Modeling the Unimolecular Decay Dynamics of the Fluorinated Criegee Intermediate, CF 3 CHOO

: When volatile alkenes are emitted into the atmosphere, they are rapidly removed by oxidizing agents such as hydroxyl radicals and ozone. The latter reaction is termed ozonolysis and is an important source of Criegee intermediates (CIs), i.e., carbonyl oxides, that are implicated in enhancing the oxidizing capacity of the troposphere. These CIs aid in the formation of lower volatility compounds that typically condense to form secondary organic aerosols. CIs have attracted vast attention over the past two decades. Despite this, the effect of their substitution on the ground and excited state chemistries of CIs is not well studied. Here, we extend our knowledge obtained from prior studies on CIs by CF 3 substitution. The resulting CF 3 CHOO molecule is a CI that can be formed from the ozonolysis of hydroﬂuorooleﬁns (HFOs). Our results show that the ground state unimolecular decay should be less reactive and thus more persistent in the atmosphere than the non-ﬂuorinated analog. The excited state dynamics, however, are predicted to occur on an ultrafast timescale. The results are discussed in the context of the ways in which our study could advance synthetic chemistry, as well as processes relevant to the atmosphere.


Introduction
Hydrofluorooelfins (HFOs) are unsaturated organofluorides that are used in modernday refrigerants [1][2][3].These fourth-generation refrigerants are of particular interest, as they have been manufactured to replace harmful chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs).HFOs are preferred over CFCs, HCFCs, and HFCs due to their very low global warming and zero ozone depletion potentials [4,5].This is a consequence of their short lifetimes in the troposphere, as they undergo oxidation via OH and Cl radicals, as well as with O 3 [6,7].
While much is known about HFO molecules themselves, little is known about the (photo)reactivities of their oxidation products.In the atmosphere, HFOs may react with ozone through ozonolysis across an olefinic C=C bond. Figure 1 provides an example of such a reaction with a common HFO refrigerant, 3,3,3-trifluoro-1-propene (HFO-1243zf), wherein ozonolysis occurs via a [3+2]-cycloaddition of the ozone across the C=C double bond of HFO-1243zf, resulting in the formation of a primary ozonide (POZ).Since the [3+2]cycloaddition reaction is highly exothermic, the POZ that is formed is highly internally excited, decaying faster than the timescale for collisional energy relaxation that would otherwise form a stabilized POZ.Its unimolecular decay occurs via the cleavage of the five-membered ring center C-C and one of the two O-O bonds, yielding two distinct products: H 2 COO + CF 3 CHO or CF 3 CHOO + H 2 CO.These H 2 COO and CF 3 CHOO products are known as Criegee intermediates (CIs), which are recognized for enhancing the oxidating capacity of the atmosphere and are implicated in secondary organic aerosol formation [8][9][10][11].
Photochem 2023, 3, FOR PEER REVIEW 2 the five-membered ring center C-C and one of the two O-O bonds, yielding two distinct products: H2COO + CF3CHO or CF3CHOO + H2CO.These H2COO and CF3CHOO products are known as Criegee intermediates (CIs), which are recognized for enhancing the oxidating capacity of the atmosphere and are implicated in secondary organic aerosol formation [8][9][10][11].Non-fluorine-containing CIs (such as H2COO) are formed in the atmosphere when volatile alkenes undergo ozonolysis.The simplest alkyl-substituted CI is acetaldehyde oxide (CH3CHOO), which is formed from the ozonolysis of molecules such as propene.It may be formed in the syn-or anti-conformer.CH3CHOO can undergo unimolecular decay via intramolecular hydrogen-atom transfer, forming a vinyl hydroperoxide (VHP), which subsequently decays to form vinoxy and OH radicals [10,[12][13][14].While this process is favored in syn-CH3CHOO, anti-CH3CHOO unimolecular decay is dominated by the formation of methyl dioxirane or acetic acid (CH3C(O)OH) via isomerization, which decays to form CH3CO + OH, CH4 + CO2, and CH3OH + CO.CH3CHOO, especially in the anticonformation, can also undergo bimolecular chemistry with trace tropospheric gases such as H2O, SO2, and HCOOH [15].The photochemical properties of CH3CHOO are also worth discussing [16,17].Using UV action spectroscopy and velocity map imaging on jetcooled CH3CHOO, Lester and co-workers showed through their studies that the photodissociation of this molecule led to the formation of CH3CHO + O products after excitation at λ < 350 nm.This dissociation resulted in two spin-allowed channels forming CH3CHO (S0) + O ( 1 D) and CH3CHO (T1) + O ( 3 P) products, with the latter becoming energetically accessible at λ ≤ 324 nm.Using trajectory surface hopping (TSH), where the energies, gradients, and non-adiabatic couplings were computed "on-the-fly" using MS-CASPT2, we previously assessed the dynamics of CH3CHOO following excitation to the S2 state.O-O bond dissociation was observed to be the dominant decay channel, forming CH3CHO (S0) + O ( 1 D) products [16].
In this manuscript, we explore the effects of fluorination on the ground state unimolecular decay and photodissociation dynamics of syn-and anti-CF3CHOO.

