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ARTICLE pubs.acs.org/est Atmospheric Degradation of Perfluoro-2-methyl-3-pentanone: Photolysis, Hydrolysis and Hydration D erek A. Jackson, C ora J. Young, M ichael D . H urley, T imothy J. W allington, and Scott A. M abury*, D epartment of C hemistry, U niversity of T oronto, 80 St. G eorge Street, T oronto, O N , C anada M 5S 3H 6 Ford M otor C ompany, M ail D rop SRL -3083, D earborn, M ichigan, 48121 U nited States bS Supporting Information ABST RAC T : Perfluorinated carboxylic acids are widely distributed in the environment, including remote regions, but their sources are not well understood. Perfluoropropionic acid (PFPrA, C F3C F2C (O )O H ) has been observed in rainwater but the observed amounts can not be explained by currently known degradation pathways. Smog chamber studies were performed to assess the potential of photolysis of perfluoro2-methyl-3-pentanone (PFM P, C F3C F2C (O )C F(C F3) 2), a commonly used fire-fighting fluid, to contribute to the observed PFPrA loadings. T he photolysis of PFM P gives C F3C F2C 3(O ) and 3C F(C F3) 2 radicals. A small (0.6%) but discernible yield of PFPrA was observed in smog chamber experiments by liquid chromatographymass spectrometry offline chamber samples. T he T ropospheric U ltravioletVisible (T U V) model was used to estimate an atmospheric lifetime of PFM P with respect to photolysis of 414 days depending on latitude and time of year. PFM P can undergo hydrolysis to produce PFPrA and C F3C FH C F3 (H FC -227ea) in a manner analogous to the H aloform reaction. T he rate of hydrolysiswas measured using 19F N M R at two different pH s and was too slow to be of importance in the atmosphere. H ydration of PFM P to give a geminal diol was investigated computationally using density functional theory. It was determined that hydration is not an important environmental fate of PFM P. T he atmospheric fate of PFM P seems to be direct photolysis which, under low N O x conditions, gives PFPrA in a small yield. PFM P degradation contributes to, but does not appear to be the major source of, PFPrA observed in rainwater. ' INTRODUCTION Perfluorinated carboxylic acids (PFC As) are ubiquitous in biotic1 and abiotic environments.2,3 L onger-chain (>C 8) PFC As are bioaccumulative4 and have attracted substantial research interest. Precipitation measurements by Scott et al.3 have demonstrated that short-chain PFC As, notably trifluoroacetic acid (T FA) and perfluoropropionic acid (PFPrA) , dominate the PFC A profile. Although these smaller compounds are not expected to bioaccumulate and are not believed to represent a threat to ecosystems, their source is unclear and requires study. T hermolysis of fluoropolymers has been suggested as a potential source of T F A observed in rainwater.5 O ther sources of T F A include the atmospheric oxidation of hydrofluorocarbons6,7 and polyfluorinated compounds.8,9 Small yields of PF PrA have been proposed from thermolysis of fluoropolymers and atmospheric oxidation of fluorotelomer alcohols5,8,9 but do not explain the levels of PF PrA observed in precipitation. Perfluoro-2-methyl-3-pentanone (PFM P) is a fire protection fluid, marketed as N ovec 1230 by 3M . It is a replacement for chlorofluorocarbons (C FC s) and H alons, which deplete stratospheric ozone. T he atmospheric lifetime of PFM P from previous work seems to be determined by photolysis and is approximately 1 week.10,11 PFM P does not contribute to stratospheric ozone depletion and has a negligible global warming potential.10,11 T he major photolysis products are C F3C (O )F and C O F2.10 T he atmospheric fate of C F3C (O )F is hydrolysis to yield T FA. It is possible that chemistry in remote environments following the photolysis of PFM P would give small yields of PFPrA or isoperfluorobutanoic acid (i-PFBA) via reactions of the corresponding perfluoroacyl radicals with H O 2 radicals.12,13 In addition to photolysis, hydration or abiotic hydrolysis may be a significant sink of