rXXXX American Chemical Society A dx.doi.org/10.1021/es104362g | Environ. Sci. Technol. XXXX, XXX, 000–000 ARTICLE

pubs.acs.org/est

Atmospheric Degradation of Perfluoro-2-methyl-3-pentanone:

Photolysis, Hydrolysis and Hydration

Derek A. Jackson,† Cora J. Young,† Michael D. Hurley,‡ Timothy J. Wallington,‡ and Scott A. Mabury*,†

†

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, Canada M5S 3H6

‡

Ford Motor Company, Mail Drop SRL-3083, Dearborn, Michigan, 48121 United States

bS Supporting Information

’INTRODUCTION

Perfluorinated carboxylic acids (PFCAs) are ubiquitous in

biotic1 and abiotic environments.2,3 Longer-chain (>C8) PFCAs

are bioaccumulative4 and have attracted substantial research

interest. Precipitation measurements by Scott et al.3 have demonstrated that short-chain PFCAs, notably trifluoroacetic acid

(TFA) and perfluoropropionic acid (PFPrA), dominate the

PFCA 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. Thermolysis of fluoropolymers has been suggested as a

potential source of TFA observed in rainwater.5 Other sources

of TFA include the atmospheric oxidation of hydrofluorocarbons6,7 and polyfluorinated compounds.8,9 Small yields of

PFPrA have been proposed from thermolysis of fluoropolymers and atmospheric oxidation of fluorotelomer alcohols5,8,9

but do not explain the levels of PFPrA observed in precipitation.

Perfluoro-2-methyl-3-pentanone (PFMP) is a fire protection

fluid, marketed as Novec 1230 by 3M. It is a replacement for

chlorofluorocarbons (CFCs) and Halons, which deplete stratospheric ozone. The atmospheric lifetime of PFMP from previous

work seems to be determined by photolysis and is approximately

1 week.10,11 PFMP does not contribute to stratospheric ozone

depletion and has a negligible global warming potential.10,11 The

major photolysis products are CF3C(O)F and COF2.

10 The

atmospheric fate of CF3C(O)F is hydrolysis to yield TFA. It is

possible that chemistry in remote environments following the

photolysis of PFMP would give small yields of PFPrA or

isoperfluorobutanoic acid (i-PFBA) via reactions of the corresponding perfluoroacyl radicals with HO2 radicals.12,13

In addition to photolysis, hydration or abiotic hydrolysis may

be a significant sink of PFMP in the environment. In the

hydration reaction, PFMP would react either reversibly or

irreversibly with water to form a geminal diol which would shut

down the photolysis pathway. Ketones are typically unreactive

toward hydrolysis because the leaving group after nucleophilic

attack is an aliphatic carbanion. Carbanions are highly basic and

are not good leaving groups. However, hydrolysis of PFMP gives

Special Issue: Perfluoroalkyl Acid

Received: December 28, 2010

Accepted: March 25, 2011

Revised: March 15, 2011

ABSTRACT: Perfluorinated carboxylic acids are widely distributed in

the environment, including remote regions, but their sources are not well

understood. Perfluoropropionic acid (PFPrA, CF3CF2C(O)OH) 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 (PFMP, CF3CF2C(O)CF(CF3)2), a commonly

used fire-fighting fluid, to contribute to the observed PFPrA loadings.

The photolysis of PFMP gives CF3CF2C3(O) and 3 CF(CF3)2 radicals.

A small (0.6%) but discernible yield of PFPrA was observed in smog

chamber experiments by liquid chromatographymass spectrometry

offline chamber samples. The Tropospheric UltravioletVisible (TUV)

model was used to estimate an atmospheric lifetime of PFMP with respect to photolysis of 414 days depending on latitude and

time of year. PFMP can undergo hydrolysis to produce PFPrA and CF3CFHCF3 (HFC-227ea) in a manner analogous to the

Haloform reaction. The rate of hydrolysis was measured using 19F NMR at two different pHs and was too slow to be of importance in

the atmosphere. Hydration of PFMP to give a geminal diol was investigated computationally using density functional theory. It was

determined that hydration is not an important environmental fate of PFMP. The atmospheric fate of PFMP seems to be direct

photolysis which, under low NOx conditions, gives PFPrA in a small yield. PFMP degradation contributes to, but does not appear to

be the major source of, PFPrA observed in rainwater.
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a perfluorinated carbanion which, because the fluorine atoms

stabilize the departing carbanion through hyperconjugation, is a

much better leaving group. This effect is the basis of the wellknown Haloform reaction, one of the oldest organic reactions

known.14

Applying the Haloform reaction mechanism to hydrolysis of

PFMP should give PFPrA and CF3CFHCF3 (HFC-227ea).

