Investigating the Photodissociation Dynamics of CF₂BrCF₂I in CCl₄ through Femtosecond Time-Resolved Infrared Spectroscopy

Abstract

The photodissociation dynamics of CF₂BrCF₂I in CCl₄ at 280 ± 2 K were investigated by probing the C−F stretching mode from 300 fs to 10 μs after excitation at 267 nm using time-resolved infrared spectroscopy. The excitation led to the dissociation of I or Br atoms within 300 fs, producing the CF₂BrCF₂ or CF₂ICF₂ radicals, respectively. All nascent CF₂ICF₂ underwent further dissociation of I, producing CF₂CF₂ with a time constant of 56 ± 5 ns. All nascent g-CF₂BrCF₂ isomerized into the more stable a-CF₂BrCF₂ with a time constant of 47 ± 5 ps. Furthermore, a-CF₂BrCF₂ underwent a bimolecular reaction with either itself (producing CF₂BrCF₂Br and CF₂CF₂) or Br in the CCl₄ solution (producing CF₂BrCF₂Br) at a diffusion-limited rate. The secondary dissociation of Br from a-CF₂BrCF₂ was significantly slow to compete with the bimolecular reactions. Overall, approximately half of the excited CF₂BrCF₂I at 267 nm produced CF₂BrCF₂Br, whereas the other half produced CF₂CF₂. The excess energies in the nascent radicals were thermalized much faster than the secondary dissociation of I from CF₂ICF₂ and the observed bimolecular reactions, implying that the secondary reactions proceeded under thermal conditions. This study further demonstrates that structure-sensitive time-resolved infrared spectroscopy can be used to study various reaction dynamics in solution in real time.

1. Introduction

Vicinal dihaloethanes are dissociated using ultraviolet (UV) energy to form haloethyl radicals that undergo various secondary reactions [1,2,3,4,5,6,7,8,9,10,11,12]. Significant efforts have been made to understand the dynamics of the photodissociation of vicinal dihaloethanes, including the reaction dynamics of haloethyl radicals, in solution and in the gas phase [1,2,3,4,5,6,7,8,9,10,11,12]. The initial reaction of vicinal dihaloalkanes, in which one halogen atom is immediately dissociated upon UV excitation to form haloethyl radicals, shows similar characteristics in the gas and condensed phases [5,8,10]. However, the secondary reactions of haloethyl radicals in solution show different kinetics from the gas-phase reaction depending on the type of substituted halogen atom and the spatial isomers of haloethyl radicals [8,10].

Vicinal diiodoethanes, such as CH₂ICH₂I, CF₂ICH₂I, and CF₂ICF₂I, were mainly investigated because the iodoethyl radicals produced upon UV excitation contain weak C−I bonds that readily proceed with secondary dissociation [1,2,3,5,7,8,10,12]. In the case of the photodissociation of CF₂ICF₂I in the gas phase, a fraction of the iodoethyl radicals dissociate the secondary C−I bond with a time constant of 26 ± 7 ps to form CF₂CF₂ and I [2]. Secondary dissociation of haloethyl radicals often occurs because of the weakness of the halogen–carbon bonds; however, these bonds strengthen upon fluorination. The C−I bond is weaker in CH₂ICH₂ than in CF₂ICF₂ because the π-bond formed concertedly upon the fission of the C−I bond is stronger in the case of CH₂ICH₂ [7]. The primary C−I bond breaking of dihaloalkanes has two dissociation channels, leading to I(²P_1/2) and I(²P_3/2) [3,4,6,7,9]. The internal energy of haloethyl radicals is larger with the reaction channel of I(²P_3/2); thus, the haloethyl radicals formed with I(²P_3/2) undergo faster and more efficient secondary dissociation in the gas phase [3,4,6,7,9].

