Reactions of H3O+ and H2O+ with several fully halogenated bromomethanes

Article abstract of DOI:10.1021/jp9719624

The bimolecular rate coefficients and ion products for the reactions of H₃O⁺ and H₂O⁺ with the bromine-containing molecules CF₃Br, CF₂Br₂, CF₂BrCl, CFBr₃, CFBr₂Cl, and CBrCl₃ at 300 K are reported. With the exception of the reactions of H₃O⁺ with CF₃Br and CF₂BrCl, the rate coefficients are near the collisional values (k ≈ 10^-9 cm³ molecule^-1 s^-1). The most efficient exit pathway for the majority of the H₃O⁺ reactions is the formation of a trihalomethyl cation, together with water and a hydrogen halide as the neutral products. In each case, more than one trihalomethyl cation can be formed. The branching ratios are largest for the products resulting from the breaking of a C-F bond. This is attributed to the high bond strength of HF relative to HCl and HBr. Similarly for CBrCl₃, the major product cation is CCl₂Br⁺. The H₂O⁺ reactions are found to proceed predominantly via charge transfer. For the reaction of H₂O⁺ with CF₃Br there is clear evidence for intimate reaction pathways in which bonds are broken and formed.

Full text of DOI:10.1021/jp9719624

J. Phys. Chem. A 1997, 101, 8489-8495
8489
Reactions of H3O+ and H2O+ with Several Fully Halogenated Bromomethanes
R. D. Thomas,† R. A. Kennedy,*,‡ C. A. Mayhew,† and P. Watts§
Chemical Physics Laboratory, School of Physics and Space Research, UniVersity of Birmingham, School of
Chemistry, UniVersity of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., and PLSD, CBD,
Porton Down, Salisbury, Wiltshire SP4 0JQ, U.K.
ReceiVed: June 16, 1997X
The bimolecular rate coefficients and ion products for the reactions of H3O+ and H2O+ with the bromine-
containing molecules CF3Br, CF2Br2, CF2BrCl, CFBr3, CFBr2Cl, and CBrCl3 at 300 K are reported. With
the exception of the reactions of H3O+ with CF3Br and CF2BrCl, the rate coefficients are near the collisional
values (k ≈ 10-9 cm3 molecule-1 s-1). The most efficient exit pathway for the majority of the H3O+ reactions
is the formation of a trihalomethyl cation, together with water and a hydrogen halide as the neutral products.
In each case, more than one trihalomethyl cation can be formed. The branching ratios are largest for the
products resulting from the breaking of a C-F bond. This is attributed to the high bond strength of HF
relative to HCl and HBr. Similarly for CBrCl3, the major product cation is CCl2Br+. The H2O+ reactions
are found to proceed predominantly via charge transfer. For the reaction of H2O+ with CF3Br there is clear
evidence for intimate reaction pathways in which bonds are broken and formed.
Introduction
H3O+ ion requires an ion-molecule complex to be first formed.
Charge transfer does not require such a complex and can occur
The monitoring of atmospheric concentrations of chlorofluo-
rocarbons (CFCs) and their replacements, the perfluorocarbons
(PFCs), the hydrofluorocarbons (HFCs), and the hydrochloro-
fluorocarbons (HCFCs) has in the past few years taken on great
importance. This is due to the adverse effects some of these
molecules have in the atmosphere. Depletion of the ozone layer
is intimately linked to increased atmospheric concentrations of
CFCs. The PFCs, HFCs, and HCFCs are significant greenhouse
gases. Gaseous ions can be used as powerful probes for the
purposes of monitoring these halogenated compounds. This in
part, together with growing interest in atmospheric ion chem-
istry, has led to much attention being paid to the ion-molecule
reactions of CFCs, PFCs, HFCs, and HCFCs with various
cations.1-5 Little attention has been given to their brominated
equivalents, yet the presence of bromine in the stratosphere can,
like chlorine, lead to the catalytic destruction of stratospheric
ozone. Although bromine compounds are present in the
atmosphere in much smaller quantities than their CFC equiva-
lents, bromine (atom for atom) leads to a much higher
destruction of ozone than chlorine.6,7
In this article we present the first results from a much larger
study of the reactions of a series of cations with a large number
of bromocarbons. Here, we concentrate on the reactions of
H3O+ and H2O+ with the bromomethanes CF3Br, CF2Br2, CF2-
BrCl, CFBr3, CFBr2Cl, and CBrCl3. The interest in the H3O+
and H2O+ reactant ions is twofold. First, H3O+ and H2O+ are
important atmospheric cations. Second, the reaction chemistry
involved will be very different for the two ions, and it was
primarily for this reason that the study was undertaken. H2O+
can react by charge transfer nondissociatively (H2O + + BC f
BC+ + H2O) and/or dissociatively (H2O + + BC f B+ + C +
H2O). H3O+ cannot react via either of these processes because
its recombination energy is far below the ionization potentials
of any of the neutral reagents. Therefore, a reaction with an
by a long-range process.
To our knowledge, of the ion-molecule reactions reported
here, only one has been previously investigated and reported.
Morris et al.3 have studied the reaction of H2O+ with CF3Br,
and our results are in good agreement with their values of the
reaction rate coefficient and branching ratios.
