Article pubs.acs.org/JPCA
Matrix Isolation Study of the Ozonolysis of 1,3- and 1,4Cyclohexadiene: Identification of Novel Reaction Pathways Laura Pinelo, Anna D. Gudmundsdottir, and Bruce S. Ault* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States S Supporting Information *
ABSTRACT: The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental and theoretical results demonstrate that these reactions predominantly do not follow the long-accepted Criegee mechanism. Rather, the reaction of O3 with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C6H6, and H2O3. These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex. There is also evidence for the formation of a small amount of the primary ozonide as a minor product, formed through a transition state that is slightly higher in energy. The reaction of O3 with 1,3cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide. In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate. This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the f irst evidence for dehydrogenation of an alkene by ozone.
■
INTRODUCTION Studies of anthropogenic and biogenic alkene ozonolysis in the atmosphere have concluded1−6 that these reactions have a complex impact on air pollution and the overall composition of the atmosphere. Many of these studies cite the need for a more complete understanding of the mechanism of the ozone− alkene reaction.1−4 The reaction mechanism first proposed by Criegee in 19497,8 is widely accepted6,9,10 on the basis of considerable indirect experimental evidence and theoretical calculations.11−21 This mechanism begins with the formation of an initial primary ozonide through addition of the ozone across the double bond followed by C−C and O−O bond rupture to form a Criegee intermediate and a carbonyl-containing species. However, other mechanisms have been proposed.9 Although the findings of solution phase studies of conjugated cyclodienes generally support the Criegee mechanism,22,23 one study noted the formation of a small amount of phenol in the ozonolysis reaction of 1,3-cyclohexadiene (1,3-CHD).22 To characterize reactive intermediates and provide verification of the Criegee or another mechanism, these species must be isolated and the excess energy must be dissipated.17,24,25 Matrix isolation has been successfully used to isolate and characterize a wide variety of reactive intermediates; such as radicals, ions, and molecular complexes.24,25 This technique has successfully led to the isolation and characterization of the primary ozonide, the secondary ozonide, and/or Criegee intermediate for several simple alkenes.16,17,26 Cyclohexadiene adds an additional level of complexity not seen in the alkenes that have been studied to date. Cyclohexadiene (CHD) has two isomers, the conjugated 1,3-CHD form and the nonconjugated © 2013 American Chemical Society
1,4-CHD form. In addition, cyclohexadiene may react with more than one ozone molecule as a consequence of the presence of two carbon−carbon double bonds.27 Consequently, a larger number of initial reaction products may be observed in the ozonolysis of 1,3- and 1,4-CHD.
■
EXPERIMENTAL SECTION
All experiments were conducted using a standard matrix isolation system that has been previously described.28 1,3Cyclohexadiene (Aldrich, 97%) and 1,4-cyclohexadiene (Acros, 97%) samples were prepared from the vapor above the liquid at room temperature after purification by two freeze−pump−thaw cycles at 77 K. Ozone was produced by the Tesla coil discharge of O2 (Wright Brothers). The discharge tube was cooled to 77 K to trap the ozone. Excess O2 was pumped off before the ozone was warmed to room temperature. Isotopically labeled 18 O3 was produced in the same manner from 18O labeled O2 (94%, Cambridge Isotope Laboratories). The purified vapor was then introduced into a mixing reservoir and diluted with argon/sample to ratios of 300/1 to 600/1. In one experiment, benzene (Aldrich, 99%) was introduced as the vapor above the room temperature liquid after purification by freeze−thaw cycles at 77 K and diluted in argon. Argon (Wright Brothers) was used as the matrix gas in all experiments, without further purification. Received: March 26, 2013 Revised: May 2, 2013 Published: May 2, 2013 4174
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Matrix samples were deposited in both the twin jet and merged jet deposition arrangements that have described previously.17,26 A 50 cm merged jet (reaction) region was used for all merged jet experiments. Samples were deposited on a CsI cold window for ∼24 h before a final spectrum was recorded. Spectra were collected with a Perkin-Elmer Spectrum One FT-IR spectrometer from 400 to 4000 cm−1 averaging 38 scans at a resolution of 1 cm−1. After deposition, the matrix was annealed by warming the cold window to between 33 and 36 K and an additional spectrum was recorded. Then, the matrix was irradiated with the H2O/quartz filtered output from a mediumpressure short arc 200 W mercury arc lamp for one hour and additional spectra recorded. Theoretical calculations were carried out on likely intermediates in this study, including those predicted by the Criegee mechanism, using Gaussian 09 and 09W suite of programs.29 Density functional calculations using the hybrid B3LYP functional30,31 were used to locate energy minima, determine structures, and calculate vibrational spectra. Final calculations with full geometry optimization employed the 6311++G(d,2p) basis set, after initial calculations with smaller basis sets were run to approximately locate energy minima. Thermodynamic functions for the reactants and potential intermediates were also calculated. All transition states were confirmed to have one imaginary vibrational frequency by analytical determination of the second derivatives of the energy with respect to internal coordinates. Intrinsic reaction coordinate calculations were used to verify that the located transition states corresponded to the attributed reactant and product.32,33
Table 1. Band Positions and Assignments for the Products in the Thermal Reaction of Ozone with 1,4-Cyclohexadiene 18
exptl bandsa
calcd bandsb
calcd shiftc
479 509 682 688 720 744 825 843 853 877 904 1039 1179 1290 1341 1474 1481 1822 1963 3097 3388 3450
451 516 682 687
−11 −20 0 0
774 823
−44 −53
O exptl shift −19 0 0 −40
−7 −2 1040 1180
0 0
1330
−7
1481 1812
0 0
3102 3431 3515
0 −12 −12
0 −4 −6 0 0 0 0 0 −15 −11
assignments benzene−H2O3 benzene−H2O3 benzene benzene−H2O3 POZ? benzene−H2O3 benzene−H2O3 POZ? POZ? POZ? POZ? benzene−H2O3 benzene−H2O3 POZ? benzene−H2O3 POZ? benzene−H2O3 benzene−H2O3 parent & benzene−H2O3 benzene−H2O3 benzene−H2O3
a Frequencies in cm−1. bBand calculated by applied calculated shift due to complexation to the literature matrix bands of parent species. c Calculated at the B3LYP/6-311G++(d,2p) level of theory.
