Carbon-13 kinetic isotope effects in the catalytic oxidation of carbon monoxide over Pd/Al2O3

Article abstract of DOI:10.1021/jp971118d

The C-13 kinetic isotope effects in the oxidation of CO by oxygen over a 0.5% Pd/γ-Al₂O₃ catalyst were experimentally determined in a temperature range of 323-413 K, and the following temperature dependence was found: 100 ln(k₁₂/k₁₃) = 3.18-831/T (±0.15). A reaction gas mixture of CO/O₂ at a ratio of 1:2 and an initial pressure in a static system ranging from 5 to 10 kPa were used. The reaction kinetics were found to be of order +1 in CO, order 0 in oxygen, and order -1 in CO₂. Under these experimental conditions, an activation energy of 56 ± 3 kJ mol^-1 was obtained. Using Bigeleisen's formalism based on the absolute rate theory of chemical reactions, kinetic isotope effects were calculated. For the transition state of the rate-determining step, a (CO₂)^? of various geometries and force constants was considered. The experimental data can be satisfactorily interpreted only with an interbond angle close to 110° and a reaction coordinate described by an asymmetric normal vibration of an asymmetric transition state.

Full text of DOI:10.1021/jp971118d

7236
J. Phys. Chem. A 1997, 101, 7236-7242
Carbon-13 Kinetic Isotope Effects in the Catalytic Oxidation of Carbon Monoxide Over
Pd/Al2O3
Nives Ogrinc,* Ivan Kobal, and Marjan Senegacˇnik
J. Stefan Institute, JamoVa 39, P.O. Box 3000, 1001 Ljubljana, SloVenia
ReceiVed: March 28, 1997X
The C-13 kinetic isotope effects in the oxidation of CO by oxygen over a 0.5% Pd/γ-Al2O3 catalyst were
experimentally determined in a temperature range of 323-413 K, and the following temperature dependence
was found: 100 ln(k12/k13) ) 3.18-831/T ((0.15). A reaction gas mixture of CO/O2 at a ratio of 1:2 and an
initial pressure in a static system ranging from 5 to 10 kPa were used. The reaction kinetics were found to
be of order +1 in CO, order 0 in oxygen, and order -1 in CO2. Under these experimental conditions, an
activation energy of 56 ( 3 kJ mol-1 was obtained. Using Bigeleisen’s formalism based on the absolute rate
theory of chemical reactions, kinetic isotope effects were calculated. For the transition state of the rate-
determining step, a (CO2)q of various geometries and force constants was considered. The experimental data
can be satisfactorily interpreted only with an interbond angle close to 110° and a reaction coordinate described
by an asymmetric normal vibration of an asymmetric transition state.
1. Introduction
is thus attached to the surface by its C atom, the CO axis being
mostly perpendicular to the surface, or slightly tilted.22,48,51 The
possibility of adsorbing via O atom or by CO lying parallel to
the surface may be excluded.39,45 At small coverages CO
molecules are randomly distributed over the metal surface.
Further adsorption forces them to arrange into regular layers
that get compressed at high coverages10,15,16,52-55 with the
appearance of domain walls56 and show a tendency for the
formation of a close-packed layer with a saturation density that
is nearly unaffected by the nature of the substrate surface and
its crystallographic orientation.15,18 Depending on conditions,
a CO molecule may be attached to one, two, or three Pd
atoms,18,25,31,40,52,54,55,57-61 which has also been supported by ab
initio quantum chemical calculations.43-46,49,62 The adsorption
energy depends slightly on the crystal planes12,18,54,63-65 as well
as on the CO coveragesit is lower at high coverages due to
the repulsion among adsorbed CO molecules.10,21,34,39,52,66,67
Smaller Pd supported particles (smaller than 5 nm) produce
higher CO binding sites, which may be explained by the high
participation of edge sites on smaller particles compared with
larger ones.34 Bridged adsorbed species are more firmly bound
than uppermost ones.31,39,45-47 The adsorption of CO may also
induce reconstruction of the Pd surface.61,68-70
The catalytic oxidation of carbon monoxide is a favorite
reaction for a large number of investigators. It is important in
locomotive and industrial pollution control.1 The removal of
CO from car exhaust is accomplished by devices using supported
Pt, Pd, and Rh catalysts. The content of CO, nitrogen oxides,
and hydrocarbons in the exhaust can be reduced by applying
three-way catalysts.2-4 Neither is CO desirable in electric fuel
cells, and its content should be addicuously controlled.5-7 In
sealed CO2 lasers the CO oxidation is carried out in order to
recombine CO and O2 formed by the dissociation of CO2 in
the laser discharge zone.8 For that purpose so-called NMRO
(noble-metal-reducible-oxide) catalysts are mainly used. These
materials may also be used in CO gas sensors.9
In addition to its technological importance, catalytic CO
oxidation is also interesting from a theoretical point of view.