PFM P in the environment. In the hydration reaction, PFM P would react either reversibly or irreversibly with water to form a geminal diol which would shut down the photolysis pathway. K etones are typically unreactive toward hydrolysis because the leaving group after nucleophilic attack is an aliphatic carbanion. C arbanions are highly basic and are not good leaving groups. H owever, hydrolysis of PFM P gives Special I ssue: Perfluoroalkyl Acid R ecei ved: A ccepted: R evi sed: Published: D ecember 28, 2010 M arch 25, 2011 M arch 15, 2011 April 05, 2011 r 2011 American Chemical Society 8030 dx.doi.org/10.1021/es104362g |Environ. Sci. Technol. 2011, 45, 8030-8036 Environmental Science & Technology ARTICLE a perfluorinated carbanion which, because the fluorine atoms stabilize the departing carbanion through hyperconjugation, is a much better leaving group. T his effect is the basis of the wellknown H aloform reaction, one of the oldest organic reactions known.14 Applying the H aloform reaction mechanism to hydrolysis of PFM P should give PFPrA and C F3C FH C F3 (H FC -227ea). H FC -227ea is a long-lived greenhouse gas and its formation would be problematic. A detailed understanding of the rate and products of PFM P hydrolysis under environmentally relevant conditions is clearly desirable. T he present work had three goals: (i) to confirm the rate of atmospheric photolysis of PFM P and to investigate the possible formation of PFPrA, (ii) to determine whether hydrolysis of PFM P is of significance in the environment, and (iii) to investigate whether hydration is an environmental fate of PFM P. ' EXPERIMENTAL DETAILS Measurements of UV Spectra and Calculations of Photo- lysis Rates. U V spectra of PFM P in the region 200400 nm were recorded in a 6-cm cell using a Perkin-Elmer U V/ vis spectrometer with a resolution of 1 nm, a slit width of 0.25 nm, and a scan speed of 15 nm min1. Photolysis rates were estimated using the T ropospheric U ltravioletVisible (T U V 4.2) package.15 All rates were calculated for 0.5 km above the surface. T he quantum yield was assumed to be 0.043, as measured by D 'Anna et al.11 Smog Chamber Methods. Experiments were performed in a 140-L Pyrex reactor interfaced to a M attson Sirus 100 FT IR spectrometer. T he reactor was surrounded by 10 fluorescent blacklamps (G E F15T 8-BL , maximum emission 360 nm) and 12 sunlamps (G E-FS40, maximum emission 310 nm), which were used to photolyze PFM P. T he spectral overlap of the emission of blacklamps and sunlamps and the absorption by PFM P has been discussed by T aniguichi et al.10 All reagents were obtained from commercial sources. C oncentrations of reactants and products were monitored by FT IR spectroscopy. IR spectra were derived from 32 coadded interferograms with a spectral resolution of 0.25 cm1 and an analytical path length of 27 m. Photolysis experiments were performed using mixtures containing 0.490.62 T orr of PFM P in 50 T orr of oxygen. W e chose a diluent pressure of 50 T orr for the photolysis experiments as T aniguichi et al.10 have established that the photolysis proceeds more rapidly at lower total pressures. After photolysis the reaction mixtures were pressurized to 750 T orr with air and sampled as described below. T he experimental conditions employed in the smog chamber experiments (296 K , 50 T orr total pressure of O 2 diluent, in the absence of water vapor, N O x, and other important atmospheric constituents) are not the same as the range of conditions in the real atmosphere. H owever, the chemical processes studied in the chamber are very similar to those which will occur in the real atmosphere. Photolysis of PFM P in the chamber will occur via the same mechanism, the fate of the radicals formed (decomposition and addition of O 2) will be similar, and subsequent reactions of the peroxy radicals are similar to those in the real atmosphere. H ence, the results from such smog chamber experiments shed important light on the behavior of PFM P in the real atmosphere. Offline Sample