■
RESULTS AND DISCUSSION 1,4-Cyclohexadiene + Ozone. After blank experiments of the two parent compounds each alone in argon, including deposition, annealing, and irradiation, were conducted many codeposition experiments with these two reagents were carried out. In an initial twin jet codeposition experiment, a large number of product bands were observed upon initial deposition of the reactants, the most intense of which appeared as a doublet at 682 and 688 cm−1 with an absorbance greater than 2.0. Product bands were also noted in the O−H stretching region between 3300 and 3500 cm−1. All of the product bands are listed in Table 1. After the matrix was annealed to 36 K, all of the initial product bands increased in intensity, and all of the bands increased by about the same amount (∼130%). Subsequent irradiation of this sample led to a slight decrease in intensity of all of the initial product bands and the formation of several new weak product bands. Figure 1 shows representative spectra. To further explore this system, this twin jet experiment was repeated a number of times, systematically varying sample concentrations of 1,4-CHD and O3, as well as the annealing temperature. Comparable results were obtained throughout, including the same initial products, the same annealing behavior and the same responses to irradiation. Finally, a twin-jet codeposition experiment was conducted with 1,4-CHD and O2. No reaction was observed, demonstrating that O3 is the reacting species, not residual O2. These observations indicate that substantial reaction is occurring between the two parent compounds during the very brief mixing time in twin jet deposition. On the basis of previous studies, one might anticipate formation of a primary ozonide and possibly either a Criegee intermediate or a
secondary ozonide. The concentrations were for the most part too low to allow for the possibility of the reaction of two ozone molecules with a molecule of 1,4-CHD, which could lead to additional products. However, the numerous product bands exhibited nearly identical behavior as a function of initial deposition, annealing, and irradiation. This would indicate that only one product is being formed or that all products are forming at the same rate. This differs from previous studies of the ozonolysis of alkenes, where multiple products were observed and showed differing behaviors with respect to annealing and irradiation. Also, the present experiments differed from previous studies in that strong peaks in the O−H stretching region were present upon initial deposition and increased in intensity after annealing in the present study.17,34 To assist the identification of products in the 1,4-CHD/O3 system, an equivalent set of twin jet experiments were conducted with samples of Ar/1,4-C6H8 and Ar/18O3 made from 18O2 containing approximately 94% 18O. Upon initial sample deposition, a large number of product bands were again observed, quite similar to the bands with 16O3. It was striking that the product bands separated into two sets, the first of which shifted substantially as a result of 18O substitution whereas the second set did not shift at all. The most intense band, at 688 cm−1, did not shift at all with this isotopic substitution. For the set of bands that did shift, 18O product bands could be identified as the counterparts of most of the 16O bands that had been observed. 18O band shifts are also listed in Table 1 for each product band. The 18O product bands showed the same behavior with respect to annealing and irradiation as did their 16O counterparts. These result again different from previous ozonolysis studies of small alkenes where the most 4175
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Figure 1. Infrared spectra from 604 to 1490 cm−1 of matrixes formed by the twin jet codeposition of samples of Ar/1,4-C6H8 and Ar/O3. Red (middle) is initial deposition and blue (top) is after annealing to 35 K, compared to a blank spectrum of Ar/1,4-C6H8 (pink, bottom). Bands marked with an asterisk are product bands.
