Dissociation Chemistry of 3-Oxetanone in the Gas Phase - The

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Dissociation Chemistry of 3-Oxetanone in the Gas Phase Sumitra Godara, Pooja Verma, and Manikandan Paranjothy J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06880 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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The Journal of Physical Chemistry

Dissociation Chemistry of 3-Oxetanone in the Gas Phase Sumitra Godara, Pooja Verma, and Manikandan Paranjothy∗ Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur 342011 Rajasthan, India E-mail: [email protected] Phone: +91 291 244 9082. Fax: +91 291 251 6823

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Abstract 3-Oxetanone is a strained cyclic molecule which plays an important role in synthetic chemistry. A few studies exist in the literature about the equilibrium properties of this molecule and the dissociation patterns of substituted 3-oxetanones. For the unsubstituted 3-oxetanone, formation of ketene (CH2 CO) and formaldehyde (HCHO) was considered to be the major dissociation pathway. In a recent work, pyrolysis products of 3-oxetanone molecule in the gas phase were investigated by Fourier transform infrared spectroscopy and photoionization mass spectrometry. In this study, an additional dissociation channel forming ethylene oxide (c−C2 H4 O) and carbon monoxide CO was reported. In the present work, gas phase dissociation chemistry of 3-oxetanone was investigated by electronic structure theory, ab initio classical chemical dynamics simulations, and Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant calculations. The barrier height for the ethylene oxide channel was found to be much higher than the ketene pathway. The dynamics simulations were performed at three different total energies viz., 150, 200, and 300 kcal/mol, and multiple reaction pathways and varying branching ratios observed. A new dissociation channel involving a ring-opened isomer of ethylene oxide was identified in the simulations. This pathway has a lower energy barrier and was dominant in our dynamics simulations.

Introduction In organic synthesis, small heterocyclic molecules have found a very important place both as starting material and target molecules. 1–3 One such molecule is 3-oxetanone (1,3-epoxy2-propanone, c−C3 H4 O2 ) which is a highly strained molecule useful in ring expansion and addition reactions. 4–6 This molecule has also been proposed as a decomposition product of acetonylperoxy radical in the atmosphere. 7 A few electronic structure theory and spectroscopic studies focusing on the equilibrium properties such as geometry, ring puckering vibration, and non-bonded interactions, have been reported in the literature. Semiempiri-


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cal 8 and ab initio 9,10 calculations indicated a planar geometry with C2v symmetry for this molecule (see Figure 1), which is consistent with infrared 11,12 and microwave 13 spectroscopic predictions. Synchrotron based high resolution infrared investigations characterizing the ground state vibrational bands of 3-oxetanone molecule have been reported recently. 14,15

+ CH2CO

HCHO

+

c-C3H4O2

c-C2H4O

CO

Figure 1: Equilibrium geometry of 3-oxetanone molecule and the two considered thermal decomposition pathways forming ketene + formaldehyde (red) and ethylene oxide + carbon monoxide (blue).

Several studies have been reported in the literature on the decomposition reactions of small heterocyclic molecules and their substituted variants. For the thermal decomposition of 3-oxetanone, two different dissociation patterns 16,17 can be considered as shown in Figure 1. The first one involves the formation of ketene (CH2 CO) + formaldehyde (HCHO) and the second one leads to ethylene oxide (oxirane, c−C2 H4 O) + carbon monoxide (CO). Photolysis of 3-oxetanone with all four hydrogen atoms substituted by either methyl 18 or phenyl 19 groups leads to the formation of substituted ketene and ethylene oxide - reactions similar to the ones for 3-oxetanone shown in Figure 1. β-propiolactone or 2-oxetanone (having oxygen atom adjacent to the carbonyl group) is an isomer of 3-oxetanone molecule and its decomposition reactions have been subjected to many experimental studies. 20–22 Cyclobutanone, a molecule similar to 3-oxetanone, forms analogous thermal dissociation products, cyclopropane (ethylene oxide in case of 3-oxetanone) + CO and ethylene + ketene. Previous studies 23–25 on cyclobutanone molecule have shown that the cyclopropane channel requires a higher activation energy in comparison to the ketene pathway. Further, Rice-Ramsperger3 ACS Paragon Plus Environment


