journal of
MOLECULAR
LIQUIDS
ELSEVIER
Journal of Molecular Liquids, 61 (1994) 133-152
Comparisons Between the Reactivity of Chlorine Dioxide in the
Gas Phase and Water Solution
V. Vaida 1., K. Goudjil 1, J. D. Simon 2. and B. N. Flanders 2
1Department of Chemistry and Biochemistry, University of Colorado
Campus Box 215, Boulder, Colorado 80309
2Department of Chemistry, University of California at San Diego
9500 Gilman Drive, La Jolla, California 92093-0341
Abstract
The thermal and photochemical reactivity of the symmetric isomer of chlorine
dioxide, OC10, is examined under isolated molecule conditions and in water
solution. Reaction in the ground state leads to CI(2Pu)+ O2(3Zg-). Within
experimental error, the barrier to reaction in water solution is the same as that
estimated for the reaction in gas phase. Chemistry on the excited electronic
manifold occurs via multiple pathways leading to C10(2H) + O(3pg), CI(2Pu) +
O2(3Eg -) and CI(2Pu) + O2(1Ag). The excited state reactivity of OC10 in water
solution is significantly different from the gas phase process. The effect of
solvent on the excited state photoreactivity of OC10 is discussed in detail.
I. Introduction
The effect of solvent on chemical reactions is a topic of long standing
interest. In most cases, either the thermal or photochemical behavior of a
given solute molecule is accessible to experimental study. It is rarely the case
that solvent effects on ground and excited state reactivities of a single solute
can be examined and compared. In this study we examine the gas phase and
solution chemistry of the symmetric isomer of chlorine dioxide, OC10. Both
the ground state and excited state reactivities are delineated and compared.
While similar behavior is found for the thermal reactivity of OC10 in gas
phase and solution, significant differences occur in the reactions on the excited
state surface as a function of the environment. These similarities and
differences form the central issues that are addressed in this paper.
Solvent effects on ground state reactions are often associated with the
influence of the liquid on the heights of activation barriersJ This issue was
discussed by Fonseca in regard to enzyme catalysis, although several of the
points raised by her apply to the reactivity of small molecules in solution. 2 In
particular, one needs to introduce the idea of a potential of mean-force. This
concept defines how the reaction coordinate is affected by the average
interactions with the solvent environment. Solvent effects on the barrier and
SSD10167-7322 (94) 00757-N
134
dynamics of isomerization reactions have been reported for many molecular
systems. 3 For many molecules, e.g. stilbenes, large changes in reaction rates
and activation barriers are observed. Such cases generally encompass
molecules whose motions associated with isomerization require displacement
of significant solvent. Consequently, there is solvent friction on the reaction
coordinate and quantifying these effects provides detailed information about
the role of solvents in impeding or accelerating motion along the reaction
coordinate. In the limit of small molecular motions, the reaction rates can be
insensitive to solvent friction. As an example of this effect, Barbara and
coworkers found that the rotational dynamics for a series vinylanthracenes
were insensitive to the solvent viscosity when the rotating group was small,
e.g. CH3. 3c
The reactivity of OC10 is a complicated yet trackable problem. In the
ground state, reaction must lead to CI(2pu) and O2(3Eg-). From a combination
of gas phase 46, solution7'8, and matrix isolation9 experiments, we established
that in the near UV excited state OC10 may undergo the four competitive
reactions given by equations (1) to (4).
OCIO + hv ~
--,
-~
-~
CIO0 ~ CI(2Pu) + 02(3y.g -)
ClO(2I-I) + O(3pg)
C1(2p.) + 02 QAg)
CI(2Pu) + 0 2 (3y.g-)
(1)
(2)
(3)
(4)
The relative importance of these competing photochemical pathways for
gaseous, solvated and matrix isolated OC10 is a topic of current research. This
effort, in part, results from a desire to understand the chlorine driven
chemistry that occurs in the stratosphere. 1° Chlorine dioxide, C1OO is
commonly invoked in mechanisms which lead to net ozone depletion over
Antarctica. However, other chlorine oxides, e.g. C10, OC10, are observed in
high mixing ratio in the polar stratosphere. Concem over the photoreactivity
of chlorine oxides is a topic of intense discussion, in efforts to identify
addition channels for stratospheric ozone loss.
This paper is organized as follows. In section II, the ground state
chemistry is examined. This section examines the results from gas phase
spectroscopy, ab-initio calculations and gas phase kinetic measurement. From
this information, the gas phase barrier is extracted. This is compared to newly
measured values of the activation barrier in water. Section III examines the
similarities and differences between the excited state photoreactivity of OC10
in the gas phase and water solution. Section IV presents a discussion of the
electronic states and symmetry considerations on the reaction dynamics. The
effect of solvent on the observed molecular behavior are discussed in detail.
Section V summarizes the important conclusions from this work.
135
II. Ground State Reactivity
To date, there is very little quantitative data concerning the reactivity of
chlorine dioxide on the ground potential energy surface. The energetics
associated with the thermal decomposition of OC10 in the gas phase can be
obtained by evaluating the results from spectroscopic studies, ab-initio theory,
and the kinetics of decomposition and isomerization reactions of this reactive
radical. We first review the available information relevant to the gas phase
reaction and thereby obtain an estimate of the gas phase barrier. This is
followed by reporting the experimental measurements of the activation barrier
in water solution.
