Environmental effects on the formation of the primary and secondary ozonides of ethylene at cryogenic temperatures

Article abstract of DOI:10.1021/ja9532852

Ethylene and ozone were co-deposited in a cryogenic matrix of argon at 15-26 K and of CO₂ at 12-20 K. In the argon matrix, no perceptible reaction took place at any temperature below the softening onset of the matrix, while formation of ethylene ozonides (both primary and secondary) was observed at temperatures as low as 25 K in the amorphous CO₂ matrix. The ozonides were identified by their infrared spectra, whose assignments were confirmed with the help of an ab initio calculation. The same reaction is imperceptible in a crystalline CO₂ matrix deposited at 65 K, becoming observable only at 77 K and higher temperatures. Molecular dynamics simulations carried out for the solid argon host indicate that a highly organized crystalline structure does not allow the motions required for the reactions. These restrictions are apparently less stringent in the amorphous solid CO₂, suggesting it as a suitable solvent for the study of reactions having a low activation barrier.

Full text of DOI:10.1021/ja9532852

J. Am. Chem. Soc. 1996, 118, 3687-3693
3687
Environmental Effects on the Formation of the Primary and
Secondary Ozonides of Ethylene at Cryogenic Temperatures
†
†
,†
‡
‡
U. Samuni, R. Fraenkel, Y. Haas,* R. Fajgar, and J. Pola
Contribution from the Department of Physical Chemistry and the Farkas Center for
Light-Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel, and
Institute for Chemical Processes Fundamentals, The Czech Academy of Sciences,
Suchdol, Prague, Czech Republic
X
ReceiVed September 25, 1995
Abstract: Ethylene and ozone were co-deposited in a cryogenic matrix of argon at 15-26 K and of CO2 at 12-20
K. In the argon matrix, no perceptible reaction took place at any temperature below the softening onset of the
matrix, while formation of ethylene ozonides (both primary and secondary) was observed at temperatures as low as
2
5 K in the amorphous CO2 matrix. The ozonides were identified by their infrared spectra, whose assignments were
confirmed with the help of an ab initio calculation. The same reaction is imperceptible in a crystalline CO2 matrix
deposited at 65 K, becoming observable only at 77 K and higher temperatures. Molecular dynamics simulations
carried out for the solid argon host indicate that a highly organized crystalline structure does not allow the motions
required for the reactions. These restrictions are apparently less stringent in the amorphous solid CO2, suggesting
it as a suitable solvent for the study of reactions having a low activation barrier.
Introduction
Scheme 1. Criegee Mechanism of Ethylene Ozonolysis
The Criegee mechanism1,2 of the ozonolysis of olefins is
generally accepted as the best way to account for the charac-
teristics of the reaction, which leads to the cleavage of the double
bond and the formation of aldehydes (or ketones) and acids.
The three-step mechanism involves three intermediates, the
primary ozonide, I (POZ), the secondary ozonide, III (SOZ),
and a carbonyl oxide, II, sometimes referred to as the Criegee
intermediate (Scheme 1). Criegee was able to isolate and study
3
some SOZ’s, which were later fully characterized by micro-
wave,4 and infrared spectroscopies. The more reactive POZ’s
,5
6
7
were first directly observed by NMR and by infrared spec-
troscopies in low-temperature matrices,8
-10
and more recently
the microwave spectrum of the ethylene POZ was obtained in
a seminal paper by Gillies et al.11 The Criegee intermediate
turned out to be much more elusive, and has not yet been directly
observed in an ozonolysis reaction, though the carbonyl oxide
was spectroscopically identified in other systems.12 Recently,
the structure of the van der Waals complex between ethylene
and ozone, which is presumably the precursor of the POZ, was
determined by microwave spectroscopy.13 Dioxirane was
identified by microwave spectroscopy in the gas-phase reaction
14
of ozone and ethylene at low temperatures, but not in
1
2
solution. It has also never been reported as a product of the
8
-10
reaction in low-temperature matrices.
Experimental attempts to verify Criegee’s mechanism and
identify the intermediates were accompanied by a considerable
15
16
theoretical effort. Semiempirical and ab initio methods were
used to study this mechanism and the properties of the
intermediates involved in it: the relative energies of these
species, the structures of the transition states, and the barriers
of the different stages of the reaction. It was generally agreed
that the formation of the POZ is the rate determining step, since
it usually involves the higher barrier. The failure to observe
the Criegee intermediate was explained by showing that the
energy barrier for its transformation to the secondary ozonide
(or directly to the final products) is smaller than the one required
for its formation from the POZ. Equilibrium structures of the
ozone-ethylene van der Waals complex and of the primary and
†
The Hebrew University of Jerusalem.
The Czech Academy of Sciences.
Abstract published in AdVance ACS Abstracts, March 15, 1996.
‡
X
(
(
(
1) Criegee, R. Rec. Chem. Prog. 1957, 18, 111.