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Kassel-Marcus (RRKM) theory 26–28 calculations have predicted a reaction rate ∼2 orders of magnitude slower for the cyclopropane channel as compared to the ketene channel. 16 Only a limited number of studies are available in the literature on the decomposition of unsubstituted 3-oxetanone. Almost four decades ago, there was a paper published 16 wherein unimolecular dissociation rate of 3-oxetanone was investigated by RRKM theory. In this work, only the ketene pathway was taken into account and the activation energy was approximated by that of cyclobutanone. The other pathway leading to ethylene oxide and carbon monoxide was not considered because the reaction rate for similar pathway in cyclobutanone was much smaller (mentioned above). Recently, matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry techniques were used to investigate the pyrolysis products of 3-oxetanone in the gas phase. 17 Reaction products identified were ketene, formaldehyde, ethylene oxide, and carbon monoxide along with other species such as ethylene and methyl radical. In these experiments, determining the branching ratio between the two primary dissociation pathways and the quantification of reaction products such as CO were not possible. In the present work, gas phase dissociation chemistry of 3-oxetanone was investigated using electronic structure theory, ab initio classical trajectory simulations - a methodology also known as direct dynamics, 29,30 and RRKM rate constant calculations. Reaction profiles of the dissociation pathways of 3-oxetanone were characterized using different levels of electronic structure theory. The direct dynamics calculations were performed at the density functional B3LYP/6-31G* level for three different initial conditions. The aim of the present work is to characterize the stationary points on the decomposition pathways of 3-oxetanone, study the atomic level reaction mechanisms, and compute the branching ratios. In addition to the two dissociation channels shown in Figure 1, a new pathway involving a ring-opened isomer of ethylene oxide was identified in the simulations. Electronic structure calculations predicted a lower energy barrier for this dominant dissociation channel. Features of the potential energy surface and the computational methodology are described


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in the next section followed by a detailed Results and Discussion on the dynamics simulations. The article is concluded in the last section. Also, Supporting Information is provided wherein optimized geometry of various stationary points, energy conservation in sample trajectories, a few trajectory snapshots, etc., are given.

Potential Energy Surface and Computational Methods Stationary points on the two primary dissociation pathways of 3-oxetanone molecule shown in Figure 1, were characterized using different levels of electronic structure theory. The geometries of the reactant, products, and the corresponding transition states were optimized and the results are summarized in Table 1. Reported values in the table are electronic energies without zero point corrections. For all the stationary points, CCSD(T)/6-311G* single point calculations (using MP2 optimized geometries) were performed as benchmark. The optimized geometries were identified as equilibria or saddle point by computing their normal mode frequencies. Intrinsic reaction coordinate (IRC) calculations were performed to confirm whether the transition states connect the correct products with the reactant. The calculations were carried out using the NWChem 31 electronic structure theory program. It can be observed from the table that both the pathways are endoergic and the ketene pathway has a lower potential energy barrier as compared to the ethylene oxide channel in all the calculations. The barrier heights for the ketene and ethylene oxide pathways vary from 51.6 to 73.6 kcal/mol and 94.8 to 100.0 kcal/mol, respectively. In comparison to the benchmark CCSD(T) calculations, wavefunction based MP2 method overestimates the reaction barriers though the overall reaction endoergicity agrees with the benchmark values. The hybrid density functional B3LYP predicts barrier heights closer to the CCSD(T) values using both the 6-31G* and 6-311G* basis sets. To perform the ab initio classical trajectory simulations (described below), B3LYP/6-31G* level of theory is chosen because of the accuracy and the less computational time requirements for this method.