The participating molecules in the oxidation contain only a few
atoms each, and the surface species formed during the reaction
is expected to be relatively simple. It is thus assumed that the
reaction mechanism will be understood in some detail. This
reaction is thus an excellent model reaction for other catalytic
systems. Among other catalysts for CO oxidation, well-defined
single crystalline as well as polycrystalline Pd surfaces (foils
or supported on oxides or metals) have been very frequently
investigated.10 Despite some indications of an Eley-Rideal
mechanism,11-14 it has been widey accepted that the oxidation
proceeds via the Langmuir-Hinshelwood reaction between the
adsorbed CO and adsorbed O atoms.10,12,15-32
Adsorption of oxygen to the Pd surface has also been
intensively studied using a wide variety of techniques. In
summarizing the results we may conclude that oxygen adsorbs
molecularly at low temperatures (150 to 200 K) and dissocia-
tively at higher temperatures.15,18,71-78 It also diffuses into metal
producing subsurface oxides10,15,18,23,74,77,79,80 and may also be
dissolved in palladium bulk.31,81 As with the CO, at small
coverage the adsorbed O atoms are distributed randomly over
the Pd surface. With increasing coverage the adsorbed species
form regular layers that are compressed at higher cover-
age.15,72,81,82 In addition to temperature, the borders between
different adsorption mechanisms also depend on oxygen
coverage.83-85 Adsorption may also induce reconstruction of
the metal surface.76,83
The adsorption of CO to the Pd surfaces has been studied
through the application of numerous complementary methods.
Only the findings essential for our work are summarized below.
CO mainly adsorbs undissociatively,10,15,16,33 although there is
an evidence for dissociative adsorption on small Pd particles
(edge atoms).34-36 A small amount of CO may also diffuse
into substrate.34,36-38 The bond between CO and Pd is formed
by a mutual donation of electrons from the 5σ orbital of CO
into Pd, and back-donation of electrons from the 5d Pd orbitals
into the CO 2π* antibonding orbitals.10,39-50 The CO molecule
In contrast to CO and oxygen, adsorption of CO2 is very
weak; it is impossible to get any measurable amounts of this
molecule adsorbed to the surface.15,18
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
S1089-5639(97)01118-3 CCC: $14.00 © 1997 American Chemical Society

Catalytic Oxidation of CO over Pd/Al2O3
J. Phys. Chem. A, Vol. 101, No. 39, 1997 7237
Bearing in mind the characteristics of adsorbed CO and
oxygen separately, we shall now explain their coadsorption
patterns and interactions resulting in surface reactions. Pread-
sorbed CO inhibits the dissociative chemisorption of oxygen,
while the adsorbed O atoms only slightly affect86 or do not at
all affect the possibility for additional uptake of
CO.10,15,16,18,20,22,78,85,87 As is evident from our experimental
conditions below, we are primarily interested in the coadsorption
when CO and oxygen are admitted to the surface at the same
moment. At lower coverages during the coadsorption the
adsorbed species of both reactants are distributed randomly over
the solid surface.68,88 As the coverage increases, adsorbed CO
and oxygen atoms form ordered isolated domains or is-
lands.10,15,16,20,22,25,30,64,85,87,89,90 Because of such complex coad-
sorption patterns, the chemical kinetics of the CO oxidation can
not be expected to be very simple. Even though in the majority
of investigations reaction orders of about +1 and -1 were found
for oxygen and carbon monoxide respectively, these orders
substantially depended on the experimental conditions, mainly
on temperature, CO and oxygen coverage, and the O2:CO
pressure ratio. They may have zero or even the opposite values
with respect to the above ones.13,16,21,25,31,91 The reaction is
always zero order with respect to CO2. The activation energy
of the oxidation depends on the temperature: it is around 50
kJ mol-1 at temperatures less than 400 K and around 100 kJ
mol-1 above.10,12,15,16,31,91,92 It falls with increasing oxygen
coverage: if it is about 100 kJ mol-1 at high coverage, it falls
to about 50 kJ mol-1 at low coverage.28 It also depends on the
particle size: if it is 32 kJ mol-1 for 27 nm Pd supported
particles, it falls to 19 kJ mol-1 for 2.5 nm particles.93 As on
