Laser (248 nm) flash photolysis and pulse radiolysis of CF2Br2 in aqueous solution: Atmospheric implications

Article abstract of DOI:10.1246/bcsj.75.1699

Henry's law constant (K_h) of CF₂Br₂ was measured to be 0.058 M atm^-1 at room temperature. The steady state as well as laser (248 nm) flash photolysis of CF₂Br₂ was carried out in aqueous solution. The formation of Br₂^.- radicals involving two primary photodissociation channels was observed in the flash photolysis. One channel produced CF₂Br^. and Br^. radicals, and the other led to the formation of Br^- and H⁺. The spectrum and reactions of CF₂Br^. were studied in aqueous solution by pulse radiolysis. The CF₂Br^. radical was found to absorb in the 230-270 nm region with a molar absorbance (ε) of 710 M^-1 cm^-1 at λ_max = 245 nm. The rate constants of some of the reactions were measured at 299 K as follows: k (e_aq^- + CF₂Br₂) = 1.20 ± 0.13 × 10¹⁰, 2κ (CF₂Br^. + CF₂Br^.) = 1.4 ± 0.4 × 10⁹, κ(CF₂Br^. + O₂) = 1.9 ± 0.5 × 10⁹ M^-1 s^-1. The CF₂BrO₂^. radical absorbed in the 245-360 nm region with ε = 593 M^-1 cm^-1 at λ_max = 265 nm. An apparatus used to measure K_h, and a closed-loop flow system used to carry out the flash photolysis/pulse radiolysis of sparingly soluble gases, like CF₂Br₂, are reported.

Full text of DOI:10.1246/bcsj.75.1699

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1699–1705 (2002) 1699
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Laser (248 nm) Flash Photolysis and Pulse Radiolysis of CF2Br2 in
Aqueous Solution: Atmospheric Implications
Flash Photolysis and Pulse Radiolysis of CF2Br2
R. D. Saini et al.
Rameshwar D. Saini,* Suresh Dhanya, and Tomi Nath Das
02
99
05
SJA8
09-2673
227
Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre,
Trombay, Mumbai - 400 085, India
(Received July 11, 2001)
Henry’s law constant (Kh) of CF2Br2 was measured to be 0.058 M atm−1 at room temperature. The steady state as
well as laser (248 nm) flash photolysis of CF2Br2 was carried out in aqueous solution. The formation of Br2·− radicals
involving two primary photodissociation channels was observed in the flash photolysis. One channel produced CF2Br·
and Br· radicals, and the other led to the formation of Br− and H+. The spectrum and reactions of CF2Br· were studied
in aqueous solution by pulse radiolysis. The CF2Br· radical was found to absorb in the 230–270 nm region with a molar
absorbance (ε) of 710 M−1 cm−1 at λmax = 245 nm. The rate constants of some of the reactions were measured at 299 K
as follows: k (eaq− + CF2Br2) = 1.20 0.13 × 1010, 2k (CF2Br· + CF2Br·) = 1.4 0.4 × 109, k(CF2Br· + O2) = 1.9
0.5 × 109 M−1 s−1. The CF2BrO2· radical absorbed in the 245–360 nm region with ε = 593 M−1 cm−1 at λmax = 265
nm. An apparatus used to measure Kh, and a closed-loop flow system used to carry out the flash photolysis/pulse radio-
lysis of sparingly soluble gases, like CF2Br2, are reported.
01
02
.3
.5
There is continuing interest in the UV photodissociation dy-
namics of CF2Br2 (halon-1202) for two reasons: 1) an under-
standing of the dissociative pathways of simple molecules like
CF2Br2 is of fundamental importance,1 2) CF2Br2 belongs to a
family of halomethanes, whose ultraviolet photodissociation in
the stratosphere initiates the well-known ozone-depletion
cycle2,3 involving ClO· and BrO· radicals; although its concen-
tration in the atmosphere has been found to be increasing over
the past few years, it is not covered by the Montreal Proto-
col.4,5 The photodissociation dynamics has been studied ex-
perimentally using a wide variety of methods and wavelengths
ranging from 223 nm to 260 nm,6–12 as well as theoretically.13
The primary step has been identified as
However, to our knowledge, all studies on the ultraviolet pho-
todissociation of CF2Br2 have so far been carried out in the gas
phase, and no attempt has been made to investigate the photo-
chemistry of CF2Br2 in the condensed aqueous phase, which
remains present in the atmosphere in the form of clouds, fog,
rain, dew, and wet aerosol particles, so much so that clouds
cover approximately 60% of the Earth’s surface.3 Any gaseous
species released into the atmosphere has to distribute itself be-
tween the gaseous phase and the condensed aqueous phase;
our experiments show that CF2Br2 is not an exception to this
rule. Upon dissolution in a condensed aqueous phase, the sub-
sequent thermal or photochemistry of a chemical species is ex-
pected to be quite different, not only due to subsequent favor-
able energetics for newer reaction pathways, but also due to the
possibility of self-ionization, pH or ionic effects and the en-
hanced concentration and proximity to other reactive species,
e.g., dissolved O2.16 Further, it is now well known that hetero-
geneous reactions on the surfaces of polar stratospheric clouds
play a pivotal role in the formation of the ozone hole.3,17 We
have therefore measured the Henry’s law coefficient (Kh)18 of
CF2Br2, and checked the effect of UV-light on its aqueous
chemistry, both under steady state conditions using a low-pres-
sure Hg-lamp as well as with 248 nm laser flash photolysis.
Due to the complex chemistry observed in the presence of UV
light, e-beam pulse radiolysis studies were also undertaken to
study the properties of the primary photodissociation product,
CF2Br·. In this case, the CF2Br· radical could be generated by
the well-known dissociative electron-capture reaction of halo-
carbons,19 reaction (5), without any interference from other
CF2Br2 + hν(248 nm) → CF2Br· + Br·,
(1)
with a quantum yield of almost unity.6,14 In addition, it has
been observed that about 16% of the CF2Br· radical produced
by reaction (1) has sufficient energy to spontaneously dissoci-
ate into the
radical and the Br atom8,11,12, i.e.
CF2Br††
→
+ Br·.
(2)
Also, Dhanya et al., using 248 nm laser flash photolysis, have
shown the formation of the BrO· radical by the following
route:15
CF2Br· + O2 → CF2BrO2 ,
·
k3 = (1.6 0.5 ) × 10−12 molecule−1 cm3 s−1
CF2BrO2· + Br· → CF2BrO· + BrO·,
,
(3)
(4)
species (like Br· and
radical transients), thereby allowing
the measurement of its UV absorption spectrum and also some
relevant reaction kinetics,
k4 = (2.4 0.9 ) × 10−11 molecule−1cm3 s−1
.

