Proton affinity and absolute heat of formation of trifluoromethanol

Article abstract of DOI:10.1021/jp961135n

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 CF₃ OH₂⁺ 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 → CF₃OH + HCO⁺. The measured enthalpy change, 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. The dissociation energy for the loss of water from CF₃OH₂⁺ was measured 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. 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.

Full text of DOI:10.1021/jp961135n

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
X
ReceiVed: April 18, 1996; In Final Form: July 2, 1996
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 CF
3
OH
2
2
in the flow tube of a flowing
+
afterglow triple-quadrupole instrument. The isomeric cluster ion, (HF)CF
OH , was shown to be more stable
+
+
+
than CF
3
OH
2
by the base-catalyzed conversion of CF
3 2 2 2
OH to (HF)CF OH using either SO or OCS as the
catalyst. The proton affinity of CF
3
+
OH at oxygen was determined from the enthalpy change for the endothermic
+
proton transfer reaction CF
kcal mol , was combined with the known value for the proton affinity of CO (141.9 kcal mol ) to yield a
3
OH
2
+ CO f CF OH + HCO . The measured enthalpy change, 9.2 ( 1.4
3
-
1
-1
-1
value for the oxygen proton affinity of CF OH of 151.1 ( 1.7 kcal mol . The dissociation energy for the
3
+
-1
loss of water from CF
3
OH
2
was measured to be 36.6 ( 2.1 kcal mol by energy-resolved collision-induced
dissociation. This value was used in a thermochemical cycle along with the measured proton affinity of
-1
3 3
CF OH to derive the gas-phase heat of formation of CF OH of -220.7 ( 3.2 kcal mol . This experimental
value is slightly lower than, but in good agreement with, the 298 K heat of formation of CF
predicted by high-level molecular orbital calculations.
3
OH that is
-
1 6
Introduction
as methanol (DH298(CH3O-H) ) 104.2 kcal mol ). Molec-
5
,7,8
ular orbital calculations at high levels of theory
predict that
The depletion of stratospheric ozone by chlorofluorocarbons
the O-H bond strength in CF3OH is equal to, or greater than,
(CFCs) has motivated the development of alternative compounds
-
1 6
that in water (DH298(HO-H) ) 119.3 kcal mol ), while bond-
additivity estimates suggest a somewhat lower value (DH298-
that are less toxic to the environment. In particular, hydro-
fluorocarbons (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, the mecha-
nisms for degradation of these compounds in the troposphere
are not fully understood. HFCs that contain a trifluoromethyl
1
9
(
CF3O-H) ) 109 kcal mol- ). A recent experimental
determination based on negative ion thermochemical cycles
-1 10
provides a value of 124.7 ( 3.6 kcal mol .
1
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 CF3-
OH, CF3O , and CF2O. At present, there are no experimental
values for the heats of formation of CF3OH and CF3O .
suggest that the
experimental value for ∆Hf,298(CF2O) of -152.7 ( 0.4 kcal
•
•
group such as CF3CF2H will produce CF3 in a series of reactions
•
2
•
that is initiated by hydroxyl radicals. Conversion of CF3 to
•
•
3
CF3O is believed to occur by reactions 1 and 2.
8
,11,12
Furthermore, recent MO calculations
1
3
•
•
CF + O + M f CF O + M
(1)
(2)
3
2
3
2
-1
-1
mol is too low by at least 7 kcal mol
.
•
•
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
CF O + NO f CF O + NO
2
3
2
3
•
While the processes leading to CF3O formation are well-
established, 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 CF3-
OH (reaction 3) or with NO to yield CF2O and FNO (reaction
•
4
6
).
4
).
+
+
•
•
CF OH + H f CF OH
∆H(5) ) -PA(CF OH) (5)
CF O + RH f CF OH + R
(3)
(4)
3
3
2
3
3
3
CF OH2+ f CF + + H O ∆H(6) ) DH(CF -OH ) (6)
+
3
•
CF O + NO f CF O + FNO
3
3
2
2
3
2
•
+
+
It was recently reported that CF3O will abstract a hydrogen
CF OH + H f CF + H O
3
3
2
5
atom from water under thermal conditions at room temperature,
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
∆
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.