intense bands were associated with oxygen atom motions in the products and all showed substantial shifts. The short time available for reaction in twin jet deposition suggested the observed products in the ozonolysis of 1,4-CHD are likely initial intermediates in the reaction sequence as observed in previous studies. In particular, the primary ozonide is believed to form initially, then the Criegee intermediate followed by the secondary ozonide. However, the product bands, particularly those around 3400 cm−1, indicate that a different reaction pathway than that followed for most alkenes is predominant. The bands between 3300 and 3500 cm−1 are clearly identified as O−H stretches on the basis of position and 18 O shift. Further, the bands at 682, 688, 1179, 1481, and 3097 cm−1, which did not shift upon 18O substitution, all lie very close to the most intense infrared absorptions of benzene, C6H6. The proximity to known bands of benzene and corroborated here by in a blank experiment with benzene, combined with the anticipated lack of any 18O shifts, strongly suggests that a dehydrogenation reaction has occurred leading to benzene formation. Literature studies35,36 of the gas phase reaction of 1,4-CHD with metal atoms and metal ions also led to dehydrogenation. For example, Davis et al. reported that the reaction of Y atoms with 1,4-CHD in crossed molecular beams led to the formation of YH2 and C6H6. This suggests two likely reaction channels, 1,4-CHD + O3 → C6H6 + H2O3 and 1,4CHD + O3 → C6H6 + H2O + O2. These channels must account for the set of bands at 509, 744, 1341, 3388, and 3450 cm−1 bands shifted −19, −40, −6, −15, and −11 cm−1, respectively, to lower energy upon 18O substitution. The infrared spectrum of H2O3 was first reported in argon matrixes by Engdahl and Nelander37 in 2002 whereas the infrared spectra of H2O and the C6H6−H2O complex in argon matrixes are well-known.38
This indicates that both channels are viable possibilities. However, the experimental spectra observed here are not at all consistent with the known spectra of H2O and the C6H6− H2O complex in argon matrixes with respect to number of bands, band locations, and 18O shifts. Thus, the 1,4-CHD + O3 → C6H6 + H2O3 channel needs examination. If C6H6 and H2O3 are formed through a dehydrogenation reaction during the matrix deposition process, then it is likely that they may interact and form a complex in the matrix cage. Therefore, DFT (B3LYP/6-311++g(d,2p)) calculations were carried out for the separated benzene and H2O3 species as well as a benzene−H2O3 complex (BHC). Two conformers of this complex were found to be stable. One conformer has one hydrogen of H2O3 hydrogen bonded to the π electron density on the C6H6 ring in a manner similar to that for complexes of the hydrogen halides with C6H6, whereas the second conformer has both hydrogens interacting with the π electron density, as shown in Figure 2. The calculated energies of these two conformers are similar within computational error. In addition, the calculated spectra for the two are quite similar, other than a reversal of intensities for the two O−H stretching modes. Though one could compare the spectrum calculated for the complex with the experimental spectrum, there are always systematic computational errors and perturbations caused by interactions with the argon matrix. These could be reduced by determining the calculated shift of the two subunits in the complex from the calculated positions of the uncomplexed molecules as shown in Tables 1 and S1 (Supporting Information). Then, these calculated shifts are applied to the known experimental bands of C6H6 and H2O3 in argon matrixes. This methodology shows clearly that the reaction of 4176
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
1,4-CHD form an initial weak complex with the O3 subunit positioned directly over the ring of 1,4-CHD. From there, the pathway to double hydrogen abstraction to form H2O3 and C6H6 is essentially barrierless. Once these two species are formed within the matrix cage, they interact to form a weak complex in the cage. The reaction to form this complex is over 70 kcal/mol exothermic with respect to the parent species. It is noteworthy that the barrier to reaction to form the primary ozonide of 1,4-CHD is low as well, perhaps just slightly higher than the barrier to form H2O3 and C6H6. Therefore, DFT calculations were then carried out to determine the stability and vibrational spectrum of the potential intermediates. Each structure was optimized using the B3LYP hybrid functional and basis sets as high as 6-311++G(d,2p) to obtain accurate energies and vibrational frequencies. Further, because of the presence of two double bonds and results from solution phase studies,22,23,27,39 additional possible intermediates were calculated. All of these species optimized to energy minima on their respective potential energy surfaces, with all positive vibrational frequencies. 18O isotopic shifts and intensities were also calculated. Several structural conformations of the intermediates were also calculated and the energies relative to the reactants are given in Table S2 (Supporting Information). Comparing the few weak, unassigned product bands to these calculations, several bands of the primary ozonide are calculated to come near these product bands. However, in most cases the 18 O counterparts are not observed and may be obscured by strong parent bands of bands of the C6H6−H2O3 complex in the region. Thus, such assignments are tentative and noted by “POZ?” in Table 1. Finally, it is noteworthy that in merged jet experiments, intense bands of H2O3 and C6H6 were observed, indicating that these are gas phase products for this system and not a consequence of the matrix itself. Overall, it is clear that the pathway leading to dehydrogenation and formation of C6H6 + H2O3 dominates the reaction of O3 with 1,4-CHD. 1,3-C6H8 + O3, Twin Jet. After blank experiments of the two parent compounds, including deposition, annealing, and irradiation were conducted, many codeposition experiments with these two reagents were carried out. In an initial twin jet codeposition experiment, a large number of product bands, with intensities ranging from medium to very weak, were observed upon initial deposition as listed in Table 2. In contrast to the 1,4-CHD system, after the matrix was annealed, some of the initial product bands increased in intensity while other bands decreased. Increases ranged from 100 to 200% and decreases ranged from 90 to 60%. Also unlike the 1,4-CHD system, subsequent irradiation of this sample led to a decrease in intensity of some of the initial product bands and an increase in intensity of other product bands. In addition, a few new bands were seen that were not present on initial deposition. Figure 4 shows representative spectra. Comparison of the bands observed for this system to the product bands in the reaction of O3 with 1,4-CHD discussed above clearly established that dehydrogenation of 1,3-CHD does not occur, e.g., that C6H6 and H2O3 are not observed. Certainly, a different reaction pathway (or pathways) is being followed. Further, these results allow the sorting of product bands into groups based on their behavior with respect to initial deposition, annealing, and irradiation. The significant growth of a number of these product bands upon annealing to 35 K indicates that the barrier to reaction for this pathway must be very low (3/2RT ∼ 0.1 kcal/mol at 35 K). Although the activation barrier for the reaction of 1,3-cyclohexadiene and
Figure 2. Calculated structure of the H2O3−C6H6 complex formed in the reaction of 1,4-CHD with ozone. The calculated energy relative to the reactants is given in parentheses.