In the condensed phase, CF₂ICF₂ radicals are also formed by the UV excitation of CF₂ICF₂I [5,8,10]. However, the secondary reaction of CF₂ICF₂ radicals in solution exhibits very different kinetics from those of gas-phase reactions, depending on their stereochemical characteristics and the surrounding solvents. CF₂ICF₂I in solution has two stereoisomers, anti and gauche, and the corresponding CF₂ICF₂ radicals formed from these isomers are a-CF₂ICF₂ and g-CF₂ICF₂, respectively [13]. According to the time-resolved infrared (TRIR) spectroscopy experiments [8], all the nascent g-CF₂ICF₂ produced upon 267 or 310 nm excitation reacted with the dissociated I atom within 1 ps to form I₂∙∙C₂F₄—a weakly associated iodine tetrafluoroethylene complex. Furthermore, some of the nascent a-CF₂ICF₂ also reacted with an I atom with a time constant of 1.5 ps to form I₂∙∙C₂F₄. The formed I₂∙∙C₂F₄ dissociated with a time constant of 180 ± 5 ps in CCl₄ and 46 ± 3 ps in CH₃CN and CD₃OH to produce CF₂CF₂ + I₂. The remaining a-CF₂ICF₂ in solution underwent secondary dissociation with a time constant of 3–6 ns to produce CF₂CF₂. As all of the excess internal energy in the haloethyl radicals dissipates into the solvent on the picosecond time scale [8], the haloethyl radicals in solution are in thermal equilibrium when they dissociate the second I atom, unlike the reaction in the gas phase. Consequently, the kinetics related to the two dissociation channels, I(²P_1/2) and I(²P_3/2), were not exhibited in the photodissociation of CF₂ICF₂I in solution [8].

The CF₂ICF₂ radical in solution has rich reaction dynamics; however, it exhibits only a simple dissociative reaction of the weak C−I bond on the nanosecond time scale. The C–I bond has a lifetime of a few nanoseconds; thus, any reaction longer than that, such as a bimolecular reaction (BR) involving the radical, cannot be observed unless the concentration of the radical is sufficiently high to expedite the BR. Haloethyl radicals with more stable C−X bonds, such as C−Br or C−Cl, can survive sufficiently long to participate in a BR involving the radical and to exhibit reactions other than the simple dissociation of the C−X bond.

By dissociating I or Br atoms, UV-excited CF₂BrCF₂I can generate CF₂BrCF₂ or CF₂ICF₂ radicals, respectively [1,2,3,7,9,12]. In the gas phase, the photodissociation of CF₂BrCF₂I at 248 or 266 nm produces CF₂BrCF₂, while photodissociation at 193 nm produces 63% CF₂BrCF₂ and 37% CF₂ICF₂ [4]. The Br-dissociated radical CF₂CH₂I was produced by the 266 nm excitation of CF₂BrCH₂I in the gas phase [6], suggesting that the primary C−Br dissociation is feasible through 266 nm excitation. The secondary reaction of CF₂ICF₂ is well established in solution, as well as in the gas phase; however, the available spectroscopic and dynamical information on CF₂BrCF₂ is scarce [7,14]. Akesson and coworkers investigated the photodissociation dynamics of CF₂BrCF₂I in solution after excitation at 266 nm by probing the formation of IBr using time-resolved electronic spectroscopy [10]. The haloethyl radical was not monitored, but its formation and dynamics were conjectured by the kinetics of the electronic absorption signal of IBr. This is the only reported time-resolved study on CF₂BrCF₂I.

Recently, we measured the photodissociation dynamics of CF₂BrCF₂I in CCl₄ at 280 K from 0.3 to 320 ps after excitation with a 267 nm pulse to study the rotational isomerization of the haloethyl radical about the C−C bond [15]. The 267 nm excitation immediately dissociates Br or I atoms from CF₂ICF₂Br in CCl₄ at 280 K, producing nascent radicals consisting of 33 ± 3% a-CF₂BrCF₂, 49 ± 3% g-CF₂BrCF₂, and 18 ± 3% a-CF₂ICF₂ [15]. While no geminate rebinding of the dissociated halogen atom with nascent radicals or secondary reactions of a-CF₂ICF₂ occur within 320 ps, almost all g-CF₂BrCF₂ rotationally isomerizes into the more stable a-CF₂BrCF₂ with a time constant of 47 ± 5 ps [15]. The excess energy in the nascent radical is thermalized with a time constant of 15 ± 3 ps [15], implying that any secondary reaction proceeding slower than 15 ps becomes a reaction in thermal equilibrium. The radicals a-CF₂BrCF₂ and a-CF₂ICF₂ survive long after rotational isomerization, and the former is expected to survive sufficiently long to participate in a BR involving the radical. Therefore, comparative reaction dynamics between the two radicals can be probed by observing the entire reaction dynamics of CF₂BrCF₂I excited at 267 nm in a CCl₄ solution. Owing to the sensitivity of IR spectroscopy to molecular structure, the conformer-specific reactions of the radicals obtained from the photodissociation of CF₂BrCF₂I in solution can be observed in detail, similar to the case of the photodissociation of CF₂ICF₂I in solution [8].

