J. Phys. Chem. 1996, 100, 16435-16440
16435
The Proton Affinity and Absolute Heat of Formation of Trifluoromethanol Leonard J. Chyall† and Robert R. Squires* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393 ReceiVed: April 18, 1996; In Final Form: July 2, 1996X
The proton affinity and absolute heat of formation of trifluoromethanol have been derived from translational energy threshold measurements for reactions involving oxygen-protonated trifluoromethanol. The reaction of ionized iodotrifluoromethane with water was used to prepare CF3OH2+ in the flow tube of a flowing afterglow triple-quadrupole instrument. The isomeric cluster ion, (HF)CF2OH+, was shown to be more stable than CF3OH2+ by the base-catalyzed conversion of CF3OH2+ to (HF)CF2OH+ using either SO2 or OCS as the catalyst. The proton affinity of CF3OH at oxygen was determined from the enthalpy change for the endothermic proton transfer reaction CF3OH2+ + CO f CF3OH + HCO+. The measured enthalpy change, 9.2 ( 1.4 kcal mol-1, was combined with the known value for the proton affinity of CO (141.9 kcal mol-1) to yield a value for the oxygen proton affinity of CF3OH of 151.1 ( 1.7 kcal mol-1. The dissociation energy for the loss of water from CF3OH2+ was measured to be 36.6 ( 2.1 kcal mol-1 by energy-resolved collision-induced dissociation. This value was used in a thermochemical cycle along with the measured proton affinity of CF3OH to derive the gas-phase heat of formation of CF3OH of -220.7 ( 3.2 kcal mol-1. This experimental value is slightly lower than, but in good agreement with, the 298 K heat of formation of CF3OH that is predicted by high-level molecular orbital calculations.
Introduction The depletion of stratospheric ozone by chlorofluorocarbons (CFCs) has motivated the development of alternative compounds that are less toxic to the environment. In particular, hydrofluorocarbons (HFCs) have been proposed to be acceptable substitutes for chemical applications that previously employed CFCs. Although there is good evidence that HFCs do not pose a serious threat to stratospheric ozone depletion,1 the mechanisms for degradation of these compounds in the troposphere are not fully understood. HFCs that contain a trifluoromethyl group such as CF3CF2H will produce CF3• in a series of reactions that is initiated by hydroxyl radicals.2 Conversion of CF3• to CF3O• is believed to occur by reactions 1 and 2.3
CF3• + O2 + M f CF3O2• + M
(1)
CF3O2• + NO f CF3O• + NO2
(2)
While the processes leading to CF3O• formation are wellestablished, the fate of this radical in the atmosphere is less certain. Laboratory experiments suggest that CF3O• will react with hydrocarbons present in the troposphere to produce CF3OH (reaction 3)4 or with NO to yield CF2O and FNO (reaction 4).
as methanol (DH298(CH3O-H) ) 104.2 kcal mol-1).6 Molecular orbital calculations at high levels of theory5,7,8 predict that the O-H bond strength in CF3OH is equal to, or greater than, that in water (DH298(HO-H) ) 119.3 kcal mol-1),6 while bondadditivity estimates suggest a somewhat lower value (DH298(CF3O-H) ) 109 kcal mol-1).9 A recent experimental determination based on negative ion thermochemical cycles provides a value of 124.7 ( 3.6 kcal mol-1.10 One impediment to the resolution of this controversy, and to achieving a complete understanding of the atmospheric reactivity of CF3O•, is the lack of reliable thermochemical data for CF3OH, CF3O•, and CF2O. At present, there are no experimental values for the heats of formation of CF3OH and CF3O•. Furthermore, recent MO calculations8,11,12 suggest that the experimental value13 for ∆Hf,298(CF2O) of -152.7 ( 0.4 kcal mol-1 is too low by at least 7 kcal mol-1. Our approach to the thermochemistry of these species involves the measurement of the enthalpy changes for selected gas-phase ion/molecule reactions. Thermochemical cycles are then employed to derive the heats of formation for the pertinent neutral species. For example, ∆Hf,298(CF3OH) can be derived from the proton affinity of the neutral molecule (reaction 5) and the water binding energy of trifluoromethyl cation (reaction 6).
