Temperature‐Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

Abstract The rate coefficients for gas‐phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH2OO + CF3COOH)=(3.4±0.3)×10−10 cm3 s−1 and k((CH3)2COO + CF3COOH)=(6.1±0.2)×10−10 cm3 s−1 at 294 K exceed estimates for collision‐limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre‐reactive complex. Fits to a model incorporating this complex formation give k [cm3 s−1]=(3.8±2.6)×10−18 T2 exp((1620±180)/T) + 2.5×10−10 and k [cm3 s−1]=(4.9±4.1)×10−18 T2 exp((1620±230)/T) + 5.2×10−10 for the CH2OO + CF3COOH and (CH3)2COO + CF3COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.

Halogenated organic acids such as trifluoroacetic acid (TFA) form in the Earthst roposphere by oxidation of anthropogenically produced hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluoro-olefins (HFOs), [1] and also have natural sources. [2] They react only slowly with hydroxyl radicals and do not photolyse at actinic wavelengths. [3] Current atmospheric models therefore incorporate surface deposition and rain-out as their main loss processes. [1b, 2] However,r ecent evidence from laboratory studies indicates that organic acids,a nd other trace atmos-pheric molecules,r eact with Criegee intermediates with room-temperature rate coefficients that approach (or exceed) the expected gas-kinetic limits predicted by collision rates. [4] Barrierless reaction pathways have been identified computationally, [5] corroborating the experimental measurements.These reactions might therefore represent asignificant chemical loss mechanism for halogenated organic acids from the troposphere.
Here,w ee xamine the temperature dependence of the reactions of CH 2 OO and (CH 3 ) 2 COO with TFA, which we selected as representative of Criegee intermediate reactions with halogenated organic acids.W ep resent rate coefficients measured over ar ange of temperatures spanning those encountered in the lower troposphere.B imolecular rate coefficients were determined by the pseudo-first-order kinetic method for CH 2 OO + CF 3 COOH (k 1 ), CH 2 OO + CF 3 COOD (k 2 )and (CH 3 ) 2 COO + CF 3 COOH (k 3 )reactions for temperatures from 240 to 340 Kand pressures from 10 to 100 torr. Them easurements used cavity ring-down spectroscopy methods described previously [6] and in Supporting Information (SI).
Complementary quantum chemistry calculations provided energies and structures along the reaction pathways to aid interpretation of the kinetic measurements,a nd to guide predictions of rates of as-yet unstudied reactions. Stationary points involved in the reactions of CH 2 OO, (CH 3 ) 2 COO, anti-C((trans-CH 3 )=CH 2 )-CHOO (anti-methacrolein oxide) and syn-CH 3 -trans-(CH=CH 2 )COO (synmethyl vinyl ketone oxide) with CF 3 COOH were calculated at the DF-HF//DF-LCCSD(T)-F12a/aug-cc-pVTZ//B3LY P/ 6-31 + G(d) level of theory.The former two reactants serve as model systems,w hereas the latter two were selected as possible Criegee intermediate products of the ozonolysis at each of the C=Cbonds of isoprene,animportant tropospheric constituent with biogenic sources. [7] Their structures are shown in the SI. Similarities between the calculated reaction paths allow predictions of rates of reaction of TFAwith the Criegee intermediates from isoprene ozonolysis which we incorporate into atmospheric chemistry models. Figure 1s hows an example of the method for determination of k 2 for the CH 2 OO + CF 3 COOD reaction. The CH 2 OO decay traces in the presence of different CF 3 COOD concentrations were fitted with as imultaneous first-and second-order decay fit function: [6] Dk t ðÞ¼ k p kp Dk t0 In Equation (1), Dk t ðÞis the change in the cavity ring-down rate coefficient at different time delays and k' = k obs /s 355 nm is the second-order decay rate coefficient for the bimolecular self-reaction of the Criegee intermediate scaled by its absorption cross section at ap robe wavelength of 355 nm. Thep arameter k p is the rate coefficient for the TFA + Criegee intermediate reaction under pseudo-first-order conditions, L and d are the cavity length and the overlap length of the photolysis and probe lasers,and c is the speed of light. The first-order component accounts for both unimolecular decomposition and reaction with excess CF 3 COOD.T he bimolecular self-reaction of CH 2 OO was observed to have atemperature dependence,w hich was included in the fitting model. Thegradients of plots of k p against CF 3 COOD concentration provide the T-dependent bimolecular reaction rate coefficients,w hose statistical errors varied from 1.5 to 5.7 %. Similar measurements were undertaken for the CH 2 OO + CF 3 COOH reaction. At all the temperatures studied, H/D substitution of the TFAh ad no significant effect on the measured rate coefficients.
Within the 10-100 torr range examined at T = 294 K, there is no significant pressure dependence,a nd ar ate coefficient k 1 (294 K) = (3.4 AE 0.3) 10 À10 cm 3 s À1 is obtained by taking an average and 2s uncertainty range of all the measurements.T his rate coefficient is greater than the gaskinetic limiting value of 1.9 10 À10 cm 3 s À1 at 294 Kcalculated from collision theory using B3LYP/6-31 + G(d) optimized CH 2 OO and CF 3 COOH geometries.