Methodology
The ground state minimum energy geometries of CH3CHOO and CF3CHOO, along with their respective dioxirane products and transition states, were optimized at the m06-2x/aug-cc-pVTZ level of theory using the Gaussian computational package [18].Using the CCSD(T)-F12/cc-pVTZ-F12 level of theory, single-point energy calculations were computed on the optimized structures, which allowed us to obtain more accurate energies.These latter calculations were carried out in Molpro [19,20].This combination of m06-2x/aug-cc-pVTZ//CCSD(T)-F12 was successfully applied to develop a structure-activity relationship of the unimolecular decay kinetics of substituted Criegee intermediates [21,22].Non-fluorine-containing CIs (such as H 2 COO) are formed in the atmosphere when volatile alkenes undergo ozonolysis.The simplest alkyl-substituted CI is acetaldehyde oxide (CH 3 CHOO), which is formed from the ozonolysis of molecules such as propene.It may be formed in the synor anti-conformer.CH 3 CHOO can undergo unimolecular decay via intramolecular hydrogen-atom transfer, forming a vinyl hydroperoxide (VHP), which subsequently decays to form vinoxy and OH radicals [10,[12][13][14].While this process is favored in syn-CH 3 CHOO, anti-CH 3 CHOO unimolecular decay is dominated by the formation of methyl dioxirane or acetic acid (CH 3 C(O)OH) via isomerization, which decays to form CH 3 CO + OH, CH 4 + CO 2 , and CH 3 OH + CO.CH 3 CHOO, especially in the anti-conformation, can also undergo bimolecular chemistry with trace tropospheric gases such as H 2 O, SO 2 , and HCOOH [15].The photochemical properties of CH 3 CHOO are also worth discussing [16,17].Using UV action spectroscopy and velocity map imaging on jet-cooled CH 3 CHOO, Lester and co-workers showed through their studies that the photodissociation of this molecule led to the formation of CH 3 CHO + O products after excitation at λ < 350 nm.This dissociation resulted in two spin-allowed channels forming CH 3 CHO (S 0 ) + O ( 1 D) and CH 3 CHO (T 1 ) + O ( 3 P) products, with the latter becoming energetically accessible at λ ≤ 324 nm.Using trajectory surface hopping (TSH), where the energies, gradients, and non-adiabatic couplings were computed "on-the-fly" using MS-CASPT2, we previously assessed the dynamics of CH 3 CHOO following excitation to the S 2 state.O-O bond dissociation was observed to be the dominant decay channel, forming CH 3 CHO (S 0 ) + O ( 1 D) products [16].
In this manuscript, we explore the effects of fluorination on the ground state unimolecular decay and photodissociation dynamics of synand anti-CF 3 CHOO.