2) Kuczkowski, R. Chem. Soc. ReV. 1992, 79.
3) Criegee, R. In Peroxide Reaction Mechanisms; Edwards, J. O., Ed.;
Wiley-Interscience: New York, 1960; p 29.
4) Gillies, C. W.; Kuczkowski, R. L. J. Am. Chem. Soc. 1972, 94, 6337,
609.
(
7
(
5) Latimer, R.; Kuczkowski, R. L., Gillies, C. W. J. Am. Chem. Soc.
1
974, 96, 348.
(
(
6) K u¨ hne, H.; G u¨ nthard, Hs. H. J. Phys. Chem. 1976, 80, 1238.
7) Greenwood, F. L. J. Org, Chem. 1965, 30, 3108. Greenwood, F.
L.; Durham, L. J. J. Org. Chem. 1969, 34, 3363.
8) Hull, L. A.; Hisatsune, I. C.; Heicklen, J. J. Am. Chem. Soc. 1972,
4, 4856.
(
(13) Gillies, C. W.; Gillies, J. Z.; Suenram, R. D.; Lovas, F. J.; Kraka,
E.; Cremer, D. J. Am. Chem. Soc. 1991, 113, 2412.
(14) Lovas, F. J.; Suenram, R. B. Chem. Phys. 1977, 51, 453. Suenram,
R. B.; Lovas, F. J. J. Am. Chem. Soc. 1978, 100, 5117.
(15) Dewar, M. J. S.; Hwang, J. C.; Kuhn, D. R. J. Am. Chem. Soc.
1991, 113, 735.
9
(
(
(
9) Nelander, B.; Nord, L. Tetrahedron Lett. 1977, 32, 2821.
10) Kohlmiller, C. K.; Andrews, L. J. Am. Chem. Soc. 1981, 103, 2578.
11) Gillies, J. Z.; Gillies, C. W.; Suenram, R. D.; Lovas, F. J. J. Am.
Chem. Soc. 1988, 110, 7991.
12) Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344.
(
(16) Cremer, D. J. Chem. Phys. 1978, 69, 4456; 1978, 70, 1898.
0
002-7863/96/1518-3687$12.00/0 © 1996 American Chemical Society

3
688 J. Am. Chem. Soc., Vol. 118, No. 15, 1996
Samuni et al.
secondary ozonides calculated by ab initio methods13,17 were
found to agree satisfactorily with experimental data.
Table 1. Parameters of the Atom-Atom Pairwise Lennard-Jones
a
6
-12 Potentials Used in the Molecular Dynamics Simulation
atom pair
σ (Å)
ꢀ (cm-1)
In this paper we report that the primary and secondary
ozonides of ethylene can be formed at much lower temperatures
than previously observed, and that at a given temperature the
reaction’s progress depends on the matrix surrounding the
reacting pair. Thus, no reaction was found to take place in an
argon matrix at temperatures up to 35 K, while in a CO2 matrix
reaction products were identified at temperatures as low as 25
K. Molecular dynamics (MD) simulations of the trapping sites
in an argon matrix are reported, and it is shown that their
structure is unsuitable for ozonide formation. The apparently
more open structure of amorphous CO2 appears to allow the
reaction to proceed at temperatures as low as 25 K.
In all experiments both the primary and secondary ozonides
of ethylene were observed by their infrared spectrum, whenever
reaction took place. The assignment of the spectra was aided
by an ab initio calculation of the vibrational frequencies and
infrared intensities of these two compounds. The implication
of the simultaneous appearance of the ozonides (and the failure
to observe directly the Criegee intermediate) on the applicability
of the Criegee mechanism to this system is briefly discussed.
Ar-Ar
C-Ar
C-Ar
H-Ar
H-Ar
O-Ar
H-O
3.40
83
3.357
3.375
3.207
3.105
3.175
2.88
3.15
4.03
3.69
3.42
42 (for ethylene)
54 (for POZ)
18 (for ethylene)
22 (for POZ)
59
16
39
154
74
30
81
C-O
Xe-Xe
C-Xe
H-Xe
O-Xe
3.49
a
Sources: Reference 19. Hallam, H. E. Vibrational Spectroscopy
of Trapped Species; Wiley: London, 1973; p 45. The potential
parameters for unlike atoms, were derived from the literature atom-
19
atom parameters using the usual combination rules.
approach the surface under the action of the interaction potentials.
Periodic boundary conditions were imposed on the crystal surfaces
perpendicular to the growing surface, thus simulating an infinite crystal.