Collection and Analysis. O ffline samples were collected by bubbling approximately 5 L of chamber air through 10 mL of sodium carbonate solution (pH 11) after dilution of the chamber contents with air ( P T = 750 T orr). T riplicate samples were collected following 45 min of irradiation. Sodium carbonate solutions were acidified to pH 4 using H C l and analyzed using a W aters 616 L C pump and 600 controller with detection by a M icromass Q uattro M icro M S/ M S detector. Analytes were separated on a G enesis C 8 column (2.1 mm 50 mm 4 m) using a 5-min isocratic run of 40% methanol and 60% water, both containing 10 mM ammonium acetate. T riplicate 10 L injections were made using a W aters 717 autosampler. PF C As were analyzed using a cone voltage of 17 V and collision energy of 9 eV and the following transitions were monitored: PF BA 213 > 169, PF PrA 163 > 119, and T F A 113 > 69. Analytes were quantified using external calibration. Hydrolysis Kinetic Experiments. T o quantify the rate of PFM P hydrolysis and identify the final products of hydrolysis, 19F N M R was used. T he rates of hydrolysis were measured at pH values of 5.6 anad 8.5 to simulate the upper and lower boundaries of realistic environmental pH conditions. T o achieve this, either 50 mM potassium hydrogen phthalate (pH 5.6) or 50 mM sodium borate (pH 8.5) buffer was used. A Varian 400 spectrometer equipped with an AT B8123-400 autoswitchable probe tuned to 19F (376.14 M H z) was used. Reaction solutions were composed of 600 L of buffer solution and 200 L of D 2O in a 5-mm N M R tube. Immediately prior to sample insertion, 20 L of PFM P was added to the N M R tube followed by inversion. Reaction kinetics were followed by acquiring N M R spectra using a preacquisition delay program such that one complete spectrum was obtained every 2030 min. Each spectrum consisted of 12 scans with an acquisition time of 1 s. T o ensure quantitive results, a standard of PF PrA under the same reaction conditions as PFM P hydrolysis was subject to a pulse inversionrecovery T 1 relaxation experiment to determine a suitable relaxation delay time for PFM P. In this case, a delay time of 20 s for PFM P was chosen. After acquisition, data analysis was performed using the VnmrJ software (Agilent T echnologies) as it possesses abuilt-in kinetic analysis module. Each arrayed spectrum was Fourier transformed with a line broadening apodization of 6 H z to improve its signal-to-noise ratio. K inetic data were obtained by fitting an exponential growth or decay function to the fluorinated signals in the N M R spectra (see Figure S5, Supporting Information). All hydrolysis experiments were performed at 25 C . Computational Method. C alculations to determine the significance of P F M P hydration were performed using the G aussian 03 program16 with the W ebM O interface. A ll computations were performed using D ensity F unctional T heory (D FT ) with B3L YP functionals using the 6-311G (d,p) basis set. C omputations were carried out for both the gas phase and the aqueous phase using a polarizable continuum solvent model (P C M ).17 Equilibrium constants for hydration were computed using the relative method of G omezBombarelli et al.18 and using a training set calibration of fluoroacetone, trifluoroacetone, and hexafluoroacetone (see SI for details). Reagents. All chemicals were used as received. PFM P was purchased from Synquest L aboratories (Alachua, FL ). D isodium tetraborate and potassium hydrogen phthalate were purchased from BD H (T oronto, O N ). T rifluoroacetic acid and heptafluoropropionic acid (PFPrA) were purchased from Sigma-Aldrich (O akville, O N ) . 8031 dx.doi.org/10.1021/es104362g |Environ. Sci. Technol. 