Methodology
The ground state minimum energy geometries of CH 3 CHOO and CF 3 CHOO, along with their respective dioxirane products and transition states, were optimized at the m06-2x/aug-cc-pVTZ level of theory using the Gaussian computational package [18].Using the CCSD(T)-F12/cc-pVTZ-F12 level of theory, single-point energy calculations were computed on the optimized structures, which allowed us to obtain more accurate energies.These latter calculations were carried out in Molpro [19,20].This combination of m06-2x/aug-cc-pVTZ//CCSD(T)-F12 was successfully applied to develop a structure-activity relationship of the unimolecular decay kinetics of substituted Criegee intermediates [21,22].
The fate of the excited states of CF 3 CHOO was modeled using TSH and static electronic structure calculations.TSH simulations were carried out using the Newton-X computational package [23,24].Initial conditions were acquired using a Wigner distribution based on the B2PLYP-D3/cc-pVTZ equilibrium geometry of synand anti-CF 3 CHOO and their respective harmonic normal mode wavenumbers.As shown in prior findings, this level of theory performs well in obtaining the geometries and normal modes of CIs and their associated reaction profiles [25][26][27][28][29][30][31][32].In TSH, the nuclei were propagated by integrating Newton's equation using the velocity Verlet method, while the electronic coordinates were treated quantum-mechanically by numerically solving the time-dependent Schrodinger equation using Butcher's fifth-order Runge-Kutta method in steps of 0.025 fs [33].Trajectories were initiated on the bright S 2 state.Energies, forces, and non-adiabatic coupling matrix elements were computed on-the-fly, using the single-state, single-reference complete active space second-order perturbation theory (SS-SR-CASPT2) method along with the cc-pVDZ basis set.These energies/forces were computed via the BAGEL interface to Newton-X [34,35].SS-SR-CASPT2 not only produced energies and analytical gradients at the MS-CASPT2 quality at a reduced computational cost, but also performed well near electronic state degeneracies.SS-SR-CASPT2 calculations were based on a state-averaged complete active space self-consistent field (CASSCF) method reference wavefunction and an active space composed of 10 electrons in 8 orbitals.Additional potential energy profiles were computed in order to assess the excited state reaction paths along the O-O stretch coordinate.These were computed using Molpro at the CASPT2/aug-cc-pVTZ level of theory and using the same 10/8 active space as the previously mentioned TSH simulations.Due to the highly multi-reference nature of the excited-state dynamics of the Criegee intermediate, these methods were required for modeling the excited states of our present systems.Our previous studies have highlighted the success of SS-SR-CASPT2 in assessing the excited state dynamics and energy profiles of Criegee intermediates [29].

Results and Discussion
Figure 2 displays the ground state minimum energy geometry of the synand anticonformers of CF 3 CHOO.These conformers are distinguishable by the orientation of the terminal oxygen atom relative to the CF 3 group.Anti-CF 3 CHOO is the most stable conformer and is predicted to be ca.0.6 kcal mol −1 more stable than syn-CF 3 CHOO).This observation is in contrast to CH 3 CHOO [12], which showed a preference for forming synconformers due to the weak intramolecular hydrogen-bonding interaction between the CH 3 -centered H-atoms and the terminal oxygen atom.Since this interaction does not exist in syn-CF 3 CHOO, anti-CF 3 CHOO shows less steric hindrance (cf.syn-CF 3 CHOO).The equivalent energy difference between synand anti-CH 3 CHOO was ca.3.7 kcal mol −1 , with a barrier height of ca.38 kcal mol −1 .
The fate of the excited states of CF3CHOO was modeled using TSH and static electronic structure calculations.TSH simulations were carried out using the Newton-X computational package [23,24].Initial conditions were acquired using a Wigner distribution based on the B2PLYP-D3/cc-pVTZ equilibrium geometry of syn-and anti-CF3CHOO and their respective harmonic normal mode wavenumbers.As shown in prior findings, this level of theory performs well in obtaining the geometries and normal modes of CIs and their associated reaction profiles [25][26][27][28][29][30][31][32].In TSH, the nuclei were propagated by integrating Newton s equation using the velocity Verlet method, while the electronic coordinates were treated quantum-mechanically by numerically solving the time-dependent Schrodinger equation using Butcher s fifth-order Runge-Kutta method in steps of 0.025 fs [33].Trajectories were initiated on the bright S2 state.Energies, forces, and non-adiabatic coupling matrix elements were computed on-the-fly, using the single-state, single-reference complete active space second-order perturbation theory (SS-SR-CASPT2) method along with the cc-pVDZ basis set.These energies/forces were computed via the BAGEL interface to Newton-X [34,35].SS-SR-CASPT2 not only produced energies and analytical gradients at the MS-CASPT2 quality at a reduced computational cost, but also performed well near electronic state degeneracies.SS-SR-CASPT2 calculations were based on a stateaveraged complete active space self-consistent field (CASSCF) method reference wavefunction and an active space composed of 10 electrons in 8 orbitals.Additional potential energy profiles were computed in order to assess the excited state reaction paths along the O-O stretch coordinate.These were computed using Molpro at the CASPT2/aug-cc-pVTZ level of theory and using the same 10/8 active space as the previously mentioned TSH simulations.Due to the highly multi-reference nature of the excited-state dynamics of the Criegee intermediate, these methods were required for modeling the excited states of our present systems.Our previous studies have highlighted the success of SS-SR-CASPT2 in assessing the excited state dynamics and energy profiles of Criegee intermediates [29].