Experimental Details
This study was carried out using a selected ion flow tube
(SIFT) apparatus. The SIFT technique and the analysis of the
data to obtain product ion distributions and reaction rate
coefficients have been described in detail by Smith and Adams.8
Therefore, only a few points pertinent to the present study will
be mentioned here. Ions were generated in an enclosed high-
pressure electron impact ion source. H2O was used to create
both the ions: H2O+ via primary ionization and H3O+ via proton
transfer from H2O+ to H2O. The reactant ions were mass
selected by a quadrupole mass filter and then injected into a
fast flowing (∼150 Torr L s-1) helium buffer gas (0.5 Torr)
where they were collisionally thermalized to 300 K. Measured
quantities of the neutral reactant of interest were introduced into
the carrier gas/reactant ion flow downstream from the ion inlet.
All the reactant samples, CF3Br, CF2Br2, CF2BrCl, CFBr3,
CFBr2Cl, and CBrCl3, were obtained commercially. CF2Br2
and CFBr2Cl have stated purities of 97% and 90%, respectively.
CF3Br, CFBr3, and CBrCl3 have stated purities of 99%. The
purity of the sample of CF2BrCl was not reported by the
supplier. CF2Br2, CFBr3, CFBr2Cl, and CBrCl3 are liquids at
room temperature and therefore prior to use were subjected to
several freeze-pump-thaw cycles to remove dissolved gases.
Parent and product ions enter a second quadrupole mass filter
where they are detected by a channeltron electron multiplier.
The product ion distribution and reaction rate coefficient are
obtained by relating the neutral flow rate to the product ion
and reactant ion count rates, respectively. The accuracy of the
measured reaction rate coefficients is (20%. The mass
discrimination of the detection system was taken into account
in the usual way.9
* Corresponding author. E-mail: R.A.Kennedy@bham.ac.uk. Fax: 0121
414 3722.
† School of Physics and Space Research.
‡ School of Chemistry.
§ Porton Down.
A key aspect of the SIFT technique is the injection of mass-
selected ions of a given single m/e ratio into the flow tube. This
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5639(97)01962-2 CCC: $14.00 © 1997 American Chemical Society

8490 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Thomas et al.
TABLE 1: Total Reaction Rate Coefficients, the Product Ions, and the Branching Percentages for the Reactions of H3O+ and
H2O+ with CF3Br, CF2Br2, CF2BrCl, CFBr3, CFBr2Cl, and CBrCl3 at 300 K
CF3Br
CF2Br2
CF2BrCl
CFBr3
CFBr2Cl
CBrCl3
(IP ) 11.40)a
(IP ) 11.07)a
(IP ) 11.21)a,b
(IP ) 10.67)a
(IP ) 10.87)a,c
(IP ) 10.60)a
Reaction of H3O+
0.254
d
kexp
1.23
1.90
1.78
1.64
1.84
1.99
kcd,e
1.72
product ionsf
CFBr2+ (73)
CFBr2+‚H2O (23)
CF2Br+‚H2O (4)
CFBrCl+ (55)
CBr3+ (90)
CBr2Cl+ (96)
CFBr2+ (4)
CCl2Br+ (70)
CCl3+ (30)
CFBrCl+‚H2O (42)
CFBr2+ (10)
CF2BrCl‚H3O+ (3)
Reaction of H2O+
1.72
d
kexp
1.56
1.76
1.69
1.78
1.74
2.06
2.05
kcd,e
1.95
product ionsf
CF3+ (71)
CF2Br+ (16)
CF3Br+ (8)
CF2OH+ (5)
CF2Br+ (100)
CF2Cl+ (86)
CFBrCl+ (8)
CF2Br+ (6)
CFBr2+ (100)
CFBrCl+ (97)
CFBr2+ (3)
CCl3+ (76)
CCl2Br+ (24)
a Ionization potentials of the neutral molecules are in eV. b The value for the ionization potential of CF2BrCl has been taken from the paper by
Wang and Leroi.14 c The ionization potential for CFBr2Cl was estimated from that of CFBr3 (10.67 eV). Replacing Br by Cl increases the ionization
potential by ∼0.20 eV, a value obtained by comparing the ionization potentials of the pairs of molecules CF2Br2/CF2BrCl (0.23 eV), CH2Br2/
CH2BrCl (0.27 eV), and CHBr3/CHBr2Cl (0.11 eV). d The reaction rate coefficients (both calculated and experimental) are given in units of 10-9
cm3 molecule-1 s-1. The measured rate coefficients and the product ion distributions are considered to be accurate to (20%. e The calculated 300
K collisional reaction rate coefficient is given for those molecules with known polarizabilities and dipole moments. f Branching percentages are in
parentheses.
was possible for H3O+ but not for H2O+. Significant quantities
of H3O+ were produced under the best ion source conditions
for H2O+ production, and the injection quadrupole mass
resolution was not sufficient to reject all the H3O+. A smaller
additional contribution to the H3O+ concentration in the flow
tube resulted from the reaction of H2O+ with trace water in the
helium buffer gas. This was minimized by passing the helium
gas through a liquid nitrogen trap. In practice, when tuned to
inject H2O+, a mixture of 70% H2O+ and 30% H3O+ was
recorded using the second quadrupole mass filter and channel-
tron electron multiplier. By use of the SIFT results for the
reactions of H3O+, the ion products of the reactions of H2O+
were extracted from the data.
suggesting that the proton affinities of the molecules are all less
than that of H2O (697 kJ mol-1). Thermodynamic data are
available for only one of the protonated moleculessCF3BrH+,
and for CF3Br the proton-transfer reaction is endothermic:
H3O+ + CF3Br f CF3BrH+ + H2O
∆H ) +112 kJ mol-1 (1)
(ii) Charge Transfer.