In this study, we investigated all the subsequent reactions after the photodissociation of CF₂BrCF₂I in a CCl₄ solution at 267 nm through TRIR spectroscopy for up to 10 μs. With the support of quantum-chemical calculations and available spectra, all the transient absorptions in the TRIR spectra of CF₂BrCF₂I in CCl₄ were assigned, and their kinetics were determined. Upon excitation, CF₂BrCF₂I immediately produces a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂ [15]. Although a-CF₂ICF₂ undergoes a simple dissociation of the C−I bond, most of the g-CF₂BrCF₂ isomerize into a-CF₂BrCF₂ which undergoes a bimolecular reaction with itself or the dissociated Br atom in the solution. Structure-sensitive TRIR spectroscopy enabled us to determine the real-time reaction dynamics of UV-excited CF₂BrCF₂I in CCl₄ at 280 K.

2. Results and Discussion

The equilibrium electronic spectrum of CF₂BrCF₂I dissolved in CCl₄ showed a weak absorption band at 274 nm and a strong band above 200 nm (Figure 1), which was slightly red-shifted from the peaks at 268 and 193 nm in the gas phase [9]. The weak absorption at 274 nm, assigned to the n → σ* transition of the C−I bond, leads to the immediate dissociation of the I atom [1,2,3,12], while the strong absorption above 200 nm, assigned to the n → σ* transition of the C−Br bond, leads to the immediate dissociation of the Br atom [9]. As the wing of this strong absorption band extends up to 300 nm and accounts for 18 ± 3% of the absorption at 267 nm, the photoexcitation of CF₂BrCF₂I at 267 nm can lead to the dissociation of not only the I atom but also the Br atom. Specifically, 82 ± 3% of the absorption intensity at 267 nm arises from the weak absorption band at 274 nm; thus, excitation at 267 nm would promote 82 ± 3% (18 ± 3%) of CF₂BrCF₂I to the antibonding state of the C−I (C−Br) bond. Consequently, 82 ± 3% (18 ± 3%) of the excited CF₂BrCF₂I at 267 nm is expected to dissociate into CF₂BrCF₂ + I (CF₂ICF₂ + Br). The contribution from the strong band to the absorption at 267 nm in the gas phase was estimated to be 9 ± 4% [9], which would result in 9 ± 4% CF₂ICF₂ production upon excitation at 267 nm in the gas phase, which is smaller than that in CCl₄.

The TRIR spectra of CF₂BrCF₂I in CCl₄ at 280 ± 2 K were measured in the spectral region of 1370−1020 (1020−870) cm⁻¹ that covers all of the C−F stretching modes of the molecule over a broad time span from 0.3 ps to 10 μs (1 ns) after excitation at 267 nm, encompassing the entire excitation-induced reaction. As the photodissociation reaction was found to be complete at 1 µs in previous experiments on similar dihaloalkanes in solution [8,10], the time range of 0.3 ps to 10 μs was expected to be sufficient to observe the fates of all of the intermediates and products induced by the photoexcitation of CF₂BrCF₂I in CCl₄. The absorption bands in the 1300−1100 cm⁻¹ region were congested, while those in the 1020−870 cm⁻¹ region (vide infra) were well resolved. Thus, the TRIR spectra in the 1020−870 cm⁻¹ region were collected up to 1 ns to confirm the band assignment [15] and dynamics of the CF₂BrCF₂ radical (vide infra), even though this was time-consuming because of the inferior sensitivity of our TRIR spectrometer in such a long-wavelength spectral region.