2011, 45, 8030-8036 Environmental Science & Technology T able 1. K inetic D ata for PFM P D egradation via Photolysis and H ydrolysis (N = 2) at T wo D ifferent pH Values (For C omplete Photolysis D ata, Refer to the SI ) mechanism conditions rate constant k (s1) lifetime ( hours) photolysis 45 N 3.5 106a 79.4 June 21 photolysis 45 N 5.9 107a 471 D ecember 21 photolysis 45 N 2.1 106a 132 M arch 21/ Sept 21 hydrolysis pH 5.6 1.9 104 ( 1 105b 1.5 ( 0.1 hydrolysis pH 8.5 3.1 104 ( 3 105b 0.9 ( 0.1 a Averaged 24 h rate constant. b Pseudo-first-order rate constant. ARTICLE ' RESULTS AND DISCUSSION Photolysis Kinetics. Photolysis lifetimes of PFM P have been determined experimentally in two studies, yielding lifetimes of approximately 12 weeks10 and 1 week.11 U sing the flux conditions and quantum yield (0.043) previously described,11 and the U V spectrum measured in this study (see SI), photolysis rates for PFM P were estimated. T he photolysis rates, presented in T able 1, agree to within a factor of 2 with those reported in previous studies as well as previous measurements of the U V spectrum.10,11 T he 24-h averaged rate constants for photolysis of PFM P for different latitudes are given in the SI. Annual averaged photolytic rate constants (J) ranged from 3.1 106 to 8.2 107 s1 corresponding to lifetimes of 414 days depending on latitude and time of year. Photolytic Production of PFCAs under Low NOx Conditions. O ffline samples were taken to determine if photolytic degradation of PF M P could yield PF C As. T he formation of PF C As in the chamber is only expected under low-N O x conditions in the presence of H O 2 radicals. M aterial was not added specifically for the purpose of forming H O 2 radicals, but it is likely that H O 2 radicals are present in small quantities in all chamber experiments because of hydrocarbon residue on the walls of the chamber (the chamber has been used at Ford for many years to study hydrocarbon oxidation mechanisms). L arge initial concentrations of PFM P (520 mT orr) were used in the smog chamber to facilitate detection of small yields of PFC As. Photolysis of PFM P can generate two different perfluoroacyl radicals: C F3C F2C O C FC F32 hv f C F3C F2 3 3C O C FC F32 1 C F3C F2C O C FC F32 hv f C F3C F2C 3O 3C FC F32 2 Perfluoroacyl radicals either decompose via elimination of C O or add O 2 to give perfluoroacyl peroxy radicals. In low-N O x environments, perfluoroacyl peroxy radicals can react with H O 2 to give PFC As:12 C F3C F2xC 3O O 2 f C F3C F2xC O O O 3 3 C F3C F2xC O O O 3 H O 2 f C F3C F2xC O O H O 3 4 Figure 1. Proposed mechanistic pathways leading to the formation of PFC As after photolysis of PFM P in the absence of N O x. Reaction numbers from the text are given in parentheses. C ompounds in black boxes represent stable degradation products observed experimentally. C ompounds in red boxes represent stable degradation products that were predicted but not observed. T he perfluoroacyl radicals derived from the photolysis of PFM P can react to give i-PFBA or PFPrA as follows: C F32C FC 3O O 2 f C F32C FC O O O 3 5 C F32C FC O O O 3 H O 2 f C F32C FC O O H i-PFBA O 3 6 C F3C F2C 3O O 2 f C F3C F2C O O O 3 7 C F3C F2C O O O 3 H O 2 f C F3C F2C O O H PFPrA O 3 8 PFPrA was observed in offline samples taken after 45 min of irradiation, at a concentration of 0.061 mT orr. i-PFBA was not detected in any of the samples. T he high concentrations of PFM P used in the experiments saturated the FT IR signal and a direct determination of the amount of PFM P photolyzed was not possible. T he amount of PFM P lost via photolysis was estimated from the photolysis halflife (determined to be 26 h by the observed formation of C F3C (O )F as discussed previously10) in the chamber conditions. W e estimate that 10.4 mT orr of PFM P was photolyzed after 45 min and hence the yield of PFPrA is 0.6%. T he low PFPrA yield observed in this experiment probably reflects the low level of 8032 dx.doi.org/10.1021/es104362g |Environ. Sci. Technol. 