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Table 1: Energies of the stationary points on the 3-oxetanone dissociation pathways shown in Figure 1. Numbers are in units of kcal/mol and are relative to the reactant energy at a given level of theory. c−C3 H4 O2

ts1a

CH2 CO+HCHO

ts2b

c−C2 H4 O+CO

B3LYP/6-31G*c

0.0

58.6

10.1

95.9

12.4

B3LYP/6-311G*

0.0

57.2

4.2

94.8

8.5

M06-2X/6-31G*

0.0

69.7

15.9

96.6

8.8

M06-2X/6-311G*

0.0

68.4

11.1

95.3

5.5

PBE0/6-31G*

0.0

64.0

18.8

97.6

17.1

PBE0/6-311G*

0.0

63.0

14.2

96.7

14.9

MP2/6-31G*

0.0

73.6

10.4

100.0

4.8

MP2/6-311G*

0.0

73.4

6.6

97.1

2.7

CCSD(T)/6-31G*d

0.0

52.1

10.5

98.9

4.4

CCSD(T)/6-311G*d

0.0

51.6

7.3

95.8

2.2

theory

a

Transition state for the ketene pathway

b

Transition state for the ethylene oxide pathway

c

Direct dynamics calculations were performed at this level of theory

d

CCSD(T) energies computed by single point calculations using the corresponding MP2 optimized geometries

Reaction profiles, geometries, and energies of the stationary points on the two primary dissociation pathways, computed at the B3LYP/6-31G*, are shown in Figure 2. Optimized coordinates of the reactant, transition states, and products at all level of theories are presented in the Supporting Information. Geometry of the ketene transition state ts1 is nonplanar and the ethylene oxide ts2 is planar. At the CCSD(T)/6-311G* level, the ethylene oxide channel exhibits a barrier height of 95.8 kcal/mol in comparison to 51.6 kcal/mol for the ketene pathway. It is to be noted that in case of cyclobutanone, the cyclopropane channel (similar to ethylene oxide pathway in 3-oxetanone) has a higher activation energy as compared to the ketene pathway 25 and a lower reaction rate predicted by RRKM theory. 16 6 ACS Paragon Plus Environment

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Hence, it is expected that the ketene pathway will be the dominant dissociation channel in the thermal decomposition of 3-oxetanone.

(95.9)

100 ts2

E (kcal/mol)

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(58.6)

50

+ CO

c-C2H4O

(12.4) c-C3H4O2

0

ts1

(10.1)

(0.0)

+ CH2CO

HCHO

Figure 2: Potential energy profile of 3-oxetanone dissociation pathways computed at the B3LYP/6-31G* level of theory. The numbers in bracket are energies in kcal/mol relative to the reactant 3-oxetanone molecule. The corresponding optimized geometries are also shown.

Though features of the potential energy surface provide some clues to the reactivity of a molecule, the actual reaction dynamics might vary from the predictions of electronic structure theory. 32 In order to understand the reaction mechanisms and product branching ratios among the dissociation pathways of 3-oxetanone, Born-Oppenheimer direct dynamics simulations 29,30 were performed at the B3LYP/6-31G* level of theory and the methodology of the simulations is described here. For the trajectory initial conditions, 3-oxetanone molecule was excited with classical microcanonical sampling technique 33,34 at three different total energies, viz. Etot = 150, 200, and 300 kcal/mol, and no rotational energy was added. At each total energy, 150 trajectories (amounting to a total of 450) were propagated. Because of the high ring-strain, 3-oxetanone is expected to be highly reactive and the trajectories were integrated only until 3 ps with a stepsize of 1 fs. Integrations of the classical equations of motion were carried out using 6th order symplectic integrator. 35,36 Though this integrator is computationally expensive in comparison to methods such as velocity Verlet, 37 it provided very 7 ACS Paragon Plus Environment


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good energy conservation in the trajectories. This is evident from Figure 1 in Supporting Information. The classical trajectory calculations were performed using the general chemical dynamics program VENUS 38,39 interfaced with NWChem 31 through a tight coupling algorithm. 40 Default values for the convergence criteria available in the NWChem program were used in all the calculations. On a LENOVO 10 core workstation, approximately 48 hours of computing time was required to integrate one trajectory to 3 ps.