Pt, also on Pd under some specific circumstances the reaction
turns to oscillation mode.21,23,24,26,29,30,61,94-99 Unlike with Pt,
here the phenomenon may be explained by the formation of
subsurface oxygen or oxygen oxide,23,24,26 the formation of two-
dimensional transition in chemisorbed phase,96 or a combination
of both.97 Nevertheless, this explanation has not been fully
proven.29 In connection with our work it should also be
mentioned that under the steady state conditions, the kinetics
of CO oxidation are practically independent of the surface
orientation of Pd catalysts.16,25,54,58,68 CO oxidation has been
studied under very different conditions: low and high presurres
(pressure gap), single crystals, polycrystals, and supported metals
(material gap). It has been shown that there is no significant
material gap and that with caution an extrapolation may be made
from low- to high-pressure data.31 This will allow us to compare
our results with the wealth of findings in the literature. The
overall CO oxidation over Pd may be written in the following
steps:
CO molecules are much more mobile than adsorbed O atoms
and may thus reach the O site by climbing and moving over
the Pd surface.10,16,20 There have been some suggestions for
the transition state of the reaction 3 above.18,31,89,90,100-102 The
most valuable information about the reaction site and thus about
the transition state comes from angular distribution and velocity
distribution studies of the desorbing CO2 formed during the
reaction20,89,90,103-105 and from infrared studies of CO2
formed.106,107 The transition state may be a linear,89,104 but more
probably a bent, CO2.18,31,85,100,101 Neither CO-Pd-O pos-
sibility should be excluded.25,31,87 The aim of our work was to
get additional information on the transition state by studying
carbon-13 kinetic isotope effects.
2. Experimental Work
Equipment, Materials, Procedure. All the experiments
were carried out in a Pyrex glass vacuum system described
elsewhere.110 Its main parts are 10 dm3 vessels for storing CO,
oxygen, and CO/O2 mixtures, traps for purification of CO2, a
mercury manometer, a Toepler mercury pump, and a 220 cm3
cylindrical quartz reaction vessel (L 40 mm). The desired
reaction temperature was provided by an electric tube kanthal
furnace and controlled within (1 K by means of a thermoregu-
lator connected with a NiCr-Ni thermocouple attached to the
outer surface of the reaction vessel. A commercial Pd catalyst
was taken: L 6 mm × 3 mm pallets of 0.5% Pd on a γ-Al2O3
support, produced by Baker, batch No.3107. Before use the
catalysts was outgassed and conditioned for 1 h in hydrogen
gas at 40 kPa and a temperature of 573 K. A 99.9% CO,
produced by L’Air Liquide, Paris, France, was used without
any purification. A commercial oxygen, produced by Rusˇe,
Slovenia, was distilled at liquid nitrogen temperature, and the
medium fraction was retained. The main impurity, as deter-
mined by mass spectrometry, was nitrogen, below 0.2%. Both
gases were mixed to obtain a reaction mixture with a CO/O2
ratio of 1:2.
3. Reaction Kinetics
The kinetics were studied at temperatures of 323 and 373 K
and with 0.56 and 1 g of catalyst. For that purpose, the reaction
vessel was connected to a mercury manometer and the total
pressure was followed during the reaction. Two typical pressure
runs are shown in Figure 1. The following two kinetic laws
were tested
PO
dP
Pt - 2(P0 - P∞)
2(Pt - P∞)
2
-
CO ) k
) k
(5)
(6)
dt
PCO
O2(g) f 2O(a)
(1)
(2)
dP
PCO
PCO
Pt - P∞
P0 - Pt
-
CO ) k
) k
CO(g) f CO(a)
dt
2
CO(a) + O(a) f CO2(a)
CO2(a) f CO2(g)
(3)
(4)
in which the partial pressures are expressed via the total pressure
according to the following relations
PCO ) 2(Pt - P∞)
(7)
(8)
It is evident from the above coadsorption presentation that
the Langmuir assumption of an energetically homogeneous
surface is not obeyed and that we can hardly talk about a true
Langmuir-Hinschelwood mechanism for reaction step 3 be-
tween adsorbed CO and O atoms.10 The reaction takes place
along the boundaries between adsorbed CO and adsorbed O
domains,10,15 so that not all the adsorbed O atoms are reactive
but only those close enough to the CO domains.12,20 Adsorbed
PO ) Pt - 2(P0 - P∞)
2
PCO ) 2(P0 - Pt)
(9)
2
P0, Pt, and P∞ being the total pressures at times 0, t, and ∞.