Results and Discussion
Figure 2 displays the ground state minimum energy geometry of the syn-and anticonformers of CF3CHOO.These conformers are distinguishable by the orientation of the terminal oxygen atom relative to the CF3 group.Anti-CF3CHOO is the most stable conformer and is predicted to be ca.0.6 kcal mol −1 more stable than syn-CF3CHOO).This observation is in contrast to CH3CHOO [12], which showed a preference for forming synconformers due to the weak intramolecular hydrogen-bonding interaction between the CH3-centered H-atoms and the terminal oxygen atom.Since this interaction does not exist in syn-CF3CHOO, anti-CF3CHOO shows less steric hindrance (cf.syn-CF3CHOO).The equivalent energy difference between syn-and anti-CH3CHOO was ca.3.7 kcal mol −1 , with a barrier height of ca.38 kcal mol −1 .Guided by previous studies on the ozonolysis of CIs, CF3CHOO is expected to undergo bimolecular chemistry with trace gases in the atmosphere or unimolecular decay either thermally or after UV excitation.Here, we focused on the latter two unimolecular Guided by previous studies on the ozonolysis of CIs, CF 3 CHOO is expected to undergo bimolecular chemistry with trace gases in the atmosphere or unimolecular decay either thermally or after UV excitation.Here, we focused on the latter two unimolecular decay paths.Figure 3a,c displays the energy profiles that are associated with the two most prominent unimolecular decay paths of CH 3 CHOO-i.e., isomerization to form vinyl hydroperoxide (VHP) or methyldioxirane.VHP formation involves 1,4-hydrogen-atom migration and is the dominant unimolecular decay path for the more stable (syn) conformer of CH 3 CHOO, and is predicted to contain a transition state barrier of ca.18 kcal mol −1 -which is in excellent agreement with prior high-level quantum chemical studies [36].The equivalent isomerization in CF 3 CHOO involves a 1,4-fluorine atom migration (Figure 3d).As expected, the energy profile associated with the F-atom migration contained a transition state barrier that was ca.60 kcal mol −1 higher than the equivalent H-atom migration in syn-CH 3 CHOO-manifesting from an unfavorable C-F bond fission and an unfavorable OF bond formation.In contrast, the cyclization of CF 3 CHOO to form dioxirane contained a transition state barrier that was comparable to that of CH 3 CHOO (Figure 3a,b).We, therefore, expect cyclization to be the only competitive thermal removal process in CF 3 CHOO, which is expected to proceed with a low rate constant.Under low humidity conditions, we expect CF 3 CHOO to be long-lived when compared to the typical sub-second atmospheric lifetime of CIs.As such, it could absorb UV radiation and become photo-excited.decay paths.Figure 3a,c displays the energy profiles that are associated with the two most prominent unimolecular decay paths of CH3CHOO-i.e., isomerization to form vinyl hydroperoxide (VHP) or methyldioxirane.VHP formation involves 1,4-hydrogen-atom migration and is the dominant unimolecular decay path for the more stable (syn) conformer of CH3CHOO, and is predicted to contain a transition state barrier of ca.18 kcal mol −1which is in excellent agreement with prior high-level quantum chemical studies [36].The equivalent isomerization in CF3CHOO involves a 1,4-fluorine atom migration (Figure 3d).As expected, the energy profile associated with the F-atom migration contained a transition state barrier that was ca.60 kcal mol −1 higher than the equivalent H-atom migration in syn-CH3CHOO-manifesting from an unfavorable C-F bond fission and an unfavorable OF bond formation.In contrast, the cyclization of CF3CHOO to form dioxirane contained a transition state barrier that was comparable to that of CH3CHOO (Figure 3a,b).We, therefore, expect cyclization to be the only competitive thermal removal process in CF3CHOO, which is expected to proceed with a low rate constant.Under low humidity conditions, we expect CF3CHOO to be long-lived when compared to the typical sub-second atmospheric lifetime of CIs.As such, it could absorb UV radiation and become photo-excited.Figure 4 presents the orbitals and orbital promotions that are associated with electronic excitation to the S 1 and S 2 states.As with other CIs, excitation to the S 1 state arises via a π* ← n orbital promotion and contains zero oscillator strength.In contrast, excitation to the S 2 state involves a π* ← π electron promotion, which is the transition that dominates the near-UV absorption.This aligns with previous observations of CIs [17].
via a π* ← n orbital promotion and contains zero oscillator strength.In contrast, excitation to the S2 state involves a π* ← π electron promotion, which is the transition that dominates the near-UV absorption.This aligns with previous observations of CIs [17].Following excitation to the bright S2 state, our TSH simulations (Figure 5) revealed fast deactivation of the S2 state within 40 fs.An analysis of the TSH simulations reveals that both the syn-and anti-conformers dominantly undergo O-O bond elongation within 40 fs, leading to the formation of CF3CHO + O products with a unity quantum yield.In the higher energy syn-conformer (Figure 5b), prompt internal conversion at early time revealed partitioning of the population to all seven electronic states.By contrast, only the S0, S1, S2, S3, and S4 states are populated in the anti-conformer-i.e., the lower energy conformer.As the CASPT2 PE profiles along the O-O elongation coordinate (Figure 6) reveal, the S0, S1, S2, S3 and S4 states correlate with the lower energy asymptote, corresponding to O( 1 D) + CF3CHO(S0) products, while the S5 and S6 states correlate with the second asymptote, which corresponds to the O( 3 P) + CF3CHO(T1) product asymptote.The electronic state character of the S0-S6 states was the same as those reported in our earlier work [37], which are briefly outlined in the following text.In the former cluster of states, S0 involves Following excitation to the bright S 2 state, our TSH simulations (Figure 5) revealed fast deactivation of the S 2 state within 40 fs.An analysis of the TSH simulations reveals that both the synand anti-conformers dominantly undergo O-O bond elongation within 40 fs, leading to the formation of CF 3 CHO + O products with a unity quantum yield.In the higher energy syn-conformer (Figure 5b), prompt internal conversion at early time revealed partitioning of the population to all seven electronic states.By contrast, only the S 0 , S 1 , S 2 , S 3, and S 4 states are populated in the anti-conformer-i.e., the lower energy conformer.