1,4-CHD with O3 to form the C6H6−H2O3 complex occurs and is the dominant, if not exclusive, reaction channel. Specifically, the intense antisymmetric O−O−O stretching mode of H2O3 is calculated to come at 774 cm−1 (unscaled) with a −44 cm−1 shift with 18O, which is in very reasonable agreement with a strong product band at 744 cm−1 with −40 cm−1 18O shift. Similarly, the antisymmetric and symmetric O−H stretches are calculated to come at 3431 and 3515 cm−1 with 18O shifts of −12 cm−1 each. These compare well to the experimental values of 3388 and 3450 cm−1, with shifts of −15 and −11 cm−1. For the C6H6 subunit in the complex, the most intense band is calculated at 696 cm−1, in good agreement with the very intense product band at 688 cm−1. Table 1 shows the complete comparison of experimental bands and shifts with those calculated using this methodology. From the above, it is clear that the reaction of O3 with 1,4-CHD leads to dehydrogenation, with the formation of C6H6 and H2O3 which are weakly complexed in the argon cage, as shown in Scheme 1. To the best of our knowledge, this is an unprecedented reaction pathway for the ozonolysis of an alkene. Scheme 1
The transition state for the hydrogen elimination from 1,4CHD to form BHC was calculated and verified with IRC calculations. The transition states were also calculated for the Criegee mechanism and for other products observed in the spectra. Each transition state was optimized to (TS) Berny and the frequency calculation found one imaginary frequency. Then IRC calculations were performed on each transition state showing that they connected to the correct reactants and products. The resulting potential energy surface is shown in Figure 3 and supports the conclusion that formation of the H2O3−C6H6 complex is an energetically viable pathway. O3 and 4177
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Figure 3. Reaction diagram (kcal/mol) for the conversion of the initial weak complex of 1,4-CHD and ozone to the secondary ozonide via the Criegee mechanism; as well as the conversion of the initial weak complex of 1,4-CHD and ozone to the benzene−H2O3 complex (BHC).
jet experiments, where most of the products were known or anticipated stable oxidation products.16,17,34 The arguments presented in the preceding paragraph strongly support assignment of the product bands formed in thermal twin jet reaction of O3 with 1,3-CHD, including upon annealing, to early intermediates in the reaction of ozone. On the basis of the results of previous studies on similar alkenes and the relatively well-established Criegee mechanism, likely initial (or “early”) intermediates that could be observed in the twin jet experiments include the primary ozonide, Criegee intermediate, and the secondary ozonide. These possibilities are compounded by the presence of two double bonds meaning that ozone can react with one or both the double bonds, as well multiple structural isomers. Further complexity arises from the observations that the ring-opened Criegee intermediate can form two isomers, because there are two possible positions for the remaining double bond relative to the carbonyl oxide and carbonyl groups. To aid in the identification of products in this system, twin jet experiments were conducted with samples of Ar/1,3-C6H8 and Ar/18O3. Upon initial sample deposition, a large number of product bands were observed, similar to the bands with 16O3, although shifted somewhat in a number of cases. Most 18O product bands could be identified as the counterpart of a 16O band observed above; 18O band shifts are listed in Table 2 for each product band. As anticipated, the 18O product bands showed the same behavior with respect to annealing and irradiation as did their 16O counterparts. Given the results described above for the reaction of O3 with 1,4-CHD, a search for absorptions of H2O3, C6H6 and the H2O3−C6H6 complexes was made, particularly for the very intense band of the H2O3− C6H6 complex at 688 cm−1. Comparison of Tables 1 and 2 clearly show that H2O3, C6H6 and the H2O3−C6H6 are not formed in the reaction of O3 with 1,3-CHD. To sort out the numerous possible products for the O3 + 1,3CHD system, theoretical calculations were essential. These calculations provide the relative energies of all of these possible intermediates as shown in Figures 5 and S1 and in Table S3 (Supporting Information), along with the computed vibrational spectra, band intensities, and 18O isotopic shifts as listed in Table S4 (Supporting Information).