2011, 45, 8030-8036 Environmental Science & Technology H O 2 radicals available in the system. Sulbaek Andersen et al.19 reported that the yield of PFPrA from C F3C F2C 3(O ) via reactions 7 and 8 was 24 ( 4%. Similarly, the yield of n-PFBA in reactions analogous to 5 and 6 was shown to be 10 ( 2%.12 W e expect the yield of i-PF BA following formation of (C F3)2C FC 3(O ) radicals in an excess of H O 2 radicals to be similar to that reported for n-PFBA. T he low yield of PFPrA observed in the present experiments suggests either that the yield of C F3C F2C 3(O ) radicals in the photolysis of PFM P is low, or that the concentration of H O 2 radicals in the channel is low and conversion of C F3C F2C 3(O ) radicals into PFPrA is inefficient. T he absence of any discernible formation of i-PFBA suggests the latter explanation is more probable. Figure 1 illustrates some mechanistic pathways leading to PFC As following photolysis of PFM P in the absence of N O x. N ote, as indicated in Figure 1, decomposition via elimination of C O is a significant fate of C F3C F2C 3( O ) radicals.13 T he presence of PFPrA and the absence of i-PFBA suggests reaction 2 is favored over reaction 1. T his observation was surprising given the perfluoroisopropyl radical is less stable than the perfluoroethyl radical. In the perfluoroethyl radical, two fluorine atoms are able to donate electron density from their lone pair orbitals into the singly occupied p orbital on the radical center, resulting in stabilization. Because the experimental results suggest this is happening, it is proposed that the stabilities of the corresponding perfluoroacyl radicals must allow this observed regioselectivity to occur. Hydrolysis Kinetics. T he hydrolysis reaction of PFM P can proceed by the well-known H aloform reaction mechanism in which a ketone with a suitable alkyl leaving group reacts with water to form a carboxylate and an alkane. In a typical H aloform reaction, a methyl ketone is tri-iodinated followed by treatment with base to produce a carboxylic acid and iodoform which precipitates as a yellow solid. T his reaction is the classic qualitative test for a methyl ketone. T he following steps mechanistically describe the H aloform reaction: RC O C X 3 O H f RC O O H C X 3 9 RC O O H C X 3 f RC O O H C X 3 10 RC O O H C X 3 f RC O O C H X 3 11 where X = C l/ Br/ I. T he final step in the H aloform reaction is a rapid and exothermic proton transfer from the carboxylic acid to the carbanion which makes the whole process irreversible. T he overall mechanism of the reaction is BAC2 which consists of separate addition and elimination steps via a tetrahedral intermediate. T he rate determining step is reaction 9. T he final two products expected from PFM P hydrolysis are PFPrA and H FC 227ea. T he mechanism for hydrolysis of PFM P is shown in Figure 2. T he hydrolysis of PFM P and the detection of both products has been reported by Saloutina et al.20 however this study was performed at pH values much higher than those relevant for the environment. T he objective of the present study was to investigate whether PFM P could undergo hydrolysis at pH values more typical of those found in the environment. T he two pH values chosen were 5.6 and 8.5 because pH 5.6 is typical of atmospheric water.21 A pH of 8.5 was also chosen to represent the upper ARTICLE Figure 2. H ydrolysis mechanism of PFM P to produce PFPrA and H FC -227ea under mildly basic conditions. Reaction numbers in the text are given in parentheses. environmental limit where hydrolysis would likely be the fastest since the H aloform reaction is base catalyzed. T o measure the kinetics of PFM P degradation, an analytical method to quantify PFM P is required. L iquid chromatography is not suitable to analyze PFM P because authentic standards would rapidly degrade in any protic solvent. It was confirmed in a separate study (data not shown) that PFM P reacts rapidly with methanol to produce H FC -227ea and the methyl ester of PFPrA. PFM P is certainly volatile enough to be analyzed by gas chromatographymass spectrometry. H owever aqueous samples would need to be extracted into a suitable G C solvent prior to injection. PFM P was found to have very low solubilities in almost every solvent tested. In addition, such a procedure would require a separate extraction for every time point, greatly increasing the amount of material needed. W e decided that 19F N M R would offer the best capability for measuring the kinetics of PFM P in situ. T he hydrolysis reaction was performed in an N M R tube and scans of the reaction mixture were taken at various time intervals for subsequent analysis. 