via a π* ← n orbital promotion and contains zero oscillator strength.In contrast, excitation to the S2 state involves a π* ← π electron promotion, which is the transition that dominates the near-UV absorption.This aligns with previous observations of CIs [17].Following excitation to the bright S2 state, our TSH simulations (Figure 5) revealed fast deactivation of the S2 state within 40 fs.An analysis of the TSH simulations reveals that both the syn-and anti-conformers dominantly undergo O-O bond elongation within 40 fs, leading to the formation of CF3CHO + O products with a unity quantum yield.In the higher energy syn-conformer (Figure 5b), prompt internal conversion at early time revealed partitioning of the population to all seven electronic states.By contrast, only the S0, S1, S2, S3, and S4 states are populated in the anti-conformer-i.e., the lower energy conformer.As the CASPT2 PE profiles along the O-O elongation coordinate (Figure 6) reveal, the S0, S1, S2, S3 and S4 states correlate with the lower energy asymptote, corresponding to O( 1 D) + CF3CHO(S0) products, while the S5 and S6 states correlate with the second asymptote, which corresponds to the O( 3 P) + CF3CHO(T1) product asymptote.The electronic state character of the S0-S6 states was the same as those reported in our earlier work [37], which are briefly outlined in the following text.In the former cluster of states, S0 involves As the CASPT2 PE profiles along the O-O elongation coordinate (Figure 6) reveal, the S 0 , S 1 , S 2 , S 3 and S 4 states correlate with the lower energy asymptote, corresponding to O( 1 D) + CF 3 CHO(S 0 ) products, while the S 5 and S 6 states correlate with the second asymptote, which corresponds to the O( 3 P) + CF 3 CHO(T 1 ) product asymptote.The electronic state character of the S 0 -S 6 states was the same as those reported in our earlier work [37], which are briefly outlined in the following text.In the former cluster of states, S 0 involves an intuitive long-range attractive interaction that arises via the interaction between the CF 3 CHO-centered O-2p lone pair and the vacant 2p orbital on O( 1 D).At long R OO , this same 2p orbital is either singly or doubly occupied in the S 1 -S 4 state configurations and manifests in the long-range repulsive interaction that is observed for these states in Figure 6.The S 5 and S 6 states contain a long-range attractive interaction, which can be understood by considering that the T 1 state of formaldehyde is of the nπ* character, with an odd electron in the oxygen-centered 2p lone pair.This odd electron forms a bonding pair with one of the two odd electrons in the 2p orbital of the O( 3 P) atom.This agrees with previous observations of CI's photochemistry [17].As noted in the above description, in the anticonformer, a negligible portion of the starting population partitions to the higher energy S 5 and S 6 states, manifesting in a negligible yield of O( 3 P) + CF 3 CHO(T 1 ) products.By contrast, for the syn-conformer, ca.15% of the initially excited population partitions across the S 5 and S 6 states, giving ca.15% yield of O( 3 P) + CF 3 CHO(T 1 ) products within 40 fs.This latter observation is in line with a previous observation of the simplest CI, CH 2 OO, which returned an equally low fraction of the overall population into O( 3 P) + CH 2 O(T 1 ) products [29].This observation is striking and may be plausibly explained by the weaker interaction between S 3 /S 4 and S 5 /S 6 states or by a perturbation caused by the CF 3 moiety on the O atom leaving group.
an intuitive long-range attractive interaction that arises via the interaction between the CF3CHO-centered O-2p lone pair and the vacant 2p orbital on O( 1 D).At long ROO, this same 2p orbital is either singly or doubly occupied in the S1-S4 state configurations and manifests in the long-range repulsive interaction that is observed for these states in Figure 6.The S5 and S6 states contain a long-range attractive interaction, which can be understood by considering that the T1 state of formaldehyde is of the nπ* character, with an odd electron in the oxygen-centered 2p lone pair.This odd electron forms a bonding pair with one of the two odd electrons in the 2p orbital of the O( 3 P) atom.This agrees with previous observations of CI s photochemistry [17].As noted in the above description, in the anticonformer, a negligible portion of the starting population partitions to the higher energy S5 and S6 states, manifesting in a negligible yield of O( 3 P) + CF3CHO(T1) products.By contrast, for the syn-conformer, ca.15% of the initially excited population partitions across the S5 and S6 states, giving ca.15% yield of O( 3 P) + CF3CHO(T1) products within 40 fs.This latter observation is in line with a previous observation of the simplest CI, CH2OO, which returned an equally low fraction of the overall population into O( 3 P) + CH2O(T1) products [38].This observation is striking and may be plausibly explained by the weaker interaction between S3/S4 and S5/S6 states or by a perturbation caused by the CF3 moiety on the O atom leaving group.The dominant nuclear motions associated with these internal conversions can be understood by assessing the PE profiles in Figure 6, which show two avoided crossings.These avoided crossings will develop into conical intersections when motions along orthogonal motions are considered.The first avoided crossing is at ROO~1.7 Å, which involves a four-state intersection involving the S1, S2, S3, and S4 states.The early time internal The dominant nuclear motions associated with these internal conversions can be understood by assessing the PE profiles in Figure 6, which show two avoided crossings.These avoided crossings will develop into conical intersections when motions along orthogonal motions are considered.The first avoided crossing is at R OO ~1.7 Å, which involves a four-state intersection involving the S 1 , S 2 , S 3, and S 4 states.The early time internal conversation observed in the TSH results arises via an internal conversion at these crossing points.A second avoided crossing can be observed at R OO ~2.2 Å between the S 3 , S 4 , S 5, and S 6 states.The ultimate branching into the O( 1 D) + CF 3 CHO(S 0 ) and O( 3 P) + CF 3 CHO(T 1 ) products is governed by the rate of internal conversion at this crossing point.