CF3O• + RH f CF3OH + R•
(3)
CF3OH + H+ f CF3OH2+ ∆H(5) ) -PA(CF3OH) (5)
CF3O• + NO f CF2O + FNO
(4)
CF3OH2+ f CF3+ + H2O ∆H(6) ) DH(CF3+-OH2) (6)
It was recently reported that CF3O• will abstract a hydrogen atom from water under thermal conditions at room temperature,5 which would render the general process shown in reaction 3 exothermic for a multitude of hydrogen-containing molecules. This is surprising since it implies an O-H bond strength for CF3OH that is much greater than those of simple alcohols such † Present address: Great Lakes Chemical Corporation, P.O. Box 2200, West Lafayette, IN 47906. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01135-5 CCC: $12.00
CF3OH + H+ f CF3+ + H2O ∆H(7) ) ∆H(5) + ∆H(6) (7) In this paper, we describe the application of energy-resolved ion beam techniques in measurements of the enthalpy changes for reactions 5 and 6 and a derivation of ∆Hf,298(CF3OH). In addition, the reactivity of CF3OH2+ with neutral bases is examined, and the stability of this ion relative to other isomers is discussed. © 1996 American Chemical Society
16436 J. Phys. Chem., Vol. 100, No. 40, 1996
Chyall and Squires
Experimental Section The flowing afterglow triple quadrupole apparatus used for this study is described elsewhere.14 Oxygen-protonated trifluoromethanol was prepared in the helium flow reactor at room temperature by allowing the molecular ion of iodotrifluoromethane, CF3I•+ (prepared by electron ionization of the neutral molecule), to react with water. The ions present in the flow tube were extracted through a 1-mm orifice into a differentially pumped chamber that houses the triple-quadrupole mass analyzer. The CF3OH2+ ions (m/z 87) were mass-selected with the first quadrupole (Q1) and then injected into the rf-only second quadrupole (Q2). Collision-induced dissociation (CID) experiments and translationally driven ion/molecule reactions were performed in Q2, which functions as a gas-tight reaction chamber. Reagent gases present in Q2 were maintained at pressures between 0.035 and 0.100 mTorr. Argon and neon targets were used for the CID threshold measurements, with the lighter target being employed for the lower energy dissociations because of the greater spread in the laboratory energy scale relative to the ion beam energy distribution (Vide infra). The intensities of the reactant and product ion signals were monitored as a function of the axial kinetic energy of the reactant ions. The kinetic energy of the reactant ions in Q2 is determined by the dc offset voltage applied to the quadrupole rods, which is calibrated by retarding potential analysis. The full width at halfmaximum (fwhm) of the ion beam kinetic energy distribution is typically in the range of 0.6-1.5 eV. The reactant and product ions were mass-analyzed with the third quadrupole and detected with an electron multiplier operating in pulse-counting mode. Absolute cross sections for formation of an ion/molecule reaction product in Q2, σp, were calculated using the relation σp ) Ip/INl, where Ip and I are the intensities of the product and reactant ions, respectively, N is the number density of the neutral reactant gas, and l is the effective reaction path length. This relationship holds as long as the total extent of reaction is less than ca. 5%. The effective path length in Q2, l, is measured to be 24 ( 4 cm based on calibration experiments14 with the reaction Ar+ + D2 f ArD+ + D, which has a well-known cross section.15 The reported absolute cross sections have estimated uncertainties of (50%, while the relative cross sections have estimated uncertainties of (20%. The ion/molecule collision energy in the center-of-mass (CM) frame is calculated from the relation ECM ) Elab[m/(m + M)], where Elab is the reactant ion axial kinetic energy in the lab frame and M and m represent the masses of the ion and neutral reactants, respectively. A plot of product ion yield vs the center-of-mass collision energy provides an appearance curve from which the activation energy for the ion/molecule reaction can be derived. The modeling procedures used to analyze the product ion appearance curves have been described previously14,16 and are similar to the methods developed by Armentrout and co-workers for investigating the translational energy dependence of ion/molecule reactions by ion beam mass spectrometry.17,18 The product ion appearance curve is fit with the assumed model function shown in eq 8,19
