Introduction

3-Oxetanone (1,3-epoxy-2-propanone, c-C₃H₄O₂) is a highly strained molecule useful in ring expansion and addition reactions. Most studies of 3-oxetanone have focused on how ring strain and the electronic interactions of atoms in the ring relate to its planar equilibrium geometry. The occurrence of 3-oxetanone is suspected in the decomposition of the acetonylperoxy radical, an intermediate formed during the atmospheric processing of acetone.This molecule has also been proposed as a decomposition product of acetonylperoxy radical in the atmosphere. A few electronic structure theory and spectroscopic studies focusing on the equilibrium properties such as geometry, ring puckering vibration, and nonbonded interactions have been reported in the literature. Semiempirical and ab initio calculations indicated a planar geometry with C2v symmetry for this molecule (see Figure 1), which is consistent with infrared and microwave spectroscopic predictions. Synchrotron-based high-resolution infrared investigations characterizing the ground state vibrational bands of 3-oxetanone molecule have been reported recently.

Dissociation Chemistry of 3-Oxetanone in the Gas Phase

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 can be considered as shown in Figure 1. The first one involves the formation of ketene (CH₂CO) + formaldehyde (HCHO), and the second one leads to ethylene oxide (oxirane, c-C₂H₄O) +carbon monoxide (CO). Photolysis of 3-oxetanone with all four hydrogen atoms substituted by either methyl or phenyl 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. Cyclobutanone, a molecule similar to 3-oxetanone, forms analogous thermal dissociation products: cyclopropane (ethylene oxide in the case of 3-oxetanone) + CO and ethylene + ketene. Previous studies on cyclobutanone molecule have shown that the cyclopropane channel requires a higher activation energy in comparison to the ketene pathway. Further, Rice?Ramsperger?Kassel?Marcus (RRKM) theory calculations have predicted a reaction rate~2 orders of magnitude slower for the cyclopropane channel as compared to the ketene channel.

For the unsubstituted 3-oxetanone, formation of ketene (CH₂CO) 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-C₂H₄O) 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.[1]

Pyrolysis Reactions of 3-Oxetanone

The pyrolysis products of gas-phase 3-oxetanone were identified via matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry. Pyrolysis was conducted in a hyperthermal nozzle at temperatures from 100 to 1200 °C with the dissociation onset observed at ~600 °C. The pyrolysis of 3-oxetanone produces the anticipated products H₂CO + H₂CCO, as well as ethylene oxide + CO. Secondary reactions involving ethylene oxide lead to the products CH₃, H, and CH₂CH₂, in addition to CO and H₂CCO. While it would be desirable to quantify the product branching for the two primary reactions of 3-oxetanone, it is impractical to measure the relative amounts of products from the two primary reactions. While formaldehyde or ketene could easily be quantified via FTIR, ethylene oxide undergoes further reactions, which also can lead to carbon monoxide. Therefore,it would be impossible to know how much ethylene oxide or carbon monoxide came directly from pyrolysis of 3-oxetanone.The detection of two pyrolysis pathways in 3-oxetanone in these experiments provides motivation to revisit the RRKM analysis of 3-oxetanone with accurate transition-state energies.The dissociation of 3-oxetanone requires temperatures such that it should not occur in the atmospheric processing of acetone via the acetonyl radical. However, 3-oxetanone and its dissociation reactions should be considered in combustion reactions that include the acetonyl radical as an intermediate.

The ring strain in the cyclic structure of 3-oxetanone causes the molecule to decompose at relatively low temperatures. Previously, only one dissociation channel,producing formaldehyde and ketene, was considered as significant in photolysis. This study presents the first experimental measurements of the thermal decomposition of 3-oxetanone demonstrating an additional dissociation channel that forms ethylene oxide and carbon monoxide. Major products include formaldehyde, ketene, carbon monoxide, ethylene oxide, ethylene, and methyl radical. The first four products stem from initial decomposition of 3-oxetanone, while the additional products, ethylene and methyl radical, are believed to be due to further reactions involving ethylene oxide.[2]

References

[1] Godara S, Verma P, Paranjothy M. Dissociation Chemistry of 3-Oxetanone in the Gas Phase. J Phys Chem A. 2017;121(36):6679-6686. doi:10.1021/acs.jpca.7b06880

[2] Wright EM, Warner BJ, Foreman HE, McCunn LR, Urness KN. Pyrolysis Reactions of 3-Oxetanone. J Phys Chem A. 2015;119(29):7966-7972. doi:10.1021/acs.jpca.5b04565