7238 J. Phys. Chem. A, Vol. 101, No. 39, 1997
Ogrinc et al.
Figure 1. Two examples of the Pt total pressure runs in the catalytic
oxidation of CO over a Pd/Al2O3 catalyst: (O) 323 K (P0 ) 26.6 kPa,
P∞ ) 22.5 kPa); (4) 373 K (P0 ) 23.7 kPa, P∞ ) 19.8 kPa).
Figure 3. Reaction kinetics for various masses of catalyst at 373 K:
(+) 0.56 g (P0 ) 24.1 kPa, P∞ ) 20.3 kPa); (O) 1 g (P0 ) 22.9 kPa,
P∞ ) 19.3 kPa); (b) 1 g (P0 ) 22.6 kPa, P∞ ) 19.1 kPa).
in which kabs stands for the absolute rate constant reduced to
the mass of catalyst, m, and the reaction volume, V.
If in expression 6 the differences of the total pressures are
expressed by P0CO (the initial CO partial pressure), P∞, and the
extent of reaction, f, reached within a time t, then we may write
V
tm
1
kabs
)
P0CO ln
- f
(13)
(
)
1 - f
4. Kinetic Isotope Effects
At least these two isotopic reactions run simultaneously in
the catalytic oxidation of carbon monoxide of a natural isotopic
composition
Figure 2. Checking the experimental data for the kinetic laws F1(P0,
Pt, P∞) (4), eq 10, and F2(P0, Pt, P∞) (O), eq 11; (b) Pt (P0 ) 26.6
kPa, P∞ ) 22.5 kPa).
k12
The above kinetic equations become after integration respec-
tively
12CO + 1/2O2
13CO + 1/2O2
9
8 12CO2
Pt - 2(P0 - P∞)
k13
9
8 13CO2
(14)
F1 ) (P0 - Pt) + [P∞ - 2(P0 - P∞)] ln
)
P0 - 2(P0 - P∞)
1
4
F1(P0, Pt, P∞) ) kt (10)
and the carbon-13 kinetic isotope effect (KIE-13) may be
defined as follows
P0 - P
Pt - P∞
F2 ) (P0 - P∞) ln
∞ - (P0 - Pt) )
KIE-13 ) 100 ln(k12/k13)
(15)
1
F2(P0, Pt, P∞) ) kt (11)
2
For the measurement of the kinetic isotope effects, the
reaction vessel with the catalyst was heated to a desired
temperature, the reactant gas mixture was added, and the reaction
was allowed to run up to a desired extent of reaction. Carbon
dioxide formed was isolated in a trap cooled with liquid nitrogen
by means of the Toepler pump and then further purified by
several distillations between two traps cooled with liquid
nitrogen (77 K) and a mixture of CCl4-CHCl2-liquid nitrogen
(195 K) respectively. The kinetic isotope effects were calculated
via the equation111
When the experimental data are input into the above expressions,
only F2 gives a straight line, as shown in Figure 2. This is also
confirmed by Figure 3 for two masses of catalyst. Thus we
may conclude that under our experimental conditions, the
reaction orders are +1 for CO, -1 for CO2, and 0 for O2. It is
well-known that CO2 desorbes immediately after being
formed;15,18 thus, the inhibition of CO2 found in our steady state
experiments may be explained by a readsorption of CO2.