σ(E) ) σ0∑gi(E + Ei - ET)n/E
(8)
where σ(E) is the cross section at center-of-mass energy E, ET is the reaction threshold energy, σ0 is a scaling factor, and n is an adjustable parameter. The vibrational energy distributions of the ionic and (where appropriate) neutral reactants are also included as a summation over i vibrational states having internal energy Ei and population gi where, ∑gi ) 1. Optimization of the fit is carried out by an iterative procedure in which ET, n, and σ0 are varied so as to minimize the deviation between the
calculated model function and the steeply rising portion of the experimental appearance curve.20 Convoluted into the fit are the reactant ion kinetic energy distribution, approximated by a Gaussian function with a 1.5 eV (lab) fwhm, and a Doppler broadening function developed by Chantry21 to account for random thermal motion of the neutral target. The internal energy distributions for the ionic and neutral reactants were calculated either from the experimental vibrational frequencies13 or from scaled harmonic vibrational frequencies obtained from molecular orbital calculations. Selected data sets were used to evaluate the magnitude of “kinetic shifts” in the CID threshold data22 due to slow dissociation of the activated species. These effects were found to be negligibly small for all of the dissociation reactions examined in this work, which obviated the need for this extended form of data analysis. The thresholds derived in the above manner correspond to the 0 K activation energies. These can be equated with the reaction endothermicities, provided there are no significant reverse activation energies. The 0 K activation energies are converted to 298 K reaction enthalpies by adding the difference in 0-298 K integrated heat capacities of the reactants and products. The stationary electron convention is employed in this work.23 Ab initio molecular orbital calculations were performed on an IBM RISC/6000 computer using the Gaussian 92 series of programs.24 Geometries were optimized employing the appropriate symmetry constraint with a HF/6-31G(d) basis set and were determined to be true minima by the absence of negative eigenvalues in the matrix of computed force constants. To be consistent with previous results from another laboratory,25 the calculated frequencies were scaled by a factor of 0.893 for all zero-point energy and thermal corrections. Single-point energies were obtained using second-order Møller-Plesset perturbation theory (frozen-core) with a 6-311G(d) basis set, i.e., MP2/6-311G(d)//HF/6-31G(d). All reagents were obtained from commercial suppliers and used as received. The hydrofluoric acid used for the flow tube chemistry was added as a pre-formed mixture of 0.5% HF in helium. Extreme caution was exercised when handling hydrofluoric acid and carbonyl fluoride. Results and Discussion Synthesis and Characterization of CF3OH2+. The relatively high pressures employed in the flowing afterglow technique (≈400 mTorr) are conducive to termolecular ion/ molecule association reactions. However, addition of water to CF3+ (generated by electron ionization of CF4) failed to generate appreciable amounts of CF3OH2+. This reaction was characterized by the formation of abundant quantities of hydronium ion (H3O+) and the corresponding hydrates, H3O+(H2O)n. This behavior is consistent with initial formation of CF3OH2+ followed by rapid deprotonation of this ion by the background water. The resulting hydronium ions then cluster to additional water molecules that are present in the flow tube. Morris et al. have shown that CF3OH2+ can be prepared in a selected ion flow tube (SIFT) by the reaction of ionized water with CF3Cl.26 Although the formation of CF3OH2+ is highly efficient, this reaction also produces CF2Cl+ as a side product. The 37Cl isotopomer of this ion (m/z 87) has the same nominal mass as CF3OH2+, which would complicate reactivity studies of CF3OH2+. We have avoided this problem by preparing CF3OH2+ by the reaction between ionized iodotrifluoromethane and water (reaction 9). This reaction is mechanistically similar to
CF3I•+ + H2O f CF3OH2+ + I•
(9)
that between H2O•+ and CF3Cl. The reaction involving CF3I•+
PA and ∆Hf of Trifluoromethanol
J. Phys. Chem., Vol. 100, No. 40, 1996 16437
Figure 1. Product ion spectrum obtained from collision-induced dissociation of CF3OH2+ at a center-of-mass collision energy of 6.2 eV with argon target.