HFC-227ea is a long-lived greenhouse gas and its formation

would be problematic. A detailed understanding of the rate and

products of PFMP hydrolysis under environmentally relevant

conditions is clearly desirable.

The present work had three goals: (i) to confirm the rate of

atmospheric photolysis of PFMP and to investigate the possible

formation of PFPrA, (ii) to determine whether hydrolysis of

PFMP is of significance in the environment, and (iii) to

investigate whether hydration is an environmental fate of PFMP.

’EXPERIMENTAL DETAILS

Measurements of UV Spectra and Calculations of Photolysis Rates. UV spectra of PFMP in the region 200400 nm

were recorded in a 6-cm cell using a Perkin-Elmer UV/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 Tropospheric UltravioletVisible

(TUV 4.2) package.15 All rates were calculated for 0.5 km above

the surface. The 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 Mattson Sirus 100 FTIR

spectrometer. The reactor was surrounded by 10 fluorescent

blacklamps (GE F15T8-BL, maximum emission 360 nm) and 12

sunlamps (GE-FS40, maximum emission 310 nm), which were

used to photolyze PFMP. The spectral overlap of the emission of

blacklamps and sunlamps and the absorption by PFMP has been

discussed by Taniguichi et al.10 All reagents were obtained from

commercial sources. Concentrations of reactants and products

were monitored by FTIR 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 Torr of PFMP in 50 Torr of oxygen. We chose a

diluent pressure of 50 Torr for the photolysis experiments as

Taniguichi et al.10 have established that the photolysis proceeds

more rapidly at lower total pressures. After photolysis the

reaction mixtures were pressurized to 750 Torr with air and

sampled as described below.

The experimental conditions employed in the smog chamber

experiments (296 K, 50 Torr total pressure of O2 diluent, in the

absence of water vapor, NOx, and other important atmospheric

constituents) are not the same as the range of conditions in the

real atmosphere. However, the chemical processes studied in the

chamber are very similar to those which will occur in the real

atmosphere. Photolysis of PFMP in the chamber will occur via

the same mechanism, the fate of the radicals formed

(decomposition and addition of O2) will be similar, and subsequent reactions of the peroxy radicals are similar to those in the

real atmosphere. Hence, the results from such smog chamber

experiments shed important light on the behavior of PFMP in the

real atmosphere.

Offline Sample Collection and Analysis. Offline 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 (PT = 750 Torr).

Triplicate samples were collected following 45 min of irradiation. Sodium carbonate solutions were acidified to pH 4 using

HCl and analyzed using a Waters 616 LC pump and 600

controller with detection by a Micromass Quattro Micro MS/

MS detector. Analytes were separated on a Genesis C8 column

(2.1 mm  50 mm  4 μm) using a 5-min isocratic run of 40%

methanol and 60% water, both containing 10 mM ammonium

acetate. Triplicate 10 μL injections were made using a Waters

717 autosampler. PFCAs were analyzed using a cone voltage

of 17 V and collision energy of 9 eV and the following

transitions were monitored: PFBA 213 > 169, PFPrA 163 >

119, and TFA 113 > 69. Analytes were quantified using

external calibration.

Hydrolysis Kinetic Experiments. To quantify the rate of

PFMP hydrolysis and identify the final products of hydrolysis,

19F NMR was used. The 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. To 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 ATB8123-400 autoswitchable probe

tuned to 19F (376.14 MHz) was used. Reaction solutions were

composed of 600 μL of buffer solution and 200 μL of D2O in a

5-mm NMR tube. Immediately prior to sample insertion, 20 μL

of PFMP was added to the NMR tube followed by inversion.