Results and Discussion For each of the excitation energies, Etot = 150, 200, and 300 kcal/mol, a total of 150 trajectories were generated. In all the trajectories, the total energy was conserved within Etot ± 0.5 kcal/mol and plots of total energy as a function of time for a few trajectories is presented in the Supporting Information. The trajectories were animated to establish atomic level reaction mechanisms and to compute the branching ratios. At the highest total energy (300 kcal/mol) all the 150 trajectories showed dissociation and at Etot = 200 and 150 kcal/mol, 147 and 59 trajectories, respectively, were reactive during the integration time. This amounts to 39, 98, and 100 % reactivity for 150, 200, and 300 kcal/mol total energies, respectively. Different dissociation behaviors were observed and a summary of trajectory events is presented in Table 2.


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Table 2: Overall trajectory events following excitation of the 3-oxetanone molecule 150a

200a

300a

CH2 CO + HCHO

1

29

38

c−C2 H4 O + CO

0

0

9

CH2 OCH2 b + CO

57

107

75

minor pathways

1

11

28

no reactionc

91

3

0

total

150

150

150

pathway

a

Excitation energy Etot in kcal/mol. A total of 150 trajectories were generated for each energy

b

Ring opened isomer of ethylene oxide

c

Trajectories that did not dissociate during the entire integration time of 3 ps

Ketene Channel: CH2 CO + HCHO Figure 2 shows that the ketene pathway has the smaller reaction barrier (58.6 kcal/mol) as compared to the ethylene oxide pathway (95.9 kcal/mol) and 1, 29, and 38 trajectories dissociated via this pathway at Etot = 150, 200, and 300 kcal/mol, respectively. The dissociation products CH2 CO + HCHO have been observed in earlier pyrolysis experiments. 17 The fraction of trajectories showing this pathway is small given the low energy barrier. Subsequent dissociation of the reaction products did not happen in the 150 and 200 kcal/mol simulations. At Etot = 300 kcal/mol, dissociation of CH2 CO to form CH2 and CO were observed in 24 (out of 38) trajectories. In Table 3, a summary of subsequent reactions observed for the primary reaction products is presented. The CH2 CO −−→ CH2 + CO reaction is barrierless and endoergic (by 101.5 kcal/mol computed at B3LYP/6-31G*). The potential energy profile



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for this reaction is shown in Figure 3(d). Dissociation of ketene in the singlet state to form CH2 + CO is known to be barrierless and endoergic. 41,42 This reaction was not seen in the low energy (150 and 200 kcal/mol) dynamics simulations due to less available energy. Shock tube experiments on ketene thermal dissociation have been reported in the literature. 43,44 These experiments have indicated that ketene is stable up to 1300◦ C and dissociation happens at higher temperatures. In our simulations also, ketene dissociation was observed only at the highest energy excitation. An interesting point to be noted here is that, during the thermal decomposition of 3-oxetanone, in addition to the direct CO production via the ethylene oxide pathway (discussed below), CO formation can happen via ketene pathway also. Snapshots of a typical trajectory showing this pathway are given in Figure 4(a). Around 30 fs, dissociation of 3-oxetanone to form CH2 CO + HCHO can be observed. Ketene was stable for approximately 1 ps in the trajectory and then the CH2 CO −−→ CH2 + CO dissociation happened close to 1200 fs. In several trajectories, surprisingly long lifetimes were observed for the ketene molecule. Also, subsequent reactions of formaldehyde were not seen in any of the trajectories.

Ethylene Oxide Channel: c−C2 H4 O + CO In the 150 and 200 kcal/mol simulations, none of the trajectories showed dissociation via the high energy pathway forming ethylene oxide and CO. Only for Etot = 300 kcal/mol, nine trajectories dissociated via this pathway (see Table 2) and this small fraction is expected given the high energy barrier. Note that strong experimental evidences for the formation of ethylene oxide from 3-oxetanone have been reported in the literature. 17 Ethylene oxide underwent further dissociation to form HCHO, CO, CH2 , CH2 CO (ketene), and H2 . Production of ketene from ethylene oxide in the thermal decomposition of 3-oxetanone was suspected in the experiments 17 but could not be confirmed. As shown in Table 3, a small fraction of ketene (three trajectories) resulted from ethylene oxide subsequent decomposition with elimination of H2 . Also, in five trajectories formaldehyde was observed with elimination of 10 ACS Paragon Plus Environment