As shown in Figure 2b, negative absorption bands (bleach) appeared immediately after excitation at the positions of the absorption bands of the equilibrium spectrum of CF₂BrCF₂I. The bleach signal arises from the depletion of the population of CF₂BrCF₂I in the ground state upon excitation. The four main absorption bands (negative absorption) of CF₂BrCF₂I near 1225, 1172, 1115, and 994 cm⁻¹ appeared at 0.3 ps and maintained their magnitude up to 10 μs. This suggests that the photoreaction proceeds within 0.3 ps and none of the nascent photofragments return to the reactant for up to 10 μs. New absorption bands can be categorized into three groups according to their kinetics: (1) bands appearing immediately after excitation, (2) bands growing around 50 ps and decaying near 50 ns, and (3) bands growing at ~50 ns and maintaining their amplitudes up to 10 μs, which is the last pump-probe delay time in our experiment. This suggests that the nascent products may undergo secondary reactions to produce intermediates that can further react to generate the final photoproducts. The absorption bands immediately appearing after excitation (group (1)) were assigned to the nascent photoproducts [15] and those maintained up to 10 μs after growing near 50 ns to the final products produced by secondary reactions of the reaction intermediates. Absorption bands near 1278 and 1128 cm⁻¹ grew near 50 ps and decayed around 50 ns; thus, they were assigned to the reaction intermediates that produced the final products through the secondary reactions. The immediately appearing absorption bands near 1260, 943, and 888 cm⁻¹ were initially broad and shifted toward blue with time, suggesting that the nascent photofragments were produced with excess energy that relaxed via the anharmonically-coupled lower-frequency modes (thermal relaxation) [15,16,17,18]. The absorption bands at 1324 and 1175 cm⁻¹, assigned to CF₂CF₂ in CCl₄ [8,13,14,19], grew near 50 ns and maintained their intensity up to 10 μs, indicating that CF₂CF₂ is the final product formed in tens of nanoseconds.

Notably, TRIR spectra beyond 1 μs did not evolve with time, indicating the completion of the photoreaction. Thus, new absorption bands in the TRIR spectra beyond 1 μs should arise from the final products and bleach the signal from the depleted reactant. The TRIR spectra from 1 to 10 μs were averaged to obtain more reliable spectra for the final products. As shown in Figure 3c, the averaged TRIR spectrum overlapped well with the difference spectrum obtained by subtracting the absorption spectrum measured before the pump-probe experiment from that measured after the experiment, confirming that the photoreaction was complete by ~1 μs. Apart from the absorption spectrum of CF₂CF₂ and the inverted absorption spectrum of CF₂BrCF₂, the averaged TRIR spectrum contained an additional absorption spectrum, which was assigned to CF₂BrCF₂Br based on the reported spectrum of CF₂BrCF₂Br in CS₂ (Figure 3a) [20]. Clearly, the photodecomposition of CF₂BrCF₂I leads to two products: CF₂CF₂ and CF₂BrCF₂Br. The decomposition of the averaged TRIR spectrum suggests that the photoexcitation of CF₂BrCF₂I in CCl₄ at 267 nm produces CF₂CF₂ (50 ± 3%) and CF₂BrCF₂Br (50 ± 3%).