19F N M R offers acceptable signal-to-noise ratios and since neither water nor the buffers used contain fluorine atoms, apurely deuterated solvent does not need to be used. For N M R data to be considered quantitative to 99% accuracy, it is necessary that the relaxation delay between scans be five times that of the longest spinlattice relaxation time (T 1) in the compound of interest. Because T 1 values can depend on the solvent used, a standard of PFPrA in the aqueous buffer of interest was used as a surrogate for the T 1 of PFM P. It was found that the longest observed T 1 (PFPrA) = 4.0 s, thus making a suitable relaxation delay for PFM P to be 20 s. T he current Varian N M R processing software, VnmrJ, is capable of analyzing kinetic data using a nonlinear fitting algorithm, provided the fit is exponential. Because the concentration of both O H and water are constant in each experiment, pseudofirst-order conditions with respect to PFM P were achieved and an exponential decrease in [PFM P] is expected due to hydrolysis. After fitting the datapoints, the software provides the 1/ e lifetime of the compound (), which is simply the reciprocal of the pseudo-first-order rate constant, kobs. T he 19F N M R spectrum of PFM P consists of four peaks: each a multiplet due to the long-range J coupling observed for fluorine atoms. T he spectra of PFPrA and H FC -227ea combined resemble that of PFM P but the chemical shifts are distinct and the coupling patterns are very different. T hus, it is straightforward to distinguish PFM P from its degradation products. For examples 8033 dx.doi.org/10.1021/es104362g |Environ. Sci. Technol. 2011, 45, 8030-8036 Environmental Science & Technology of 19F N M R spectra of both PFM P and its photolysis products, please see the SI. In summary, we observed the 1/ e lifetime of PFM P hydrolysis to vary depending on the pH of the buffer used. T he reaction was faster in more alkaline solutions as shown in T able 1. T his is entirely reasonable given the base-catalyzed nature of the reaction. Both PFPrA and H FC -227ea were positively identified as the sole products of the hydrolysis reaction. PFPrA was confirmed by comparison to an authentic standard. H FC -227ea was confirmed by comparison to a literature spectrum.22 Saloutina et al.20 did not report any kinetics in their study on the hydrolysis of PFM P, however it is not surprising they observed high yields in PFPrA and H FC -227eagiven the strongly basic conditions of their reaction solutions. A further study by the same group23 elaborated on the specificity of the C C bond cleavage step to exclusively produce the more stable carbanion leaving group. In the case of perfluorinated compounds, the more substituted anion is more stable because of increased hyperconjugation from the anion lone pair to the vicinal antibonding C F orbitals. Sykes et al.24 confirmed the regioselectivity of the H aloform reaction for a number of polyfluorinated ketones. H ence, there is precedent for the H aloform reaction to occur regioselectively in fluorinated ketones to produce the less substituted carboxylate and the more substituted hydrofluoroalkane. Hydration of PFMP. Aldehydes and ketones can react with water to produce a geminal diol by the following reaction scheme. RC O R 0 H 2O f RC O H O H R 0 12 T he reaction is catalyzed by both acids and bases and is usually a reversible equilibrium reaction. For most aldehydes and ketones, the equilibrium constant, K hyd, usually strongly favors the carbonyl compound due to the high strength of the C d O bond. Exceptionally, formaldehyde (H C (O ) H ) exists in aqueous solution purely as the gem-diol (H C (O H ) (O H )H ) due to its high K hyd value (log K hyd = 