Gas-Phase Spectroscopy and Ab-lnitio C~llculotions
Spectroscopic studies of chlorine dioxide provide the ground state near
equilibrium structural information available for OC10 and C1OO. There are
two isomers of C102: the symmetrical, stable OCIO and the asymmetric,
reactive C1OO. The first infrared spectroscopic observation of C1OO was
reported by Arkell and Schwager. 9b The vibrational frequencies reported
were 373, 407, and 1441, 407 and 373 cm -1, corresponding to the symmetric
stretch, bend, and anti-symmetric stretch, respectively. The electronic ground
state was determined to have 2A" symmetry. The equilibrium geometry had a
bond angle of 110", an elongated C10 bond of 1.83/~, and an O-O bond of
1.23 A, almost similar to that in the free 02 molecule. Theoretical studies of
the ground and excited state properties of C1OO have also appeared. 11 For
example, the recent work of Peterson and Werner it" calculated a geometry
with a bond angle of 115.7", a C10 bond length of 2.139/~ and a O-O bond
length of 1.201 /~. These calculations are in fair agreement with the
experimental results. The agreement also holds for the computed vibrational
frequencies: 181.2, 390.8, and 1505.6 cm -l.
Extensive experimental and theoretical effort focussed on the
characterization of the more stable isomer OC10 both near equilibrium and in
the chemically relevant higher energy, vibrational overtone region) 1~'5 The
first infrared spectrum of the OC10 ground state was reported by Bailey and
Cassie. ~2 A more complete infrared study on OC10 ground state was
performed by Nielsen and Woltz) 3 The results reported were confirmed by
Richardson et al) 4 Microwave spectra of 35C102, 37C102 in the vibrational
states vl, v2, v3 and 2v2 were observed and used to obtain the equilibrium
structure of the ground state) 5 These studies showed that OC10 in its ground
state is a near prolate asymmetric top with re(C10) = 1.469 ~, and qe = 117.4 °.
In table 1, the different vibrational frequencies are summarized. It is worth
noting, from the intensity measurement, that the overtone 2v2 as well as the
combination band vl+v2 have not been observed. In contrast, the 2v~ band
has significant intensity. One would expect in a harmonic potential that, given
the almost identical intensities of the fundamental bands vl and v2, the 2v2
136
band will present comparable intensity to the 2vl band. Furthermore, the
combination band vl+v2+v3 is less intense than the combination band 2vl+v 3
even though the 2vl+v3 band is higher in energy. This lack of intensity for
the bending overtone and combination bands may be an indication of the
involvement of bending in the OC10 ground state dynamics. Indeed, if the
ground state PES is highly anharmonic along this coordinate, the overlap
between the ground vibrational state and the bending overtone or combination
bands will be smaller.
Vibrational
Mode
Vl
v2
v3
2V2
Vl+V2
Infrared Electronic Ab-initio
Absorption Absorption Calculation
945.2
945.3
955.4
447.3
447.9
455.6
1110.8
1095
895.5
1388.7
2V1
1881.3
1881.9
vl+v3
2041.1
2v3
2208.4
vl+v2+v3
2482.1
2Vl+V3
2960.7
3v3
3294.2
Table 1. Fundamental, overtone and combination
bands for OC10 obtained from infrared and
electronic absorption spectra. 5,x2-14 The vibrational
energies determined from the electronic absorption
data are derived from hot bands observed for the
2A2.-- ZB1 transition.
The calculations of Werner and Peterson 11a find a bond length of 1.476
~, and a bond angle of 117.9", in good agreement with the microwave results.
The computed fundamental harmonic frequencies vl, v2, and v3 are given in
Table 1. The vl and v2 frequencies show good agreement with experiment.
The calculated and experimental values of v3 are not as close, presumable
because of the smaller basis set used in evaluating v3. Peterson and Werner 11a
also calculated the absolute infrared intensities, S, at 300 K for the
fundamental vibrational modes of the OC10 ground state. They found
$1=44.8 cm -2 atm-1, $2= 66.8 cm-2 atm-1 and $3= 480 cm -2 atm -1. From these
data the relative intensities $1/$3 and $2/$3 are calculated to be 0.094 and
0.139, respectively. The experimentally determined ratios are S1/$3= 0.44,
$2]S3= 0.4.13 There is a significant difference between theory and experiment
in this respect, most likely connected to the reactivity of OC10.
137
All of the data discussed in this section clearly points to the fact that the
ground state decomposition of OC10 proceeds with involvement of the
bending mode.
Kinetics of chlorin¢ dioxide in the ~round state
At present, the barrier height for the reaction of ground state OC10 in
the gas phase is not accurately known. Nevertheless, information leading to
reasonable estimates of this barrier are available. In a kinetic study of the
chlorine dioxide thermal decomposition initiated by a shock wave, an
activation energy of about 0.67 eV was deduced) 6 These shock tube
experiments were done in different diluted mixtures (1% to 10% of OCIO in
Ar) at ambient initial temperature over an initial pressure range of 5-50
Torr. For dilute mixtures at high initial pressures (> 25 Torr), the wave
propagation was unstable. At low dilution and at low initial pressures the
possibility of a bimolecular reaction between two OC10 is unlikely. Thus, the
experimental observations suggest that a collisional induced dissociation of
OC10 occurs via the following reaction, equation (5).