Reaction kinetics were followed by acquiring NMR 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. To ensure quantitive results,

a standard of PFPrA under the same reaction conditions as

PFMP hydrolysis was subject to a pulse inversionrecovery T1

relaxation experiment to determine a suitable relaxation delay

time for PFMP. In this case, a delay time of 20 s for PFMP was

chosen. After acquisition, data analysis was performed using the

VnmrJ software (Agilent Technologies) as it possesses a built-in

kinetic analysis module. Each arrayed spectrum was Fourier

transformed with a line broadening apodization of 6 Hz to

improve its signal-to-noise ratio. Kinetic data were obtained

by fitting an exponential growth or decay function to the

fluorinated signals in the NMR spectra (see Figure S5, Supporting Information). All hydrolysis experiments were performed

at 25 C.

Computational Method. Calculations to determine the

significance of PFMP hydration were performed using the

Gaussian 03 program16 with the WebMO interface. All

computations were performed using Density Functional

Theory (DFT) with B3LYP functionals using the 6-311G(d,p)

basis set. Computations were carried out for both the gas

phase and the aqueous phase using a polarizable continuum

solvent model (PCM).17 Equilibrium constants for hydration

were computed using the relative method of GomezBombarelli 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. PFMP was

purchased from Synquest Laboratories (Alachua, FL). Disodium

tetraborate and potassium hydrogen phthalate were purchased

from BDH (Toronto, ON). Trifluoroacetic acid and heptafluoropropionic acid (PFPrA) were purchased from Sigma-Aldrich

(Oakville, ON).
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’RESULTS AND DISCUSSION

Photolysis Kinetics. Photolysis lifetimes of PFMP have been

determined experimentally in two studies, yielding lifetimes of

approximately 12 weeks10 and 1 week.11 Using the flux

conditions and quantum yield (0.043) previously described,11

and the UV spectrum measured in this study (see SI), photolysis

rates for PFMP were estimated. The photolysis rates, presented

in Table 1, agree to within a factor of 2 with those reported in

previous studies as well as previous measurements of the UV

spectrum.10,11 The 24-h averaged rate constants for photolysis of

PFMP for different latitudes are given in the SI. Annual averaged

photolytic rate constants (J) ranged from 3.1  106 to 8.2 

107 s

1 corresponding to lifetimes of 414 days depending on

latitude and time of year.

Photolytic Production of PFCAs under Low NOx Conditions. Offline samples were taken to determine if photolytic

degradation of PFMP could yield PFCAs. The formation of

PFCAs in the chamber is only expected under low-NOx

conditions in the presence of HO2 radicals. Material was not

added specifically for the purpose of forming HO2 radicals, but

it is likely that HO2 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).

Large initial concentrations of PFMP (520 mTorr) were used

in the smog chamber to facilitate detection of small yields

of PFCAs.

Photolysis of PFMP can generate two different perfluoroacyl

radicals:

CF3CF2CðOÞCFðCF3Þ2 þ hv f CF3CF2 3 þ 3 CðOÞCFðCF3Þ2 ð1Þ

CF3CF2CðOÞCFðCF3Þ2 þ hv f CF3CF2C 3ðOÞ þ 3CFðCF3Þ2 ð2Þ

Perfluoroacyl radicals either decompose via elimination of CO or

add O2 to give perfluoroacyl peroxy radicals. In low-NOx

environments, perfluoroacyl peroxy radicals can react with

HO2 to give PFCAs:12

CF3ðCF2ÞxC3ðOÞ þ O2 f CF3ðCF2ÞxCðOÞOO 3 ð3Þ

CF3ðCF2ÞxCðOÞOO 3 þ HO2 f CF3ðCF2ÞxCðOÞOH þ O3 ð4Þ

The perfluoroacyl radicals derived from the photolysis of PFMP

can react to give i-PFBA or PFPrA as follows:

ðCF3Þ2CFC 3ðOÞ þ O2 f ðCF3Þ2CFCðOÞOO 3 ð5Þ

ðCF3Þ2CFCðOÞOO 3 þ HO2 f

ðCF3Þ2CFCðOÞOH ði-PFBAÞ þ O3 ð6Þ

CF3CF2C 3ðOÞ þ O2 f CF3CF2CðOÞOO 3 ð7Þ

CF3CF2CðOÞOO 3 þ HO2 f CF3CF2CðOÞOH ðPFPrAÞ þ O3

ð8Þ

PFPrA was observed in offline samples taken after 45 min of

irradiation, at a concentration of 0.061 mTorr. i-PFBA was not

detected in any of the samples.