Conclusions
In this article, we have reported the details of the ground and excited state unimolecular decay of CF 3 CHOO, which can be formed from the ozonolysis of commonly used HFOs.Unlike CH 3 CHOO, the two synand anti-conformers of CF 3 CHOO are energetically close, manifesting from the absence of hydrogen bonding that is present in syn-CH 3 CHOO.Our results show that the unimolecular decay of CF 3 CHOO is expected to follow an analogous cyclization to form dioxirane products but with a slightly lower energy barrier.Unlike CH 3 CHOO, however, syn-CF 3 CHOO does not have an analogous hydrogen atom transfer that forms a vinyl hydroperoxide.As demonstrated in our results above, the analogous fluorine atom transfer is, as expected, unfavorable.As with other CIs, the electronic absorption of CF 3 CHOO is dominated by the S 2 state, which arises via a π* ← π transition.Excitation to this state leads to prompt O-O bond fission, forming O( 1 D) products in both syn-CF 3 CHOO and anti-CF 3 CHOO.Unlike CH 2 OO, the O( 3 P) channel was unobserved in anti-CF 3 CHOO.
These results shed light on the expected ground and excited state chemistry of an HFO-derived CI.When HFO refrigerants are emitted into the atmosphere, their primary removal is via a reaction with OH radicals; however, reaction with O 3 is relevant in ozonerich environments.Our studies are also relevant to the synthetic chemistry community, since the results highlight the effect of a CF 3 substitution on the ground and excited state dynamics of a substituted CI.When compared to our previous studies on CFHOO, the effect of CF 3 substitution was much less dramatic than F substitution, which is most likely due to the weaker π-perturbing effect of CF 3 versus F.
Our future studies aim to focus on the bimolecular chemistry of CF 3 CHOO and how its chemistry compares to CH 3 CHOO.Since some HFOs contain a chlorine substituent, we would also be excited to explore the effect of chlorination on the ground and the excited state chemistry of CIs.