requires the addition of relatively small amounts of water to the flow tube, such that secondary reactions between CF3OH2+ and water are minimized. Nonetheless, small amounts of hydronium ion and its hydrates are still observed. Collisional activation of CF3OH2+ using argon target and 5-10 eV (CM) collision energies results in CF3+ formation (m/z 69) and, to a lesser extent, formation of CF2OH+ (m/z 67). At a collision energy of 6.3 eV (CM), the ratio of fragment ion signal intensities, I(m/z 69)/I(m/z 67), is 95:5, as shown in Figure 1. Energy-resolved CID measurements indicate that the HFloss channel is a relatively low-energy process with a threshold energy of about 0.6 eV, while the water-loss channel has an onset at about 1.5 eV (Vide infra). This behavior suggests that the formation of CF2OH+ does not result from elimination of HF from CF3OH2+, because such a reaction would require a high-energy, kinetically disfavored, four-centered transition state.25 The ratio of CF2OH+ to CF3+ remains unchanged at higher argon target pressures (up to 0.20 mTorr) and at higher collision energies (up to 10 eV, CM), which eliminates the possibility that the CF3OH2+ ions are isomerized in the activated collision complex prior to dissociation. Therefore, the possibility was addressed that the CF3OH2+ ions prepared in the flow tube were contaminated with an isomeric impurity that dissociates to CF2OH+ upon collisional activation. The processes that lead to the formation of the CF2OH+ fragment ion were elucidated with the assistance of molecular orbital calculations. Grandinetti et al. have examined the CF3OH2+ potential energy surface at the MP2/6-311G(d,p)//HF/631G(d) level of theory.25 Their calculations predict that the transition state for loss of HF from oxygen-protonated trifluoromethanol lies above the energy threshold for loss of water, which would lead to behavior inconsistent with our observation that HF loss occurs at collision energies below that for H2O loss. Additional calculations of the CF3OH2+ potential energy surface by Grandinetti et al. have identified a lower energy structure corresponding to an ion with an H-F bond (Figure 2, 2). Although 2 can be viewed as fluorine-protonated trifluoromethanol, the rather long C-F distance to the proton-bearing fluorine atom indicates that this species is better described as a protonated carbonyl fluoride-hydrofluoric acid cluster ion. In this structure, the interaction between the HF molecule and CF2OH+ is largely electrostatic in nature, and the computed binding energy is quite low, 14.4 kcal mol-1. We have reproduced these computational results in the present study and have located a third and even more stable isomer, corresponding to a cluster where the HF molecule is electrostatically bound to the oxygenbearing proton of CF2OH+ (Figure 2, 3). Ion 3 is computed to be lower in energy than the oxygen-protonated form, CF3OH2+ (1), by 14.3 kcal mol-1 (Table 1). Grandinetti et al. also compute values of 150.9 and 160.1 kcal mol-1 for the proton affinity of CF3OH at oxygen and
Figure 2. Calculated structures for oxygen-protonated trifluoromethanol and the two (HF)CF2OH+ cluster ions. Structural details are provided as supporting information.