The high concentrations of PFMP used in the experiments

saturated the FTIR signal and a direct determination of the

amount of PFMP photolyzed was not possible. The amount of

PFMP lost via photolysis was estimated from the photolysis halflife (determined to be 26 h by the observed formation of

CF3C(O)F as discussed previously10) in the chamber conditions.

We estimate that 10.4 mTorr of PFMP was photolyzed after 45

min and hence the yield of PFPrA is 0.6%. The low PFPrA yield

observed in this experiment probably reflects the low level of

Table 1. Kinetic Data for PFMP Degradation via Photolysis

and Hydrolysis (N = 2) at Two Different pH Values

(For Complete Photolysis Data, Refer to the SI)

mechanism conditions rate constant k (s1)

lifetime τ

(hours)

photolysis 45 N 3.5  106a 79.4

June 21

photolysis 45 N 5.9  107a 471

December 21

photolysis 45 N 2.1 106a 132

March 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.

Figure 1. Proposed mechanistic pathways leading to the formation of

PFCAs after photolysis of PFMP in the absence of NOx. Reaction

numbers from the text are given in parentheses. Compounds in black

boxes represent stable degradation products observed experimentally.

Compounds in red boxes represent stable degradation products that

were predicted but not observed.
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HO2 radicals available in the system. Sulbaek Andersen et al.19

reported that the yield of PFPrA from CF3CF2C 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

We expect the yield of i-PFBA following formation of

(CF3)2CFC 3(O) radicals in an excess of HO2 radicals to be

similar to that reported for n-PFBA. The low yield of PFPrA

observed in the present experiments suggests either that the yield

of CF3CF2C3(O) radicals in the photolysis of PFMP is low, or

that the concentration of HO2 radicals in the channel is low and

conversion of CF3CF2C 3(O) radicals into PFPrA is inefficient.

The absence of any discernible formation of i-PFBA suggests the

latter explanation is more probable. Figure 1 illustrates some

mechanistic pathways leading to PFCAs following photolysis of

PFMP in the absence of NOx. Note, as indicated in Figure 1,

decomposition via elimination of CO is a significant fate of

CF3CF2C3(O) radicals.13

The presence of PFPrA and the absence of i-PFBA suggests

reaction 2 is favored over reaction 1. This 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. The hydrolysis reaction of PFMP can

proceed by the well-known Haloform 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 Haloform 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. This reaction is the

classic qualitative test for a methyl ketone. The following steps

mechanistically describe the Haloform reaction:

RCðOÞCX3 þ OH f RCðOÞðOHÞCX3 ð9Þ

RCðOÞðOHÞCX3 f RCðOÞðOHÞ þ CX3

 ð10Þ

RCðOÞðOHÞ þ CX3

 f RCðOÞðOÞ þ CHX3 ð11Þ

where X = Cl/Br/I.

The final step in the Haloform reaction is a rapid and

exothermic proton transfer from the carboxylic acid to the

carbanion which makes the whole process irreversible. The

overall mechanism of the reaction is BAC2 which consists of

separate addition and elimination steps via a tetrahedral intermediate. The rate determining step is reaction 9. The final two

products expected from PFMP hydrolysis are PFPrA and HFC227ea. The mechanism for hydrolysis of PFMP is shown in

Figure 2.

The hydrolysis of PFMP 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. The objective of the present study was to investigate whether PFMP could undergo hydrolysis at pH values more

typical of those found in the environment. The 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

environmental limit where hydrolysis would likely be the fastest

since the Haloform reaction is base catalyzed.

To measure the kinetics of PFMP degradation, an analytical

method to quantify PFMP is required. Liquid chromatography is

not suitable to analyze PFMP because authentic standards would

rapidly degrade in any protic solvent. It was confirmed in a

separate study (data not shown) that PFMP reacts rapidly with

methanol to produce HFC-227ea and the methyl ester of PFPrA.

PFMP is certainly volatile enough to be analyzed by gas

chromatographymass spectrometry. However aqueous samples would need to be extracted into a suitable GC solvent prior

to injection. PFMP 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. We decided that 19F

NMR would offer the best capability for measuring the kinetics of

PFMP in situ. The hydrolysis reaction was performed in an NMR

tube and scans of the reaction mixture were taken at various time

intervals for subsequent analysis. 19F NMR offers acceptable

signal-to-noise ratios and since neither water nor the buffers used

contain fluorine atoms, a purely deuterated solvent does not need

to be used. For NMR data to be considered quantitative to 99þ%

accuracy, it is necessary that the relaxation delay between scans

be five times that of the longest spinlattice relaxation time (T1)

in the compound of interest. Because T1 values can depend on

the solvent used, a standard of PFPrA in the aqueous buffer of

interest was used as a surrogate for the T1 of PFMP. It was found

that the longest observed T1 (PFPrA) = 4.0 s, thus making a

suitable relaxation delay for PFMP to be 20 s.