Table 2. Band Positions and Assignments for the Initial Intermediates in the Thermal Reaction of Ozone with 1,3Cyclohexadiene 18
exptl bands 441 476 639 672 710 723 766 770 784 825 863 876 895 956 967 989 1011 1318 1340 1365 1380 1740 3443 3540
a
O
calcd bands
exptl shift
calcd shiftb
assignments
411 488
−6 −11 −3
−4 −11
PI-B PI-A c CI CI POZ POZ PI-A PI-B c c c c POZ CI CI POZ c c PI-B c CI PI-A PI-B
666 738 702 755 772 790
978 951 999 1001
1407 1806 3664 3706
−7 −14 −11 −9 −12 −3 −1 −8 −14
−5 −1 −7 −5 0 −40 −15 −13
−1 −9 −13 −9 −7 −8
−15 −29 −13 −4
−8 −36 −12 −12
Frequencies in cm−1. bCalculated at the B3LYP/6-311G++(d.2p) level of theory. cCould be assigned to POZ, PI-A, or PI-B; see text. a
ozone has not been experimentally measured, it is anticipated to be similar to the Ea for the reaction of ozone with cycloalkenes such as cyclopentene and cyclohexene, ∼2 kcal/ mol.40 For the reaction of O3 with cyclopentene, similar growth of product bands upon annealing was observed. Additionally, the product bands observed in the thermal twin jet experiments are in many cases different from those observed in the merged 4178
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Figure 4. Infrared spectra from 750 to 1460 cm−1 of matrixes formed by the twin jet codeposition of samples of Ar/1,3-C6H8 and Ar/O3. Red (middle) is initial deposition and blue (top) is after annealing to 35 K, compared to a blank spectrum of Ar/1,3-CHD (pink, bottom). Bands marked with an asterisk are product bands.
eliminated included the primary ozonide, Criegee intermediate and secondary ozonide from the trans form of 1,3-CHD as well as those products requiring two O3 molecules (e.g., the double POZ). The former result is anticipated because the trans forms are approximately 7 kcal/mol higher in energy that the cis forms. The latter result is not surprising in that it would be unlikely that two ozone molecules would react with both double bonds simultaneously because both the ozone and cyclohexadiene are very dilute in argon. This information, along with the intensity ratio changes during annealing, allow for assignment of a number of the product bands to the primary ozonide and Criegee intermediate arising from ozonolysis of one of the carbon−carbon double bonds through a Criegee mechanism as shown in Scheme 2. Specifically, intense product bands at 723, 766, 895, 967, 1011, 1340, and 1380 cm−1 were present upon initial deposition, decreased at the same rate upon annealing (by about 70%), and were all reduced by about 50% upon irradiation. The bands at 723, 766, 956, 1011, and 1340 cm−1 shifted −14, −11, −14, −5, and −7 cm−1, respectively, to lower energy on 18O substitution, whereas the 1380 cm−1 band did
Figure 5. Calculated structures for early intermediates from the monoozonolysis reaction of 1,3-CHD. The calculated energies for each intermediate relative to the reactants are given in parentheses.
First, a number of possibilities could be eliminated due to a serious mismatches between experimental band positions and calculated positions. The possibilities that were initially Scheme 2
4179
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Figure 6. Reaction diagram (kcal/mol) for the conversion of the initial weak complex of 1,3-CHD and ozone to the secondary ozonide via the Criegee mechanism, as well as the conversion of the initial weak complex of 1,3-CHD and ozone to intermediate PI (hydroperoxy intermediate).