After the molecules were deposited, more argon atoms were deposited,
at least as many as used for the template. The total number of argon
atoms used in a typical simulation was 600. Many repetitions of the
simulations were performed, in which the initial locations of the
deposited atoms and molecules were varied at random, but always
chosen from a 300 K Boltzmann distribution. Different trapping sites
could be generated in different runs, and the most frequently encoun-
tered ones were considered the most stable ones, as discussed in more
detail elsewhere.18
Experimental and Computational Details
Ozone was prepared from a purified oxygen (98.5%, main impurity
N
2
) by electric discharge. Ethylene (99.5%) from Aldrich was used
as received; each gas was mixed separately with a carrier gas (argon
or CO ) to the desired concentration using standard manometric
2
techniques. An all-glass vacuum line was used for ozone, and a
stainless steel one for ethylene. The gases were codeposited (at a rate
of 2-10 mmol/h) on a KBr window attached to the cold tip of a closed
cycle helium cryostat (Air Products Model CS202). The infrared
spectrum of the resulting matrix was recorded by a Fourier transform
Ab initio calculations were carried out using the Gaussian 92 program
21
package at the HF or HF-MP2 levels. Several basis sets were used:
preliminary calculations were performed with a 6-31G, and more
22
sophisticated ones with a 6-311G* basis set. Complete structural
-
1
spectrometer (Nicolet Model 520, 0.5-cm resolution) at any desired
temperature higher than 12 K. The temperature was controlled by a
Lake Shore Cryogenics Temperature Controller (Model 330 Autotuning)
to within (0.5 K. Three characteristic temperatures were used in the
optimization was performed for each species, and vibrational frequen-
cies were calculated from the Hessian matrix using the harmonic
approximation at the optimized geometries. No imaginary frequencies
were found in the calculation, showing that the optimized structure
was indeed a minimum. The resulting frequencies were uniformly
experiments:
Tdep, the deposition temperature; Treac, the reaction
temperature; and Tspec, the temperature at which the spectra were
recorded. The reaction temperature was defined as the highest
temperature at which a reaction could be observed in a sample by
monitoring its infrared spectrum. The time intervals, treac, at which
the matrix was exposed to Treac varied between a few minutes and
23
scaled by 0.9427 for the smaller basis set and by 0.954 for the larger.
Results
Figure 1 shows a portion of spectra obtained in an argon
-
1
several hours. In a typical experiment, ozone in CO
2
(1:265) and
matrix between 920 and 980 cm . Ozone has no absorption
bands in this range, and ethylene was found to exhibit two
ethylene in CO (1:310) were codeposited at 15 K. The matrix was
2
warmed to 26.5 K in 128 min; during the heating period, spectra were
-1
absorption features at 946.9 and 959.7 cm , which are
recorded in the temperature intervals 16-17.4, 19-20.4, 22-23.4, and
apparently due to two different trapping sites of monomeric
2
5-26 K. The temperature was held at 26.5 K for 18 h; most of the
24
ethylene in an argon matrix. These two bands were observed
spectral changes occurred in the first 2 h, and after 10 h further changes
were minimal. The matrix was then warmed to 27 K in 10 min, and
held at that temperature for 12 h, without showing any further changes.
Warming gradually to higher temperatures at a very slow rate did not
for highly diluted ethylene in argon matrices (argon/ethylene
ratio ∼ 6000). A new band, which appeared only when both
ozone and ethylene were codeposited, is clearly seen at 951.1
-
1
cause further reaction until the CO
2
began to evaporate.
cm ; this band was not due to an oligomer of either ethylene
or ozone, as verified by depositing the pure compounds at
Molecular dynamics simulations were performed as previously
18
described, except that the molecules were treated as rigid bodies using
the RATTLE algorithm.1 Lennard-Jones pairwise atom-atom interac-
tion potentials were used, with the potential parameters listed in Table
(21) Frisch, M. J.; Trucks, G. W.; Gordon-Head, M.; Gill, P. M. W.;
9
Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M.
A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley,
J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.;
Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision F2; Gaussian, Inc.:
Pittsburg, PA, 1992.
1
. Full technical details of the simulations can be found in a separate
2
0
paper; briefly, the geometry of the molecules was taken from
microwave experimental data. The simulation was begun by construct-
ing a small crystal lattice, consisting of a few hundred atoms, that serves
as a template for further growth. Argon atoms were allowed to
approach the surface of the template, which is kept at a low constant
temperature. The molecules to be deposited were also allowed to
(
22) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2
257.
(
23) Pople, J. A.; Scott, A. P.; Wong, M. W.; Radom, L. Isr. J. Chem.
1
993, 33, 345.
(24) Surprisingly little information is available on the spectra of ethylene
in an argon matrix. To our knowledge, the most dilute spectra (argon/
ethylene ratio ) 999) were reported by: Rytter, E.; Gruen, D. M.
Spectrochim. Acta 1979, 35A, 199. They found an incompletely resolved
(
(
(
17) McKee, M. L.; Rohlfig, C. L. J. Am. Chem. Soc. 1989, 111, 2497.
18) Fraenkel, R.; Haas, Y. Chem. Phys. 1994, 186, 185.
19) Allen, M. P.; Tildesley, D. J. Computer Simulations of Liquids;
-1
doublet at 948 and 959.5 cm . As Figure 1 shows, we find a well-resolved
-
1
Oxford University: New York, 1987.