The current Varian NMR processing software, VnmrJ, is

capable of analyzing kinetic data using a nonlinear fitting algorithm, provided the fit is exponential. Because the concentration

of both OH and water are constant in each experiment, pseudofirst-order conditions with respect to PFMP were achieved and

an exponential decrease in [PFMP] is expected due to hydrolysis.

After fitting the data points, the software provides the 1/e lifetime

of the compound (τ), which is simply the reciprocal of the

pseudo-first-order rate constant, kobs.

The 19F NMR spectrum of PFMP consists of four peaks: each

a multiplet due to the long-range J coupling observed for fluorine

atoms. The spectra of PFPrA and HFC-227ea combined resemble that of PFMP but the chemical shifts are distinct and the

coupling patterns are very different. Thus, it is straightforward to

distinguish PFMP from its degradation products. For examples

Figure 2. Hydrolysis mechanism of PFMP to produce PFPrA and

HFC-227ea under mildly basic conditions. Reaction numbers in the text

are given in parentheses.
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of 19F NMR spectra of both PFMP and its photolysis products,

please see the SI.

In summary, we observed the 1/e lifetime of PFMP hydrolysis

to vary depending on the pH of the buffer used. The reaction was

faster in more alkaline solutions as shown in Table 1. This is

entirely reasonable given the base-catalyzed nature of the reaction. Both PFPrA and HFC-227ea were positively identified as

the sole products of the hydrolysis reaction. PFPrA was confirmed by comparison to an authentic standard. HFC-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 PFMP, however it is not surprising they

observed high yields in PFPrA and HFC-227ea given the strongly

basic conditions of their reaction solutions. A further study by the

same group23 elaborated on the specificity of the CC 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 CF

orbitals. Sykes et al.24 confirmed the regioselectivity of the

Haloform reaction for a number of polyfluorinated ketones.

Hence, there is precedent for the Haloform 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.

RCðOÞR0 þ H2O f RCðOHÞðOHÞR0 ð12Þ

The reaction is catalyzed by both acids and bases and is usually

a reversible equilibrium reaction. For most aldehydes and

ketones, the equilibrium constant, Khyd, usually strongly favors

the carbonyl compound due to the high strength of the CdO

bond. Exceptionally, formaldehyde (HC(O)H) exists in aqueous

solution purely as the gem-diol (HC(OH)(OH)H) due to its

high Khyd value (log Khyd = 3.36).25 The vast majority of ketones

are quite unreactive to hydration (acetone has log Khyd = 2.8525)

and the geminal diol form is usually neglected in reaction

schemes. Other exceptions can be halogenated ketones; trichloroacetaldehyde has a log Khyd = 4.45 and hexafluoroacetone-

(HFA) has log Khyd = 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, HFA is provided

commercially as the geminal diol derivative.

It might be reasonable to assume that the electron withdrawing groups of HFA impart a strong partial positive charge

at the fluorinated carbonyl carbon, resulting in enhanced electrophilicity and hence, a greater degree of hydration. However,

Linderman et al.26 used HartreeFock computational methods

and found most of the positive electric potential in HFA is found

on the CF3 carbons and not the carbonyl carbon. Instead, they

propose the lower energy level of the LUMO of fluorinated

ketones compared to their hydrogenated analogs is the major

cause of the enhanced reactivity toward water. Because PFMP is

perfluorinated like HFA, it would be a reasonable assumption

that PFMP could rapidly and irreversibly form a geminal diol in

the environment. The PFMP hydrate would not absorb actinic

radiation due to the absence of the CdO 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. Thus,

it is important to determine whether PFMP could form a stable

hydrate in the same manner as HFA.

The hydration reaction of PFMP was investigated in our

laboratory and no evidence of such a hydrated species was

observed (data not shown). This was puzzling as other fluorinated ketones are known to hydrate readily.25 To provide further

insight, computational methods were used to estimate Khyd for

PFMP and compare it to other fluorinated carbonyls using the

methods of Gomez-Bombarelli et al.18 The effects of including an

aqueous PCM solvent model were also studied although Guthrie

et al.27 previously noted difficulties of applying the PCM model

to halogenated compounds. For full details on the computational

work, please see the SI.

