Article pubs.acs.org/JPCA
Stability of Criegee Intermediates Formed by Ozonolysis of Different Double Bonds Jaroslaw Kalinowski,† Petri Heinonen,† Ilkka Kilpelaï nen,† Markku Ras̈ an̈ en,† and R. Benny Gerber*,†,‡,§ †
Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1 (P.O. BOX 55), FI-00014 Helsinki, Finland Institute of Chemistry, Hebrew University, Jerusalem 91904, Israel § University of California, Irvine, California 92697-2025, United States ‡
ABSTRACT: The formation of Criegee intermediates by ozonolysis of different species containing CN and CP bonds is studied computationally. Electronic structure calculations are carried out for the energetics of ozonolysis, and the lifetime of the Criegee intermediate formed is computed by transition state theory. All calculations are carried out for formation of CH2OO, the simplest Criegee intermediate. Extremely large differences are found for the lifetime of CH2OO depending on the specific CN, CP, and CC precursor, due to the great variations in the exoergicity of the ozonolysis. The largest lifetimes of CH2OO are found to be up to a millisecond range for a Schiff base precursor, being orders of magnitude greater than for CC and CP precursors at the same conditions. The results provide insights into the role of the precursor in determining the stability of the Criegee species formed and suggest an approach for preparing Criegee intermediates of relatively long lifetimes.
I. . INTRODUCTION Because a large fraction of the tropospheric oxidation of unsaturated hydrocarbons is initiated by reaction with ozone,1,2 the detailed mechanism of these reactions has been of major interest in atmospheric chemistry for years. In 1949, Rudolf Criegee proposed a mechanism for ozonolysis of alkenes.3 This mechanism is generally accepted now.4,5 According to Criegee, the ozonolysis of alkenes goes through carbonyl oxides, and in recent years the carbonyl oxides have become a more popular subject than ozonolysis itself. It turned out that these carbonyl oxides, which are now commonly called the Criegee intermediates (CI), are very elusive. Difficulty in observing and characterizing CI’s arises from the very low stability of these species.6−8 Many theoretical studies have shown that CI’s are in fact fascinating species with very complicated electronic structure and low stability. The nature of a hybrid of biradical and zwitterion6−8 make theoretical studies of CI’s very difficult and challenging. CI’s became so popular among chemists not only because of their nature and structure but also due to their role in oxidation reactions of atmospheric relevance. By now it is confirmed that CI’s can oxidize species in the atmosphere by a direct reaction9−12 (oxidizing SO2, NOx) and they constitute an important source of the key to the oxidizing capacity at the troposphere; i.e., their decomposition is a source of OH radicals,13,14 which are important atmospheric oxidants.15 The assumed major role of CI’s in atmospheric chemistry has stimulated numerous theoretical and experimental studies.16 However, their direct identification was not made until very recently.17,18 Due to their very low stability, the identification of these species requires sophisticated tools and approaches. One of the breakthroughs that made the identification of CI’s © XXXX American Chemical Society
possible was a neat idea for synthesizing these. The big problem with the use of classic ozonolysis of alkenes for laboratory synthesis of CI’s is the fact that this reaction is highly exoergic,5 making the observation of these species excessively difficult. The new idea was to make the simplest possible CI by a reaction of CH2I with O2.19−21 The success of this method has resulted in many studies that include discovering complicated photochemistry of CI’s.22−24 Searching for other methods of synthesizing CI’s with improved stability remains important. In this study we explore the possibility of synthesizing CI’s from ozonolysis of double bonds other than CC, analyzing lifetimes of CI’s formed. It will be shown that ozonolysis of other compounds may result in stability orders of magnitude higher than in the case of alkenes leaving the possibility of other sources of stabilized CI’s in the atmosphere. It is because of their assumed role in the atmosphere that it is important to understand not only the nature of CI’s but also both their formation and their decomposition channels. They were proposed from the very beginning to be intermediates for reactions of ozonolysis of CC and CC bonds, but it has been already indirectly established that other double bonds can also lead to CI’s.25 Therefore, the relative stability of CI’s formed in different reactions is important for proper understanding of the atmospheric role of Criegee intermediates. In this paper we first discuss the methods used for studying the details of the reaction. Then we will study different Special Issue: Markku Räsänen Festschrift Received: July 1, 2014 Revised: September 4, 2014