TABLE 1: Calculated MP2/6-311G(d)//HF/6-31G(d) Electronic Energies, Zero-Point Energies, Temperature Corrections, and Relative Enthalpies species
Eelec, hartrees
CF3OH2+ (1) -412.957 56 HF+-CF2OH (2) -412.969 29 CF2O-H+-FH (3) -412.978 66 a
ZPE,a H298 - H0, Hrel,298, kcal mol-1 kcal mol-1 kcal mol-1 24.70 22.78 22.81
3.77 4.84 4.65
(0) -8.2 -14.3
Computed vibrational frequencies were scaled by 0.893.
SCHEME 1
fluorine, respectively.25 This suggests that a base with a proton affinity near 150.9 kcal mol-1 should be able to isomerize oxygen-protonated trifluoromethanol to a more stable (HF)CF2OH+ cluster ion (Scheme 1).27 We have verified this reactivity experimentally when either SO2 (PA ) 152.1 kcal mol-1)23 or OCS (PA ) 150.7 kcal mol-1)23 is employed as the base catalyst. Addition of either of these reagents to the flow tube in which CF3OH2+ was prepared by reaction 9, followed by CID in the triple-quadrupole analyzer, produces appreciable amounts of CF2OH+ as well as CF3+ product ions. The abundance of CF2OH+ relative to CF3+ was observed to increase with increasing concentrations of the base catalyst in the flow tube (Figure 3 for the reaction with SO2), which is consistent with the proposed isomerization of oxygen-protonated trifluoromethanol to a more stable structure (reaction 10). Complete isomerization of the
CF3OH2+ + SO2 f (HF)CF2OH+ + SO2
(10)
reactant ion could not be achieved due to the extensive attenuation of the reactant ion signal intensity that results from
16438 J. Phys. Chem., Vol. 100, No. 40, 1996
Chyall and Squires TABLE 2: Threshold Energies, Fitting Parameters, and Derived Enthalpy Changes for the Reactions of (HF)CF2OH+ and CF3OH2+ reaction
ET,a eV
nb
∆Hrxn,c kcal mol-1
(HF)CF2OH+ f CF2OH+ + HF 0.62 ( 0.07 1.2 ( 0.2 14.4 ( 1.7 CF3OH2+ f CF3+ + H2O 1.53 ( 0.08 1.6 ( 0.1 36.6 ( 2.1 0.42 ( 0.05 1.2 ( 0.3 9.2 ( 1.4 CF3OH2+ + CO f HCO+ + CF3OH a Average threshold energy (0 K). b Average fitting parameter from eq 8. c Reaction enthalpy (298 K) derived from ET.
Figure 3. Product ion spectrum obtained from collision-induced dissociation of the ions with m/z 87 as function of increasing concentrations of SO2 in the flow tube.
Figure 4. Cross section for collision-induced dissociation of CF2OH+ from (HF)CF2OH+ as a function of center-of-mass collision energy with neon target. The solid line is the convoluted model appearance curve calculated using eq 8.