Depending onthe method usedto calculateKhyd (see SI), different

values for PFMP were obtained (2.09 < log Khyd < 0.43) but the

range suggests hydrate formation is not significant for PFMP.

Given the perfluorinated nature of PFMP and that even partially

fluorinated ketones such as trifluoroacetone (log Khyd = 1.54)25

have large Khyd values we were surprised by the low values of Khyd

calculated for PFMP. The computational methods used in the

present study (B3LYP/6-311þþG(d,p)) were able to compute

Khyd 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 PFMP in

the gas phase (B3LYP/6-311þþG(d,p)) gave the energy level of

the LUMO (84.1 kcal/mol) as almost identical to the LUMO

of HFA (81.4 kcal/mol), especially when compared to the

computed LUMO of acetone (19.3 kcal/mol). This supports

the results of Linderman et al.26 who proposed these energy

differences as the reason for the enhanced reactivity of HFA to

hydration. In the case of PFMP, steric hindrance could be the

reason for its decreased hydration equilibrium constant compared to HFA. We conclude that geminal diol formation is not a

significant environmental fate for PFMP.

’ENVIRONMENTAL IMPLICATIONS

We have investigated the potential for photolysis, hydrolysis,

and hydration to contribute to the environmental fate of PFMP.

As discussed in the previous section, hydration is not a significant

fate for PFMP. While 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 HFC-227ea produced would not be significant and

that photolysis dominates hydrolysis as the atmospheric fate of

PFMP. Cahill and Mackay29 came to the same conclusion in their

modeling study. Interestingly, Cahill and Mackay 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. Hence, our work suggests hydrolysis is an even less

important fate for PFMP compared to photolysis than the ratio

Cahill and Mackay29 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. This ratio is so great

that even at night time during a heavy rain event it is unlikely

hydrolysis will occur to any significant extent.
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Photolysis is the dominant mechanism by which PFMP is

removed from the atmosphere. It has been established

previously10,11 that photolysis of PFMP in the presence and

absence of NOx will lead to the formation of CF3C(O)F in a

molar yield of approximately 100%. The atmospheric fate of

CF3C(O)F is hydrolysis to give trifluoroacetic acid (TFA). TFA

is a ubiquitous naturally occurring component of the hydrosphere and the additional burden associated with PFMP photolysis is not significant. In the present work, we also show that

small amounts of PFPrA are also formed as a result of PFMP

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 PFMP we applied the

following logic (see SI for details). The production of PFMP

by 3 M is 1001000 t year1 and began in approximately 2003.30

Given PFMP is used entirely in fire-protection systems that are

released by an alarm, it can be considered stored emission

potential with releases averaging 13% year1

.

30 Combining

the upper limit of production (1000 t year1

) with a 3% emission

factor provides an upper limit of 30 t of PFMP released into the

atmosphere each year. Reactions subsequent to the formation of

CF3CF2C(O) radicals in air in the presence of excess HO2 give

PFPrA in a molar yield of 24%.12 Using a simple model (see SI),

and assuming the photolysis of PFMP proceeds exclusively via

reaction 2, we derive an upper limit of 0.6 ng L1 for the average

concentration of PFPrA in precipitation resulting from PFMP

oxidation. Perfluoropropionic acid has been detected in rainwater at several sites in North America at concentrations on the

order of 110 ng L1 and these levels have yet to be explained.3

The photolysis of PFMP contributes to, but does not appear to

be the major source of, PFPrA observed in precipitation.

’ASSOCIATED CONTENT

bS Supporting Information. All computational experiment

methodologies, photolysis rate constants from the TUV model,

full modeling calculations on the environment fate of PFMP, 19F

NMR spectra, and the UVvis spectrum of PFMP. This material

is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author

*Phone: 416-978-1780; e-mail: smabury@chem.utoronto.ca.

’ACKNOWLEDGMENT

We thank Prof. Mark Taylor and Dr. Mima Staikova for

assistance in the computational modelling. Funding to D.A.J.

was provided through an Ontario Graduate Scholarship. Funding

to C.J.Y. was provided by an NSERC Canada Graduate

Scholarship.

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