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double excitations without any interactions between them, making results for systems with partial biradical character rather questionable. Another step in the MP method, MP3, may deviate further from correct results than MP2, due to behavior of the MP series. However, by introducing interaction between double excitations, MP3 describes ozone chemistry already in a reasonable way. Another step, MP4 has been proven to give enough electron correlation to describe bond elongation up to the transition state for homolytic bond breaking32 of processes of the type studied here. The problem with MP4 is that its computational cost scales exactly as that of CCSD(T) whereas the latter is much better. But if one uses just MP4(SDQ) omitting triple excitations, one gets a method much better than MP2 for the ozone-related system, and it is still relatively fast. To validate whether or not MP4(SDQ) is capable of describing at least a shape of potential energy surface (PES), we study in the following section a well-known reaction of ozonolysis of ethylene and compare MP4(SDQ) results with previous high level calculations.5 One of the known problems with higher orders of Møller−Plesset perturbation theory is the spin contamination for open-shell system, but even though systems studied here may contain a substantial amount of biradical contribution, spin contamination does not occur because of the use of RHF method. In this work all structures were optimized using MP4(SDQ) quantum chemistry potential and relative energies were then validated by CCSD(T) single point energies calculated for optimized geometries. For all calculations Dunning’s aug-ccpVTZ basis set was used33,34 and aug-cc-pV(T+d)Z for phosphorus atoms.35 For all MP4 calculations, we used a code we developed. The MP4(SDQ) energies can be evaluated directly from atomic integrals, in the spirit of the direct Hartree−Fock method.36 However, in the present work the calculation was done using a parallelized algorithm where the integrals are kept in memory, distributed over the available CPUs. The approach avoids a bottleneck due to transfer of data from the hard disk. This massively parallelized algorithm proved very efficient for the calculations reported here. CCSD(T) calculations were done using the Molpro code.37 Transition state structures were confirmed by Hessian matrix calculation and a presence of just one imaginary frequency. Calculations of the exoergicity of the ozone reactions with double-bond molecules studied here were carried out by the MP4(SQD) algorithm described above. In computing the lifetime of the CI formed, we introduced an additional simplification. We considered the released energy as distributed thermally among the vibrational modes of the CH2OO product and carried out a unimolecular TST calculation to obtain the lifetime. A full RRKM calculation38,39 for the ozone−reagent molecule complex seems unnecessary here, because the energy released into the CH2OO formed is the decisive factor in determining the lifetime. The question of interest here are just the orders of magnitude of the different lifetimes obtained for the different precursors. Unfortunately, the system is too large for proper investigation of multiconfigurational character like in our previous work.26
ozonolysis reactions where we explain the influence of the precursor on the lifetime of the CI formed. First we will present the classic ozonolysis of ethylene as a validation for the method used. Next, we will present the mechanism of ozonolysis of CN bond in two different variants. Finally we will present the mechanism for CP bond ozonolysis.
II. COMPUTATIONAL APPROACH Reactions in which CI species are involved are described by complex electronic states, which creates difficulties for treating such processes. Thus, even at equilibrium geometry, CI molecules have some measure of radical and zwitterionic character,6−8,26 and these and other contributions to the electronic state can become much greater at certain geometries along the reaction pathways.26 Thus, in a recent study of the isomerization and decomposition of CH2OO, the simplest CI, classical trajectories using a multireference algorithm for the potential surface were computed.26 The analysis of the process in this case shows that the multiconfigurational nature of the wave function manifested itself in some regions along the trajectory. Clearly, trajectories propagating from reagents to products for such systems may go through regions requiring multiconfigurational description of the electronic state. Because “on the fly” simulations with such high-level treatments of the electronic state are computationally very demanding,27 this is an option only for the simplest CI, such as CH2OO.26 In this work, instead of going for direct molecular dynamics, we are going for transition state theory calculations (TST) that are less demanding than molecular dynamics, therefore more practical when many reactions of larger systems are explored for comparison purposes. In addition, TST calculations can always be supplemented by single point energies with a much more accurate method than the one used for PES analysis because TST employs only stationary points of the PES. When it comes to studying reactions, it is always important to have as much electron correlation as