OC10 + Ar --* CI(2Pu) + O2(3y'-g") + Ar
(5)
From the small 0.67 eV (5400 cm -1) barrier and the low intensity of
bending overtones and combination bands involving the bend in the infrared
absorption spectrum, one can conclude that OC10 ground state dissociation to
generate CI(2pu) + O2(3Eg -) is a favorable process. There are two possible
reaction coordinates that can be followed during the reaction. First, the
reaction may proceed by a symmetrical process in which a C2v symmetry is
preserved throughout the reaction. Second, the reaction can process
asymmetrically, with the possible involvement of C1OO as a reaction
intermediate. To distinguish these possibilities, one need to further examine
what is known about the isomerization of OC10 to C1OO.
The thermal isomerization of C1OO to OC10 involves a barrier between
the ground states of OC10(2BI) and C1OO(2A"). While the height of this
barrier is not known, it is important in delineating the chemistry of OC10. If
the barrier is sufficiently low to allow a reasonable rate at ambient
temperatures, the ground state isomerization of OC10 to C1OO will result in
the production of CI(2Pu) + O2(3Yg -) via the decomposition of C1OO, a
reaction known to have an negligible barrier. The observation of both thermal
isomerization and photoisomerization have been reported by Eachus et al. 17
In their work, the photolysis of OC10 in rigid glasses and solid matrices has
been investigated. In rigid sulfuric acid matrices, OC10 was photolytically
converted into C 1 O O . 9¢ Upon melting the matrix, approximately half of the
original OC10 was regenerated. This data clearly establishes a ground state
reaction channel linking OCIO and C1OO. The same phenomenon was also
138
observed when rigid matrices of KC104 were used. 9 These data argue that the
0.67 eV barrier observed in the kinetic measurements could correspond to the
activation energy needed for the OC10 --* CIOO isomerization reaction.
Reactivity of chlorine dioxide in water solution
The thermal decomposition of OC10 in water solution was examined by
monitoring the time dependent disappearance of the absorption spectrum. The
absorption spectrum of OC10 in water is very similar to that observed in the
gas phase. The thermal decomposition kinetics of OC10 were followed by
monitoring the evolution of the absorption signal for samples of OC10 in
water. For all the temperatures studied (294, 331, and 346 K), the decay of
the absorption signal revealed a first order kinetic process. From the
temperature dependence of the rate constants, an activation energy of 0.60 eV
was determined. This value is essentially identical to the gas phase barrier
estimated above.
III. Photochemistry of OCIO
The excited state photoreactivity of OC10 is more complicated than the
chemistry exhibited from the ground state. This results, in part, from the
complicated nature of the excited state potentials. Significant differences are
observed between the excited state reactivity of OC10 in the gas phase and in
solution. This results from a combination of static and dynamic solvent effects
on the evolution of the electronically excited molecule. In this section, we
examine the photoprocesses of electronically excited OC10.
Gas Phase Photoreactivitv of OCIO
Three excited state of chlorine dioxide, 2A2, 2B2, and 2A 1 are accessed
in the near UV. Electronic dipole selection role predict that the transition into
the 2B2 state is dipole forbidden, whereas the other two transitions are
allowed. The experimentally observed transition is the parallel polarized 2A2
,--- 2B 1. The perpendicular polarized 2A1 4-- 2B1 transition is expected to be
weaker and has not been observed experimentally. The most intense
progression in the spectrum of OC10 in the near UV (the 2A2 state) is caused
by excitation of the symmetric stretch (Vl).5 Other less intense progressions
involve the bending (v2) and the asymmetric stretch (v3). The splitting in each
band is due to the 35C1 and 37C1 isotopes. These high resolution spectra were
used to revise the excited state structure, s The geometries of the electronic
ground state and the 2A2 excited state are somewhat different. On excitation,
the bond length increases from 1.47 ,~ to 1.63 ~, and the bond angle decreases
from 117" to 107". 5
The spectra have further been interpreted to describe the excited state
dynamics: b In absorption, in a sample cooled in a supersonic expansion, all
lines are lifetime broadened because of the efficient photochemistry of OC10.
139
The magnitude of this broadening is independent of the rotational quantum
numbers, but depends strongly on the vibrational quantum numbers. From the
spectra line widths, the lifetime of the 2A2 excited state is estimated to range
from 20 picoseconds to hundreds of femtosecond. The line widths associated
with the corresponding combinations of Vl with the bending mode v2 and the
asymmetric stretch v3 are always considerably larger than those of vl alone.
As a result, both v2 and v3 are thought to serve as promoters for the UVphotochemistry of OC10. The bending mode in the 2A2 state may become
especially important in bringing the oxygen atoms closer together, which
could result in the formation of C1OO and ultimately produce C1 and 02.