1700 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
eaq− + CF2Br2 → CF2Br· + Br−.
Flash Photolysis and Pulse Radiolysis of CF2Br2
calculate the exact partial pressure.
The preparation as well as the handling of the aqueous solu-
tions was carried out in a closed-loop system using the apparatus
shown in Fig. 2. Briefly, a solution of known composition was
prepared in flask A, and flowed through reaction cell C; the spent
solution was collected in flask D. Reaction cell C was made from
a SuprasilTM tube of 1 cm × 1 cm square bore. All operations
were carried out at sub-atmospheric pressures and in an air-free
environment, unless otherwise stated.
(5)
The CF2Br2− type radical formed after electron capture has not
been reported in measurements on the µs time scale. Instead,
concurrent dehalogenation takes place in solution; in this case
bromine is preferentially released over fluorine. A closed-loop
continuous flow system was designed to maintain the desired
concentration of dissolved CF2Br2 and other reactants during
the course of the study.
UV Photolysis. Steady-state photolysis was carried out in a
stoppered cuvette using a Pen-Ray low-pressure mercury lamp.
Another cuvette containing methanol was placed between the
lamp and the sample to cut off the mercury lines of λ ꢀ 184.9 nm.
The experimental setup for the KrF (248 nm) laser flash photolysis
was described earlier.15 The laser intensity was measured with re-
spect to the yield of biphenyl triplet using an oxygen-free solution
of biphenyl in cyclohexane having an absorbance equal to that of
the experimental solution of CF2Br2 at 248 nm. The values for the
quantum yield and the molar absorbance of the biphenyl triplet
were taken as 0.84 and 4.28 × 104 M−1 cm−1, respectively.21 The
maximum laser intensity was 1.74 × 1016 photon cm−2, and it was
varied by placing several flat plates of technical-grade quartz, each
having a transmission of about 85% at 248 nm.
Experimental
General. All experiments were carried out at an ambient
temperature of 299 K unless otherwise stated. Triply distilled wa-
ter was used to prepare all aqueous solutions, where distilled wa-
ter was further distilled over (a) acidic dichromate and (b) alkaline
permanganate. The gases and chemicals were of reagent grade.
Atmospheric air was purified by passing through a column of
freshly activated charcoal, and CF2Br2 from Aldrich (> 97%) was
degassed before use. A grease-free vacuum setup for attaining a
vacuum of 1 × 10−3 Torr was used when required. All pressure
measurements were made with capacitance manometers (MKS
Baratrons, Type 122A), and the absorption spectra were recorded
using a Hitachi-330 spectrophotometer.
Determination of Henry’s Law Constant of CF2Br2. Mea-
surements of Kh were carried out using the apparatus schematical-
ly shown in Fig. 1. In principle, our method involved measuring
the concentration of gaseous CF2Br2 spectrophotometrically in cu-
vette B, before and after equilibration with a known quantity of
degassed water contained in cuvette A. We carried out spectro-
photometric measurements on a number of gas mixtures, each
containing 21 Torr of water vapor in the presence of CF2Br2 with
its partial pressure ranging from 1 to 15 Torr. We found that the
presence of water vapor did not have any effect on the absorption
spectrum and the absorption cross section (σ) of gaseous CF2Br2.
Hence, the reported values of the absorption cross section (σ) of
CF2Br2 at different wavelengths20 in the gas phase were used to
Pulse Radiolysis Setup. Pulse radiolysis studies were carried
out using an experimental setup described earlier.22,23 Briefly, the
solution was exposed to an electron pulse from a linear accelerator
Fig. 2. Apparatus for the preparation and manipulation of
aqueous solutions of CF2Br2 for pulse radiolysis (schemat-
ic). A: Reservoir for the preparation and storage of feed
solution, total capacity = 0.6 L.; B: Ampoule to store the
measured amount of CF2Br2 before mixing; C: Flow cell
for pulse radiolysis, made from Suprasil tube of 10 mm ×
10 mm internal cross section, D: Receiver; 1–9: Grease
free high vacuum stopcocks.
Fig. 1. Apparatus for determining the Henry’s law constant
(Kh) (schematic). A and B: Far UV Grade spectrophoto-
metric cuvetts of 10 mm path length, A for liquid and B for
gas; C: equilibration chamber; 1, 2: Grease free high vacu-
um stopcocks; 3: sampling port with viton/silicone rubber
septum for gas chromatographic analysis; D: O-ring con-
nector.