possible. Behavior of PES when extended bond geometry comes close to a homolytic bond breaking cannot in principle be described accurately using a spin restricted determinants28 as done in popular DFT functionals. Classic post Hartree−Fock (HF) ab initio methods usually include much more electron correlation, giving good results also when bonds are extended. The problem with post HF methods is that computational cost may be high for reactions such as studied here. CC methods, for example, should offer an outstanding level of accuracy29 but scale relatively poorly when one tries to perform calculations on many CPU’s. It was actually shown in our previous study26 that some regions of potential energy surfaces (PES) of CI, including transition states, are well described by a single reference approximation. One cannot expect that single reference approximation would perform well in the case of molecular dynamics describing hybrid zwitterionic/biradical character,6−8,26 but for the study of just stationary points and exploring PES, it can most likely suffice. Among single reference post HF methods, MP230 is famous for offering a good level of accuracy for a modest computational cost; MP2 can go significantly further than DFT for extended bonds but still lacks sufficient accuracy for our purpose. Most importantly MP2 is incapable of describing some aspects of ozone chemistry.31 However, Møller−Plesset perturbation theory30 gives us a powerful tool for studying chemistry by defining a systematic and iterative way of improving results where MP2 is just a first step. The problem with MP2 is that it includes only
III. RESULTS AND DISCUSSION III.1. Ozonolysis of Alkenes: H2CCH2. The mechanism of ozonolysis of alkenes has been a subject of many studies, and the accepted mechanism is the following.5 Ozone quickly attaches to the double CC bond making a rather unstable species called primary ozonide (POZ). Reaction B
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Figure 1. Overview of selected stationary point geometries and mechanism for first steps of ozonolysis of ethylene. Geometry optimizations were done using MP4(SDQ)/aug-cc-pVTZ quantum chemistry potential. Selected distances for main transitions states are given in angstroms. Numbers for structures are the same as used in the text and Figure 4.
of the primary ozonide formation goes through a rather early symmetric transition state (TS) with C−O distances around 2.3 Å (Figure 1). POZ can decompose in a few different ways. The lowest energy path for POZ decomposition leads directly to a CI (CI) and a carbonyl species through TS defined by structure TS-2 (Figure 1). Such an early TS with O−O distance around 2.1 Å and C−C distance below 2 Å already suggests that CI formation is highly exoergic in this case. The transition state for this decomposition constitutes a barrier of 19.0 kcal/mol, according to MP4(SDQ) theory, which is in good agreement with 18.7 kcal/mol obtained from the hybrid CASSCF/CCSD(T) approach.5 After passing through the TS, the system goes down by 15.73 kcal/mol to CI and a carbonyl species according to MP4, which is again in good agreement with a hybrid approach published before (16.8 kcal/mol).5 The energy diagram for these first steps of ethylene ozonolysis is shown in Figure 2 with summary of relative energies for stationary points in Table 1. After starting with ozone and ethylene, the system went down by 52.7 kcal/mol to a stage of CI and a carbonyl species. Assuming that the process takes place in the gas phase, the
Table 1. Relative Potential Energies for Selected Stationary Pointsa E [kcal/mol] C2H4 + O3 TS-1 1 TS-2 2+3 5 + O3 TS-3 6 TS-4 8+3 2+9 2+7 10 11+O3 TS5 12 TS-6 3 + 13 (complex) 3 + 13 (isolated) 14 2 + 15 16 + O3 17 TS-7 3 + 18
0.0 4.0 −56.0 −37.0 −52.7 0.0 1.6 −40.0 −33.3 −39.2 −108.7 −38.5 −64.9 0.0 2.8 −43.9 −20.3 −20.9 −14.8 −66.2 −105.9 0.0 −99.9 −74.0 −85.3
a
Numbers for the structures correspond to structures from Figures 1, 3, and 5. Energies are divided corresponding to reactions of which they are part and are relative to the energy of the initial reactants.
Figure 2. Schematic potential energy diagram showing relative energies of different minima involved in the first steps of ozonolysis of ethylene. Relative energies are given in parentheses in kcal/mol and were obtained by single point energy calculation with the CCSD(T)/ aug-cc-pVTZ method on geometry obtained with MP4(SDQ)/aug-ccpVTZ. Numbers correspond to structures from Figure 1.
system has no time to cool down, and all 52.7 kcal/mol is accumulated as the kinetic energy raises the temperature of the system considerably. Under these conditions, assuming that the process started at 300 K, CI decomposes into dioxirane on a picosecond time scale according to transition state theory, and C
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Figure 3. Overview of selected stationary points geometries and mechanism for ozonolysis of O-methyloxime. Geometry optimizations were done using the MP4(SDQ)/aug-cc-pVTZ quantum chemistry potential. Selected distances for main transitions states are given in angstroms. Numbers for structures are the same as used in the text and Figure 4.