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Figure 3: Potential energy profiles for (a) 3-oxetanone dissociating to CH2 OCH2 + CO (b) isomerization of CH2 OCH2 to ethylene oxide (c) dissociation of CH2 OCH2 to HCHO + CH2 and (d) ketene dissociation to CH2 + CO. The numbers in brackets are energies in kcal/mol, relative to the corresponding reactants.



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CO and CH2 . These are indirect pathways but result in the same products as the primary ketene channel. (a)

14 fs

29 fs

37 fs

1283 fs

1483 fs

6 fs

28 fs

474 fs

524 fs

531 fs

64 fs

81 fs

98 fs

216 fs

269 fs

(b)

(c)

Figure 4: Snapshots of trajectories showing dissociation of 3-oxetanone to (a) HCHO + CH2 CO −−→ HCHO + CH2 + CO (b) CO + CH2 OCH2 −−→ CO + c−C2 H4 O −−→ CO + CH3 CHO and (c) CO + CH2 OCH2 −−→ CO + CH2 + HCHO.

Besides the trajectories described above, there are several other trajectories that resulted in ethylene oxide via another pathway different than the one shown in Figure 2. In this pathway, 3-oxetanone dissociated to form CO and a ring-opened isomer of ethylene oxide, CH2 OCH2 , which isomerized to ethylene oxide or underwent further dissociation. This pathway was major in our simulations and was not considered in any of the previous studies. These trajectory events are discussed in detail below.


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Table 3: Various different dissociation pathways observed for the 3-oxetanone molecule 150a

200a

300a

CH2 CO + HCHO

1

29

14

CH2 CO + HCHO −−→ CH2 + CO + HCHO

0

0

24

CO + c−C2 H4 O −−→ CO + HCHO + CH2

0

0

5

CO + c−C2 H4 O −−→ CO + H2 + CH2 CO

0

0

3

CO + c−C2 H4 O −−→ 2 CO + H2 + CH2

0

0

1

CO + CH2 OCH2

46

9

0

CO + CH2 OCH2 −−→ CO + CH2 + HCHO

0

44

55

CO + CH2 OCH2 −−→ CO + c−C2 H4 O

11

47

0

CO + CH2 OCH2 −−→ CO + c−C2 H4 O −−→ CO + CH3 CHOb

0

5

7

pathway

a

Excitation energy Etot in kcal/mol

b

CH3 CHO underwent further dissociation to form CO, CH4 , etc.

CH2 OCH2 + CO Channel In our direct dynamics simulations, a major fraction of trajectories showed dissociation of 3-oxetanone into CO and the ring-opened ether like isomer of ethylene oxide, CH2 OCH2 (see Table 2). This planar, C2v molecule is an energetically much less stable 45–47 isomer of ethylene oxide (by 46.1 kcal/mol computed at B3LYP/6-31G*). A total of 57, 107, and 75 trajectories showed dissociation via this pathway at Etot = 150, 200, and 300 kcal/mol, respectively. The computed potential energy profile for this reaction is shown in Figure 3(a). The reaction products lie 58.4 kcal/mol above the reactant and are connected to the reactant via a loose transition state with a barrier height of 57.1 kcal/mol. This pathway is energetically much more favorable than the direct ethylene oxide pathway. Upon formation, CH2 OCH2 underwent many different subsequent reactions. Among these, two were major which are isomerization to ethylene oxide and dissociation to HCHO + CH2 . Ring closure of