20) Fraenkel, R.; Haas, Y. To be submitted for publication.
pair at 946.9 and 959.7 cm (these values vary somewhat with deposition
conditions).
(

EnVironmental Effects on the Formation of Ozonides
J. Am. Chem. Soc., Vol. 118, No. 15, 1996 3689
Figure 1. A portion of the absorption spectrum of an argon matrix
taken at 26 K in which ozone and ethylene were codeposited at the
same temperature (solid line). The dashed line shows the spectrum
obtained when ethylene only was deposited under the same conditions.
different dilutions. It cannot be due to a cluster of ethylene
with either water or CO2, the most common impurities, as
checked in separate experiments. It is also not likely to arise
from a reaction product between ethylene and ozone: it was
formed under conditons where no other new bands were
observed, and the IR bands due to the first reaction product,
the primary ozonide, were clearly seen upon subsequent heating
of the matrix. Therefore, this band is tentatively assigned to
the ethylene-ozone van der Waals adduct. (This conclusion
is supported by the observation of weaker bands assignable to
Figure 2. Infrared spectra of solid argon matrices obtained upon
codeposition of ethylene and ozone (a baseline correction was applied
to all spectra). (a) The spectrum of a matrix deposited at 15 K, recorded
at 30 K. (b) The spectrum of the same sample after heating to 130 K
and evaporating both argon and unreacted ozone and ethylene. Bands
due to the POZ are marked by P and those due to the SOZ by S.
-
1
-1
the adduct at 1440.2 cm , very close to the 1440.7 cm
-
1
ethylene band, and at 2992.2 cm ; the latter is weak and
-
1
broader than the other twosfwhm ) 1.4 cm . The fact that
all three bands are found to be close to a strong ethylene
absorption band is consistent with their assignment to an
ethylene adduct.)
after the deposition, as long as the matrix was kept at
temperatures below 25 K, even for elongated periods of time
(several days).
Warming the matrix to 35 K did not result in the appearance
of new bands, nor in the decrease of the intensity of any of the
old bands. Attempts were made to begin the reaction by
irradiating the matrix by an infrared laser tuned to the absorption
Warming the matrix to 25 K or to a higher temperature led
to the appearance of SOZ and POZ bands; Figure 3 shows a
spectrum recorded after the matrix was warmed to 30 K.
Spectra similar to that shown in Figure 3 were recorded under
different experimental conditions in which the concentrations
of ethylene and ozone, the various characteristic temperatures
of the experiment (Tdep, Treac, and Tspec), as well as treac, were
varied over a large range. Table 2 summarizes some of the
results obtained under different experimental conditions; the data
show that the SOZ bands were found to be more intense than
the POZ ones for all experimental conditions used, with either
argon or CO2 as the matrix material. It is seen that the SOZ/
POZ concentration ratio, as measured from the IR peaks’
intensities, was remarkably constant for all of these experiments.
This result was obtained under a large variety of different
experimental conditions; in particular, the time between the
deposition and the recording of the spectrum varied between 1
and 26 h. During the intervening period, the matrix was either
kept at a constant temperature or subjected to programmed
heating/cooling procedures.
-
1
peak (the P(12) line of a pulsed CO2 laser at 951.2 cm ), but
no change in the IR spectrum could be discerned after a
prolonged irradiation at 100 mJ/pulse (few hours at 1 Hz).
New bands appeared only when the temperature was raised
beyond 44 K (a temperature at which the vapor pressure of the
argon matrix becomes high and rapid evaporation is observed),
and the previously observed absorption bands showed a marked
decline. At that temperature argon was quickly pumped away,
and the infrared spectra of the remaining species was found to
contain a mixture of the primary (POZ) and secondary (SOZ)
ozonides of ethylene, as determined by comparison with
previously published spectra.6
,8-10
Warming the sample to 130
K and pumping away the remaining ozone and ethylene led to
the spectrum shown in Figure 2, in which almost all absorption
bands can be assigned to one of the two ozonides.
This experiment was repeated in a CO2 matrix, deposited at
temperatures between 12 and 20 K. Two ethylene bands were
-
1
observed in the 950-cm region, as in the case of an argon
matrix, but they were less well resolved due to their smaller
The fact that a reaction was observed at a temperature as
low as 25 K in CO2 but not in argon might be due to the poorer
heat conductivity of the amorphous solid CO2. A possible
scenario is that the actual temperature of some parts of the
sample is higher than 25 K (the thermocouple was in thermal
contact with the window, not with the deposited matrix). A
reaction starts, and local heating (due to the exothermicity of
the reaction) leads to the appearance of products. This
-
1
separation (∼5 cm ) and rather large widths. In this matrix
an absorption feature assignable to a van der Waals cluster band
was also observed upon the codeposition of ozone and ethylene,
although only as a small shoulder on one of the strong ethylene
bands; its intensity varied between different runs, and often it
could not be clearly discerned from the stronger pure ethylene
bands. No IR bands due to either POZ or SOZ were observed

3
690 J. Am. Chem. Soc., Vol. 118, No. 15, 1996
Samuni et al.