The photochemistry of OC10 in the gas phase has been studied since the
1950's. However, results to date have been complicated by efficient seconda.ry
reactions. Studies in our lab 5a .6a "c and more recently, in several others 5b -'f 6 d ,j
were aimed at understanding the primary photochemistry of OC10. Secondary
reactions are minimized by sample preparation in the collision free conditions
of the supersonic jet expansion. These studies showed that excitation of the
near UV manifold of states produces several photofragments: C10(21-I)+
O(3pg), CI(2Pu) + O2(3Y.g-), and CI(2Pu) + O2(]Ag). The photoreactive
electronic states of OC10 have been investigated theoretically using ab-initio
methods. These calculations find evidence for both reactive channels leading to
CIO(2II) + O(3pg) and CI(2Pu) + O2(3y.g-,lAg), consistent with the picture we
derive from experimental results. Figure 1 depicts the photodynamics
following excitation to the 2A2 state.
Recently, experiments have been performed to derive the quantum yield
for chlorine formation in the 360 nm region but these results remain
controversial. Lawrence et al. 6s find a quantum yield of 5x10 -4. Bishenden et
al. 6d have shown that the quantum yield is considerably higher, on the order of
10%, while Davis and Lee 6f suggest it to be below 4%. While the accurate
quantum yield is still to be determined, it is clear from the recent studies that
C10 + O is the predominant reaction channel in the gas phase. Nascent C10 is
formed in this photochemical reaction in vibrationally excited states (v=l-8),
carrying a substantial amount of internal energy. Bishenden and Donaldson 6e
have recently shown that C10 is formed predominantly by excitation of the
asymmetric stretch while Davis and Lee 6f found that C1 + 02 is generated
predominantly on excitation of the bending mode.
The recent spectroscopic, photofragment and theoretical results
therefore show that chlorine dioxide in the gas phase reactions
photochemically in a complex yet mode specific manner to yield the
dissociation products C10 + O (high quantum yield) and C1 + 02 with a low
quantum efficiency.
These results are striking when compared to photolysis studies of OC10
in matrices of argon 9a, nitrogen 9b, and sulfuric acid 9c. Photolysis in matrices
find exclusive rearrangement of OC10 to form C1OO when the photolysis
140
wavelength is near the maximum of the near-UV transition (-360 nm). Recent
results suggest that in the matrix OC10 may dissociation along a symmetric
C2v reaction coordinate to form C1 and 02, which then recombine to form
C1OO. 9d=If this is the case, there would be an order of magnitude change in
the quantum efficiency for this process in the matrix compared to that in the
gas phase.
ZA 1
ZA z
2B z
0
0
el :~o
; C I ¢) 117"
Cl ,/) 107"
~0
spin orbit
"~0
vibronic
coupling
clo
7z" ~
coupling
(Zrz)+ o (3p)
---~ Cl(Zp) + 0 z(1Ag, SZg")
Reactive Chemistry
h'~
t
0
~O--Cl
~
cI(zP)+ 0 z(3Zg-)
0
CI ~1) 11 O"
h
o
ZB 1
Figure 1. A schematic representation of the photoreactive processes of OC10.
Photochemistry of OC10 in Water Solution
The photochemistry of OC10 in water solutions was studied using a
variety of time resolved spectroscopic techniques. 7 By taking advantage of the
well characterized electronic spectroscopy of OC10 7, C1z8, and C1018 in
water solution, the quantum yields for generation of C10 + O and C1 + 02
can be determined using picosecond absorption spectroscopy.
In the experiments described, OC10 was excited at 355 nm, near the
maximum of the absorption band. Unlike the behavior exhibited in the gas
phase, there is no wavelength dependence, 355 nm < ~.ex < 420 nm, to the
solution photochemistry. This indicates that vibrational relaxation of the 2A2
state is fast compared to the predissociation process. In figure 2, the dynamics
at various probe wavelengths following the photolysis of OC10 at 355 nm are
shown.
141
A variety of different kinetic processes are revealed by the transient
data. The solid lines through the data are calculated results of a kinetic model
where OC10 undergoes competitive photochemical reactions whereby 90% of
the molecules dissociate into C10 and O, with the remaining 10% generating
C1 and 02. Based on the gas phase absorption spectrum of ClOO 19, the
dynamics observed in the UV portion of the spectrum, and the kinetics of C1
formation, the data indicate that C1 production involves C1OO as a reaction
intermediate.
0.05
...................
0.1Z
( a ) ZSO nm
0.04
0.1
i
0.08
0.03
0,06
O.OZ
0.04
0.01
0,02
0-
0
-0,01 . . . . . . . . . . . . . . . . . .
-500
0
500
O,OZ
. . . . .
-
.
.
.
.
.
.
.
.
.
.
1000
.
.
.
.
.
.
.
1500
.
.
.
.
-O,OZ . . . . . . . . . . . . . .
0
-500
0.01
1500
..............
(e) 385 n m
0
500
ZSO0
(b 4z0 .m
0
-O.OZ
-0.01
-0,04
-O,OZ
-0.06
S
-0,0:
-0.08
-0.0'
-0.1
-0.12
-500
.
.
.
.
.
0
.
.
.
.
.
.
500
,i
. . . .
1500
i
.
.
.
.
-0.05' "
Z500
.
.
.
-500 0
.
, , i
. . . .
5001500
i
. . . .
i
. . . .