termolecular association reactions and other competing processes. Addition of excess water (PA ) 166.5 kcal mol-1)23 to the flow tube during the synthesis of CF3OH2+ leads to diminished intensities of this ion with a concomitant increase in the intensities of H3O+ and H3O+(H2O)n cluster ions. However, the ratio of product ions obtained from CID of the ions with m/z 87 remains unchanged with increasing concentrations of water in the flow tube. Therefore, while water is able to deprotonate CF3OH2+, it apparently does not isomerize this ion to (HF)CF2OH+. In separate experiments, (HF)CF2OH+ cluster ions were prepared in the flow tube by allowing a 0.5% mixture of hydrofluoric acid in helium to react with CF2OH+, which in turn was prepared by reaction of CF2O with the ions produced upon electron ionization of methane. CID of (HF)CF2OH+ with neon target produces CF2OH+ (reaction 11) as the sole product
(HF)CF2OH+ f CF2OH+ + HF
(11)
with a maximum cross section of 15 Å2 at 3.2 eV (CM). Energy-resolved CID measurements for reaction 11 were carried out and fit to the model function given in eq 8. A representative product ion appearance curve is shown in Figure 4.28 The energy threshold for CF2OH+ was determined from replicate measurements to be 0.62 ( 0.07 eV, with an average value of the fitting parameter n of 1.2 ( 0.2 (Table 2). The uncertainty in ET is derived from the precision of the measurements (1 standard deviation) and a 0.15-eV (lab) uncertainty in the absolute energy scale. When the appropriate thermal corrections are applied to this threshold energy, a 298 K enthalpy of 14.4 ( 1.7 kcal mol-1 is obtained for the binding energy of HF to CF2OH+ (Table 2). An identical HF binding energy is obtained from the energy-resolved CID of the (HF)CF2OH+ ion produced by isomerization of CF3OH2+ with SO2 (reaction 10). The measured HFCF2OH+ binding enthalpy is consistent with the value predicted by ab initio calculations for isomer 2.25
Figure 5. Cross section for collision-induced dissociation of CF3+ from CF3OH2+ as a function of center-of-mass collision energy with argon target. The solid line is the convoluted model appearance curve calculated using eq 8.
Nevertheless, the long bonds between the HF molecule and protonated carbonyl fluoride predicted for ions 2 and 3 suggest that interconversion among HF cluster ions of various structures should be facile. Heats of Formation of CF3OH2+ and (HF)CF2OH+. The loss of water from CF3OH2+ was examined by energy resolved CID. The cross section for CF3+ reaches a maximum value of 6 Å2 at a collision energy of 4.0 eV (CM) with argon target (Figure 5). The average threshold energy for this dissociation determined from replicate analyses is 1.53 ( 0.08 eV (n ) 1.6 ( 0.1), where the uncertainty in ET is obtained in the manner described previously. We take the measured activation energy to be equal to the thermochemical bond dissociation energy since the reverse reaction, addition of CF3+ to H2O, occurs without a barrier.29 The 0 K energy threshold and the appropriate thermal corrections for reaction 6 provide a 298 K dissociation enthalpy, DH(CF3+-OH2), of 36.6 ( 2.1 kcal mol-1 (Table 2). This value was used with the pertinent reference data given in Table 3 to derive ∆Hf,298(CF3OH2+) ) -6.1 ( 2.4 kcal mol-1, where the uncertainty represents the root-square sum of the component uncertainty intervals. An estimate of the heat of formation of (HF)CF2OH+ can be made using the measured threshold energy for loss of HF from this ion and values (Table 3) for the proton affinity and heat of formation of CF2O (eq 12). The value for ∆Hf,298(CF2O) )
∆Hf,298((HF)CF2OH+) ) ∆Hf,298(CF2O) + ∆Hf,298(H+) + ∆Hf,298(HF) - PA(CF2O) - ∆H(11) (12) -145.3 kcal mol-1 derived from MO theory,8,33 the experimental value for PA(CF2O) ) 160.5 kcal mol-1, and the HF binding energy of 14.4 kcal mol-1 obtained in this study (Table 2) lead to ∆Hf,298((HF)CF2OH+) ) -19.6 kcal mol-1. This is 13.5 kcal mol-1 lower than the measured heat of formation for CF3OH2+, which confirms the experimental observation that CF3OH2+ is thermodynamically unstable with respect to a lower energy isomer (reaction 13). In addition, this energy difference is close to the theoretically predicted difference in stability
PA and ∆Hf of Trifluoromethanol
J. Phys. Chem., Vol. 100, No. 40, 1996 16439
TABLE 3: Supplemental and Measured Thermochemical Data (kcal mol-1)a species
∆Hf,298
ref
H H 2O HF CF3+ CF3OH2+ (HF)CF2OH+ CF3OH
365.7 -57.8 -65.1 ( 0.2 88.3 ( 1.6 -6.1 ( 2.4 -19.6 -220.7 ( 3.2 -213.5 ( 1.5 -215 ( 1 -217.7 ( 2.0 -217.7 -152.7 ( 0.4 -145.3 ( 1.7 -144.8 ( 1.0
23 23 23 23,30 this work this work this work 31 9 11 7 13 8 11
+
CF2O
species
PA
ref
CO OCS SO2 H 2O CF3OHb
141.9 ( 1.0 150.7 152.1 166.5 151.1 ( 1.7 150.9 160.5 ( 2.0 159.0 ( 2.0
23 23 23 23 this work 25 23 32
CF2O
a Values obtained from MO calculations are given in italics. b Proton affinity at the oxygen atom.