present a mechanism for a CI formation in the ozonolysis reaction of H2CNOCH3 (5). Geometry for the most important stationary points in this reactions are shown in Figure 3, and energy profiles are shown in Figure 4a with summary of relative energies for stationary points in Table 1. The first TS on the reaction pathway between ozone and Omethyloxime (structure TS-3 in Figure 3) with a C−O distance lower than 2 Å appears slightly later than its counterpart from ozonolysis of alkenes. This first TS for the ozonolysis of the Omethyloxime constitutes the 1.6 kcal/mol barrier for the reaction. After reaching this first TS, the system quickly falls down on the potential energy surface into POZ defined by structure 6 (Figure 3). The POZ molecule in this case is 40.0 kcal/mol lower in energy than the initial ozone and O-methyloxime system. POZ formed in this case has two possibilities to decompose. The lowest energy path goes through TS defined by structure TS-4 (Figure 3) and leads to a CI (3) and CH3ONO (8) molecule. CH3ONO is a well-known molecule. Just like in the case of ozonolysis of alkenes, CI and the second product can react again, making the SOZ (10). SOZ in this case is 64.9 kcal/ mol lower in energy than the initial system and is relatively stable. The lowest energy path leads to SOZ decomposition through a barrier of 4.0 kcal/mol into formaldehyde (2) and CH3ONO2 (9) molecules.
this decomposition is most likely followed by immediate decomposition into CO + H2O.26 This very simple mechanism connected with high exoergicity of ozonolysis of alkenes makes CI identification very difficult. Very low stability of CI’s and very high exoergicity of the reaction (E = 52.7 kcal/mol) result in the lifetime of the CI product in the range of picoseconds. For such fast reactions, transition state theory cannot be trusted quantitatively. However, the exact lifetime is not so important here; the order of magnitude of this lifetime is of central interest. TST tells us that the CI in this reaction is formed extremely hot and most likely decomposes immediately. In principle, one can try to cool down the formed CI by the environment but unfortunately the process is most likely too fast for an extensive amount of CI’s to cool down even in the condensed phase. Another way to stabilize CI’s formed in the ozonolysis would be to use larger and asymmetric precursors; unfortunately in the case of ozonolysis of alkenes, this would result in a mixture of different CI’s that could even react with one another,40 making identification or isolation of a specific CI an extremely difficult task. III.2. Ozonolysis of a O-Methyloxime: H2CNOCH3. Ozonolysis of methyloximes has already been studied experimentally and indirect evidence for formation of CI’s has been provided.25 However, no theoretical studies on the mechanism of this reaction have been reported so far. Here we D
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The other way of decomposition of POZ leads to formaldehyde and CH3ONOO (7) molecule, which is very high in energy and quickly relaxes into CH3ONO2 (9). This POZ decomposition channel is associated with a barrier of 35.8 kcal/mol, which makes the channel leading to the Creigee intermediate formation considerably favored. Assuming that, after reaching the initial transition state for POZ formation, the system has a temperature of 300 K, the reaction of POZ → H2COO + CH3ONO takes about 0.1 ps; for such time scales, TST is certainly not quantitatively reliable. However, we use it merely to point out that this species involved certainly does not correspond to our searched longlived CI’s. The reaction of POZ → H2CO + CH3ONOO takes 4 min. Because of the much higher reaction rate of the first channel nearly all POZ molecules decompose into CI’s. Because of the high speed of this reaction it is reasonable to assume that even in the condensed phase the system will not have time to cool down at the stage of POZ, and the whole kinetic energy, gathered when falling from the initial high energy transition state, remains in the system, making the system and the CI itself very hot. At this stage the system has gathered 39.2 kcal/mol of kinetic energy. H2COO decomposes into dioxirane molecule (H2CO2) (4), and transition state theory predicts a 3 ns lifetime for the CI under these conditions. Considering that this lifetime is again too short for the system to cool down, it is reasonable to assume that indeed this is the lifetime of the CI formed in the ozonolysis of a formaldehyde O-methyloxime.