Table 2. The Relative Concentrations of POZ and SOZ Formed in
the Reaction
matrix composition
tempa
C
2
H
4
ozone
CO
2
T
dep
T
spec
T
reac
SOZ/POZ ratiob
CO
15
15
15
65
65
15
15
20
20
20
2
Matrices
26
1
9
7200
5200
5200
5200
5200
600
600
600
600
600
26
13.7
11.1
11.4
11.8
13.3
11.2
13.6
12.5
11.8
12.3
1
1
1
1
1
1
1
1
1
8.6
8.6
8.6
8.6
1
1
1
29.5
32.5
65
65
26.5
80
40
65
80
29.5
32.5
77
90
26.5
26.5
40
65
80
1
1
1
2.3(1.0)d
Neat Filmc
1
1
1
1
1
2
2
2
2
5
5
5
5
5
0
0
0
0
43
53
60
90
100
44
60
90
130
43
53
60
52
52
44
60
60
60
7.1
7.7
7.5
5.6
7.3
9.1
9.9
7.7
7.7
7
.73(1.2)d
a
Tdep, deposition temperature; Treac, highest temperature at which
reaction was observed before recording the spectrum; Tspec, the
b
temperature at which the spectrum was recorded. The ratio of the areas
-1
-1
Figure 3. Infrared spectra of solid CO
codeposition of ethylene and ozone. (a) The spectrum of a CO
deposited at 15 K, taken at 15 K. (b) The spectrum of the same matrix
recorded at 30 K, after being kept at that temperature for 30 min.
2
matrices obtained upon
matrix
under the most intense lines of SOZ (1072 cm ) and POZ (926 cm ).
c
Prepared by co-depositing ethylene and ozone in an argon matrix and
2
evaporating the argon. d Average (standard deviation).
Table 3. Observed and Calculated Vibrational Frequencies and
Infrared Intensities of the Primary Ozonide of Ethylene (POZ)
possibility was checked by repeating the experiment many times,
under widely varying conditions: deposition rates and durations
freq (cm-1)
IR intensitya
(leading to different sample thicknesses), different concentrations
expc
approx descriptionb
calc
exp
calc
and different concentration ratios (that should lead to different
local final temperatures). Under all these conditons, the reaction
first became perceptible at the same temperatures25 K.
4
6
6
7
08
46
86
25
399
668
682
717
13
92
7
16
53
6
94
7
ring puckering
OOO out-of-plane skeletal deform.
OOO asymmetric stretch
CO in-ring deform.
OOO symmetric str + CH bend
57
In order to further check the temperatures and environmental
dependence of the reaction, CO2 matrices were also prepared
by depositing the same mixtures at 65 K, a temperature at which
8
37
8
46
18
8
45
3
34
100
6
OOO symmetric str + bend
CO + CC stretch + CH wag
CO stretch
CH in-plane bend
CH bend
25,26
the compound is known to form a crystalline solid.
These
9
26
927
983
100
98
9
conditions were used by Nelander and Nord in their early
experiments, and we repeated them in order to enable a proper
comparison with the lower temperature results. In this case, a
temperature of 77 K was required to observe a perceptible
product absorption band as reported by ref 9. The second surge
in POZ and SOZ formation reported by these authors at 90 K
was also clearly observed. The SOZ/POZ signal intensity ratio
under all these conditions was essentially the same as that
observed for the low-temperature deposited matrix. Warming
up the matrix to a temperature at which CO2 evaporates (120-
983
1214d 1214
1
328
1313
23
6
a
Normalized to the most intense bands926 cm-1 in the experiment,
1
-
983 cm in the calculation. The experimental values are based on
b
the integrated area under the absorption band. Based on the ab initio
calculation. c In a neat film. d Congested band region in our experiments,
value taken from ref 8.
satisfactory, allowing proper assignment of the spectrum. A
more detailed discussion of the calculation and assignment will
be published separately.
relative IR intensities of the SOZ were also calculated by the
same method, the results agreeing very well with the analysis
1
30 K) resulted in diminution of the SOZ signal, probably due
27
The vibrational frequencies and
to evaporation.
Some codepositions of ozone and ethylene at 15 K without
any solvent were also made. Warming to 44-55 K resulted in
the observation of POZ and SOZ absorption bands, in this case
with a somewhat different ratio than in the CO2 matrix case, as
seen from Table 2. The experimental vibrational frequencies
and relative IR intensities of the POZ are listed in Table 3 and
compared with the values calculated by the ab initio quantum
chemical method. Agreement with experiment is seen to be
6,28
of G u¨ nthard and co-workers.