H3O+ + M f M+ + H + H2O
The recombination energy of H3O+ (+ e- f H2O + H, 615 kJ
mol-1 ≡ 6.37 eV) is much less than the ionization potential of
any of the reactant molecules of this study, and therefore, charge
transfer is impossible.
Results and Discussion
The measured rate coefficients (kexp), the product ions, and
their branching ratios for all the reactions included in this study
are summarized in Table 1. The reactant molecules are listed
at the top of the table in order of decreasing fluorination (CF3-
Br f CBrCl3), and their ionization potentials are given in
brackets below each molecule. Also included in the table are
the collisional rate coefficients (kc) when calculable. These have
been determined using the trajectory method.10 It is clear from
Table 1 that, with the exception of the reactions of H3O+ with
CF3Br and CF2BrCl, the measured rate coefficients are close
to the expected collisional values.
For the analysis of the reaction pathways, thermochemical
data have been taken from Lias et al.11 and Lide,12 unless
indicated otherwise in the text. Enthalpies of formation of some
of the observed ion products have not been reported. The
observation of a reaction pathway to one of these ions implies
that the enthalpy of the reaction is less than 0, and so this allows
the determination of an upper limit to its enthalpy of formation.
These upper limits can then be used to place bounds on the
enthalpies of other reactions.
(iii) Attack of H3O+ on a Halogen Atom Followed by the
Release of Water and the Formation of a Hydrogen Halide.
H3O+ + M f [M-X]+ + HX + H2O
This is the only pathway observed for CFBr3, CFBr2Cl, and
CBrCl3 and is the predominant pathway for CF2Br2 and CF2-
BrCl. Only CF3Br is found to be unreactive with H3O+. In
Table 2 we list all the channels of this type, together with the
reaction enthalpies. Two very different values for the enthalpy
of formation of CF2Br+ have been reported. A photoionization
mass spectrometric (PIMS) study of CF3Br by Clay et al.13
yielded a value of ∆fH(CF2Br+) ) 640 kJ mol-1 at 0 K, while
PIMS studies by Wang and Leroi lead to an upper limit of
∆fH(CF2Br+) e 544 kJ mol-1 14
There is no obvious explana-
.
tion for the large difference between these values. The table
contains entries derived from both values. In the construction
of Table 2 an estimate for the heat of formation of CFBr2Cl
(-181 ( 10 kJ mol-1) has been taken from an earlier study of
ours.15
For simplicity, in the following discussion “M” will be used
to indicate the halogenated methane molecule.
+
Reactions with H3O+. Four reaction pathways need to be
considered.
With the exception of the formation of CBr3 from CFBr3,
the existing thermodynamic data are consistent with our
observations. The enthalpy of formation of CBr3+ reported by
Lias et al., ∆fH(CBr3+) ) 1000 kJ mol-1, appears to be too
high. Our results require ∆fH(CBr3+) < 869 kJ mol-1. This
and other bounds established from the identification of the
product ions are collected in Table 3.
(i) Proton Transfer.
H3O+ + M f MH+ + H2O
This was not observed with any of the molecules of this study,

Reactions of H3O+ and H2O+
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8491
TABLE 2: Ion and Neutral Products and Enthalpies for the Reaction of H3O+ with the Bromomethanes Presented in This
Study
ion products
observed?
bromomethane
CF2Br2
products
∆H/kJ mol-1
CFBr2+ + H2O + HF
CF2Br+ + H2O + HBr
CFBrCl+ + H2O + HF
CF2Br+ + H2O + HCl
CF2Cl+ + H2O + HBr
CBr3+ + H2O + HF
∆fH(CFBr2+) -726
150 or e 54
yes
no
yes
no
CF2BrCl
∆fH(CFBrCl+) - 667
153 or e 57
e75a
no
CFBr3
∆fH(CBr3+)b - 869
∆fH(CFBr2+) - 633
∆fH(CBr2Cl+) - 924
∆fH(CFBr2+) - 744
∆fH(CFBrCl+) - 688
∆fH(CBrCl2+) - 882
5 ( 7
yes
yes
yes
yes
no
yes
yes
CFBr2+ + H2O + HBr
CBr2Cl+ + H2O + HF
CFBr2+ + H2O + HCl
CFBrCl+ + H2O + HBr
CBrCl2+ + H2O + HCl
CCl3+ c + H2O + HBr
CFBr2Cl
CBrCl3
a ∆fH(CF2Cl+) e 506 kJ mol-1, taken from Morris et al.,3 is used in the calculation. b CBr3+ is observed with a large branching ratio. This is only
possible if the heat of formation of CBr3+ reported by Lias et al.11 (∆fH(CBr3+) ) 1000 kJ mol-1) is incorrect (and it should be noted that the heat
reported by Lias et al. has been estimated). c The heat of formation of CCl3+ reported by Lias et al.11 (∆fH(CCl3+) ≈ 831 kJ mol-1) is between 824
and 837 kJ mol-1 owing to the experimental observation of CCl3+ from the reaction H3O+ + CFCl3 (f CCl3+ + H2O + HF) and the nonobservation
+
+
+
of the reaction sec-C3H7 + CFCl3 (f CCl3 + C3H7F). The observation of CCl3 from the reaction of H3O+ + CCl3Br would seem to indicate
that ∆fH(CCl3+) lies closer to 824 kJ mol-1 than to 837 kJ mol-1
.