Another explanation would be the formation of carbon on the
surface followed by a decrease of reaction rate,34,35,37,108,109
which could then be manifested as a CO2 inhibition.
k12
k13
ln(1 - f)
In order to enable a comparison of reaction rates for different
masses of catalyst, the following relation is useful
)
(16)
ln 1 - f‚S 13
(
(12))
m
k ) k
(12)
absV
where S(13) stands for the C-13 isotopic enrichment of the CO2
12

Catalytic Oxidation of CO over Pd/Al2O3
J. Phys. Chem. A, Vol. 101, No. 39, 1997 7239
TABLE 1: Experimental Conditions and Experimental
Results of the C-13 Kinetic Isotope Determination in the
Oxidation of CO over Pd/Al2O3 Catalyst
13
12
T,
K
catalyst
mass, mg kPa min
P0CO
,
t,
f
S( )
100 ×
ln(k12/k13) m2 s-3
103 kabs
,
323
560
560
7.98 90 0.1038 0.9931
9.87 95 0.1072 0.9938
8.70 90 0.1173 0.9940
8.70 90 0.1249 0.9937
0.73
0.67
0.65
0.68
0.81
1.00
1.16
0.95
0.61
0.97
0.96
0.95
0.95
0.95
0.93
1.39
1.45
1.08
1.29
3.3
4.3
4.6
5.3
353
8.26
8
0.0670 0.9922
15.4
18.4
28.6
40.0
59.3
8.81 10 0.0780 0.9905
8.30 12 0.1095 0.9890
5.91 15 0.1668 0.9915
8.35
7
0.1200 0.9943
383
413
80
40
9.67 40 0.1370 0.9910
9.99 35 0.1360 0.9912
10.08 30 0.1180 0.9912
9.74 35 0.1345 0.9913
10.31 30 0.1130 0.9912
9.75 39 0.1340 0.9915
113
130
114
125
105
112
434
617
264
314
Figure 4. Arrhenius plot of the overall rate constants kabs (eq 13).
9.26
6
0.0770 0.9867
state were obtained by solving Wilson’s FG matrix equa-
tion116,117
9.34 15 0.1410 0.9866
8.88 10 0.0790 0.9896
8.95 10 0.0860 0.9877
GFL ) LΛ
(20)
formed up to an extent of reaction f and is defined by
in which G is the Wilson matrix, F is the force-constant matrix,
L is the eigenvector matrix, and Λ is a diagonal matrix of
eigenvalues λii ) 4π2νi2 with νi equal to the frequency of the
ith normal vibration.
[13C16O2]f
[12C16O2]f
[13C16O2]1
[12C16O2]1
13
12
For the rate-determining elementary step (eq 3 above) of the
reaction mechanism, for which Bigeleisen’s formula is valid, a
bent (CO2)q transition state was supposed. For its internal
coordinates, changes of both C-O bond lengths, designated by
D and d, and of the interbond angle R were taken. In
calculations the values of diagonal F matrix elements were
varied in the following ranges: FD and Fd from 100 to 2000 N
m-1 in steps of 100 N m-1; FR from 50 to 300 N m-1 in steps
of 50 N m-1 (100 N m-1 equals 1 mdyn A-1). The interbond
angle R took values between 80 and 170° in 10° steps. The
off-diagonal elements were set zero, with the only exception
of the FDd element needed to get a zero determinant of the F
matrix, thus resulting in one zero normal frequency belonging
to the reaction coordinate. Its value was calculated from the
equation
S
)
(17)
( )
and obtained by analyzing the CO2 at the extent of reaction f )
f and f ) 1 (CO2 at f ) 1 has the same isotopic composition as
the unreacted CO), respectively, on a double-collector mass
spectrometer.112
In Table 1 the main experimental data are collected. The
temperature dependence of the kinetic isotope effects may be
expressed by
k12
831
T
100 ln
) 3.18 -
((0.15)
(18)
( )
k13
FDd ) ( (F ‚F )
(21)
If the kabs values from Table 1, as calculated by using eq 13,
are plotted in the Arrhenius diagram, Figure 4 is obtained
x
D
d
resulting in an activation energy of 56 ( 3 kJ mol-1
.