In order to convert the relative values to absolute ones, it is
necessary to know the relative molar absorption coefficients of
at least one band each for the POZ and the SOZ; this information
is not available at the moment. A rough estimate may be made
based on the computed results, according to which the absolute
(
27) Samuni, U.; Haas, Y. Spectrochim. Acta, in press.
(
(
25) Falk, M.; Seto, P. F. Can. J. Spectrosc. 1986, 31, 134.
26) Falk, M. Can. J. Chem. Phys. 1987, 86, 560.
(28) The results of the SOZ ab initio calculations are available from the
authors on request.

EnVironmental Effects on the Formation of Ozonides
J. Am. Chem. Soc., Vol. 118, No. 15, 1996 3691
intensity of the strongest SOZ band at 1072 cm- is about 20
1
-
1
times larger than that of the strongest POZ band at 926 cm .
This result leads to a SOZ/POZ concentration ratio of about
1
1
:2 in all the CO2 matrix experiments reported above, and of
:3 in the neat film experiments.
Attempts to identify absorption bands due to carbonyl oxide
(the Criegee intermediate) and formaldehyde (which sould be
formed together with it) were made throughout this work. In
line with previous reports,9 no such bands were observed. In
some experiments, a strong and narrow absorption band
,10
-
1
appeared in the 1715-1722-cm range; it is almost certainly
due to a carbonyl compound. These bands were usually
observed only after the matrix was subjected to relatively high
temperatures, and the solvent was evaporated. Other known
strong formaldehyde bands2
9-31
at 1500 and 2797 cm were
-1
sought, but were either not observed at all or observed at a much
smaller relative intensity than observed for authentic formal-
dehyde samples. It is concluded that formaldehyde monomer
are not formed under the experimental conditions used in this
-1
work, and the identity of the 1720-cm band remains uncertain.
One possibility is a polymeric form of a carbonyl compound.
Analysis and Discussion
The structure of the ozone-ethylene van der Waals adduct
was recently determined in the gas phase by microwave
spectroscopy;13 the two molecules are separated by about 3.24
Å and lie roughly parallel to each other. In the POZ, the oxygen
atoms are much closer to the carbons, about 1.42 Å.2
,11
The
molecular dynamics simulations performed on an fcc-based
argon matrices show that several trapping sites are possible for
ozone in argon, the most stable ones being mono- and
disubstitutional sites (ozone replacing one or two adjacent argon
atoms), in agreement with a recent analysis of the infrared
spectrum of ozone in rare gas matrices.3
2,33
The simulation
shows that the most stable trapping site for ethylene is a
disubstitutional one, a trisubstitutional one being also formed
but with a smaller probability.
Over 20 simulation runs by POZ depositions were made,
Figure 4. The structure of the most frequently obtained trapping site
of POZ deposited in an argon fcc lattice at 25 K as calculated by the
molecular dynamics simulation. Only a small portion of the actually
calculated structure is shown, to facilitate the exposition of the geometry
near the trapped molecule. The molecule is seen to displace three argon
atoms from a 111 plane of the crystal lattice, the displaced atoms
forming a triangle. The upper part shows a three-dimensional
representation of the site. The bottom part shows cuts taken along the
lattice planes in which argon atoms were replaced by the molecule.
The key is as follows: argon atoms are shown as open large circles,
hydrogen atoms as smaller open circles, oxygen atoms are black, and
carbon atoms are gray.
1
1
using the known gas-phase structure of the molecule. The
structure of the most frequently generated trapping site of POZ
is shown in Figure 4. The molecule is seen to occupy a tri-
substitutional site, the argon atoms being removed from a 111
plane. The MD simulation of the codeposition of ozone and
ethylene leads to several trapping sites in which the two
molecules lie close together; the calculated structures of two
commonly encountered sites are shown in Figure 5. In both
cases, the structure is found to be quite different from the most
stable gas-phase one,13 probably since it is determined pre-
dominantly by the argon lattice. It is immediately apparent from
the trapping sites’ structures shown that a considerable geo-
metrical rearrangement is needed to attain a configuration that
can evolve to the POZ structure. This rearrangement would
require a major movement of the matrix argon atoms, which is
extremely unlikely unless the matrix contains an appreciable
number of vacancies. Thus, the absence of a reaction between
ethylene and ozone in low-temperature argon matrices is
accounted for by the MD simulations. Only when the matrix
is severely softened, or eroded by evaporation of argon atoms,
does the reaction to form the POZ become observable.
It is reported that ethylene reacts with ozone in a xenon matrix
10
at temperatures exceeding 50 K. Xenon, as argon, forms a
fcc crystal, and we ran simulations on this solid in a similar
fashion. The main difference is the larger lattice constant of
34
xenon (6.136 Å at 20 K as compared to 5.318 Å for argon),
which allows more space in substitutional sites. Molecular
dynamics simulations were run for a fcc xenon lattice held at
50 K. It was found that in most runs, ethylene, as well as ozone,
occupies a single substitutional site in solid xenon; the van der
Waals pair occupies a disubstitutional site, and the POZ, a single
(!) substitutional site, which tends to displace neighboring matrix
atoms from their equilibrium positions somewhat, but occupies
a volume smaller than that engaged by the van der Waals pair.