and from H3O+ + CBrCl3:
TABLE 3: Thermodynamic Upper Limits Established from
the Thermochemistry of the Reported Reactions by
Requiring That for Any Observed Reaction ∆H < 0
+
∆fH(CBrCl2 ) < 882 kJ mol-1
species
∆H(300 K)/kJ mol-1
The inequalities can also be expressed in terms of bond
strengths. Consider, for example, the reaction between H3O+
and CFBr3:
+
CBr3
e869
e633
e667
e924
e882
+
CFBr2
CFBrCl+
CBr2Cl+
CCl2Br+
+
H3O+ + CFBr3 f CBr3 + H2O + HF
(4a)
(4b)
The failure of H3O+ to react with CF3Br is seen to be due to
the endothermicity of the dissociative pathways leading to both
CF2Br+ and CF3+. For example,
f CFBr2+ + H2O + HBr
+
H3O+ + CF3Br f CF2Br+ + H2O + HF
∆H ) +185 or e 89 kJ mol-1 (2)
∆H(4b) - ∆H(4a) ) D(CBr2 -Br) +
+
D(H-F) - D(CBr2 -F) - D(H-Br)
Channel 4a will be preferred if
is the least endothermic dissociative pathway. The reactions
of H3O+ with CF2Br2, CFBr3, CFBr2Cl, and CBrCl3 have rate
coefficients close to the collisional values, so for these systems
there is evidently no significant potential barrier on the exit
pathway to the trihalomethyl cations. The rate coefficient for
H3O+ reacting with CF2BrCl is at least a factor of 5 below any
reasonable estimate of the collisional rate coefficient; in this
case there must be some barrier to the exothermic pathway:
+
+
D(H-F) - D(H-Br) > D(CBr2 -F) - D(CBr2 -Br)
The difference between the HF and HBr bond strengths is 204
kJ mol-1. Our observation that channel 4a is preferred implies
that
+
+
D(CBr2 -F) - D(CBr2 -Br) < 204 kJ mol-1
H3O+ + CF2BrCl f CFBrCl+ + H2O + HF
∆H ) ∆fH(CFBrCl+) - 667 kJ mol-1 (3)
and that the greater difference between the HF and HBr bond
strengths compared to the CF and CBr bonds in CBr2F+ and
CBr3+, respectively, is the enthalpic driving force that distin-
guishes the two channels.
The same analysis can be applied to the product branching
ratios for the reaction of H3O+ with CFBr2Cl and with CBrCl3,
giving
Only one trihalomethyl cation product is observed from CF2-
Br2 (CFBr2+) and CF2BrCl (CFBrCl+), and this is in accord
with the available thermochemical data. Where more than one
trihalomethyl cation product is observed, the pathways to the
different trihalomethyl cations will be similar. On this basis,
we suggest that the branching ratios will correlate with the
relative exothermicities of the channels. This cannot be tested
with the available thermochemical data. However, if this
assertion is correct, then the following inequalities can be
obtained from our observations.
+
+
D(CBr2 -F) - D(CBr2 -Cl) < 139 kJ mol-1
D(CBrCl+-F) - D(CBrCl+-Br) < 204 kJ mol-1
D(CFBr+-Cl) - D(CFBr+-Br) < 65 kJ mol-1
from H3O+ + CFBr3:
and
+
+
∆fH(CFBr2 ) > ∆fH(CBr3 ) - 236 kJ mol-1
+
+
D(CCl2 -Cl) - D(CCl2 -Br) < 65 kJ mol-1
from H3O+ + CFBr2Cl:
(iV) Other Pathways. Two additional products are observed
+
∆fH(CFBr2 ) > ∆fH(CBr2Cl+) - 180 kJ mol-1
from the reaction of H3O+ with CF2Br2:

8492 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Thomas et al.
+
H3O+ + CF2Br2 f CFBr2 ‚H2O + HF
(5a)
(5b)
the helium buffer gas. The resulting association product CF2-
BrCl‚H3O+ is a minor product, and no attempt was made to
investigate the dependence of the association rate on the helium
pressure. With CF3Br no bimolecular products were observed,
and attempts to detect CF3Br‚H3O+ were unsuccessful. Both
kinetic and thermodynamic reasons can be advanced to explain
the failure to observe CF3Br‚H3O+. A kinetic argument is that
the initially formed collision complex may break up to CF3Br
and H3O+ at a rate much faster than it can be stabilized by
collisions with He at 0.5 Torr. Thermodynamically, although
the formation of CF3Br‚H3O+ from CF3Br and H3O+ is
exothermic, the process will be opposed by the large decrease
in entropy accompanying the association reaction. As a result,
the equilibrium (maximum) concentration of CF3Br‚H3O+ may
be too small to be detectable. However, it is curious that
association was observed between CF2BrCl and H3O+, but under
the same conditions CF3Br is not observed to associate with
H3O+.
f CF2Br+‚H2O + HBr
The observation of these ion products supports the proposal that
the reactions of H3O+ with the bromomethanes proceed via an
ion-molecule complex. In the bimolecular pathways, reactions
5a and 5b, the H2O is not released as the reaction proceeds
from the initially formed ion-molecule complex to the products.