When CF₂BrCF₂I is excited by UV light, it dissociates I or Br atoms, producing CF₂BrCF₂ or CF₂ICF₂ radicals, respectively [4,9,10,15]. As mentioned, because 82 ± 3% (18 ± 3%) of the excited CF₂BrCF₂I in CCl₄ at 267 nm is promoted to the antibonding of the C−I (C−Br) bond, the majority of the produced radicals are expected to be CF₂BrCF₂. As shown in our previous TRIR spectroscopic experiment on CF₂BrCF₂I in the time range of 0.3−320 ps, the TRIR spectra consisted of the spectra of three nascent radicals (a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂) and the inverted spectrum of CF₂BrCF₂I for the bleach signal [15]. The final photoproducts were 50 ± 3% CF₂CF₂ and 50 ± 3% CF₂BrCF₂Br, which were produced via secondary reactions of the reaction intermediates. Therefore, the TRIR spectra should include the spectra of the reactant (CF₂BrCF₂I), final products (CF₂CF₂ and CF₂BrCF₂Br), and reaction intermediates (a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂). The TRIR spectra were globally fitted using the basis spectra of CF₂BrCF₂I, CF₂CF₂, CF₂BrCF₂Br, a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂, as shown in the upper panel of Figure 4. The basis spectra of CF₂BrCF₂I, CF₂CF₂, and CF₂BrCF₂Br were obtained from equilibrium FTIR measurements, and those of a-CF₂ICF₂, a-CF₂BrCF₂, and g-CF₂BrCF₂ were obtained from our previous experiments [8,15]. The decomposition of CF₂BrCF₂I and CF₂BrCF₂Br into their conformer-specific spectra was not necessary for the fitting because their basis spectra have contributions from both conformers, and these contributions were maintained throughout the experiment. As shown in Figure 2c and the lower panel of Figure 4, the TRIR spectra were well reproduced by the sum of the basis spectra for CF₂BrCF₂I, CF₂CF₂, CF₂BrCF₂Br, a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂ shown in the upper panel of Figure 4. The time-dependent amplitude changes in the basis spectra were obtained by global fitting of the TRIR spectra. As mentioned, the magnitude of the bleach spectrum did not change throughout the experimental time span, implying that there was no rebinding of the dissociated halogen atom with its counter radical for up to 10 μs. The amplitudes of the remaining five basis spectra revealed rich kinetic information related to the reaction dynamics of the 267-nm-excited CF₂BrCF₂I in CCl₄ at 280 K.

The amplitude of a basis spectrum (amp) is related to the population of the corresponding compound by $amp \propto \epsilon \times n$ [15,19], where ε and n represent the integrated extinction coefficient and population of the compound, respectively. The integrated extinction coefficient (ε) of a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂ were obtained from our previous measurements [8,15], and those of CF₂BrCF₂I, CF₂CF₂, and CF₂BrCF₂Br were determined from equilibrium FTIR measurements. Time-dependent fractional population changes of CF₂BrCF₂I, CF₂CF₂, CF₂BrCF₂Br, a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂ were derived from time-dependent amplitude changes for the basis spectra of the corresponding compounds using the relation, $n \propto amp∕\epsilon$. As shown in Figure 5, three nascent radicals (a-CF₂BrCF₂, g-CF₂BrCF₂, and a-CF₂ICF₂) appeared immediately, implying that the photodissociation of CF₂BrCF₂I occurred within 0.3 ps [15]. The decay of g-CF₂BrCF₂ correlated with the growth of a-CF₂BrCF₂ because g-CF₂BrCF₂ isomerizes into a-CF₂BrCF₂ at a time constant of 47 ± 5 ps [15]. Moreover, the decays of a-CF₂ICF₂ and g-CF₂BrCF₂ correlated with the growth of CF₂CF₂ and CF₂BrCF₂Br, implying that these products were formed by the secondary reactions of a-CF₂ICF₂ and a-CF₂BrCF₂.

A kinetic scheme (Scheme 1) was introduced to reproduce the time-dependent fractional population changes, as shown in Figure 5, by optimizing the rate constants of all transitions. As shown in Figure 5, Scheme 1 reproduced the time-dependent fractional population changes.

Scheme 1 indicates that all the excited CF₂BrCF₂I at 267 nm dissociated one halogen atom to produce CF₂ICF₂ or CF₂BrCF₂ (18 ± 3% a-CF₂ICF₂, 33 ± 3% a-CF₂BrCF₂, and 49 ± 3% g-CF₂BrCF₂) [15]. Almost all nascent g-CF₂BrCF₂ isomerizes into a-CF₂BrCF₂ with a time constant of 47 ± 5 ps [15]. All of the nascent a-CF₂ICF₂ undergoes secondary dissociation to produce CF₂CF₂ + I with a time constant of 56 ± 5 ns. The nascent a-CF₂ICF₂ obtained from CF₂ICF₂I in CCl₄ undergoes secondary dissociation with a time constant of 5.5 ns [8]. Although a secondary dissociation time of 56 ns for a-CF₂ICF₂ is less than that observed previously, it clearly demonstrates that the secondary dissociation of the C−I bond is feasible and can occur on the nanosecond time scale in solution.