Figure 4. Schematic potential energy diagram showing relative energies of different minima involved in ozonolysis of (a) H2CNOCH3 O-methyloxime, (b) H2CNCH3 Schiff base, and (c) H2CP(O)OCH3, which are shown in the figure. Relative energies are given in parentheses in kcal/mol and were obtained by single point energy calculation with the CCSD(T)/aug-cc-pVTZ method on geometry obtained with MP4(SDQ)/aug-cc-pVTZ. Numbers correspond to structures from Figures 1, 3, and 5. The presentation of two structures in square parentheses means the two structures form a complex and are not isolated from each other.
Figure 5. Overview of selected stationary points geometries and mechanism for ozonolysis of a Shiff base. Geometry optimizations were done using MP4(SDQ)/aug-cc-pVTZ quantum chemistry potential. Selected distances for main transitions states are given in angstroms. Numbers for structures are the same as used in the text and Figure 4. E
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Figure 6. Overview of selected stationary points geometries and mechanism for ozonolysis of the CP bond. Geometry optimizations were done using the MP4(SDQ)/aug-cc-pVTZ quantum chemistry potential. Selected distances for main transition states are given in angstroms.
the CI. The decomposition of POZ into CI in this case goes through a rather late TS defined by structure TS-6 (Figure 5) with a C−N distance around 2.3 Å and OO distance above 2.5 Å, suggesting lower exoergicity than in the case of Omethyloxime. The CI can again recombine with the other product (in this case CH3NO (13)) to form a secondary ozonide, which is very low in energy but smoothly decomposes into stable products, i.e., formaldehyde and nitromethane (15). In the case of Schiff base ozonolysis, the POZ is 43.9 kcal/ mol in energy below initial reactants, with a 2.8 kcal/mol energy barrier for POZ formation. In the case of Schiff bases, there is only one POZ decomposition channel that yields a CI and a CH3NO molecule. This channel is associated with a 23.6 kcal/mol barrier, which in the presence of 43.9 kcal/mol kinetic energy accumulated in the system after going down to the POZ stage results in only a 0.4 ps lifetime according to transition state theory. Another difference between the ozonlysis of O-methyloxime and Schiff base appears at the stage of the CI. CH3NO is much lower in energy than previous CH3ONO, which makes the CI stage of the reaction much colder than in the case of Omethyloxime ozonolysis. After POZ decomposition, the CI and CH3NO form a complex with an interaction energy around 6 kcal/mol, which seems to decompose without an additional transition state. The complex of CI + CH3NO will most likely decompose immediately after POZ decomposition, as its interaction energy is relatively small (6 ckal/mol) compared to kinetic energy accumulated in the system (20.9 kcal/mol). Isolated CI and CH3NO have less than 15 kcal/mol of kinetic energy accumulated, and modification of the precursor so that
The lifetime of the Crigee intermediate synthesized from Omethyloxime is much longer than in the case of alkenes, but it is still too short for practical applications. Another problem, especially in condensed phases, may be that the CI formed will meet CH3ONO to form H2CO + CH3ONO2 either by a slow recombination through SOZ or in a fast reaction of oxidation: H 2COO + CH3ONO → H 2CO + CH3ONO2
The barrier for SOZ formation is equal to 2.1 kcal/mol whereas for a direct oxidation reaction it is equal to 14.0 kcal/ mol. Transition state theory predicts 66 and 33 μs lifetimes for these channels, respectively. Hence, even though the barrier for SOZ formation is lower, the entropy effect is large enough to make the oxidation reaction faster. SOZ ozonide in this case is 64.9 kcal/mol lower than the initial system, making the lifetime of its decomposition into H2CO + CH3ONO2 on a picosecond time scale through the low barrier of 4.0 kcal/mol. In these conditions the SOZ molecule is actually unlikely to form because the time scale for CI decomposition is much faster than for SOZ formation. III.3. Ozonolysis of a Schiff Base: H2CNCH3. After exploring the mechanism of reaction between H2CNCH3 (11) and ozone, we present the mechanism shown in Figure 4b. Ozonolysis of a double CN bond in Schiff bases goes through a mechanism similar to that in the case of Omethyloximes. First, the primary ozonide (12) is formed through a TS similarly to the case of O-methyloxime (Figure 5) with CO and NO distances around 2 Å. The POZ in the case of a Schiff base can decompose only in one way, leading to F