The binding between the cation and the H2O increases the
exothermicity of the reactions:
∆H(5a) )
+
+
∆fH(CFBr2 ) - D(CFBr2 ‚H2O) - 726 kJ mol-1
∆H(5b) ) -D(CF2Br+‚H2O) + (150 or < 54) kJ mol-1
The observation of CF2Br+‚H2O is particularly interesting, since
bare CF2Br+ is not a product of the reaction of H3O+ with CF2-
Br2. The binding of the product CF2Br+ with H2O releases
sufficient energy to make the pathway to CF2Br+‚H2O exo-
thermic, while the pathway to CF2Br+ is endothermic:
Reactions with H2O+. All the molecules reacted with H2O+
with rate coefficients at or close to the collisional values. The
recombination energy of H2O+ (1217 kJ mol-1 ≡ 12.61 eV) is
far higher than that of H3O+ and considerably exceeds the
ionization potential of each of the molecules in this study. As
a consequence, the major reaction products can all be identified
as resulting from processes initiated by charge transfer. A
detailed mechanistic scheme has been proposed by Morris et
al. from their results for the reaction of H2O+ with various CF3X
molecules.3
D(CF2Br+‚H2O) > ∆fH(CF2Br+) - 490 kJ mol-1
Using the two PIMS values for ∆fH(CF2Br+) gives either
∆fH(CF2Br+) - 490 ) 150 kJ mol-1
or
Three reaction pathways need to be considered.
(i) NondissociatiVe Charge Transfer.
M + H2O+ f M+ + H2O
∆fH(CF2Br+) - 490 e 54 kJ mol-1
The binding between H2O and CF2Br+ is unlikely to be greater
than 150 kJ mol-1. Thus, the detection of CF2Br+‚H2O indicates
that the best value for ∆fH(CF2Br+) is the upper bound reported
by Wang and Leroi:14
This channel is exothermic for all the molecules. If sufficient
excess energy remains in the ion M+, fragmentation of the parent
ion may occur. Nondissociative charge transfer is thus most
likely to be seen for CF3Br, which has the highest ionization
potential of the molecules investigated in this study. But even
for CF3Br it is observed to be a relatively minor channel:
∆fH(CF2Br+) e 544 kJ mol-1
This is confirmed by our studies of the reactions of H2O+ with
CF3Br (see later) and is in accord with the bound reported by
CF3Br + H2O+ f CF3Br+ + H2O ∆H ) -117 kJ mol-1
(9)
Morris et al., e566 kJ mol-1 3
.
In the case of the reaction of H3O+ with CF2BrCl, there are
in addition to CFBrCl+ two other ion products. One is due to
a bimolecular reaction
A further requirement for the formation of the parent cation is
the existence of an adequate energy range over which the cation
is stable to fragmentation. The recent threshold photoelectron-
photoion coincidence (TPEPICO) study of CF3Cl and CF3Br16
shows that a clear region of stability exists for CF3Br+, and
included in this region is the energy corresponding to the
recombination energy of H2O+, but such a region does not exist
for CF3Cl+. This correlates with the observations of Morris et
al.;3 CF3Cl+ is not formed in the reaction of CF3Cl with H2O+,
but CF3Br+ is a product of the reaction of CF3Br with H2O+.
TPEPICO results have not been reported for the other molecules
of this study, and the question of an adequate region of stability
remains open.
H3O+ + CF2BrCl f CFBrCl+‚H2O + HF
∆H ) ∆fH(CFBrCl+) - D(CFBrCl+‚H2O) - 667 kJ mol-1
(6)
The other is due to a three-body associative process:
H3O+ + CF2BrCl
9
He8 CF2BrCl‚H3O+
(7)
Based on our thermochemical analysis for the formation of CF2-
(ii) DissociatiVe Charge Transfer. From the list of observed
ion products (Table 1), M+ is evidently formed with energy in
excess of one or more dissociative ionization limits:
Br+‚H2O from the reaction of H3O+ with CF2Br2, the pathway
H3O+ + CF2BrCl f CF2Br+‚H2O + HCl
(8)
M + H2O+ f (M+)* + H2O
(M+)* f [M-X]+ + X
is likely to be exothermic, but CF2Br+‚H2O was not observed.
The overall reaction between H3O+ and CF2BrCl is relatively
slow. There is some barrier to reactive breakup of the
intermediate ion-molecule complex. This causes the complex
lifetime to be much longer than for the other systems, and a
small fraction of the complexes are stabilized by collisions with
In Table 4 are collected the enthalpies for all the dissociative
charge-transfer pathways of the type above, that is, for

Reactions of H3O+ and H2O+
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8493
TABLE 4: Enthalpies for All the Dissociative Charge-Transfer Pathways of the Type M + H2O+ f (M+)* + H2O f [M-X]+
+ X + H2O and for All the Pathways M + H2O+ f [M-X]+ + HX + OHa
∆H/kJ mol-1
∆H/kJ mol-1
M + H2O+
M + H2O+
[M-X]+
observed?
M
X
f [M-X]+ + H2O + X
f [M-X]+ + HX + OH
CF3Br
Br
F
Br
F
Br
Cl
F
Br
F
Br
Cl
F
Br
Cl
-47
77
yes (71)
yes (16)
yes (100)
no
yes (86)
yes (6)
yes (8)
yes (100)
no
yes (97)
yes (3)
no
yes (76)
yes (24)
e57
e-15
e-49
e-197
e-28 b
e-46
e-104
e-103
e-104
e-124
e-214
e-104
-98
CF2Br2
e-182
e-125
e-161 b
e-114
e-32
e-236
e-32
e-257
e-282
e-32
-231
CF2BrCl
CFBr3
CFBr2Cl
CBrCl3
e-171
e-104
a Note that the heat of formation taken for CF2Br+ is that reported by Wang and Leroi.14 Other inequalities are a result of the heat of formation
of the product ion being taken as the upper limit derived from the H3O+ study (Table 3). The numbers in parentheses are the branching ratios to
the observed [M-X]+ products. b ∆fH(CF2Cl+) e 506 kJ mol-1, reported by Morris et al.,3 is used in the calculation.