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CH2 OCH2 to form ethylene oxide has an energy barrier of 18.3 kcal/mol through a counterrotatory transition state 46 and the corresponding energy profile is shown in Figure 3(b). In our simulations 11, 52, and 7 trajectories, at Etot = 150, 200, and 300 kcal/mol, respectively, showed isomerization reaction. It appears that this pathway is the major source of ethylene oxide observed in the experiments. 17 Snapshots of a typical trajectory forming ethylene oxide via this pathway are shown in Figure5(a). Relevant C1 −C2 distance (in Å) and ∠C1 −O−C2 angle (in degree) are shown in Figures 5(b) and (c), respectively. In this trajectory, the dissociation of 3-oxetanone happened around 1400 fs and the ring-closure of CH2 OCH2 to form ethylene oxide at 1700 fs. At this time, the C1 −C2 bond distance and ∠C1 −O−C2 angle started oscillating about their equilibrium values of 1.47 Å and 61.9◦ , respectively, computed for the equilibrium geometry of ethylene oxide. Figure 5 clearly establishes the indirect ethylene oxide pathway discussed here. In a smaller fraction of trajectories (a total of 12), the ethylene oxide molecule underwent [1,2]-H shift to form acetaldehyde. In the pyrolysis experiments of 3-oxetanone, acetaldehyde was suspected as a thermal intermediate but was not confirmed. 17 This is a high energy pathway 47 and was observed only in a smaller fraction of trajectories. Snapshots of a trajectory showing acetaldehyde formation are given in Figure 4(b). The [1,2]-H shift in ethylene oxide to form acetaldehyde happened around 530 fs which can be observed in the last two frames of the figure. The other major subsequent reaction observed for the CH2 OCH2 molecule was CH2 OCH2 −−→ HCHO + CH2 . This dissociation reaction was seen in 44 (Etot = 200 kcal/mol) and 55 (300 kcal/mol) trajectories and might be a major source of formaldehyde observed in the experiments 17 along with the primary CH2 CO + HCHO pathway. Figure 4(c) shows trajectory snapshots of a typical trajectory and the dissociation reaction is indicated in the 98 fs frame. This subsequent reaction is also endoergic with a barrier height of 50.5 kcal/mol and the reaction profile is shown in Figure 3(c). Among the two major subsequent reactions of CH2 OCH2 , dissociation has a larger reaction barrier as compared to the isomerization and


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(a)

213 fs

1413 fs

1434 fs

3 2

C2

C1

(b) O

2157 fs

1 1757 fs

1660 fs 150 (c)

C2

100

C1 O

50 0

1

t (ps)

2

3

Figure 5: (a) Snapshots of a typical trajectory showing dissociation of 3-oxetanone to CO and CH2 OCH2 which isomerizes to ethylene oxide (b) C1 −C2 distance (in Å) (c) ∠C1 −O−C2 angle (in degree) as a function of time. Isomerization of CH2 OCH2 to ethylene oxide happens around 1700 fs.

the former dominated in high energy simulations and the latter in low energy simulations. Our computed values of barrier heights are higher than the CCSD(T)/cc-pVTZ//B3LYP/6311++G(d,p) values (38.8 kcal/mol for dissociation and 10.9 kcal/mol for isomerization) reported earlier 47 but qualitatively similar. In the same earlier work, few other isomerization/dissociation pathways of CH2 OCH2 molecule leading to formation of CH3 OCH, CHOH, etc., were considered but these are energetically not favorable in our dynamics simulations.

Minor Pathways In addition to the major dissociation pathways of 3-oxetanone molecule discussed above, several other minor channels were observed in our simulations and they are briefly described here. Total number of trajectories were less than 10 for each of these pathways and a summary is provided in Table 4. These were observed only in the 200 and 300 kcal/mol simulations. Among the six reactions listed in the table, the ring-opening isomerization pathways leading to CH3 COCHO (2-oxopropanal, five trajectories in total) and CHOCH2 CHO (propanedial, three trajectories) are exoergic owing to the ring-strain in 3oxetanone molecule. 2-oxopropanal further dissociated because of the high available energy 15 ACS Paragon Plus Environment


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yielding CH2 CO, CO, CH2 , CHO, etc. The other four reactions listed in the table are endoergic dissociations forming c−COCH2 CO, CHOCHO (glyoxal), CHCOH, and CH2 CHOH (vinyl alcohol).