not shift. The 723, 766, 956, and 1011 cm−1 bands are in regions where primary ozonides are known to absorb strongly.16,17,34 In addition, the 702, 755, 978, 1001, and 1340 cm−1 bands of the primary ozonide were calculated to have 18O shifts of −12, −9, −15, −4, and −2 cm−1, which is in reasonable agreement with the observed shifts of −14, −11, −14, −5, and −7 cm−1. The experimental band at 1380 cm−1 that did not shift is consistent with a C−H bending vibration of the primary ozonide and is in good agreement with the computed primary ozonide band at 1373 cm−1 with no 18O shift. Taken altogether, the agreement of calculated and experimental bands, 18O shifts, and the decrease of these bands upon annealing all support assignment of these bands to the primary ozonide. The reaction of cyclohexadiene and O3 to form the primary ozonide is calculated to be about 51 kcal/mol exothermic, a value that is in line with experimental measurements40 on a number of related systems. With this large excess energy, it is possible for the reaction to proceed over the barrier to the ringopened Criegee intermediate, with a carbonyl oxide (CO O) on one end an aldehyde on the other end. The barrier has been estimated to be around 20 kcal/mol for similar systems,12,15−17,19−21,40 suggesting that unless the argon matrix rapidly deactivates the energetically excited primary ozonide, reaction to the Criegee intermediate may occur. There are many different structural conformations of the Criegee intermediate and further complexity arises in that the ringopened Criegee intermediate can form two isomers. There are two possible positions for the remaining double bond: in form (CI-A) the carbon−carbon double bond is located at the carbonyl oxide end of the molecule and in the other form (CIB) this bond is located at the carbonyl end (Figure 5b,c). Solution phase studies on the ozonolysis of cyclic 1,3-dienes showed that the CI-A is preferentially formed.23,27 The infrared spectrum is most consistent with the CI-A form. The infrared spectra of carbonyl oxides are dominated by the COO antisymmetric stretch near 900 cm−1 whereas aldehydes are dominated by the CO stretch in the 1700 cm−1 region. Both should show strong 18O red shifts. Such characteristic bands were observed here, at 967 and 1740 cm−1 with a −40 cm−1 18O shift for the 1740 cm−1 product band (no 18O counterpart of the 967 cm−1 band was seen due to spectral
congestion in the region). Calculations for the most likely configuration of the Criegee intermediate (given the rigidity of the surrounding argon matrix) predict the COO and C O stretching modes to come at 951 and 1806 cm−1 with 18O shifts of −28 and −36 cm−1, respectively. This reasonable agreement supports identification of the Criegee intermediate formed in the reaction of 1,3-CHD with O3. Additional product bands at 672, 710, and 989 cm−1 showed annealing and irradiation behavior similar to that of the 967, 1740 cm−1 pair, and on the basis of calculations may also be assigned to this conformer of the Criegee intermediate. The entries in Table 2 make it clear, however, that identifications of the POZ and CI of 1.3-CHD do not account for all of the observed product bands. In particular, two product bands were observed in the O−H stretching region, bands that cannot be attributed to the POZ or CI. On the basis of the results for the 1,4-CHD/O3 system, above, they also cannot be attributed to H2O3 or the complex of H2O3 with C6H6. As shown in Figure 6, detailed exploration of the potential energy surface for this pair of reagents leads to another possibility. With only a 2 kcal/mol barrier, ozone can interact with carbon C1 and the axial hydrogen of C5 in transition state TS-4 shown in Figure 6 en route to the formation of a hydroperoxy (O−O−O−H) species PI attached to the ring at C1. This barrier is similar to that for the formation of the primary ozonide, suggesting that both pathways should be accessible. This hydroperoxy intermediate is 42 kcal/mol exothermic with respect to the parent species, providing it with sufficient energy to settle into one of several conformers. PI-A and PI-B were identified as two such conformers. The calculated infrared spectra of PI-A and PI-B match up well with several of the asyet unassigned bands in Table 2, in particular with the two bands in the O−H stretching region and their 18O counterparts. All of the remaining unassigned bands are in regions in which the POZ, PI-A, and PI-B all have calculated bands. Although definitive assignments cannot be made due to the lack of observation of 18O counterparts due to band overlap, they can all very likely be assigned to one or more of these intermediate species. The findings here indicate that for the reactions of 1,3- and 1,4-cyclohexadiene with ozone, reaction mechanisms other than the standard Criegee mechanism are followed. One question 4180
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
subtleties in the potential energy surface appear to favor the dehydrogenation reaction. The ozonolysis reaction of 1,3-CHD led to the formation of both the primary ozonide and Criegee intermediate, along with two conformers of a novel hydroperoxy intermediate. At the same time, a dehydrogenation reaction to form H2O3 and C6H6 does not occur. For this system, the potential energy surface has similar, low barriers to reaction for the Criegee and hydroperoxy pathways. These findings indicate that structural configuration and position of the double bonds in the ring influence the reaction pathways for the reaction of cyclohexadiene with O3.