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With this paper we propose a new way of stable Criegee intermediate synthesis, instead of photolysis of R2CIOO. Synthesis of large and complicated Criegee intermediates by this kind of photolysis may be challangeing or impossible, but the ozonolysis reaction may offer a very simple way of the stable Criegee intermediate formation taking into account that the synthesis of even very complex Schiff bases is a routine procedure for modern organic synthesis. We have shown that the ozonolysis of Schiff bases can lead to stabilized Criegee intermediates after the proper choice of a precursor. An important conclusion from this study is the fact that not only ozonolysis of CC bonds may result in Criegee intermediate formation. We have shown not only that other double bonds act as precursors for Criegee intermediates but also that other species can lead to much higher stabilization of Criegee intermediates formed than the CC precursors. It raises additional questions about possible scenarios where CI’s can prevail in the atmosphere, and of possible unexplored sources of CI’s in both the laboratory and atmosphere.41−43
the R−NO product is larger could cool down the CI significantly, thus increasing its stability. Analogously to the previous precursors, the calculation of the lifetime of the CI gives 0.9 μs for H2CNCH3 as a precursor. However, even a slight modification of the precursor to H2CNC(CH3)3 increases the predicted CI lifetime to 0.5 ms, which actually could be enough for the system to cool down even in the gas phase, resulting in a much longer lifetime. After the CI formation, the products can again recombine, making SOZ (14) 66.2 kcal/mol lower in energy than the precursor and ozone. The SOZ decomposes through a 0.8 kcal/ mol barrier into H2CO + CH3NO2 in a picosecond time scale. Similarly to the situation in the case of O-methyloximes, the CI can oxidize CH3NO into CH3NO2 directly. This reaction has a 6.7 kcal/mol barrier and 0.2 ms lifelife. Similarly to the O-methyloxime ozonolysis, SOZ formation is much slower than decomposition of the CI. Even though the SOZ formation has a lower barrier than CI decomposition, the entropy effect, resulting from the fact that SOZ formation is a bimolecular reaction whereas CI most likely decomposes in a unimolecular reaction, makes CI decomposition favored. A short summary of relative energies for stationary points is shown in Table 1. III.4. H2CP(O)OCH3. Modification of the precursor into a Schiff base for laboratory synthesis of a CI shows that it is possible to lower the kinetic energy accumulated in the CI to increase its relative stability. However, one problem remains. For both, O-methyloxime and Schiff base, the CI can easily, especially in the condensed phase, react back with the other product, forming much more stable products. To avoid this, we explore further modification of the precursor to prevent the products of ozonolysis reacting with each other. First, H2CP(O)OCH3 (16) reacts with ozone in a similar fashion as O-methyloximes and Schiff bases with the only minor difference being that there seems to be no TS on this reaction pathway. The main difference is that the primary ozonide (17) can decompose in only one way through TS (TS7) (Figure 6), into the CI and CH3OPO2 (18) and these products cannot react with each other. The CI cannot oxidize further the CH3OPO2 species and thus the secondary ozonide does not exist. These differences simplify the mechanism of this ozonolysis reaction greatly, making the CI the final step of the reaction. However, the CI formed is extremely hot; in this case, after a rather fast reaction, there is about 85 kcal/mol (Figure 4c and Table 1) of kinetic energy accumulated in the products, making intrinsic decomposition of the CI take place on a subpicosecond time scale.
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AUTHOR INFORMATION
Corresponding Author
*R. B. Gerber. E-mail:
[email protected]. Phone: 972 2 6585732. Fax: 972 2 6513742. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Academy of Finland and the University of Helsinki for support of this work in the framework of the FiDiPro program. We thank also the Finnish CSC Center for the computational resources provided. Work was also supported by the Israel Science Foundation (grant 172/12) and the National Science Foundation (CHE-090-9227). We thank Prof. Sergey Nizkorodov for helpful discussions.
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REFERENCES
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IV. CONCLUDING REMARKS In conclusion, the ozonolysis reactions of CX double bonds for X other than C were studied and it was shown that ozonolysis of all studied CX leads to Criegee intermediate formation. The stability of the Criegee intermediates formed in different ozonolysis reactions has been estimated, and it has been shown that, with a proper design of the precursor, stabilization of Criegee intermediate can be achieved. In the synthesis of the simplest possible Criegee intermediate, the system at the stage of the Criegee intermediate in the ozonolysis of alkene, O-methyloxime, and a Schiff base has exoergicity of 52.7, 39.2, and 14.8 kcal/mol, respectively, making the ozonolysis of Schiff bases very promising in search of possible ways to synthesize stabilized Criegee intermediates. G
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