between 1 and the HF-cluster forms, 8.2 - 14.3 kcal mol-1 (Table 1).
CF3OH2+ f (HF)CF2OH+ ∆Hrxn ) -13.5 kcal mol-1 (13) The Proton Affinity and Heat of Formation of CF3OH. In principle, the proton affinity of trifluoromethanol can be measured by acid-base bracketing experiments.23,34 However, the aforementioned isomerization of CF3OH2+ to (HF)CF2OH+ would complicate the interpretation of these measurements. Misleading results may be obtained for reactions involving CF3OH2+ and bases that have proton affinities similar to that of CF3OH. For example, the proton transfer product of a mildly exothermic reaction involving CF3OH2+ and a reference base may not be observed due to the occurrence of the more favorable base-catalyzed isomerization of CF3OH2+, as shown in Scheme 1. Moreover, the greater basicity of CF3OH at fluorine would complicate any attempts to determine the oxygen proton affinity by bracketing reactions between CF3OH and reference acids. In a previous study, we demonstrated the efficacy of an alternative procedure for measuring proton affinities that are based on the translational energy thresholds for endothermic proton transfer reactions carried out in the triple-quadrupole mass analyzer.35 We have adopted this approach to determine the proton affinity of CF3OH by examining the deprotonation of CF3OH2+ by CO (reaction 14). This particular neutral base CF3OH2+ + CO f HCO+ + CF3OH
(14)
was selected because the proton affinity of CO is accurately known.23,36 In addition, the difference in the proton affinities of CF3OH and CO is expected to be within 1 eV, which would minimize complications in the data analysis resulting from competing reactions such as CID. The energy-dependent cross sections for the ionic products of the reaction between CF3OH2+ and CO are shown in Figure 6a. The cross section for the proton-transfer product, HCO+, reaches a maximum at 1.5 eV CM (Figure 6b); at higher energies, the CID channel that generates CF3+ dominates the
Figure 6. (a) Cross sections for the products obtained upon reaction of CF3OH2+ with CO as a function of center-of-mass collision energy. (b) Cross section and calculated model cross section (solid line) for the proton transfer channel.
Figure 7. Schematic summary of the relative enthalpies (kcal mol-1) for the reaction of CF3OH2+ with CO.