that arises is why these two alkenes have viable if not dominant pathways available to them that are not present in the wide range of alkenes studied to date. For 1,4-CHD, the answer likely lies with the gain in resonance stabilization energy with the formation of benzene. The dehydrogenation of 1,4-CHD is known for several reagents for very much the same reason. Second, for 1,4-CHD, the distance between the axial hydrogens on the two −CH2− groups (C3 and C6) in the ring matches very well the spacing needed to interact with the two terminal oxygen atoms of O3. In contrast, this is not the case for 1,3CHD, where the two −CH2− groups (C5 and C6) are adjacent to one another on the ring, and the hydrogens are too close together to match up well with the terminal oxygen atoms of O3. It is noteworthy that the reactions35 of Y atoms with 1,3and 1,4-CHD both lead to dehydrogenation, forming C6H6 and YH2. The smaller size of a Y atom compared to O3 may facilitate dehydrogenation of 1,3-CHD. Thus, the pathway to benzene formation is less favorable. A related question is: Why then does the reaction of 1,3-CHD and O3 form a hydroperoxy species? The answer again appears to be favorable geometry in an initially weakly bonded complex. From there, there is only a small (4 kcal/mol) barrier to a transition state in which one terminal oxygen atom of O3 position interacts with an axial hydrogen on C5 whereas the second oxygen atom spans the ring and interacts with C1. From this transition state, it is energetically very favorable for hydrogen atom abstraction from C5 and O atom addition to C1, forming the hydroperoxy species PI. A final question has to do with the branching ratio between the two pathways available in each of these systems (for 1,4CHD + O3, formation of either the primary ozonide or the C6H6−H2O3 complex and for 1,3-CHD + O3, formation of either the primary ozonide or the hydroperoxy intermediate). There are probably two factors that contribute to determining the preferred pathway for each system. One is that the angle of incidence or approach of the two parent molecules may determine which pathway is preferred. Second is that the subtle details of the potential energy surface and the transition states determine the pathway. Although the calculated barriers are similar in each case, the uncertainties in the barriers are sufficient that we cannot conclude with certainty which would be preferred pathway. To explore this point further, we looked at the relevant orbitals on ozone and the cyclohexadiene molecules. As expected, the favorable molecular orbital interactions between the ozone and the 1,3- and 1,4-alkenes allow for the concerted reactions to take place. However, this conclusion is qualitative and does not permit a determination from those interactions as to which reaction is favored. Similarly, the calculated charges on the alkenes and the ozone match but this is not sufficient to permit a determination as to which reactions are favored on the basis of charges. In the future, a study of the reactions of ozone with substituted 1,3and 1,4-cyclohexadienes may allow insights into this question.
■
ASSOCIATED CONTENT
S Supporting Information *
Additional computational results including structures, energies, and vibrational spectra of possible intermediates are provided. This information is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The National Science Foundation is gratefully acknowledged for support of this research through grant CHE 0749109. The Ohio Supercomputer Center is also acknowledged for computer time. Michael Hoops is acknowledged for his assistance in the early stages of this research.
■
REFERENCES
(1) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Products of the Gas-Phase Reactions of O3 with Alkenes. Environ. Sci. Technol. 1995, 29, 1860−1866. (2) Ariya, P.; Sander, R.; Crutzen, P. Significance of HO x and Peroxides Production Due to Alkene Ozonolysis during Fall and Winter: A Modeling Study. J. Geophys. Res.-Atmos. 2000, 105, 17721− 17738. (3) Paulson, S.; Orlando, J. The Reactions of Ozone with Alkenes: An Important Source of HOx in the Boundary Layer. Geophys. Res. Lett. 1996, 23, 3727−3730. (4) Ravishankara, A. R. Introduction: Atmospheric Chemistry−LongTerm Issues. Chem. Rev. 2003, 103, 4505−4508. (5) Hatakeyama, S.; Ohno, M.; Weng, J.; Takagi, H.; Akimoto, H. Mechanism for the Formation of Gaseous and Particulate Products from Ozone-Cycloalkene Reactions in Air. Environ. Sci. Technol. 1987, 21, 52−57. (6) Zaikov, G. E.; Rakovsky, S. Ozonation of Organic and Polymer Compounds; iSmithers: Shawbury, 2009; pp 1−389. (7) Criegee, R. The Course of Ozonization of Unsaturated Compounds. Record Chem. Progr. (Kresge-Hooker Sci. Lib.) 1957, 18, 111−120. (8) Criegee, R. Mechanism of Ozonolysis. Angew. Chem. 1975, 87, 745−752. (9) Bailey, P. S. Ozonation in Organic Chemistry Olefinic Compounds; Academic Press, Inc.: New York, 1978; Vol. 1, pp 9−82. (10) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: New York, 1993; Vol. 1, pp 96−102. (11) Chuong, B.; Zhang, J.; Donahue, N. Cycloalkene Ozonolysis: Collisionally Mediated Mechanistic Branching. J. Am. Chem. Soc. 2004, 126, 12363−12373. (12) Olzmann, M.; Kraka, E.; Cremer, D.; Gutbrod, R.; Andersson, S. Energetics, Kinetics, and Product Distributions of the Reactions of