3.36).25 T he vast majority of ketones are quite unreactive to hydration (acetone has log K hyd =2.8525) and the geminal diol form is usually neglected in reaction schemes. O ther exceptions can be halogenated ketones; trichloroacetaldehyde has a log K hyd = 4.45 and hexafluoroacetone(H FA) has log K hyd = 6.08.25 Effectively, there is quantitative conversion of the carbonyl to the hydrate in aqueous solution and the reverse reaction back to the carbonyl does not occur appreciably at room temperature. For example, H FA is provided commercially as the geminal diol derivative. It might be reasonable to assume that the electron withdrawing groups of H FA impart a strong partial positive charge at the fluorinated carbonyl carbon, resulting in enhanced electrophilicity and hence, a greater degree of hydration. H owever, L inderman et al.26 used H artreeFock computational methods and found most of the positive electric potential in H FA is found on the C F3 carbons and not the carbonyl carbon. Instead, they propose the lower energy level of the L U M O of fluorinated ketones compared to their hydrogenated analogs is the major cause of the enhanced reactivity toward water. Because PFM P is perfluorinated like H FA, it would be a reasonable assumption that PFM P could rapidly and irreversibly form a geminal diol in the environment. T he PFM P hydrate would not absorb actinic radiation due to the absence of the C d O chromophore. In addition, formation of the diol would increase water solubility and decrease vapor pressure, resulting in a greater rate of wet deposition and enhancing the potential role of hydrolysis. T hus, ARTICLE it is important to determine whether PFM P could form a stable hydrate in the same manner as H FA. T he hydration reaction of PFM P was investigated in our laboratory and no evidence of such a hydrated species was observed (data not shown) . T his was puzzling as other fluorinated ketones are known to hydrate readily.25 T o provide further insight, computational methods were used to estimate K hyd for PFM P and compare it to other fluorinated carbonyls using the methods of G omez-Bombarelli et al.18 T he effects of including an aqueous PC M solvent model were also studied although G uthrie et al.27 previously noted difficulties of applying the PC M model to halogenated compounds. For full details on the computational work, please see the SI. Dependingon themethod used to calculateK hyd (seeSI),different values for PFM P were obtained (2.09 <log K hyd <0.43) but the range suggests hydrate formation is not significant for PFM P. G iven the perfluorinated nature of PFM P and that even partially fluorinated ketones such as trifluoroacetone (log K hyd = 1.54)25 have large K hyd values we were surprised by the low values of K hyd calculated for PFM P. T he computational methods used in the present study (B3L YP/ 6-311G (d,p) ) were able to compute K hyd values for several fluorinated ketones ( fluoroacetone, trifluoroacetone, and hexafluoroacetone) with fairly good accuracy, providing confidence in the method used. A single point molecular orbital calculation on geometry-optimized PFM P in the gas phase (B3L YP/ 6-311G (d,p)) gave the energy level of the L U M O (84.1 kcal/ mol) as almost identical to the L U M O of H FA (81.4 kcal/ mol), especially when compared to the computed L U M O of acetone (19.3 kcal/ mol). T his supports the results of L inderman et al.26 who proposed these energy differences as the reason for the enhanced reactivity of H FA to hydration. In the case of PFM P, steric hindrance could be the reason for its decreased hydration equilibrium constant compared to H FA. W e conclude that geminal diol formation is not a significant environmental fate for PFM P. ' ENVIRONMENTAL IMPLICATIONS W e have investigated the potential for photolysis, hydrolysis, and hydration to contribute to the environmental fate of PFM P. As discussed in the previous section, hydration is not a significant fate for PFM P. W hile the rate constant for hydrolysis