H2O+:
M + H2O+ f [M-X]+ + H2O + X
(10)
CF3Br + H2O+ f CF2OH+ + HF + Br
∆H ) -267 kJ mol-1 (14)
The bounds of Table 3 were used to calculate many of the entries
in Table 4. With the possible exception of
CF3Br + H2O+ f CF2Br+ + H2O + F
∆H ) 153 or e57 kJ mol-1 (11)
This is the most exothermic pathway to CF2OH+. Its observa-
tion again suggests that the reaction of CF3Br with H2O, at least
in part, proceeds via an intimate mechanism within an ion-
molecule complex. The product is assumed to be protonated
carbonic difluoride rather than the difluoromethoxy cation
CHF2O+, since a comparison of the enthalpies of formation of
CH2OH+ (703 kJ mol-1) and CH3O+ (842 kJ mol-1) leads to
the conclusion that CF2OH+ will be the lower energy isomer.
Protonated carbonic dihalides are not observed as products
of any of the other reactions, although there are exothermic
pathways to CF2OH+ production for both CF2Br2 and
CF2BrCl:
these fragmentation pathways to the trihalomethyl cations are
all exothermic. For most of the molecules there are other
exothermic pathways to the trihalomethyl cations of the form
M + H2O+ f [M-X]+ + HX + OH
(12)
and the enthalpies of these pathways are also given in Table 4.
For pathway 10 the exothermicities increase in the order X
) Br < Cl < F, reflecting the relative strengths of the C-X
bonds in the trihalomethyl cations. For pathway 12 the overall
thermochemistries are the result of the balance between C-X
and H-X bond strengths, and these pathways are most
exothermic for X ) F.
CF2Br2 + H2O+ f CF2OH+ + HBr + Br
∆H ) -301 kJ mol-1 (15)
CF2BrCl + H2O+ f CF2OH+ + HCl + Br
∆H ) -298 kJ mol-1 (16)
There is a strong correlation between the exothermicity for
the simple dissociative charge-transfer mechanism 10 and the
observed branching ratios, indicating that mechanism 10 is the
preferred pathway by which the trihalomethyl cations are
formed. There is one definite exception, the formation of CF2-
Br+ from CF3Br, for which the most exothermic pathway is
There are no thermochemical data for the other protonated
carbonic dihalides. The proton affinities of carbonic difluoride
and methanal are quite similar, 672 and 718 kJ mol-1
,
CF3Br + H2O+ f CF2Br+ + HF + OH
∆H ) 82 or e-15 kJ mol-1 (13)
respectively. This suggests that the proton affinity may be
regarded as a transferable property of the carbonyl group (∼700
kJ mol-1). This value was used to make reasonable estimates
of the enthalpies of formation of CCl2OH+ (610 kJ mol-1) and
CBr2OH+ (717 kJ mol-1). The following pathways are likely
to be exothermic:
The observation of CF2Br+ as a product of the reaction between
CF3Br and H2O+ is only consistent with the second of these
two values for ∆H. This value is derived from Wang and
Leroi’s upper limit to ∆fH(CF2Br+) (e544 kJ mol-1).14 The
value of ∆fH(CF2Br+) ) 640 kJ mol-1 obtained by Clay et al.
from a PIMS study of CF3Br appears to be much too high.13 It
has thus been rejected in the construction of Table 4, and Wang
and Leroi’s bound for the heat of formation of CF2Br+ has been
used.
CF2Br2 + H2O+ f CBr2OH+ + HF + F
CFBr3 + H2O+ f CBr2OH+ + HF + Br
CFBr2Cl + H2O+ f CBr2OH+ + HF + Cl
CBrCl3 + H2O+ f CCl2OH+ + HCl +Br
The formation of CF2Br+ from CF3Br indicates the occurrence
of reaction within an ion-molecule complex, while the other
reactions could result from long-range charge transfer. The
significance of this observation is discussed further below.
(iii) Formation of Protonated Carbonic Dihalides. CF2OH+
is one of the products observed from the reaction of CF3Br with
None of these pathways were observed. Similarly, by use of
values of the heats of formations of CFBrO (-377 kJ mol-1),15
CBrClO (-167 kJ mol-1),15 and CFClO (-430 kJ mol-1),17

8494 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Thomas et al.
Figure 1. Mechanistic scheme for the reaction of H2O+ with CF3Br (adapted from ref 3).
estimates of the heats of formation of CFBrOH+ (453 kJ mol-1),
CBrClOH+ (664 kJ mol-1), and CFClOH+ (400 kJ mol-1) have
been determined. These estimated heats of formation indicate
that there are exothermic reaction pathways leading to proto-
nated carbonic dihalides for all the molecules of this study.