In our case only an asymmetric stretching vibrational normal
mode of the transition state can describe the movement along
the reaction coordinate-CO2 formation: one of the bonds is
5. Discussion
weakened from its value in CO (2000 N m-1 118,119
to a value
)
Theoretical interpretation of the experimental kinetic isotope
effects was made following Bigeleisen’s formalism113,114 based
on the absolute rate theory. The carbon-13 kinetic isotope
effects may accordingly be calculated114 from the equation
in CO2 (1600 N m-1),119 while the other bond is formed, thus
having a stretching force constant between zero and a value in
CO2. Hence in our calculations the “+” sign was used in the
above equation. An agreement between the calculated and
experimental kinetic isotope effects was sought by a graphic
method already used in the catalytic oxidation of CO over NiO
and ZnO,120,121 in which for selected values of other parameters
of the transition state, a region of acceptable values of both
stretching force constants (such as giving an agreement with
experiment) is plotted. Figure 5 shows two examples of such
regions for several values of FR and interbond angles R of 80°
and 90°, respectively. We see that higher interbond angles are
accompanied by higher stretching force constants. This is
further confirmed in Figure 6 where acceptable regions for 100°
and 110° angles are plotted together. As seen in Figure 7, for
the 120° angle, acceptable regions were found only for the
values of bending force constants being unrealistically high.122-125
q
q
q
3n-6
3nq-7
k1 νL1
u2i sinh(u1i/2)
u1i sinh(u2i/2)
u1i sinh(u2i/2)
)
(19)
∏
∏
νqL2
uq2i sinh(uq1i/2)
k2
i)1
i)1
in which q denotes the transition state. The first product runs
over all the isotopic frequencies of the reactant and the second
one over the real frequencies of the transition state, νL is the
frequency belonging to the reaction coordinate, ν ) hcω/kBT
(ω, wavenumber in cm-1; h, Planck’s constant; kB, Boltzmann’s
constant; c, speed of light; T, temperature).
The isotopic normal frequencies for the reactant CO molecule
were obtained from the literature,115 while those for the transition

7240 J. Phys. Chem. A, Vol. 101, No. 39, 1997
Ogrinc et al.
Figure 5. Ranges of acceptable values of both stretching force
constants for selected values of the bending force constant and an
interbond angle of 80° and 90°.
Figure 7. Ranges of acceptable values of both stretching force
constants for selected values of the bending force constant and an
interbond angle of 120°.
Figure 8. Agreement with the experiment for two successful examples
of the (CO2)q transition state with parameter values collected under
(1) and (2) in Table 2.
Figure 6. Ranges of acceptable values of both stretching force
constants for selected values of the bending force constant and interbond
angles of 100° and 110°.
ate. In Figure 8 agreement with experiment is shown for two
sets of the transition state parameter values listed in Table 2.
TABLE 2: Geometries, Force Constants, and Isotopic
Frequencies of Normal Vibrations of Two Successful
Examples of the (CO2)q Transition State
6. Conclusions
Our study of the C-13 kinetic isotope effects in the catalytic
oxidation of CO over Pd confirms a bent structure of the (CO2)q
transition state of the rate-determining step of the reaction
mechanism of this reaction. Its interbond angle is around 110°,
which is close to the structure suggested before on the basis of
other experimental data.85 The reaction coordinate is described
by an asymmetric motion of the transition state: one C-O bond
is weakened while the other one is being formed. Due to this
limitation only an asymmetric transition state is acceptable, even
though there are also indications of a symmetric structure.101
The resulting acceptable values of force constants of the
transition state are shown in Figure 8, while in Table 2 values
of geometric parameters and isotopic frequencies of normal
vibrations for two examples are collected.
FD,
Fd,
FR,
D,
nm
d,
nm
ω12,
ω13,
N m-1 N m-1 N m-1
cm-1
cm-1
(1)
(2)
1800
1600
1200
1200
250
300
0.112 0.121 2524.2 2472.5
853.8
0
846.4
0
0.115 0.121 2457.8 2406.0
914.9
0
907.8
0
For interbond angles above 120° no agreement with experiment
can be obtained. Because of the asymmetric motion of the
reaction coordinate, in Figures 5-7 only regions with one of
the stretching force constants above and the other below 1600
N m-1 are of interest. We see that angles R of 100° and below
do not fit our requirements. Because no acceptable ranges of
parameter values were found for angles R above 110°, only a
transition state with an interbond angle around 110° is appropri-
Acknowledgment. The authors are greatful to Dr. V. Kramer
at the J. Stefan Institute for performing mass-spectrometric

Catalytic Oxidation of CO over Pd/Al2O3
J. Phys. Chem. A, Vol. 101, No. 39, 1997 7241
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