Equilibrium geometries of these different species were computed using

B3LYP/6-31G* level of theory and are listed in the Supporting Information. Nine trajectories showed dehydrogenation to form the cyclic c−COCH2 CO, four and three trajectories yielded CHOCHO and CHCOH, respectively. These species also underwent subsequent reactions forming CH2 CO, HCHO, CO, etc., with the exception of CHCOH. Vinyl alcohol formation was observed in two trajectories which further dehydrated to form acetylene. This is such a small fraction that experiments 17 could not detect either water or acetylene in the thermal decomposition of 3-oxetanone. Note that many of these minor pathways yielded products same as the primary pathways discussed above.


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Table 4: Minor pathways observed in the dissociation of 3-oxetanone product

ΔEa

200b

300b

H2 + c−COCH2 COc

83.2

1

8

CH3 COCHOd

-15.8

1

4

CH2 + CHOCHOe

105.8

0

4

62.2

0

3

0.2

0

2

-15.5

3

0

H2 + CO + CHCOH CO + CH2 CHOHf CHOCH2 CHO a

ΔE = Eproduct - E3-oxetanone (in kcal/mol)

b

Excitation energy in kcal/mol

c

Further dissociated to CH2 CO, CH2 , and CO

d

Further dissociated to CH2 CO, CH2 , CH3 , CH4 , CHO, and CO

e

Further dissociated to HCHO, CH2 , H2 , and CO

f

Further dissociated to H2 O and C2 H2

Elimination of CO from 3-Oxetanone One of the intriguing questions about gas phase dissociation of 3-oxetanone that could not be answered in the experiments 17 was how much carbon monoxide produced through direct dissociation of 3-oxetanone and how much via secondary dissociation of the reaction products, i.e. quantification of the amount of CO produced. This information can be obtained from our dynamics simulations. Among the two primary dissociation pathways of 3-oxetanone, the ethylene oxide/CH2 OCH2 channel is the direct CO elimination pathway. Simulations showed that in addition to the primary pathway mentioned above, subsequent decomposition of ketene and products of minor dissociation channels also lead to the formation of CO. In Figure 6(a), fraction of trajectories eliminating CO via primary pathway, secondary pathways 17 ACS Paragon Plus Environment


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and their sum is presented. It can be seen from the figure that quantity of CO formed increases with the increase in total available energy and at sufficiently high energy, CO elimination happens also via secondary pathways. 0.9

(a)

total primary

0.9

(b)

CH2CO + HCHO CO + c-C2H4O/CH2OCH2

secondary

minors no reaction

0.6

0.6

fT 0.3

0.3

0

0 150

200

250

300

150

200

250

300

Etot (kcal/mol)

Figure 6: Fraction of trajectories fT as a function of Etot showing (a) CO elimination via primary dissociation of 3-oxetanone (red), secondary decomposition of reaction products (green), and the total amount of trajectories eliminating CO (black) (b) the branching of 3-oxetanone dissociation products into various different pathways.

RRKM Calculations In an earlier work, 16 RRKM theory was used to investigate the ketene dissociation channel of 3-oxetanone. The rate constants were computed using the energy barrier of analogous cyclobutanone molecule as detailed electronic structure data were not available on the dissociation pathways of 3-oxetanone. In the present work, we have computed the RRKM rate constants (kRRKM ) for the dissociation pathways of 3-oxetanone using the B3LYP/6-31G* electronic structure data. The calculations were carried out employing Beyer-Swinehart direct count algorithm 48,49 and using classical barrier heights (without zero point energy corrections) for the dissociation channels. The computed kRRKM values for the pathways forming ketene + formaldehyde, ethylene oxide + CO, and CH2 OCH2 + CO, are plotted in Figure 7 as a function of total energy. Clearly, rate constants for the CH2 OCH2 pathway are orders of magnitude larger than those for the other two. Ethylene oxide pathway has 18 ACS Paragon Plus Environment

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the smallest values for the rate constants at lower energies due to the high potential barrier. With increase in total energy kRRKM for the ethylene oxide pathway increases and becomes larger than those for the ketene pathway. Results of our dynamics simulations, summarized in Table 2 are in qualitative agreement with the RRKM theory predictions. 10

4

2

-1

10

log(kRRKM)/ps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

10

0

-2

CH2CO + HCHO

10

c-C2H4O + CO

-4

CH2OCH2 + CO

100

200

300

E (kcal/mol)

Figure 7: RRKM theory rate constants kRRKM as a function of energy for the dissociation pathways of 3-oxetanone.