The decay of a-CF₂BrCF₂ was correlated with the growth of the final products, CF₂CF₂ and CF₂BrCF₂Br. The possible reactions of a-CF₂BrCF₂ to produce the final products are as follows.

a-CF₂BrCF₂ + a-CF₂BrCF₂ → CF₂CF₂ + CF₂BrCF₂Br

(1)

a-CF₂BrCF₂ + Br → CF₂BrCF₂Br

(2)

a-CF₂BrCF₂ → CF₂CF₂ + Br

(3)

(a)

If reaction (1) does not occur, reactions (2) and (3) should proceed, resulting in the formation of the two products, CF₂BrCF₂Br and CF₂CF₂. Reaction (2), which produces CF₂BrCF₂Br, should proceed with a rate constant of (1.6 ± 0.3) × 10¹¹ M⁻¹s⁻¹, and reaction (3), which produces CF₂CF₂, with a time constant of 130 ± 5 ns.

(b)

As the time constant of 130 ns for the secondary dissociation of a-CF₂BrCF₂ (reaction (3)) is too high to be realistic (vide infra), reaction (1) should occur to produce CF₂CF₂.

(c)

Reaction (1) produces equal amounts of CF₂CF₂ and CF₂BrCF₂Br, and the secondary dissociation of a-CF₂ICF₂ produces CF₂CF₂ (18% of the reactant), resulting in the final products CF₂BrCF₂Br (41%) and CF₂CF₂ (59%) if reaction (1) is the only one to proceed. Reaction (2) should proceed in addition to reaction (1) to give the observed final products (50 ± 3% CF₂CF₂ and 50 ± 3% CF₂BrCF₂Br).

(d)

Furthermore, if a-CF₂BrCF₂ reacts bimolecularly with itself (reaction (1)), it can also react with Br atoms in solution (reaction (2)).

Therefore, reactions (1) and (2) were allowed to fit the fractional population changes of a-CF₂BrCF₂, CF₂CF₂, and CF₂BrCF₂Br, resulting in rate constants for the BR of a-CF₂BrCF₂ with itself (reaction (1)) and the Br atoms (reaction (2)), which were approximately the same, (6.5 ± 0.3) × 10¹⁰ M⁻¹s⁻¹. The recovered BR rate constant is closer to the calculated diffusion-limited rate constant of 7 × 10⁹ M⁻¹s⁻¹ than the obtained value of (1.6 ± 0.3) × 10¹¹ M⁻¹s⁻¹, assuming that only reaction (2) produces CF₂BrCF₂Br.

The excess energy in the nascent radicals (a-CF₂ICF₂, a-CF₂BrCF₂, and g-CF₂BrCF₂) produced during the photodecomposition of CF₂BrCF₂I was thermalized with a time constant of 15 ± 3 ps [15], which is consistent with the thermalization time constant observed in other reactions in solution [8,19]. The solvent acts as an energy sink for the excess energy of a molecule in solution. All nascent radicals were thermalized with a time constant of 15 ± 3 ps after the photodecomposition of CF₂BrCF₂I in CCl₄, indicating that the observed reactions became thermal within tens of picoseconds after the photodecomposition. Therefore, the radicals should gain the energy required for the secondary bond dissociation that occurs within nanoseconds or longer. The bond dissociation energy is supplied by the solvent, indicating that the solvent becomes an energy source as well as an energy sink. As the solvent can act as an energy sink and an energy source, secondary dissociation at a longer duration than thermalization is a characteristic of the reaction in solution. For the secondary dissociation of the C−X bond, the activation energy should be as high as the C−X bond energy. As the C−Br bond energy of 22.3 ± 2.5 kcal/mol is 15.2 kcal/mol higher than the C−I bond energy of 7.1 ± 2.5 kcal/mol [7], the secondary dissociation of the C−Br bond is expected to take approximately 10¹² times longer than that of the C−I bond, based on the Arrhenius equation for the rate constant at room temperature. Considering that the secondary dissociation time of thermalized a-CF₂ICF₂ is 56 ns, it is unlikely that the secondary dissociation of Br from thermalized a-CF₂BrCF₂ will take 130 ns.