†
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 © 1996 American Chemical Society

1
6436 J. Phys. Chem., Vol. 100, No. 40, 1996
Chyall and Squires
Experimental Section
calculated model function and the steeply rising portion of the
2
0
experimental appearance curve. 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
The flowing afterglow triple quadrupole apparatus used for
this study is described elsewhere.14 Oxygen-protonated tri-
fluoromethanol was prepared in the helium flow reactor at room
temperature by allowing the molecular ion of iodotrifluo-
2
1
broadening function developed by Chantry to account for
random thermal motion of the neutral target. The internal
energy distributions for the ionic and neutral reactants were
•+
romethane, 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 ana-
13
calculated either from the experimental vibrational frequencies
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
+
lyzer. 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 dissocia-
tions 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 half-
maximum (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.
2
2
data 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 endother-
micities, 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
2
3
stationary electron convention is employed in this work.
Ab initio molecular orbital calculations were performed on
an IBM RISC/6000 computer using the Gaussian 92 series of
2
4
programs. Geometries were optimized employing the ap-
propriate 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.
25
To be consistent with previous results from another laboratory,
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 hydro-
fluoric acid and carbonyl fluoride.
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
14
to be 24 ( 4 cm based on calibration experiments with the
+
+
reaction Ar + D2 f ArD + D, which has a well-known cross
1
5
section. 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 previously1 and are similar to the methods devel-
oped by Armentrout and co-workers for investigating the
Results and Discussion
+
Synthesis and Characterization of CF3OH2 . The rela-
tively 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 character-
ized 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.
4,16
translational energy dependence of ion/molecule reactions by
ion beam mass spectrometry.1
7,18
The product ion appearance
Morris et al. have shown that CF3OH2+ can be prepared in
a selected ion flow tube (SIFT) by the reaction of ionized water
1
9
curve is fit with the assumed model function shown in eq 8,
2
6
+
with CF3Cl. Although the formation of CF3OH2 is highly
+
n
efficient, this reaction also produces CF2Cl as a side product.
σ(E) ) σ ∑g (E + E - E ) /E
(8)
0
i
i
T
3
7
The Cl isotopomer of this ion (m/z 87) has the same nominal
+
mass as CF3OH2 , which would complicate reactivity studies
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
+
of CF3OH2 . We have avoided this problem by preparing CF3-
+
OH2 by the reaction between ionized iodotrifluoromethane and
water (reaction 9). This reaction is mechanistically similar to
CF I• + H O f CF OH + I
+
+
•
(9)
3
2
3
2
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 CF
3
OH
2
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.
Figure 2. Calculated structures for oxygen-protonated trifluorometha-
+
nol and the two (HF)CF
2
OH cluster ions. Structural details are
provided as supporting information.
Collisional activation of CF3OH2+ using argon target and
+
5
6
-10 eV (CM) collision energies results in CF3 formation (m/z
TABLE 1: Calculated MP2/6-311G(d)//HF/6-31G(d)
Electronic Energies, Zero-Point Energies, Temperature
Corrections, and Relative Enthalpies
+
9) 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
E
elec
,
ZPE,a
H
298 - H
0
,
rel,298
H ,
1
-1
-1
1
. Energy-resolved CID measurements indicate that the HF-
species
hartrees
kcal mol- kcal mol kcal mol
loss 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
+
CF
3
OH
2
(1)
-412.957 56
24.70
22.78
22.81
3.77
4.84
4.65
(0)
-8.2
-14.3
+
HF -CF OH (2) -412.969 29
2
+
CF O-H -FH (3) -412.978 66
2
+
the formation of CF2OH does not result from elimination of
a
Computed vibrational frequencies were scaled by 0.893.