R. D. Saini et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1701
(Forward Industries, UK) and the transient species were moni-
tored by kinetic spectrophotometry. The linear accelerator provid-
ed pulses of 7 MeV electrons with a duration that could be varied
between 50 ns and 2 µs with a corresponding dose range of ~3 Gy
to 140 Gy, respectively. The dosimetry was carried out with an
aerated aqueous solution containing 10−2 M potassium thiocyan-
ate, while taking into account that G•ε = 2.59 × 10−4 m2 J−1 at
475 nm.24 The interference due to the scattered light at 250 nm
was < 2%, measured by comparing the transmitted light intensity
with and without a Pyrex filter. The spectral measurements were
made at a wavelength resolution of about 2.5 nm.
measurements under the conditions employed (Fig. 3 Inset),
the value of Kh was determined to be 0.058 M atm−1 at 299 K.
UV Photolysis. The laser (248 nm) flash photolysis of an
air-free aqueous solution containing 6.7 × 10−3 M of CF2Br2
yielded a transient absorption spectrum with an absorption
maximum (λmax) at ~360 nm, as shown in Fig. 4. The absorp-
tion at 360 nm decayed by second-order kinetics with 2k/εl =
6.14 0.40 × 105 s−1. The formation of the transient species
was found to be quite fast, and attained its peak absorbance
value within about 2 µs after the laser pulse. A log–log plot
(Fig. 4 inset) between the peak absorbance (∆A) and the laser
intensity (I) gives a slope of 2, thereby indicating the involve-
ment of two photons in its generation. The bubbling of atmo-
spheric air into the experimental solution had practically no ef-
fect on the intensity and the decay kinetics of transient absorp-
tion at 360 nm. Upon steady state photolysis for a period of 5
minutes with the λ ꢁ 235 nm light, another portion of the
same solution showed the following changes: a) it acquired a
brown tinge, b) its pH changed from 5.8 to 2.5, and c) it gave a
precipitate of AgBr upon the addition of a solution of AgNO3.
Similar changes were observed on exposing the solution to
about 500 flashes of a 248 nm laser.
Results and Discussion
Behavior of CF2Br2 in Aqueous Solution. The addition
of CF2Br2 did not change the pH of water, and a saturated
aqueous solution of CF2Br2 did not produce any precipitate
upon the addition of AgNO3, even after keeping for several
days in the dark. These observations rule out the occurrence of
the hydrolytic reaction (6), i.e.,
CF2Br2 + H2O → CF2BrOH + Br− + H3O+.
(6)
The absorption spectrum of CF2Br2 in the gas and aqueous
phases are shown in Fig. 3. The λmax of CF2Br2 in the aqueous
solution was blue shifted by 3 nm compared to that in the gas
phase; this is expected from the solvent effect on the n-σ* tran-
sition involved.25 In addition, it was also found that the inten-
sity of absorption is higher in the gas phase than in solution. A
similar solvent effect on the absorption characteristics of a
molecule, undergoing n-σ* transition was observed earlier,
e.g., in dimethyl sulfide.26,27
Pulse Radiolysis. Transient Absorption Spectrum.
The radiolysis of water at pH 7 leads to the formation of pri-
mary radicals, eaq−, H·, (both reducing) and OH (oxidizing)
·
with respective yields of 0.28, 0.062, and 0.28 µmol J−1 28
The
.
transient absorption spectrum shown in Fig. 5(a) was obtained
from the pulse radiolysis of an air-free aqueous solution con-
taining 1.4 × 10−3 M of CF2Br2; it shows two absorption max-
ima (λmax) at ~240 nm and ~350 nm. From kinetic measure-
ments at these two wavelengths, the formation and decay rates
Henry’s Law Constant (Kh) of CF2Br2. From repeated
Fig. 3. Absorption spectra of CF2Br2 in gas phase (ꢀ) and
aqueous solution (—). Inset: Measurement of Henry’s law
constant (Kh).
Fig. 4. Transient absorption spectrum, 3 µs after 248-nm la-
ser flash. [CF2Br2] = 6.7 × 10−3 M. Inset: Dependence of
peak absorbance (350 nm) on laser intensity.