ZSO0
Time (ps)
Figure 2: Examples of the time resolved absorption dynamics of OC10 in
water solution following excitation at 355 nm. The four wavelengths
plotted are (A) 250 nm, (B) 330 nm, (C) 385 nm and (D) 420 nm.
142
In particular, at 250 nm, the instantaneous rise in absorption following
photolysis arises from the generation of C10 and C1OO. The subsequent
thermal decomposition of C1OO into C1 and 02 causes the signal to decrease
during the first 500 ps. At 330 nm, the formation kinetics of the C1 atom are
revealed; the rise time is identical to the fast decay observed at 250 nm. OC10
absorbs at 385 and 420 nm, consistent with the negative signals observed
following excitation. C1 also absorbs at 385; thus the increase in signal
observed during the first 500 ps corresponds to C1 formation from the
decomposition of C1OO, which does not absorb at this wavelength. The long
time recovery in absorption observed at 385 and 420 nm reflects the
reformation of OC10 from the O and C10 dissociation products.
As the absolute absorption cross section for C1OO is now known in
solution, the absorption data cannot be used to conclude that all chlorine
product occurs by isomerization to C1OO followed by thermal
decomposition. Using the correlation diagram shown in figure 3, vida infra,
as a guide, two allowed reaction channels exist that can produce C1 atoms
from the electronically excited 2B2 state of OC10. These are given by
equations (6) and (7).
OC10 + hv ~ C100 (2A' or 2A") -* CI(2Pu) + O2(3~'.g -)
OC10 + hv ~ CI(2Pu) + O2(1Ag)
(6)
(7)
The first mechanism, equation (6), occurs by photoisomerization of
OC10 to C1OO, which can be produced either in the ground state, 2A', of the
first electronic excited state, 2A". While thermodynamically more stable than
OC10, C1OO is kinetically unstable and readily dissociates to form CI(2Pu)
and O2(3Eg-). The second mechanism involves symmetric dissociation (along
a C2v reaction coordinate) producing CI(2Pu) + O2QAg). The electronic state
of the product oxygen molecule provides an experimental handle by which
the relative importance of these two chlorine producing mechanisms can be
distinguished.
To determine the relative extent of the two allowed mechanisms for C1
generation, the quantum yield for formation of either O2QAg) or O2(3y.g-) is
needed. This was determined by measuring the yield of 3yg- 4-- lag emission
of 02 following excitation of OCIO at 355 n m in deuterated solvents.7d By
comparing the intensityof thisemission 0270 nrn) with that of sensitizersfor
which the emission quantum yield is known 2°, the quantum yield of O2QAg)
formation in D20 was found to be 0.005 + 0.003. Thus, in water solution,
95% of the Cl is produced from the isomerizationpathway, equation (6); the
remaining 5 % results from the direct symmetric dissociation,equation (7).
The importance of the symmetric C2v dissociationincreases with decreasing
solvent polarity.The quantum yield of Cl(2Pu) + O2QAg) production in C6D6
143
(ET(30) = 34.5 kcal/mole) and carbon tetrachloride (ET(30) = 32.5
kcal/mole) is 0.02 and 0.07, respectively, much larger than in D20 (ET(30)
-- 63.1 kcal/mole). The increase in the amount of symmetric C2,, dissociation
with decreasing solvent polarity suggests that this mechanism may be
important in the gas phase chemistry of OC10. This conclusion is consistent
with translational energy distributions for the 02 product in the gas phase
photolysis of OC10 reported by Davis and Lee. 6f
The combination of picosecond transient absorption and time-resolved
infrared emission spectroscopy enable one to quantify the photoreactivity of
OC10 in water solution. The quantum yields for the three different reaction
channels are give in equations (8) to (10).
OC10 + I~ --* C1OO (A' or A") ~ CI(2Pu) + O2(3T-g-) (9.5%) (8)
CI(2Pu) + 02(lAg)
(0.5%) (9)
--* ClO(2I-I) + O(3pg)
(90%) (10)
IV: Discussion
Reactive Electronic States of OC10
In this section, the ground and excited electronic state reactivity of
chlorine dioxide is discussed within the context of the energetic and orbital
symmetry constraints. The adiabatic electronic state correlations for the two
symmetry cases considered (C2v and Cs) are shown in Figure 3. The C2 axis is
taken to be the z axis and the x axis is defined to be perpendicular to the plane
of the molecule. Only the doublet states are presented. The energy of the lowlying quartet states are calculated to be 6.82 eV (4B2), 7.96 eV (4A2), 8.08
eV(4A1) and 8.27 eV (4B1).lla These vertical excitation energies are
significantly higher than that of the 2A2 state (vertical excitation of 2.66 eV),
thus any state mixing involving the quartet states is negligible.
The ground state of OC10 molecule (2B1) correlates to two sets of
reaction products, CI(2Pu) + O2(3Eg-) and C10(21-I) + O(3pg). From
thermodynamic measurements, the OC10 ground state 2B1 lies -2.2 kcal/mole
(-0.1 eV) lower in energy than the dissociation products CI(2Pu) + O2(3Y.g-).
In contrast, the asymmetric products lie much higher in energy. The
experimental dissociation limit for formation of C10(2rI) + O(3pg) is 58
kcal/mol (2.51 eV).