SCHEME 1
+
HF from CF3OH2 , because such a reaction would require a
high-energy, kinetically disfavored, four-centered transition
2
5
+
+
state. 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 pos-
+
sibility 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 CF3-
+
OH2 potential energy surface at the MP2/6-311G(d,p)//HF/6-
3
1G(d) level of theory.25 Their calculations predict that the
transition state for loss of HF from oxygen-protonated trifluo-
romethanol 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
fluorine, respectively.25 This suggests that a base with a proton
affinity near 150.9 kcal mol- should be able to isomerize
oxygen-protonated trifluoromethanol to a more stable (HF)CF2-
1
+
27
+
OH cluster ion (Scheme 1). We have verified this reactivity
loss. Additional calculations of the CF3OH2 potential energy
-1 23
experimentally when either SO2 (PA ) 152.1 kcal mol ) or
surface by Grandinetti et al. have identified a lower energy
structure corresponding to an ion with an H-F bond (Figure 2,
-1 23
OCS (PA ) 150.7 kcal mol ) is employed as the base
catalyst.
2
). Although 2 can be viewed as fluorine-protonated trifluo-
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
romethanol, 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 CF2-
+
+
CF2OH as well as CF3 product ions. The abundance of CF2-
+
+
+
OH relative to CF was observed to increase with increasing
OH is largely electrostatic in nature, and the computed binding
3
-1
concentrations of the base catalyst in the flow tube (Figure 3
energy is quite low, 14.4 kcal mol . 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 oxygen-
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
+
bearing proton of CF2OH (Figure 2, 3). Ion 3 is computed to
CF OH2+ + SO f (HF)CF OH + SO
+
(10)
+
be lower in energy than the oxygen-protonated form, CF3OH2
3
2
2
2
-
1
(1), by 14.3 kcal mol (Table 1).
Grandinetti et al. also compute values of 150.9 and 160.1
reactant ion could not be achieved due to the extensive
attenuation of the reactant ion signal intensity that results from
kcal mol- for the proton affinity of CF3OH at oxygen and
1

1
6438 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)CF
2
OH and CF
3
OH
2
∆
H
rxn,c
-1
reaction
E
T
,a eV
nb
kcal mol
+
+
(
CF
CF
HF)CF
2
OH f CF
2
OH + HF 0.62 ( 0.07 1.2 ( 0.2 14.4 ( 1.7
2
+
+
3
OH
OH
2
f CF
+ CO f HCO +
3
+ H
O
1.53 ( 0.08 1.6 ( 0.1 36.6 ( 2.1
0.42 ( 0.05 1.2 ( 0.3 9.2 ( 1.4
+
+
3
2
3
CF OH
a
Average threshold energy (0 K). b Average fitting parameter from
eq 8. c Reaction enthalpy (298 K) derived from E
.
T
Figure 3. Product ion spectrum obtained from collision-induced
dissociation of the ions with m/z 87 as function of increasing
concentrations of SO in the flow tube.
2
+
Figure 5. Cross section for collision-induced dissociation of CF
3
from
+
3 2
CF OH 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.
Figure 4. Cross section for collision-induced dissociation of CF
from (HF)CF
2
OH+
2
OH 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.
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.
termolecular association reactions and other competing pro-
-1 23
cesses. Addition of excess water (PA ) 166.5 kcal mol )
to the flow tube during the synthesis of CF3OH2+ leads to
diminished intensities of this ion with a concomitant increase
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
+
+
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 concentra-
tions of water in the flow tube. Therefore, while water is able
2
6
Å 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
(
+
to deprotonate CF3OH2 , it apparently does not isomerize this
described previously. We take the measured activation energy
to be equal to the thermochemical bond dissociation energy since
+
ion to (HF)CF2OH .