Mechanisms of the Reactions of H2O+ with the Bro-
momethanes. The results show that the reaction of H2O+ with
CF3Br is different from the reactions of H2O+ with the other
fully halogenated bromomethanes of this study. The parent
cation product CF3Br+ is observed. Two of the other products
CF2Br+ and CF2OH+ are indicative of intimate reactions within
an ion-molecule complexstheir formation requires the making
and breaking of bonds, together with charge transfer. On the
basis of the reactions of the other bromomethanes, the only
+
product to be expected from CF3Br is CF3
. The general
mechanistic scheme proposed by Morris et al.3 provides a
framework within which to examine these observations. For
the specific case of the reaction of CF3Br with H2O+, an adapted
version of the scheme of Morris et al. is illustrated in Figure 1.
The steps in this scheme are (a) the initial formation of an ion-
molecule complex and (b) charge transfer within this complex.
The efficiency of charge transfer at long range can be assessed
by combining the separate Franck-Condon factors for the
recombination of the ion (here, H2O+ + e- f H2O) and
ionization of the molecule.5,18-20 Energy conservation requires
that the energy released by recombination is equal to that
supplied to ionize the molecule. The geometry of the ground
state of H2O+ differs little from that of H2O, and the largest
Franck-Condon (F-C) factor starting from H2O+ in its zero-
point level is to H2O in its zero-point level. The energy released
is 12.61 eV (the adiabatic and vertical ionization potential of
H2O). The relative F-C factor for ionization of the molecule
at this energy can be judged by inspection of the photoelectron
spectrum of the molecule in which relative intensities are usually
set by F-C factors for ionization. For the case of charge
transfer to H2O+, this is only likely to be efficient if there is a
peak in the photoelectron spectrum of the molecule with an
ionization energy close to 12.61 eV. An inspection of the
photoelectron spectra of CF3Br,21 reproduced in Figure 2, shows
no peak at 12.61 eV. When the match is poor, as for H2O+ +
CF3Br, a large distortion of the potential energy surface of the
neutral would be required to obtain good F-C factors. Charge
transfer must then occur within an ion-molecule complex, as
Figure 2. Photoelectron spectrum of CF3Br in the ionization energy
range 11.5-20.5 eV (adapted from ref 21). The vertical dashed line is
drawn at the recombination energy (RE) of the H2O+ ion, 12.61 eV.
indicated in the mechanism. Step b is exothermic. The energy
released is partitioned between internal energy of the cation CF3-
Br+, internal energy of the H2O, and relative motion of CF3-
Br+ and H2O. There are a number of possible fates of the
complex indicated by steps c-f. Step c is the breakup of the
complex to release the parent cation. If the parent cation
contains sufficient internal energy, it may fragment (step g).
Step c is in competition with step d, dissociation of the cation
within the complex, and steps e and f, which are reactions within
the complex.
Steps d-f lead to steps h-j, respectively; i.e., each of the
complexes formed by reactions within the initially generated
ion-molecule complex will break up to release the respective
product cations.
To observe the parent cation, step c must compete against
steps d-f and the parent cation must not break up after release
from the complex (step g). Step b is least exothermic for CF3-
Br, and CF3Br+ is known to have a fairly wide energy range
over which it is stable to dissociation. The formation of stable
parent cations is most likely for CF3Br and was only observed
for CF3Br.
Steps e and f are intimate reactions within the ion-molecule
complex, involving the making and breaking of bonds. Com-

Reactions of H3O+ and H2O+
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8495
pared to charge transfer, these are relatively slow processes,
which will only compete with steps c and d if the complex
lifetime is long and the parent cation does not rapidly dissociate.
The energy released within the complex by the initial charge
transfer, step b, will determine the lifetime of the ion-molecule
complex between the parent cation and H2O. The charge
transfer is least exothermic for CF3Br, and it is only for CF3Br
that a product corresponding to steps e and i is observed. Steps
f and j are the route to CF2Br+ for CF3Br.
For completeness, illustrated in Figure 1 are the pathways
ending in steps h′, g′, and j′, all of which are overall
endothermic, and the pathway ending in step i′, which although
overall exothermic, is not observed.
complex, the first reaction step is attack of the acidic H+ in the
H3O+ ion on a halogen in the neutral. Preferentially, an HF is
ejected together with H2O from the complex for the fluorine-
containing molecules, and HCl and H2O are ejected from the
complex for the reaction involving CBrCl3. Such a reaction
pathway is endothermic for the reaction of H3O+ with CF3Br,
and this explains the lack of an observable reaction with this
molecule. In the case of CF2Br2 and CF2BrCl, it is observed
that the H2O may bind to the trihalomethyl cation as the ion-
molecule complex breaks up.
H2O+ reacts predominantly via dissociative charge transfer,
which probably takes place at short range, since the Franck-
Condon factors connecting the ground state of the bromine-
containing molecule to any ionic state are small at the vertical
recombination energy of H2O+. Clear evidence of this has been
found from the reaction of H2O+ with CF3Br, for which other
ion products that can only result from intimate reactions
occurring within an ion-molecule complex are observed in
addition to charge transfer.