In C2v symmetry, the combination of ground state chlorine CI(2Pu) and
ground state oxygen O2(3Y.g-) correlates to electronic states of 2B1, 2A1, and
2A2 symmetry. These correspond to the ground state of OC10 and two excited
electronic states. It is important to note at this point that among all the lowlying states involved in the excited state dynamics of OC10, only the 2132 state
correlates to CI(2p,) + O2(lAg) products.
In Cs symmetry, the same combination, CI(2Pu) and O2(3Eg'), correlates
to three electronic states : two of 2A symmetry and one of 2A' symmetry.
144
70-
Cs
/
6050 - -
CIO( 2ri)
OCIO(2A") \
OC'IO((:AA:)
,
+ O(3p)
o
~ 40-~ 30
t-
--
CI(2p)+o2(lAg)
CIOO(2A')
20~
10--
\
OCIO(2A") ~ C I ( 2 p ) + o
2(3Eg)
0---
70-
C2v
OClO(2A2)
60-
OCIO(ZA1)
OCIO(2B2)
'~' 5 0 Ill
o
~ 40-
~
~30-
I..
~ 2010-
OCIO(
2B1)~
C I ( 2 p ) + o 2(lAg)
~ ~.....,~ CI(2p)+o 2(3y-g)
Figure 3. Orbital correlationdiagramsfor the reactivityof OC10
assuming a Cs(top) and C2v(bottom)reactioncoordinate.
145
The 2A" states indicate correlation to the C1OO ground state and the
dissociation products C10(2I-I) + O(3Pg). The 2A' state correlates with a lowlying exited state of C1OO. Extensive theoretical calculations on C1OO were
reported by Jafri etal. 11b That work examined eight doublet and eight quartet
states o f CIOO using ab-anitio CI techniques. The results predicted that all
doublet electronic states are repulsive along the C1-O2 coordinate. Only the
ground 2A" state is bound along this coordinate. In a related study, Gole Hc
estimated that the low-lying 12A' state is at most slightly (5 kcal/mole) more
stable than CI(2pu) + O2QAg). The second 2A" state (labeled in the figure 3 as
22A"), which correlates to the generation of C10 (21-1)+ O(3pg), lies at 2.7 eV.
For completeness, we include the combination of CI(2Pu) and O20Ag),
which in Cs symmetry gives rise to three 2A' states and three 2A" states. In
the correlation diagram only one 2A, state labeled 22A' and one 2A" labeled
32A '' are presented. Based on the calculations of Jafri et al., these two states
are positioned at 2.2 eV and 3.2 eV, respectively, well above the C1OO ground
state level. Thus, this reaction cannot contribute to the ground state
decomposition, but will need to be considered in the excited state case.
The excited state photoreactivity of OC10 involves three closely spaced
excited state surfaces (2A2, 2A 1 and 2B2). Transition to both the 2A2 and 2A 1
states are symmetry allowed but no experimental observation of the 2A1 4- 2B 1
has been reported to date. Transition to the 2B2 surface is dipole forbidden.
The 2A2 *-- 2B 1 transition has been well characterized in the gas phase and is
known to be predissociative. The optically active 2A2 state is not involved
directly in the excited state reactivity of OC10 as indicated by the structured
spectrum it generates in the near UV. Instead, coupling to the lower energy
2A1 surface occurs from which the calculations suggest two possible pathways.
OC10 may dissociate forming C10 and O through bending excitation or cross
onto the lower energy 2B 2 surface.
Theory suggests t h a t all of the photochemical activity of OC10
originates from the 2B 2 state) ~a Peterson and Werner find that on the 2B2
surface, the asymmetric stretch reaction coordinate is purely dissociative
(forming C10 and O) for angles greater than -100". The mechanism(s) by
which C1 and 02 are formed from OC10(2B2) remains unclear. One
possibility is excited state isomerization of OC10 to form C1OO followed by
thermal dissociation to produce C1 and 02. Thermodynamic cycles show
C1OO to be more stable than OCIO by - 4 kcal/mole. However, C1OO is
kinetically less stable and readily dissociates into C1 and 02 at room
temperature. To gain insight into the possible channels for C1 production, we
reconsider the orbital correlation diagram shown in Figure 3. Compared to
the energy of the photoexcited 2A 2 state, the formation of C10 + O and C1 +
02 are both exothermic reactions. In the later case, three electronic states of
02 are energetically accessible (3Eg-, lAg, and lYu). The dissociation of OC10
146
into C10 and O must produce the two photofragments in the 2I-I and 3Pa
electronic states, respectively. Symmetry considerations restrict the product
states that can be formed from the various electronic excited state of OC10. As
described above, we consider the asymmetric dissociation into C10 and O, the
direct Czv dissociation into CI(2Pu) + O2(tAg) and isomerization to C1OO. The
correlations between reactant and product states for these cases are shown in
Figure 3. From this diagram it can be seen that if formed, orbital symmetry
considerations require that C1OO be initially generated in an excited electronic
state, 2A'. C1OO(2A') can thermally dissociate or relax to its 2A" ground state
from which dissociation can take place. Both states ultimately correlate with
the formation of CI(2Pu)+ O2(3Eg-).
