Modeling the Unimolecular Decay Dynamics of the Fluorinated Criegee Intermediate, CF₃CHOO

Abstract

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 CF₃ substitution. The resulting CF₃CHOO 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.

1. Introduction

Hydrofluorooelfins (HFOs) are unsaturated organofluorides that are used in modern-day 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 O₃ [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: 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 anti-conformation, 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 jet-cooled 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 (¹D) and CH₃CHO (T₁) + O (³P) 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 (¹D) 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.

2. 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].

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 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].

3. Results and Discussion

Figure 2 displays the ground state minimum energy geometry of the syn- and anti-conformers 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 syn- conformers 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 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⁻¹—which 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 S₁ and S₂ states. As with other CIs, excitation to the S₁ state arises 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, S_3, 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(¹D) + CF₃CHO(S₀) products, while the S₅ and S₆ states correlate with the second asymptote, which corresponds to the O(³P) + 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 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(¹D). At long R_OO, 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(³P) atom. This agrees with previous observations of CI’s photochemistry [17]. As noted in the above description, in the anti-conformer, a negligible portion of the starting population partitions to the higher energy S₅ and S₆ states, manifesting in a negligible yield of O(³P) + 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(³P) + 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(³P) + CH₂O(T₁) products [29]. 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 R_OO~1.7 Å, which involves a four-state intersection involving the S₁, S₂, S_3, and S₄ 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 S₃, S₄, S_5, and S₆ states. The ultimate branching into the O(¹D) + CF₃CHO(S₀) and O(³P) + CF₃CHO(T₁) products is governed by the rate of internal conversion at this crossing point.

4. Conclusions

In this article, we have reported the details of the ground and excited state unimolecular decay of CF₃CHOO, which can be formed from the ozonolysis of commonly used HFOs. Unlike CH₃CHOO, the two syn- and anti-conformers of CF₃CHOO are energetically close, manifesting from the absence of hydrogen bonding that is present in syn-CH₃CHOO. Our results show that the unimolecular decay of CF₃CHOO is expected to follow an analogous cyclization to form dioxirane products but with a slightly lower energy barrier. Unlike CH₃CHOO, however, syn-CF₃CHOO 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 CF₃CHOO is dominated by the S₂ state, which arises via a π* ← π transition. Excitation to this state leads to prompt O-O bond fission, forming O(¹D) products in both syn-CF₃CHOO and anti-CF₃CHOO. Unlike CH₂OO, the O(³P) channel was unobserved in anti-CF₃CHOO.

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 O₃ is relevant in ozone-rich environments. Our studies are also relevant to the synthetic chemistry community, since the results highlight the effect of a CF₃ substitution on the ground and excited state dynamics of a substituted CI. When compared to our previous studies on CFHOO, the effect of CF₃ substitution was much less dramatic than F substitution, which is most likely due to the weaker π-perturbing effect of CF₃ versus F.

Our future studies aim to focus on the bimolecular chemistry of CF₃CHOO and how its chemistry compares to CH₃CHOO. 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.