■
CONCLUSIONS The deposition of ozone and 1,4-CHD into argon matrixes led to the observation of a high yield of the benzene−H2O3 complex through a dehydrogenation reaction. The formation of this complex is not consistent with the mechanism proposed by Criegee. In addition, a small yield of the primary ozonide was likely observed. Within the uncertainties of the calculations, the barriers of the two pathways are comparable; small 4181
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
The Journal of Physical Chemistry A
Article
Ozone with Ethene and 2,3-Dimethyl-2-butene. J. Phys. Chem. A 1997, 101, 9421−9429. (13) Johnson, D.; Marston, G. The Gas-phase Ozonolysis of Unsaturated Volatile Organic Compounds in the Troposphere. Chem. Soc. Rev. 2008, 37, 699−716. (14) Horie, O.; Moortgat, G. K. Decomposition Pathways of the Excited Criegee Intermediates in the Ozonolysis of Simple Alkenes. Atmos. Environ. Part A, Gen. Topics 1991, 25, 19881−19896. (15) Anglada, J. M.; Crehuet, R.; Bofill, J. M. The Ozonolysis of Ethylene: A Theoretical Study of the Gas-Phase Reaction Mechanism. Chem.Eur. J. 1999, 5, 1809−1822. (16) Clay, M.; Ault, B. S. Infrared Matrix Isolation and Theoretical Study of the Initial Intermediates in the Reaction of Ozone with cis-2Butene. J. Phys. Chem. A 2010, 114, 2799−2805. (17) Hoops, M.; Ault, B. S. Matrix Isolation Study of the Early Intermediates in the Ozonolysis of Cyclopentene and Cyclopentadiene: Observation of Two Criegee Intermediates. J. Am. Chem. Soc. 2009, 131, 2853−2863. (18) Chan, W.; Hamilton, I. Mechanisms for the Ozonolysis of Ethene and Propene: Reliability of Quantum Chemical Predictions. J. Chem. Phys. 2003, 118, 1688−1701. (19) Hendricks, M. F.; Vinckier, C. 1,3-Cycloaddition of Ozone to Ethylene, Benzene, and Phenol: A Comparative ab Initio Study. J. Chem. Phys. A 2003, 107, 7574−7580. (20) Ljubic, I.; Sabljic, A. Theoretical Study of the Mechanism and Kinetics of Gas-Phase Ozone Additions to Ethene, Fluoroethene, and Chloroethene: A Multireference Approach. J. Phys. Chem. A 2002, 106, 4745−4757. (21) Rathman, W. A Theoretical Investigation of OH Formation in the Gas-phase Ozonolysis of E-but-2-ene and Z-but-2-ene. Phys. Chem. Chem. Phys. 1999, 1, 3981−3985. (22) Griesbaum, K.; Jung, I.; Mertens, H. Difunctional and Heterocyclic Products from the Ozonolysis of Conjugated C5-C8Cyclodienes. J. Org. Chem. 1990, 55, 6024−6027. (23) Wang, Z.; Zvlichovsky, G. Selectivity in Ozonolyses of Cyclic 1,3-dienes. Tetrahedron Lett. 1990, 31, 5579−5582. (24) Cradock, S.; Hinchcliffe, A. Matrix Isolation; Cambridge University Press: Cambridge, U.K., 1975; pp 1−140. (25) Chemistry and Physics of Matrix-isolated Species; Andrews, L.; Moskovits, M., Eds.; Elsevier Science Publishers: Amsterdam, 1989. (26) Coleman, B. E.; Ault, B. S. Matrix Isolation Investigation of the Ozonolysis of Propene. J. Mol. Struct. 2010, 976, 249−254. (27) Park, S.; Huh, T. Diozonides from Coozonolyses of Cyclodienes and Carbonyl Compounds. Bull. Korean Chem. Soc. 2002, 23, 423− 426. (28) Ault, B. S. Infrared Spectra of Argon Matrix-Isolated Alkali Halide Salt/Water Complexes. J. Am. Chem. Soc. 1978, 100, 2426− 2433. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2009. (30) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (31) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (32) Gonzalez, C.; Schlegel, H. B. An Improved Algorithm for Reaction Path Following. J. Chem. Phys. 1989, 90, 2154−2161. (33) Gonzalez, C.; Schlegel, H. B. Reaction Path Following in MassWeighted Internal Coordinates. J. Phys. Chem. 1990, 94, 5523−5527. (34) Pinelo, L.; Ault, B. S. Infrared Matrix Isolation and Theoretical Study of the Initial Intermediates in the Reaction of Ozone with Cycloheptene. J. Mol. Struct. 2012, 1026, 23−29. (35) Schroden, J. J.; Davis, H. F. Reactions of Neutral Gas-Phase Yttrium Atoms with Two Cyclohexadiene Isomers. J. Phys. Chem. A. 2012, 116, 3508−3513.
(36) Tews, E. C.; Freiser, B. S. Heteronuclear Diatomic Transition Metal Cluster Ions in the Gas Phase. Reactions of CuFe+ with Hydrocarbons. J. Am. Chem. Soc. 1987, 109, 4433−4439. (37) Engdahl, A.; Nelander, B. The Vibrational Spectrum of H2O3. Science 2002, 295, 482−483. (38) Engdahl, A.; Nelander, B. A Matrix Isolation Study of the Benzene-Water Interaction. J. Phys. Chem. 1985, 89, 2860−2864. (39) Park, S.; Lee, J.; Huh, T. Unsaturated Ozonides from the Ozonolysis of Cyclodienes in the Presence of Carbonyl Compounds. Eur. J. Org. Chem. 2001, 23, 3083−3087. (40) Atkinson, R.; Carter, W. P. L. Kinetics and Mechanisms of the Gas-phase Reactions of Ozone with Organic Compounds Under Atmospheric Conditions. Chem. Rev. 1984, 84, 437−470.
4182
dx.doi.org/10.1021/jp402981n | J. Phys. Chem. A 2013, 117, 4174−4182