It follows that according to the simulation, the transformation
of the van der Waals adduct to the primary ozonide can take
(29) Lugez, C.; Schriver, A.; Levant, R.; Schriver-Mazzouli, L. Chem.
Phys. 1994, 181, 129.
(30) Thomas, S. G.; Guillory, W. A. J. Chem. Phys. 1973, 77, 2469.
(31) Khoshkhoo, H.; Nixon, E. R. Spectrochim. Acta 1973, 29A, 603.
(32) Brosset, P.; Dahoo, R.; Gauthier-Roy, B.; Abou af-Marguin, L.;
Lakhlifi, A. Chem. Phys. 1993, 172, 315.
33) Lakhlifi, A.; Girardet, C.; Dahoo, R.; Brosset, P.; Gauthier-Roy,
B.; Abouaf-Marguin, L. Chem. Phys. 1993, 177, 31.
(
(34) Pollack, G. L. ReV. Mod. Phys. 1964, 36, 748.

3
692 J. Am. Chem. Soc., Vol. 118, No. 15, 1996
Samuni et al.
Scheme 2. Proposed Reaction Sequence Leading To
Formation of POZ and SOZ
found to be essentially complete in less than 30 min at 30 K,
while at 26.5 K, about 30% conversion was found after 2 h.
However, an exact value for the activation energy could not be
established since rapid heating caused damage to the rather
fragile matrix, and when slower heating was used, contribution
from lower temperatures than 30 K to the reaction progress
could not be neglected, nor quantified. In any case, a very low
barrier (less than 2 kcal/mol, and probably as low as 0.2 kcal/
mol) is indicated for the reaction in amorphous CO2.
In the highly ordered and more densely packed crystalline
solid obtained upon deposition at 65 K, no reaction takes place.
Heating to 77 K and higher temperatures appears to allow
rotational motion in trapping sites containing the ethylene and
ozone, generating a configuration that enables the transition state
to be formed. The additional upsurge in reactivity observed at
90 K may be due to the onset of diffusion that allows molecules
initially separated by lattice CO2 molecules to approach each
other and react, as suggested in ref 9. The solid state value is
lower than the reported gas-phase activation energys4.7-4.9
36,37
kcal/mol,
which was measured at much higher temperatures
(down to 178 K). An artifact caused by local heating of the
matrix due to the exothermicity of the reaction might conceiv-
ably be the reason for this discrepancy. This was carefully
checked by varying the concentrations of the reactants over more
than an order of magnitude. Since no differences in the
temperature dependence of the reaction under these widely
different conditions could be observed, this option appears
extremely unlikely. The reason for the lower activation energy
in the low-temperature deposited solid CO2 (as compared to
the gas phase) is not clear at this point. This phase is known
to be characterized by a very large surface area. Thus, this
solid appears to contain large cavities, allowing ozone and
ethylene to approach each other. Surface catalysis by this rather
open structure is an option, but we cannot suggest a mechanism
by which the energy barrier of the reaction is reduced. Further
work is needed to clarify this issue.
Figure 5. The structure of the two most commonly obtained trapping
sites for the ozone-ethylene van der Waals adducts in a fcc argon
lattice, as obtained by the molecular dynamics simulations (25 K
deposition). See the caption of Figure 4 for an explanation of the
symbols. One (a) is a three-substitutional site, in which the ethylene
and ozone molecules substitute three argon atoms lying in a row in a
3
8
1
11 plane of the crystal lattice. The other (b) is a disubstitutional site
in which the two molecules replace two adjacent argon atoms in a 100
plane of the lattice (compare with the triangular three-substitutional
site of POZ shown in Figure 4).
A notable result of this study is the constant SOZ/POZ
concentration ratio obtained in solid matrices whenever a
reaction took place. This was found to be the case for very
different concentrations, reactant ratios, heating rates, and
reaction temperatures (Table 2). The constant ratio is seemingly
at variance with the accepted Criegee mechanism, according to
which the POZ is formed first, and subsequently transforms,
via the Criegee intermediate, to the SOZ. In the low temper-
atures used in this study, POZ is indefinitely stable, and should
not form SOZ at all. The latter must, therefore, be formed either
before POZ has a chance to become vibrationally stabilized or
in an independent route. Scheme 2 offers a model that can be
used to qualitatively account for this apparent discrepancy.
place within a trapping site, without requiring a major motion
of the surrounding solvent atoms. The agreement of the xenon
matrix simulation with the experimental result enhances the
significance of the negative result obtained by the simulation
for the argon matrix.
Simulations of CO2 matrices were not performed at this stage,
but the observed data can be qualitatively accounted for from
2
5,26,35
the known properties of solid CO2.