We first consider the information deriving from the observed T-dependence of the reaction rates,a nd then apply the resulting mechanistic understanding to further TFAr eactions of atmospheric importance.W ep reviously proposed that the self-reactions of Criegee intermediates follow dipole capture behaviour. [8] In the dipole capture model, [9] thereaction cross section is greater than the physical dimensions of the reactants,a nd the rate coefficient k d-d is: Here m D1 and m D2 are the dipole moments of the two reactants, m is their reduced mass, k B is the Boltzmann constant, and C is ac onstant dependent on the anisotropy of the capture potential. Figure 2s hows ap lot of the temperature dependence of the measured rate coefficients k 1 (T). This T-dependence is steeper than the predictions of the dipole-capture model obtained using Equation (2) with computed dipole moments (see SI). Similar behaviour is found for the temperature dependence of the rate coefficient k 3 (T)f or the (CH 3 ) 2 COO + CF 3 COOH reaction, for which the rate coefficients are approximately twice as large as for the CH 2 OO + CF 3 COOH reaction at any given T. Fore xample, k 3 (294 K) = (6.1 AE 0.2) 10 À10 cm 3 s À1 . Figure 3s hows computed energies for stationary points along the minimum energy pathways for the CH 2 OO + CF 3 COOH and (CH 3 ) 2 COO + CF 3 COOH reactions.T he features of both pathways are similar and we focus on the former reaction. Ap re-reactive complex coordinated by ah ydrogen bond precedes am ostly entropic submerged barrier to reaction. Passage over this transition state,t he properties of which are described in the SI, gives ah ydroperoxy ester (HPE), CF 3 C(O)OCH 2 OOH. In this product, the Hatom from TFAtransfers to the CH 2 OO moiety and the carbonyl Oatom of CF 3 COOH forms abond with the Catom of CH 2 OO.T his barrierless pathway is consistent with the large experimentally observed rate coefficients (Figure 2), and may account for the absence of an H/D kinetic isotope effect.
As econd pathway (not shown in Figure 3) involving adifferent pre-reactive complex, stabilized by dual hydrogen bonds (DHBs), is expected on the basis of prior computa-  (1). The inset shows the pseudo-first-order decay rate coefficients plotted against CF 3 COOD concentration. The lowest and highest concentration measurements were repeated to ensure reproducibility.T he solid line in the inset plot is alinear fit from which the bimolecularr ate coefficienti sobtained. Figure 2. Temperaturedependence of the measured rate coefficients for the CH 2 OO + CF 3 COOH and (CH 3 ) 2 COO + CF 3 COOH reactions. Dashed and solid lines are fits to Equation (2) and (5), respectively. tional studies of the CH 2 OO + HCOOH reaction. [10] The binding energy of this DHB complex may be sufficient to influence the T-dependence of the rate coefficients.T herefore,ar eaction Scheme is invoked which incorporates an equilibrium between the CH 2 OO and TFAr eactants and ad ual hydrogen-bonded CH 2 OO-CF 3 COOH complex, [10] as well as the pathway shown in Figure 3. TheDHB complex has activated routes to either the HPE or as econdary ozonide (SO) product.
This model predicts at emperature dependence to the rate coefficient of: Here, k r is the rate coefficient for the direct reaction (3b) (approximated to be temperature independent over the range of our study) and DH = DH -3a ÀDH 4 is the difference in activation enthalpies for the DHB complex to dissociate to CH 2 OO + CF 3 COOH (the reverse of (3a)) and to surmount the barrier to reaction (4). The A-factor depends on the corresponding entropy changes.E quation (5) was used to fit the CH 2 OO + CF 3 COOH T-dependent rate coefficients with ac onstrained value of the high-temperature limit (for which k = k r )e stimated from the data (see Figure 2). Thef it returns A = (3.8 AE 2.6) 10 À18 cm 3 s À1 K À2 and DH = 13.1 AE 1.5 kJ mol À1 ,t he latter corresponding to ar eaction in which the binding energy for the DHB complex is greater than the activation barrier to its reaction(s). This value is consistent with the computed enthalpy changes DH -3a % 48.5 kJ mol À1 and DH 4 % 41 kJ mol À1 (at the CBS-QB3 level) reported by Long et al. for the CH 2 OO + HCOOH reaction. [10] Asimilar analysis was conducted for the (CH 3 ) 2 COO + CF 3 COOH reaction, giving A = (4.9 AE 4.1) 10 À18 cm 3 s À1 K À2 and DH = 13.1 AE 1.9 kJ mol À1 .These fit outcomes and the corresponding entropy changes are summarized in Table S5 in the SI.
Thec omputational methodology used for reactions of TFAwith CH 2 OO and (CH 3 ) 2 COO can also be applied to its reactions with Criegee intermediates from the ozonolysis of biogenic isoprene.Computed pathways for reactions of these Criegee intermediates with CF 3 COOH are found to be analogous to those in Figure 3( see SI). Thes imilarities indicate that the isoprene-derived Criegee intermediate reactions (and, by extension, those of other biogenic Criegee intermediates) will approach dipole-capture limited values and show similar T-dependences to the CH 2 OO and (CH 3 ) 2 COO + CF 3 COOH reactions.T hese deductions allow us to predict the loss rate of TFAi nt he atmosphere by reaction with the most tropospherically abundant Criegee intermediates. Figure 4s hows computed global CF 3 COOH loss rates from reactions with Criegee intermediates,asapercentage of the overall TFAl oss rate.T he SI provides details of the STOCHEM-CRI global atmospheric model and Criegee intermediate field calculations (incorporating known production and loss mechanisms) used for these computer simulations.The outcomes suggest that rapid reactions with Criegee intermediates are the dominant sink for tropospheric TFAin forested regions around the world, and that the TFA atmospheric lifetime might be as short as 4days.R eactions of TFAw ith Criegee intermediates can form adducts with high O:Cr atios and low vapour pressures,w hich encourages  condensation to secondary organic aerosol (SOA). Competition between SOAf ormation, solar photodissociation, and adduct hydrolysis will then have consequences for the distribution of TFAa nd other halogenated organic acids in the environment.