In separate experiments, (HF)CF2OH+ cluster ions were
prepared in the flow tube by allowing a 0.5% mixture of
+
the reverse reaction, addition of CF3 to H2O, occurs without a
29
barrier. The 0 K energy threshold and the appropriate thermal
+
hydrofluoric acid in helium to react with CF2OH , which in
corrections for reaction 6 provide a 298 K dissociation enthalpy,
turn was prepared by reaction of CF2O with the ions produced
+
-1
DH(CF3 -OH2), of 36.6 ( 2.1 kcal mol (Table 2). This
+
upon electron ionization of methane. CID of (HF)CF2OH with
value was used with the pertinent reference data given in Table
to derive ∆Hf,298(CF3OH2 ) ) -6.1 ( 2.4 kcal mol , where
+
neon target produces CF2OH (reaction 11) as the sole product
+
-1
3
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)CF OH f CF OH + HF
(11)
2
2
+
2
with a maximum cross section of 15 Å 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. 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 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.
2
8
+
+
+
∆
Hf,298((HF)CF OH ) ) ∆H (CF O) + ∆H (H ) +
2 f,298 2 f,298
∆
Hf,298(HF) - PA(CF O) - ∆H(11) (12)
2
-145.3 kcal mol- derived from MO theory, the experimental
1
8,33
-1
value for PA(CF2O) ) 160.5 kcal mol , and the HF binding
energy of 14.4 kcal mol obtained in this study (Table 2) lead
to ∆Hf,298((HF)CF2OH ) ) -19.6 kcal mol . This is 13.5
kcal mol lower than the measured heat of formation for CF3-
OH2 , which confirms the experimental observation that CF3-
OH2 is thermodynamically unstable with respect to a lower
-1
-1
+
-1
(
+
-1
+
+
+
+
+
energy isomer (reaction 13). In addition, this energy difference
is close to the theoretically predicted difference in stability
2
5

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
H+
∆Hf,298
ref
365.7
-57.8
23
23
23
H
2
O
HF
-65.1 ( 0.2
88.3 ( 1.6
-6.1 ( 2.4
-19.6
+
CF
CF
3
23,30
this work
this work
this work
+
3
OH
2
(
CF
HF)CF
2
OH+
3
OH
-220.7 ( 3.2
-
-
-
-
213.5 ( 1.5
215 ( 1
31
9
11
7
13
8
11
217.7 ( 2.0
217.7
2
CF O
-152.7 ( 0.4
145.3 ( 1.7
144.8 ( 1.0
-
-
species
PA
ref
CO
OCS
141.9 ( 1.0
150.7
23
23
SO
2
152.1
166.5
23
23
this work
25
23
32
H
2
O
CF
3
2
OHb
151.1 ( 1.7
150.9
CF
O
160.5 ( 2.0
159.0 ( 2.0
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).
Figure 6. (a) Cross sections for the products obtained upon reaction
+
of CF
3
OH
2
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.
CF OH2 f (HF)CF OH+ ∆Hrxn ) -13.5 kcal mol
+
-1
3
2
(
13)
The Proton Affinity and Heat of Formation of CF3OH.
In principle, the proton affinity of trifluoromethanol can be
2
3,34
measured by acid-base bracketing experiments.
However,
the aforementioned isomerization of CF3OH2 to (HF)CF2OH+
would complicate the interpretation of these measurements.
Misleading results may be obtained for reactions involving CF3-
+
+
OH2 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. 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
Figure 7. Schematic summary of the relative enthalpies (kcal mol-1
)
+
for the reaction of CF
3
OH
2
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)CF2-
3
5
+
+
CF OH + CO f HCO + CF OH
(14)
3
2
3
+
+
was selected because the proton affinity of CO is accurately
OH that contaminate the beam of CF3OH2 reactant ions. A
2
3,36
+
known.
In addition, the difference in the proton affinities
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).
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
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.
+
of the reaction between CF3OH2 and CO are shown in Figure
+
6
a. The cross section for the proton-transfer product, HCO ,
reaches a maximum at 1.5 eV CM (Figure 6b); at higher
+
-1
energies, the CID channel that generates CF3 dominates the
A temperature correction of -0.36 kcal mol provides a 298

1
6440 J. Phys. Chem., Vol. 100, No. 40, 1996
Chyall and Squires
-1
K reaction enthalpy for reaction 14 of 9.2 ( 1.4 kcal mol .