The formation of atomic chlorine can can also occur directly from the
2B2 state through a symmetric dissociation mechanism. In this scheme, the
reaction coordinate maintains the C2v symmetry of the molecule and the
products are formed in a concerted unimolecular mechanism without first
forming C1OO. This mechanism produces CI(2pu) + O2(1Ag). It is important
to stress that this mechanism produces oxygen in its first excited state whereas
isomerization to form C1OO leads to the production of ground state oxygen.
The distribution of 02 electronic states can therefore provide information on
the mechanisms involved in the production of C1.
Solvent Effects on the Reactivitv of OC10
1. Ground State. The above data reveal a barrier for the thermal
decomposition of OC10 of 0.67 eV and 0.60 eV in the gas phase and water
solution. The nearly identical gas phase and solution reaction barrier gives
insight into the reaction coordinate and reaction mechanism. Consider the
solvation of OC10, C1OO and C1 in water. The dipole moments of ground
state OC10 and C1OO are 1.79 and 1.11 D, respectively. 11" Estimating the size
of these two bent triatomics as 6.5 A s, the difference in solvation energies
A(AG) can be approximated using Kirkwood's expression 1. This gives the
following expression
A(AG) = ((lX2cioo - ~2oclo)/aS)((e-1)/(2e+l))
(11)
where E is the dielectric constant (e = 78.3 for water1), and a is the cavity
radius. This approach indicates a difference in the free energies of solvation of
OC10 and C1OO of 1.4 kcal/mole. This reduces the exothermicity of a OC10
--* C1OO reaction from the gas phase value of 3 kcal/mole to ~ 1.5 kcal/mole.
Using the Kirkwood expression, the solvation energy of OC10 in water is on
the order of 2.2 kcal/mole. This is likely to be an underestimate as it does not
account for specific hydrogen bonding interactions. In contrast, the solvation
of C1 atom involves formation of a specific donor-acceptor complex with a
147
single solvent molecule, forming CI& and H2OS+J s The solvation energy of
chloride ion in water is -83 kcal/mole. 21 Thus, it is reasonable to conclude
that the generation of ClS- upon solvation of C1 atoms in water and the
subsequent atom-molecule dipole moment that is generated will result a
solvation energy of tens of kcal/moles, significantly greater than that
associated with OC10. Thus, while the reaction OC10 ~ C1 + 02 is
endothermic by 4 kcal/mole in the gas phase, it is expected to be exothermic in
water solution.
Effect of Solvation
on Isomerization
OCIO -'* CIO0
OCIO
AGsolv ~
CIOO
-~
~ AGsolv._[.._
CX210--* CI + 0 2
AG° ~ -3 kcal/mole
AG° ~ - 1.5 kcal/mole
Effect of Solvation
on Dissociation Reaction
C1 + 0 2
OCIO
AGs°lv II
I
AG ~ 4
AGsolv
AG° • 10
Figure 4: Solvation Effects on the Energetics for Isomerization and
Dissociation Reactions
148
The ramifications of these results on the activation energies of the two
possible reaction channels are depicted in figure 4. Consider the ground state
reaction involves isomerization to C1OO. The overall energetics of this
reaction are only weakly effected by solvation. Isomerization is likely to
involve an isopolar transition state. 1 It is also reasonable to assume that the
viscous friction on this reaction would be negligible. As a result, solvation
should have a negligible effect on the reactive potential energy surface
involved in isomerization. This is not the case for the OC10 ---, C1 + 02
symmetric dissociation reaction. While one still expects an isopolar transition
state, the solvation of the C1 product in water causes a substantial increase in
the exothermicity of this reaction compared to the gas phase. Within any
quantitative free energy relationship, this necessitates a decrease in the
activation barrier for reaction. The quantitative agreement between the gas
phase and solution data strongly support the conclusion that the ground state
chemistry involves isomerization to C1OO, which subsequently dissociates into
C1 + 02.
2. Excited State. While the present study focusses on our studies in water, we
have recently reported that photoexcitation of OC10 leads to both C10 + O
and C1 + 02 in a variety of solvents, e.g. water, methanol, ethanol, 1propanol, 2-propanol, sulfuric acid, acetonitrile and toluene, zz Unfortunately,
the lack of spectral information on the intermediates in the various solvents
prevents quantifying of the relative partitioning between these two product
channels as was done above for the case of OC10 in water. As in the gas phase,
the predominant photochemical product channel of OC10 in solution is
dissociation to form C10 and O. This is not the case for matrix isolated
OC10, where a unit quantum efficiency for formation of C1OO is observed.
These very different photochemical results in gas phase, solution, and low
temperature matrices, can at present be cast in a unified picture. As discussed
in detail in the remainder of this section, these seeming disparate results can be
understood by considering the details of (a) the C1 + 02 channel and the
mechanism(s) by which these products are formed and (2) the effects of
solvation on the energetics of the OC10 --* C10 + O pathway.