The Gibbs free energy of a-CF₂BrCF₂Br was calculated to be 0.91 kcal/mol more stable than that of g-CF₂BrCF₂Br. Therefore, when thermalized, 72% of CF₂BrCF₂Br is in the anti-conformer at 280 ± 2 K, while almost all (97%) the CF₂BrCF₂ radicals are a-CF₂BrCF₂. Although most CF₂BrCF₂Br is produced by the BR of a-CF₂BrCF₂, the produced CF₂BrCF₂Br is in both anti and gauche conformers in the equilibrium distribution. Based on the rotational isomerization time of 47 ps for CF₂BrCF₂ and the calculated rotational barriers [15], that of CF₂BrCF₂Br in CCl₄ at 280 K was estimated [15] to be 12 ns using the calculated Gibbs free energy for the rotational activation. The Gibbs free energy was found to be 6.21 (7.12) kcal/mol for gauche-to-anti (anti-to-gauche) transition by the DFT method using ωB97X-D/aug-cc-pVTZ. As shown in Figure 5, CF₂BrCF₂Br was produced with a time constant of 83 ns, which is longer than the estimated rotational isomerization time. Thus, CF₂BrCF₂Br was produced in the equilibrium distribution, even though the reactant was mainly the a-CF₂BrCF₂ radical.

3. Materials and Methods

3.1. Time-Resolved Mid-IR Spectroscopy

The TRIR spectrometer is based on a commercial Ti:sapphire oscillator/amplifier system (Spitfire Ace, Spectra Physics, Milpitas, CA, USA) that generates 800 nm, 110 fs pulses with a repetition rate of 2 kHz [8,19]. Half of the amplified femtosecond pulses were sent to a third-harmonic generator (THG), and the other half were sent to a homemade linear optical parametric amplifier (OPA). The THG produced 267 nm, 160 fs pulses with 10 μJ of energy. The OPA produced near-IR signal and idler pulses, which were difference frequency mixed in a 1-mm-thick GaSe to generate tunable mid-IR pulses with a duration of 120 fs, 160 cm⁻¹ spectral width, and 1 μJ of energy. The sample was excited by a portion of the 267 nm pulse and probed by a small fraction (~10 nJ) of the mid-IR pulse. The 267 nm excitation pulses were optically delayed up to 1 ns relative to the mid-IR probe pulses by a computer-controlled translation stage (M-415. PD, PI, Karlsruhe, Deutschland). The translation stage was impractical for optical delays beyond 1 ns because of the long physical distances for the stage. Thus, the excitation pulses were replaced with the nanosecond pulses (267 nm, 2.5 ns, and 20 μJ) produced by a commercial nanosecond tunable laser (NT240, EKSPLA, Vilnius, Lithuania) based on an optical parametric oscillator. These nanosecond excitation pulses were synchronized with the femtosecond probe pulses using an electronic digital delay generator (DG535, Stanford Research Systems, Sunnyvale, CA, USA), which produced an optical delay beyond 1 ns. The angle between the linearly polarized probe and the excitation pulses was set as the magic angle (54.7°) to obtain an isotropic absorbance proportional to the population of the corresponding compounds free from molecular rotation. The energy of the excitation pulse was reduced to 1–2 μJ to ensure linear absorbance and to minimize thermal lensing, which causes high background absorption on the nanosecond time scale due to solvent heating [21,22,23,24,25]. To measure the spectral region beyond the spectral width (160 cm⁻¹) of the probe pulses, the experiment was repeated with probe pulses centered at 1300, 1230, 1160, 1080, or 950 cm⁻¹, and the data were combined to obtain a broad spectrum. The probe pulses were passed through the sample and were detected by a 1 × 128-pixel MCT array detector (MCT-8-128, InfraRed Associates, Stuart, FL, USA) for wavenumbers > 1000 cm⁻¹ or a 1 × 64-pixel MCT array detector (MCT-16-64, InfraRed Associates, Stuart, FL, USA) for wavenumbers < 1000 cm⁻¹ in a 320 mm monochromator (HR320, Horiba, Miami, FL, USA) with a grating of 100, 75, or 50 lines/mm. The 1 × 64-pixel MCT array detector was custom-made to enhance spectral responsivity at longer wavelengths (<1000 cm⁻¹). The resulting spectra have a spectral resolution of 0.9–1.2 cm⁻¹ for the spectral regions > 1000 cm⁻¹ and 2.7 cm⁻¹ for spectral regions < 1000 cm⁻¹. The instrument response function was determined by the transient absorption of the Si wafer and was 0.2 ps in the subnanosecond experiment (0.3 ps–1 ns) and 2.5 ns in the nanosecond experiment (1 ns–10 μs). The spectra were collected at pump-probe delay times ≥ 0.3 ps to avoid signal complications due to pump-probe overlap [26,27,28].