(c) Bevilacqua, T. J.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1993,
97, 3750. (d) Turnipseed, A. A.; Barone, S. B.; Ravishankara, A. R. J.
Phys. Chem. 1994, 98, 4594.
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
(
4) (a) Barone, S. B.; Turnipseed, A. A.; Ravishankara, A. R. J. Phys.
-1
mol (Table 3). The enthalpy change for loss of H2O from
Chem. 1994, 98, 4602. (b) Jensen, N. R.; Hanson, D. R.; Howard, C. J. J.
Phys. Chem. 1994, 98, 8574. (c) Chen, J.; Zhu, T.; Niki, H.; Mains, G. J.
Geophys. Res. Lett. 1992, 19, 2215.
CF3OH2+ (reaction 6) can then be combined with this value
and the supporting thermochemical data (Table 3) to derive
(
5) Wallington, T. J.; Hurley, M. D.; Schneider, W. F.; Sehested, J.;
Nielsen, O. J. J. Phys. Chem. 1993, 97, 7606.
6) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994,
98, 2744.
-
1
∆
Hf,298(CF3OH) ) -220.7 ( 3.2 kcal mol (eq 15), where
(
+
∆
Hf,298(CF OH) ) ∆H (CF ) + ∆H (H O) +
3
f,298
3
f,298
2
(
(
(
7) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1993, 97, 12783.
8) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1994, 98, 7448.
9) Benson, S. W. J. Phys. Chem. 1994, 98, 2216.
+
PA(CF OH) - ∆H (H ) - ∆H(6) (15)
3
f,298
(
10) Huey, L. G.; Dunlea, E. J.; Howard, C. J. J. Phys. Chem. 1996,
100, 6504.
(11) Montgomery, J. A., Jr.; Michels, H. H.; Francisco, J. S. Chem. Phys.
Lett. 1994, 220, 391.
12) Schneider, W. F.; Wallington, T. J.; Hurley, M. D.; Sehested, J.;
Nielsen, O. J. J. Phys. Chem. 1994, 98, 2217.
13) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R.; Frurip, D. J.;
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
(
McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data, Suppl. 1, 1985,
14.
(14) Marinelli, P. J.; Paulino, J. A.; Sunderlin, L. S.; Wenthold, P. G.;
Poutsma, J. C.; Squires, R. R. Int. J. Mass Spectrom. Ion Processes 1994,
-1
∆
Hf,298(CF3OH) ) -217.7 ( 2 kcal mol , where the estimated
1
1
30, 89.
uncertainty represents the average error in the computed heats
(15) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1985, 83, 166.
(16) Sunderlin, L. S.; Wang, D.; Squires, R. R. J. Am. Chem. Soc. 1993,
15, 12060.
7
of formation of a test set of molecules. Related calculations
at a slightly lower level of theory also provide ∆Hf,298(CF3OH)
-
1
(17) Armentrout, P. B. In AdVances in Gas Phase Ion Chemistry; Adams,
N. G., Babcock, L. M., Eds.; JAI: Greenwich, CT, 1992; Vol. 1.
)
-217.7 kcal mol . Bond additivity estimates suggest a
9,31
somewhat higher value.
(
18) Armentrout, P. B.; Hales, D. A.; Lian, L. In AdVances in Metal
and Semiconductor Clusters; Duncan, M. A., Eds.; JAI: Greenwich, CT,
Summary
1994; Vol. 2.
(19) Chesnavich, W. J.; Bowers, M. T. J. Phys. Chem. 1979, 83, 900.
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
(20) Data analysis was carried out using the CRUNCH program
•
+
+
developed by P. B. Armentrout and co-workers.
(21) Chantry, P. J. J. Chem. Phys. 1971, 55, 2746.
+
(22) (a) Schultz, R. H.; Crellin, K. C.; Armentrout, P. B. J. Am. Chem.
Soc. 1991, 113, 8590. (b) Khan, F. A.; Clemmer, D. E.; Schultz, R. H.;
Armentrout, P. B. J. Phys. Chem. 1993, 97, 7978.