product ion spectrum. The cross section for the CF2OH+ product is characterized by a pronounced maximum near the energy axis origin, which is indicative of a reaction with a low energy barrier. This is followed by a decrease in the cross section to a nearly constant value at higher energies. The generation of CF2OH+ at higher energies (>1.5 eV, CM) is believed to be due to CID of the minor amounts of (HF)CF2OH+ that contaminate the beam of CF3OH2+ reactant ions. A plausible mechanism for the low-energy formation of CF2OH+ is the CO-catalyzed isomerization of CF3OH2+ to (HF)CF2OH+ followed by loss of HF. The energetics of this reaction are consistent with this explanation (cf. Figure 7). The average value of the threshold for reaction 14 (Figure 6b) was determined from replicate measurements to be 0.41 ( 0.07 eV, with n ) 1.2 ( 0.3 (Table 2), where the assigned uncertainty in ET is obtained in the manner described previously. A temperature correction of -0.36 kcal mol-1 provides a 298
16440 J. Phys. Chem., Vol. 100, No. 40, 1996 K reaction enthalpy for reaction 14 of 9.2 ( 1.4 kcal mol-1. Adding this value to PA(CO) ) 141.9 ( 1.0 kcal mol-1 gives the proton affinity of CF3OH at oxygen of 151.1 ( 1.7 kcal mol-1 (Table 3). The enthalpy change for loss of H2O from CF3OH2+ (reaction 6) can then be combined with this value and the supporting thermochemical data (Table 3) to derive ∆Hf,298(CF3OH) ) -220.7 ( 3.2 kcal mol-1 (eq 15), where
∆Hf,298(CF3OH) ) ∆Hf,298(CF3+) + ∆Hf,298(H2O) + PA(CF3OH) - ∆Hf,298(H+) - ∆H(6) (15) the assigned uncertainty represents the root-square-sum analysis of the component uncertainties. Figure 7 presents a schematic summary of the relative enthalpies of the pertinent ionic and neutral species. Our experimental value for ∆Hf,298(CF3OH) is slightly lower than, but in satisfactory agreement with, the values predicted by ab initio MO calculations. Montgomery et al.11 used an isodesmic reaction approach at the G2 level of theory to compute ∆Hf,298(CF3OH) ) -217.7 ( 2 kcal mol-1, where the estimated uncertainty represents the average error in the computed heats of formation of a test set of molecules. Related calculations7 at a slightly lower level of theory also provide ∆Hf,298(CF3OH) ) -217.7 kcal mol-1. Bond additivity estimates suggest a somewhat higher value.9,31 Summary Oxygen-protonated trifluoromethanol can be prepared by the reaction of CF3I•+ with water. The CF3OH2+ ions are kinetically stable but are easily isomerized to (HF)CF2OH+ cluster ions in the presence of base catalysts such as SO2 and COS. The (HF)CF2OH+ cluster ion is characterized by a relatively weak HF binding energy of 14.4 ( 1.7 kcal mol-1. The threshold for water loss from CF3OH2+ has been determined to be 36.6 ( 2.1 kcal mol-1. The endothermicity for proton transfer from CF3OH2+ to CO was determined to be 9.2 ( 1.4 kcal mol-1, which leads to an oxygen proton affinity for CF3OH of 151.1 ( 1.7 kcal mol-1. When these measurements are combined Via the thermochemical cycle given in reactions 5-7, a value of -220.7 ( 3.2 kcal mol-1 is obtained for ∆Hf,298(CF3OH). Our measured value for ∆Hf,298(CF3OH) can be used to calculate that complete hydrolysis of CF3OH to CO2 and HF is exothermic by 10.9 kcal mol-1. These thermochemical data provide additional support for the suggestion that CF3OH molecules formed by tropospheric oxidation of hydrofluorocarbons are readily degraded by reactions with water droplets and other heterogeneous processes.37 Acknowledgment. We thank Drs. Greg Huey and Carleton Howard for helpful discussions. This work was funded by the National Science Foundation and the Department of Energy, Office of Basic Energy Science. Supporting Information Available: Optimized geometries for CF3OH2+ (1), HF+-CF2OH (2), and CF2O-H+-FH (3), provided in GAUSSIAN 92 Z-matrix format, and the computed harmonic vibrational frequencies for these ions (2 pages). Ordering information is given on any current masthead page. References and Notes (1) Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S.; Mills, M.; Howard, C. J.; Solomon, S. Science 1994, 263, 71. (2) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, O. J.; Sehested, J.; Debruyn, W. J.; Shorter, J. A. EnViron. Sci. Technol. 1994, 28, 320A. (3) (a) Ryan, K. R.; Plumb, I. C. J. Phys. Chem. 1982, 86, 4678. (b) Caralp, F.; Lesclaux, R.; Dognon, A. M. Chem. Phys. Lett. 1986, 129, 433.
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