3.2. Sample Preparation

CF₂BrCF₂I was purchased from Alfa Aesar, and CCl₄ was obtained from Sigma-Aldrich and used without further purification. A flowing sample cell consisting of two 2-mm-thick BaF₂ windows and a 100-μm-thick Teflon spacer was connected to a peristaltic tubing pump (Masterflex L/S, Cole-Parmer, Vernon Hills, IL, USA), refreshed with 50 mM CF₂BrCF₂I in CCl₄ fast enough for each probe pulse at a repetition rate of 2 kHz. Concentration and path length were set to obtain the maximum transient absorption signal in the spectral region of interest. A sufficiently large volume of the sample was used to maintain the decrease in the sample concentration as <2% due to the photoreaction of CF₂BrCF₂I by 267 nm excitation. The temperature of the sample was maintained at 280 ± 2 K by passing the coolant through the sample mount block. The low temperature was beneficial in minimizing the noise in the TRIR spectra caused by the temperature gradient in the probe spot of the sample induced by the pump energy (i.e., thermal lensing) appearing on the nano- to microsecond time scale [21,22,23,24,25]. To ensure the integrity of the samples, equilibrium UV–Vis and FT-IR spectra were collected before and after the TRIR experiments.

3.3. Computational Details

All quantum-chemical calculations were performed using the Gaussian 09 software (Revision D.01). The DFT method with the ωB97X-D (long-range corrected hybrid density functionals with damped atom–atom dispersion corrections) functional was used for molecular geometry optimization and the energies of the optimized compounds. ωB97X-D has been reported to yield satisfactory accuracy in thermochemistry, kinetics, and noncovalent interactions [29,30]. The aug-cc-pVTZ basis set was used for the C, F, and Br atoms, and the aug-cc-pVTZ-PP basis set was used for the I atom. Solvent effects were incorporated into the polarizable continuum model using the integral equation formalism variant.

4. Conclusions

In this study, the photodissociation dynamics of CF₂BrCF₂I in a CCl₄ solution were determined over a broad time span, encompassing the entire reaction, after excitation at 267 nm using TRIR spectroscopy. UV excitation led to ultrafast dissociation of I or Br atoms, resulting in CF₂BrCF₂ and CF₂ICF₂ radicals, respectively, which further proceeded with conformer-specific reactions. CF₂BrCF₂ was the major photofragment with both conformers of a-CF₂BrCF₂ (33 ± 3%) and g-CF₂BrCF₂ (49 ± 3%), while a-CF₂ICF₂ (18 ± 3%) was the minor one. The less stable g-CF₂BrCF₂ was isomerized with a time constant of 47 ± 5 ps into a-CF₂BrCF₂. a-CF₂BrCF₂ further reacted bimolecularly with itself at a diffusion-limited rate to produce CF₂BrCF₂Br and CF₂CF₂, or with the dissociated Br atom to form CF₂BrCF₂Br. However, no secondary dissociation of C−Br in a-CF₂BrCF₂ was observed. In contrast, a-CF₂ICF₂ underwent simple secondary dissociation of the I atom with a time constant of 56 ± 5 ns to produce CF₂CF₂ + I. The final products contained 50 ± 3% CF₂BrCF₂Br and 50 ± 3% CF₂CF₂. Structure-sensitive TRIR spectroscopy allowed us to determine the entire reaction dynamics of photoexcited CF₂BrCF₂I in a CCl₄ solution, including the conformer-specific dynamics of the CF₂BrCF₂ radicals.