Deposition in the
temperature range of 15-30 K leads to the formation of an
1
5
amorphous solid, while careful deposition at temperatures higher
According to semiempirical calculations, the overall barrier
of the reaction is due to the formation of the POZ. In the cold
environment of the matrix, the transition state formed by the
encounter between ozone and ethylene is stabilized by collisions
to form the POZ. Once in the bottom of its potential well (i.e.,
6
h
than 55 K results in a cubic T crystalline structure. The
amorphous form is considerably more “open” than the crystalline
one, and presumably permits more translational and rotational
motion than the latter, although the ambient temperature is
lower. Several attempts to measure the temperature dependence
of the POZ and SOZ formation were made; the reaction was
(
(
36) DeMore, W. B. Int. J. Chem. Kinet. 1969, 1, 209.
37) Becker, K. H.; Schurath, U.; Seitz, H. Int. J. Chem. Kinet. 1974, 6,
7
25.
(38) Kn o¨ zinger, E.; Schuller, W.; Langel, W. J. Phys. Chem. 1993, 97,
927.
(35) L o¨ wen, H. L.; Bier, K. D.; Jodl, H. J.; L o¨ wenschuss, A.; Givan, A.
J. Chem. Phys. 1989, 90, 5309.

EnVironmental Effects on the Formation of Ozonides
J. Am. Chem. Soc., Vol. 118, No. 15, 1996 3693
vibrationally relaxed), the barrier to dissociation to the carbonyl
oxide and formaldehyde (∼20 kcal/mol) is much too large for
a thermal reaction to take place at the temperatures used in this
work. Indeed, the POZ is known to be thermally stable up to
about 180 K.11 Thus, any thermally relaxed POZ formed in
the reaction is accumulated in the matrix. The SOZ, therefore,
must be formed also upon the initial encounter between ozone
and ethylene and not from a vibrationally relaxed intermediate.
A possible SOZ precursor is the vibrationally excited POZ
initially formed by the reaction. Assuming that the same barrier
must be overcome to form both ozonides, their branching ratio
is determined, in the Arrhenius description, by the pre-
exponential factor, which according to the transition state theory
relates to the entropy of activation of the reaction.
quantitative study of the kinetics of reactions characterized by
low activation barriers. In order to realize this option, it would
be necessary to find ways to mechanically stabilize amorphous
CO2 solids, which tend to be rather fragile under the conven-
tional preparation scheme used in this work.
Summary
The reaction between ozone and ethylene was studied at low
temperatures in the solid state. No reaction was observed in
an argon matrix, while in a solid CO2 the reaction proceeded at
temperatures as low as 25 K. The two major products identified
by infrared spectroscopy were the primary and secondary
ozonides of ethylene; they were produced at a constant ratio
under a wide variety of experimental conditions. The identi-
fication of the primary ozonide was supported by an ab initio
calculation of the IR spectrum. Molecular dynamics simulation
of the deposition indicates the trapping sites formed in the rigid
argon matrix are not suitable for the formation of the primary
ozonide from the ozone-ethylene van der Waals adduct. In
contrast, the trapping sites in a xenon matrix, which has a larger
lattice constant than argon, are calculated to be able to allow a
reaction without the need to disrupt the solid. The facile reaction
in amorphous CO2 at very low temperatures is apparently due
to the open structure of this solid.
As mentioned above, all attempts to identify absorption bands
due to carbonyl oxide (Criegee intermediate) and formaldehyde
(which is expected to form alongside it) failed. Experimental
evidence for carbonyl oxide existence in the ozonation reaction
2
depends solely on trapping experiments. However, the pos-
sibility that the trapping agents react with some reactive species
(such as vibrationally excited POZ) cannot be excluded. Such
reactions could lead to the same final products as those of the
Criegee intermediate. The experimental observation of the POZ
and SOZ at low temperatures allows, in principle, the direct
assessment of the sequence of events assumed by the Criegee
mechanism. Perusal of the available literature shows that
whenever these two ozonides were observed, the SOZ was
detected simultaneously with the POZ, rather than sequentially
Acknowledgment. This research was supported under Grant
No. HRN-2078, C12-223, US-Israel Cooperative Development
Research Program, Office of the Science Advisor, US Agency
for International Development. We thank a referee for his
comments regarding the temperature dependence of the reaction
in amorphous CO2. R.F. thanks the Israel Ministry for Sciences
and Arts for a Levy Eshkol Fellowship. We thank Dr. S. Zilberg
for his help in the ab initio calculations and for many
enlightening discussions.
(see for instance ref 11 for gas-phase results). It is concluded
that as far as existing kinetic and spectroscopic evidence goes,
both ozonides may be formed from the same precursor state.
The fact that the reaction is observed in amorphous CO2 at
temperatures as low as 25 K indicates that considerable
translational and rotational motions are possible in this environ-
ment. This suggests the use of this porous “solvent” for the
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