Summary and Conclusion Gas phase dissociation chemistry of 3-oxetanone molecule was studied using electronic structure calculations, direct dynamics methodology, and RRKM theory. Previous studies have considered two primary dissociation pathways for this molecule resulting in ketene + formaldehyde and ethylene oxide + carbon monoxide. Electronic structure calculations showed that both reactions are endoergic and the ethylene oxide pathway has a much larger potential barrier as compared to the ketene pathway. To understand the actual reaction dynamics, ab initio classical trajectory simulations at B3LYP/6-31G* level of theory were performed. Using classical microcanonical sampling technique, 150 trajectories were propagated to 3 ps at three different total energies. Simulations showed that only a small fraction of trajec19 ACS Paragon Plus Environment


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tories preferred the ethylene oxide pathway as compared to the ketene pathway which is expected due to the higher reaction barrier for the former. We found another dissociation channel in the dynamics simulations that involves CO and a ring-opened isomer of ethylene oxide (CH2 OCH2 ) which later isomerized to ethylene oxide or underwent subsequent dissociation. This pathway has a much lower reaction barrier in comparison to the direct ethylene oxide formation and a major fraction of trajectories preferred this channel. Branching of 3-oxetanone dissociation products into these different paths is summarized in Figure 6(b). At all energies, ketene pathway was less preferred over CO + ethylene oxide/CH2 OCH2 formation. This is an important finding given the fact that previous studies 16,17 have considered the ketene channel as the predominant dissociation pathway for 3-oxetanone molecule. Also, different minor pathways emerged with increase in available energy. These lead to the formation of molecules such as 2-oxopropanal, propanedial, glyoxal, vinyl alcohol, etc. We did not perform detailed characterization of these minor channels. The work presented in this paper has demonstrated again 32,50 the importance of doing dynamics simulations in addition to electronic structure calculations in understanding the reactivity of a molecule. Clear evidence for the presence of molecules such as water, acetylene, and acetaldehyde during the thermal decomposition of 3-oxetanone could not be obtained in the pyrolysis experiments. 17 Consistently, these molecules were observed in a very small fraction of trajectories in our simulations. Subsequent dissociation of reaction products made the chemistry of 3-oxetanone more complicated. For example, formaldehyde formation occurred through the primary ketene channel and also the secondary CH2 OCH2 dissociation in non-negligible proportions. Similar is the scenario for carbon monoxide formation. In the work presented here, the initial conditions for the direct dynamics were chosen from a classical microcanonical ensemble in which a given amount of energy is distributed randomly among the normal modes of the molecule. 33 Due to the importance of 3-oxetanone in synthetic chemistry, it would be of interest to study the dynamics of this molecule after mode-specific excitation. Further, the RRKM theory predictions for the 3-oxetanone dissociation agreed qualitatively


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with the results of our dynamics calculations. For a quantitative comparison, one needs to estimate lifetimes and microcanonical rate constants 51 from trajectory data which were not performed in this work.

Acknowledgement Funding from Department of Science and Technology, India, through grant number SB/FT/CS053/2013 is acknowledged. SG would like to thank University Grants Commission (UGC), India, for a fellowship.

Supporting Information Available Energies and coordinates of optimized geometries at different levels of theory; Total energy conservation in trajectories; Complete author list for a few references; Trajectory snapshots for a few trajectories.

This material is available free of charge via the Internet at http:

//pubs.acs.org/.

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Graphical TOC Entry B3LYP/6-31G* +

+ +

+ 3-Oxetanone


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Dissociation Chemistry of 3-Oxetanone in the Gas Phase - The

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