Based on the correlation diagram shown in Figure 3, the experimental
observation of the 3y.g- ~ lag emission of 02 following the photolysis of
OCIO in solution substantiates a symmetric C2v dissociation pathway. The
experimental data on the 02 energy content following photolysis of gas phase
OC10 support the conclusion that O2(1/~g) is generated. In solution, however,
this channel is strongly sensitive to solvent polarity, increasing in quantum
efficiency by over a magnitude from water (0.5%) to CC14 (7%). 7d There is a
systematic increase in the quantum yield of this channel with decreasing
solvent polarity. In water solution, the quantum yield for C1 production is
149
10%; however, only 0.5% is generated by direct dissociation. The majority
results from decomposition of C1OO, producing CI(zPu) + O2(3Eg-). The C1OO
molecule is formed by an excited state photoisomerization reaction. While
definitive experimental proof has not been reported, the photoisomerization of
OC10 to C1OO has also been invoked in the gas phase photoreactivity. This
conclusion is reasonable; the solution data clearly establish that C1 is
photogenerated by both reaction mechanism.
The quantum yield for total C1 production from photoexcited OC10 in
the gas phase, water solution and low-temperature matrix is <.046 , 0.17 , and
1.09.23 respectively. Consider the effect of solvation on the energetics of the
allowed photochemical pathways. In gas phase, solution and matrices,
photoisomerization to C1OO and direct C2v dissociation to CI(2Pu) + O20Ag)
are highly exothermic reactions. As described above solvation effects will only
increase the exothermicity of channels that create atomic C1, while have a
negligible effect on the isomerization reaction. However, this is not the case
for the asymmetric C10 + O dissociation reaction. The ground state OC10
C10(21-I) + O(3pg) dissociation reaction is endothermic by 58 + 2 kcal/mole.
The calculations of Peterson and Wemer 11a indicate that the 2B2 excited state,
from which dissociation occurs, is only slightly higher in energy, - 2 - 3
kcal.mole, than the C10(2I-I) + O(3pg) products. A small barrier, on the order
of kT was also calculated. Given this small energy different, solvent effects on
the energetics of this reaction can dramatically effect the ability of this
reaction channel to compete with both isomerization to C1OO and direct
dissociation to CI(2Pu) + O2(1Ag) . The difference in solvation energies A(AG)
for C10 + O and OCIO (2B2) c a n be estimated using the Kirkwood equation.
The volume of C10 was taken to be 2/3 of that of OC10. The dipole moments
of C10 and OC10(2B2) are 1.224 and 2.46 D 25, respectively. As oxygen atoms
do not form charge transfer complexes with water the solvation energy was
assumed to be negligible. Using these values, water stabilizes OC10(2B2) by
2.8 kcal/mole more than that of the dissociation products. As a contrast, in
liquid argon at -191 K, A(AG) is estimated to be 0.74 kcal/mole, still
significant compared to thermal energy. Combining this information with the
ab-initio calculations suggests that solvation in both liquids and matrices could
make this reaction channel isoenergetic to slightly endothermic. An associated
small increase in the barrier to reaction is also likely. If this reaction barrier
were on the order of kT in room temperature solution, it would be as much as
4kT at 77 K, making this channel energetically inaccessible from the 2B2
excited state at low temperature. Such an effect can easily account for
differences in the quantum efficiency for C1 + 02 formation observed between
the gas liquid and matrix phase data. With the closing of the C10 + O, channel
in low-temperature matrices, the partitioning between direct dissociation to C1
+ 02, followed by cage recombination to form C1OO and isomerization to
CIOO becomes an interesting question. Experimental measurements should be
150
able to address the relative importance of these channels as direct dissociation
will produce O2(IAg). As CI(2Pu) + O20Ag) does not correlate with any of the
accessible states of C1OO, the oxygen molecule must electronically relax
before reacting. This will result in the generation of 3y.g 4- lag emission.
Determination of the quantum yield for this emission will provide the missing
details of the condensed phase chemistry of OC10.
V. Conclusions
The comprehensive data base available for the ground and excited state
reactions of OC10 in gas phase and in water solution is presented in this paper.
Solvent effects on the different reaction channels possible in the ground state
(OC10 ---, CI(2Pu) + O2(3Eg-)) and in the excited state (OC10 + hv --* C10(2FI)
+ O(3pg); OC10 + hx~ --* CI(2pu) + O2(3Eg-,1Ag)) are discussed in a unified
picture consistent with orbital selection rules for reactions of this radical. No
solvent effect is observed in the ground state reaction of OC10. In contrast,
significant changes occur in photoproduct quantum yields on solvation when
reaction proceeds in the excited state. These effects can be understood within a
framework of the effect of solvent on energetic and mechanistic details of the
excited state chemistry of OC10.
VI. Acknowledgements
This work is supported by grants from the National Science Foundation
to VV(CHE-9013037) and JDS (CHE-9013138). We thank Dr. Erik Richard
and Dr. Robert Dunn for their contributions to the described work. We thank
Dr. Kirk Peterson for many helpful discussion and for providing us with his
unpublished results.
.:
We dedicate this paper to the memory of Teresa Fonseca, remembering
her sense of value, her love of life, and her remarkable strength and
determination in her pursuits. She was always eager to participate in
discussions whose topics spanned the sciences, arts, and politics. Teresa has
been and continues to be an inspiration in both of our scientific and personal
lives; her contributions to the lives she touched will not be forgotten.
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