f the p e r o x i d e . 3
g
( b ) A o x y g e n is p r e d i c t e d t o b e reactive since this state o f t h e c o m p l e x correlates s m o o t h l y a n d e x o t h e r m i c a l l y w i t h the g r o u n d singlet state o f the p e r o x i d e .
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1
g
( c ) T h e l o w l y i n g e x c i t e d states o f the c o m p l e x a r i s i n g f r o m inter a c t i o n o f 0 i n its A * o f S state w i t h the g r o u n d state o f the d i e n e b o t h correlate w i t h h i g h energy excited singlet states o f t h e p e r o x i d e ; hence, A ^ * a n d * 2 / o x y g e n are p r e d i c t e d t o b e u n r e a c t i v e i n this m o d e . x
2
1
g
f7
+
X
T h e extension of these considerations to other concerted a d d i t i o n reactions is s t r a i g h t f o r w a r d . I n g e n e r a l w e find: ( 1 ) B o t h t h e g r o u n d S / a n d e x c i t e d % states o f o x y g e n are u n reactive because the states o f the c o m p l e x associated w i t h these states o f o x y g e n correlate e n d o t h e r m i c a l l y w i t h e x c i t e d states o f t h e p r o d u c t . E x c e p t i o n s are expected o n l y i f a v e r y stable p e r o x i d e o r h y d r o p e r o x i d e is f o r m e d . 3
+
( 2 ) A is i n g e n e r a l p r e d i c t e d to b e reactive since this state o f the c o m p l e x correlates s m o o t h l y w i t h t h e g r o u n d state o f t h e p e r o x i d e o r hydroperoxide. 1
g
I n the f o r m a t i o n o f a n e n d o p e r o x i d e the e x o t h e r m i c i t y is d e t e r m i n e d p r i m a r i l y b y t h e difference b e t w e e n the extra e n e r g y g a i n e d i n f o r m i n g t w o (T b o n d s at the expense o f a C = C d o u b l e b o n d a n d a c c o m p a n y i n g 00
resonance
energy.
F r o m this analysis of t h e energetics w e c a n u n d e r
s t a n d easily w h y molecules l i k e anthracene,
tetracene, a n d
pentacene
r e a d i l y f o r m p h o t o p e r o x i d e s . T h e p u z z l i n g n o n r e a c t i v i t y of n a p h t h a l e n e and phenanthrene
( 1 5 ) c a n c o r r e s p o n d i n g l y b e u n d e r s t o o d i n terms o f
the v e r y large loss o f resonance energy w h i c h a c c o m p a n i e s the f o r m a t i o n of t r a n s - a n n u l a r peroxides of these m o l e c u l e s . T h e a b o v e considerations a p p l y specifically to c o n c e r t e d a d d i t i o n reactions w h e r e the o x y g e n a d d u c t is p r e s u m e d t o h a v e a singlet g r o u n d state. W h e n r a d i c a l s are p r o d u c e d , h o w e v e r , a s o m e w h a t different p i c t u r e emerges.
S i n c e the g r o u n d state o f a p a i r o f r a d i c a l s ( R * -f- H O C ,
for example)
w i l l b e a t r i p l e t state, t h e g r o u n d state o f t h e reactants
( g r o u n d state acceptor
+
3
S / oxygen)
c a n n o w correlate
w i t h the
ground state rather t h a n w i t h a n e x c i t e d state o f t h e p r o d u c t s .
If the
energetics are not too u n f a v o r a b l e , w e m a y n o w expect t o find reactions of this t y p e w h i c h i n v o l v e 2 / g r o u n d state o x y g e n . 3
W e m a y also expect t o find S 1
f7
+
r e a c t i n g i n this m o d e ( r a d i c a l for
m a t i o n ) , p r o v i d e d t h e r a d i c a l p r o d u c t s h a v e l o w l y i n g e x c i t e d singlet states. T h i s i s a n i n t e r e s t i n g contrast t o t h e p r e d i c t e d n o n r e a c t i v i t y o f i n c o n c e r t e d a d d i t i o n reactions.
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
71.
KHAN
Oxygen
A N D KEARNS
161
Molecules
Decomposition o f Endoperoxides.
A n i n t r i g u i n g p r o p e r t y of some
e n d o p e r o x i d e s is t h e i r a b i l i t y to dissociate u p o n h e a t i n g i n t o m o l e c u l a r oxygen a n d the parent
hydrocarbon
Perhaps
(2, 15).
even more i n
t r i g u i n g a r e t h e e x p e r i m e n t a l observations w h i c h suggest t h a t t h e 0
2
f o r m e d i n a n e x c i t e d electronic
To
state ( p r e s u m a b l y
A)
1
(25, 52).
g
is
u n d e r s t a n d h o w a n d u n d e r w h a t c o n d i t i o n s this m i g h t o c c u r , w e h a v e u s e d o r b i t a l a n d state c o r r e l a t i o n d i a g r a m s t o explore possible m o d e s of endoperoxide decomposition.
F o l l o w i n g procedures outlined above, w e
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h a v e c o n s t r u c t e d t h e state c o r r e l a t i o n d i a g r a m f o r t h e d e c o m p o s i t i o n of a s i m p l e e n d o p e r o x i d e ( I ) either b y c l e a v a g e o f t h e O — O b o n d t o f o r m a d i r a d i c a l ( I I ) or b y loss of m o l e c u l a r o x y g e n w i t h f o r m a t i o n of t h e parent hydrocarbon ( I I I ) .
T h e state a n d o r b i t a l c o r r e l a t i o n
diagrams
f o r t h e latter r e a c t i o n w e r e g i v e n i n F i g u r e s 5 a n d 6, a n d t h e o r b i t a l correlation diagram for formation of the d i r a d i c a l f r o m the endoperoxide is s h o w n i n F i g u r e 7. I n c o n s t r u c t i n g t h e c o m p l e t e state c o r r e l a t i o n d i a g r a m , t h e g r o u n d state electronic c o n f i g u r a t i o n of t h e d i r a d i c a l w a s t a k e n to b e ( o ± 7 r P ) . 4
2
C
6
[ I n t h e absence o f c o n f i g u r a t i o n i n t e r a c t i o n t h e f o l l o w i n g e l e c t r o n i c c o n figurations
(for
s i m p l i c i t y these a r e a l l d e n o t e d
as ( O ± T T P ) ) a r e 4
a s s u m e d t o b e n e a r l y degenerate i n e n e r g y : i'V/r 2
P
a ! 2