(23) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin,
R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data, Suppl. 1 1988, 17. Some
data were taken from the NIST Positive Ion Energetics Database, Version
+
(HF)CF2OH cluster ion is characterized by a relatively weak
-
1
HF binding energy of 14.4 ( 1.7 kcal mol . The threshold
for water loss from CF3OH2 has been determined to be 36.6
+
-
1
(
2.1 kcal mol . The endothermicity for proton transfer from
CF3OH2 to CO was determined to be 9.2 ( 1.4 kcal mol ,
2
.0, NIST Standard Reference Database 19A, July 1993.
24) Gaussian 92, Revision D.2: Frisch, M. J.; Trucks, G. W.; Head-
+
-1
(
which leads to an oxygen proton affinity for CF3OH of 151.1
Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B.
G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres,
J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox,
D. J. Defrees, D. J.; Baker, J.; Stewart, J. J. P; Pople, J. A. Gaussian, Inc.,
Pittsburgh, PA, 1992.
(25) Grandinetti, F.; Occhiucci, G.; Crestoni, M. E.; Fornarini, S.;
Speranza, M. Int. J. Mass Spectrom. Ion Processes 1993, 127, 123. For an
earlier theoretical study at the HF/4-31G level of theory, see: Grein, F.
Can. J. Chem. 1985, 63, 1988.
-
1
(
1.7 kcal mol . When these measurements are combined
Via the thermochemical cycle given in reactions 5-7, a value
-
1
of -220.7 ( 3.2 kcal mol 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 . These thermochemical data
provide additional support for the suggestion that CF3OH
molecules formed by tropospheric oxidation of hydrofluoro-
-
1
(26) Morris, R. A.; Viggiano, A. A.; Van Doren, J. M.; Paulson, J. F.
J. Phys. Chem. 1992, 96, 3051.
(27) Bohme, D. K. Int. J. Mass Spectrom. Ion Processes 1992, 115, 95.
carbons are readily degraded by reactions with water droplets
and other heterogeneous processes.37
(28) The energy-resolved cross section obtained with argon target is
similar in appearance but displays somewhat more tailing in the threshold
region, which makes analysis more difficult. For this reason, the CID
threshold was determined from the neon experiments.
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.
(29) (a) Nguyen, M. T.; Bouchoux, G. J. Phys. Chem. 1996, 100, 2089.
b) McEwan, M. J.; Fairley, D. A.; Scott, G. B. I.; Anicich, V. G. J. Phys.
(
Chem. 1996, 100, 4032. (c) Hiraoka, K.; Nasu, M.; Fugimaki, S.; Ignacio,
E. W.; Yamabe, S. J. Phys. Chem. 1996, 100, 5245.
(
30) Fisher, E. R.; Armentrout, P. B. Int. J. Mass Spectrom. Ion
Supporting Information Available: Optimized geometries
Processes 1990, 101, R1.
(31) Batt, L.; Walsh, R. Int. J. Chem. Kinet. 1982, 14, 933.
for CF3OH2+ (1), HF -CF2OH (2), and CF2O-H -FH (3),
+
+
(
17.
32) Koppel, I. A.; Anvia, F.; Taft, R. W. J. Phys. Org. Chem. 1994, 7,
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.
7
(
33) Preliminary measurements in our laboratory give a value for ∆Hf,298-
2
(CF O) that is in agreement with the theoretical values given in Table 3.
Chyall, L. J.; Squires, R. R. Unpublished results.
(
34) Munson, M. J. B. J. Am. Chem. Soc. 1965, 87, 2332.
References and Notes
(35) Chyall, L. J.; Squires, R. R. Int. J. Mass Spectrom. Ion Processes
(
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,
8, 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.
1995, 149/150, 257.
(36) Szulejko, J. E.; McMahon, T. B. J. Am. Chem. Soc. 1993, 115,
7839.
(
(37) Lovejoy, E. R.; Huey, L. G.; Hanson, D. R. J. Geophys. Res. 1995,
2
1
00, 18775.
(
JP961135N