Considering the other molecules of this study, an inspection
of the photoelectron spectra of CF2BrCl,22 CFBr3,23 and
24
CBrCl3 shows that for these molecules there are no F-C
accessible ionic states at the vertical recombination energy of
H2O+. There is a partial match for CF2Br2.22 Charge transfer
must again occur within an ion-molecule complex, and reaction
is initiated by steps a and b of the mechanism. The very
different pattern of products, exclusive formation of trihalo-
methyl cations, observed from CF2Br2, CF2BrCl, CFBr3, CFBr2-
Cl, and CBrCl3, when compared to CF3Br, indicates a change
in the dynamics of the complex [M+ + H2O] so that the only
significant steps are step c followed by step g and/or step d
followed by step h. For CF2Br2, CF2BrCl, CFBr3, CFBr2Cl,
and CBrCl3 more energy is released by charge transfer during
step b. This energy will appear both in the internal energy of
the cation, leading to its dissociation (steps d and g), and in the
relative motion of the molecules within the complex, accelerat-
ing its destruction (steps c and h). The net effects are a short
complex lifetime and the exclusive production of trihalomethyl
cations, for which the branching ratios will parallel the exo-
thermicities of the competing dissociative charge-transfer reac-
tions. Whether the route is via steps c and g or via steps d and
h will depend on the relative rates for dissociation of the parent
cation and breakup of the ion-molecule complex.
Acknowledgment. Financial support from the PLSD (CBD),
Porton Down, U.K., is gratefully acknowledged. One of the
authors (R.D.T.) especially thanks Professor G. C. Morrison,
Head of the School of Physics and Space Research, for a School
studentship.
References and Notes
(1) Fehsenfeld, F. C.; Crutzen, P. J.; Schmeltekopf, A. L.; Howard, C.
J.; Albritton, D. L.; Ferguson, E. E.; Davidson, J. A.; Schiff, H. I. J. Geophys.
Res. 1976, 81, 4454.
(2) Raksit, A. B. Int. J. Mass Spectrom. Ion Processes 1986, 69, 45.
(3) Morris, R. A.; Viggiano, A. A.; Van Doren, J. M.; Paulson, J. F.
J. Phys. Chem. 1992, 96, 3051.
(4) Morris, R. A.; Viggiano, A. A.; Arnold, S. T.; Paulson, J. F.;
Liebman, J. F. J. Phys. Chem. 1995, 99, 5992.
(5) Jarvis, G. K.; Mayhew, C. A.; Tuckett, R. P. J. Phys. Chem. 1996,
100, 17166.
(6) McElroy, M. B.; Salawitch, R. J.; Wofsy, S. C.; Logan, J. A. Nature
1986, 321, 759.
(7) Washida, N.; Imamura, T.; Bandow, H. Bull. Chem. Soc. Jpn. 1996,
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(8) Smith, D.; Adams, N. G. AdV. At. Mol. Phys. 1988, 24, 1.
(9) Adams, N. G.; Smith, D. Int. J. Mass Spec. Ion. Phys. 1976, 21,
349.
To conclude, the observed branching ratios for the reaction
of H2O+ with CF3Br appear to be due to the high ionization
potential of CF3Br, compared to the other molecules of this
study, and the stability of CF3Br+ to dissociation at the
recombination energy of H2O+.
(10) Su, T.; Chesnavich, W. J. J. Chem. Phys. 1982, 76, 5183.
(11) 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.
(12) CRC Handbook of Chemistry and Physics, 77th ed.; Lide, D. R.,
Ed.; CRC Press Inc.: Boca Raton, FL, 1996.
(13) Clay, J. T.; Walters, E. A.; Grover, J. R.; Willcox, M. V. J. Chem.
Phys. 1994, 101, 2069.
(14) Wang, F.; Leroi, G. E. Ann. Isr. Phys. Soc. 1983, 6, 210.
(15) Thomas, R.; Liu, Y.; Mayhew, C. A.; Peverall, R. Int. J. Mass
Spectrom. Ion Processes 1996, 155, 163.
(16) Creasey, J. C.; Smith, D. M.; Tuckett, R. P.; Yoxall, K. R.; Codling,
K.; Hatherly, P. A. J. Phys. Chem. 1996, 100, 4350.
(17) Mayhew, C. A.; Peverall, R.; Watts, P. Int. J. Mass Spectrom. Ion
Processes 1993, 125, 81.
(18) Bowers, M. T.; Elleman, D. D. Chem. Phys. Lett. 1972, 16, 486.
(19) Gauglhofer, J.; Kevan, W. T., Jr. Chem. Phys. Lett. 1972, 16, 492.
(20) Laudenslager, J. B.; Huntress, W. T., Jr.; Bowers, M. T. J. Chem.
Phys. 1974, 61, 4600.
(21) Doucet, J.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1973, 58,
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Summary
There are important reasons for undertaking fundamental and
comprehensive investigations of the ion chemistry of bro-
momethanes. These include applications of ion-molecule
reactions in pollutant monitors and an understanding of atmo-
spheric ion chemistry. Furthermore, the ion chemistry of the
bromomethanes is essentially unexplored.
In this paper the results from a study of the reactions of two
important atmospheric cations, H3O+ and H2O+, with six
brominated molecules have been presented. Of the 12 reactions
detailed here, only one has previously been reported in the
literature by Morris et al.3 H3O+ reacts with a rate coefficient
close to the collisional value with four of the molecules (CF2-
Br2, CFBr3, CFBr2Cl, and CBrCl3) at a decreased efficiency
with CF2BrCl and has no observable reaction with CF3Br,
whereas H2O+ is found to react at the collisional rate with all
six of the molecules studied.
H3O+ can only react through the formation of an intimate
reaction complex. We suggest that after the formation of the
(24) Novak, I.; Cvitasˇ, T.; Klasinc, L.; Gu¨sten, H. Faraday Trans. 2
1981, 77, 2049.