Figure 1 .
Figure 1.Schematic reaction path displaying the ozonolysis of the HFO-1243zf refrigerant.The molecule of interest in this study is shown in red (trifluoroacetaldehyde oxide-CF3CHOO).

Figure 1 .
Figure 1.Schematic reaction path displaying the ozonolysis of the HFO-1243zf refrigerant.The molecule of interest in this study is shown in red (trifluoroacetaldehyde oxide-CF 3 CHOO).

Figure 2 .
Figure 2. Minimum energy structures of anti-and syn-H3CHOO and anti-and syn-CF3CHOO.

Figure 3 .
Figure 3. Reaction energy profiles associated with the unimolecular decay paths available to CH3CHOO and CF3CHOO; dioxirane formation in (a) CH3CHOO and (b) CF3CHOO, (c) 1,4-H migration in syn-CH3CHOO and (d) F-migration in syn-CF3CHOO.Panels (a,b) represent the cyclization to form dioxirane products, while (c,d) represent the energy profile associated with intramolecular hydrogen or fluorine atom transfer, respectively.

Figure 4
Figure 4 presents the orbitals and orbital promotions that are associated with electronic excitation to the S1 and S2 states.As with other CIs, excitation to the S1 state arises

Figure 3 .
Figure 3. Reaction energy profiles associated with the unimolecular decay paths available to CH 3 CHOO and CF 3 CHOO; dioxirane formation in (a) CH 3 CHOO and (b) CF 3 CHOO, (c) 1,4-H migration in syn-CH 3 CHOO and (d) F-migration in syn-CF 3 CHOO.Panels (a,b) represent the cyclization to form dioxirane products, while (c,d) represent the energy profile associated with intramolecular hydrogen or fluorine atom transfer, respectively.

Figure 4 .
Figure 4. Orbitals and orbital promotions associated with excitation to the lowest two single excited electronic states of syn-and anti-CF3CHOO.

Figure 5 .
Figure 5.The time evolution of the populations for excitation to the S2 state of (a) anti-CF3CHOO and (b) syn-CF3CHOO.

Figure 4 .
Figure 4. Orbitals and orbital promotions associated with excitation to the lowest two single excited electronic states of synand anti-CF 3 CHOO.

Figure 4 .
Figure 4. Orbitals and orbital promotions associated with excitation to the lowest two single excited electronic states of syn-and anti-CF3CHOO.

Figure 5 .
Figure 5.The time evolution of the populations for excitation to the S2 state of (a) anti-CF3CHOO and (b) syn-CF3CHOO.

Figure 5 .
Figure 5.The time evolution of the populations for excitation to the S 2 state of (a) anti-CF 3 CHOO and (b) syn-CF 3 CHOO.

Figure 6 .
Figure 6.Adiabatic potential energy profiles along the O-O stretch coordinate for the lowest seven singlet states of (a) anti-CF3CHOO and (b) syn-CF3CHOO.

Figure 6 .
Figure 6.Adiabatic potential energy profiles along the O-O stretch coordinate for the lowest seven singlet states of (a) anti-CF 3 CHOO and (b) syn-CF 3 CHOO.