is much greater than that for photolysis, the levels of liquid water in the atmosphere are usually very low. A typical cloud only contains approximately 3 107 cm3 liquid water per mL of total volume28 and we conclude that even at night, the amount of PFPrA and H FC -227ea produced would not be significant and that photolysis dominates hydrolysis as the atmospheric fate of PFM P. C ahill and M ackay29 came to the same conclusion in their modeling study. Interestingly, C ahill and M ackay predicted a hydrolysis rate constant at pH 5.6 of 2.2 s1 which is approximately 104 times larger than the hydrolysis rate constant we measured. H ence, our work suggests hydrolysis is an even less important fate for PFM P compared to photolysis than the ratio C ahill and M ackay29 predicted; the ratio of rates of photolysis to hydrolysis is approximately 980,000,000:1. For full details on this calculation, please see the SI. Photolysis will always dominate over hydrolysis. It is clear the very low fraction of liquid water in the atmosphere contributes heavily to this ratio and more than offsets the higher hydrolysis rate constant. T his ratio is so great that even at night time during a heavy rain event it is unlikely hydrolysis will occur to any significant extent. 8034 dx.doi.org/10.1021/es104362g |Environ. Sci. Technol. 2011, 45, 8030-8036 Environmental Science & Technology ARTICLE Photolysis is the dominant mechanism by which PFM P is removed from the atmosphere. It has been established previously1'11 that photolysis of PFM P in the presence and absence of N x will lead to the formation of C F 3C(0 )F in a molar yield of approximately 100%. The atmospheric fate of C F 3C (0 )F is hydrolysis to give trifluoroacetic acid (T FA). T FA is a ubiquitous naturally occurring component of the hydrosphere and the additional burden associated with PFM P photolysis is not significant. In the present work, we also show that small amounts of PFPrA are also formed as a result of PFM P photolysis. To provide a crude upper limit estimate for the amount of PFPrA that might be expected in precipitation as a result of atmospheric degradation of PFM P we applied the following logic (see SI for details). The production of PFM P by 3 M is100-1000 t year--l and began in approximately 2003.3 Given PFM P is used entirely in fire-protection systems that are released by an alarm, it can be considered stored emission potential with releases averaging 1-3% year-1.3 Combining the upper limit of production (1000 t year-1) with a 3% emission factor provides an upper limit of 30 t of PFM P released into the atmosphere each year. Reactions subsequent to the formation of C F 3C F 2C (0) radicals in air in the presence of excess H O2 give PFPrA in a molar yield of 24%.12 U sing a simple model (see SI), and assuming the photolysis of PFM P proceeds exclusively via reaction 2, we derive an upper limit of 0.6 ng L -1for the average concentration of PFPrA in precipitation resulting from PFM P oxidation. Perfluoropropionic acid has been detected in rainwater at several sites in N orth America at concentrations on the order of 1-10 ng L -1and these levels have yet to be explained.3 The photolysis of PFM P contributes to, but does not appear to be the major source of, PFPrA observed in precipitation. ' ASSOCIATED CONTENT b Supporting Information. All computational experiment methodologies, photolysis rate constants from the T U V model, full modeling calculations on the environment fate of PFM P, 19F N M R spectra, and the U V --visspectrum of PF M P.T his material is available free of charge via the Internet at http:// pubs.acs.org. ' AUTHOR INFORMATION Correspondin Author *Phone: ; e-mail: @chem.utoronto.ca. ' ACKNOWLEDGMENT We thank Prof. M ark Taylor and Dr. M ima Staikova for assistance in the computational modelling. Funding to D.A.J. was provided through an 0 ntario G raduate Scholarship. Funding to C.J.Y. was provided by an NSERC Canada Graduate Scholarship. 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