1702 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
Flash Photolysis and Pulse Radiolysis of CF2Br2
been found to be dose dependent, as would be expected from
the above reaction scheme. For example, with a 2 µs pulse
(dose = 108 Gy), the peak absorption was reached within the
duration of the pulse, whereas with a 500 ns pulse (dose = 56
Gy) it took as much as 21 µs to attain the peak-absorbance at
350 nm. The above mechanism is further supported by a simi-
lar experiment in the presence of 0.2 M 1,1-dimethylethanol
(2-Methyl-2-propanol), wherein the transient absorption with
λ
max at 350 nm disappeared completely, and the one with λmax
at 240 nm became modified [Fig. 5b]. This is due to the fact
that reaction (13) is suppressed by the following reaction:
(CH3)3COH + ·OH → (CH3)2(·CH2)COH + H2O, (15)
and the resultant β-hydroxy radical, (CH3)2(·CH2)COH, ab-
sorbs in the 220–300 nm30 region. Upon subtracting the con-
tribution of the (CH3)2(·CH2)COH radical, and making a cor-
rection for the loss of absorption due to depletion of the parent
compound, CF2Br2, (using spectrum in Fig. 3), we obtained the
corrected absorption spectrum of CF2Br· radical, as shown in
Fig. 5 (Inset).
−
Rate Constant for the Reaction of eaq with CF2Br2.
The kinetics of hydrated electron decay, monitored at 720 nm,
was found to become faster in the presence of CF2Br2, as
shown in Fig. 6. The observed rate constant of eaq− decay in-
creases with increasing concentration of CF2Br2 from 1 to 6 ×
10−4 M, as shown in Fig. 6 (Inset). From the slope of this plot,
the bimolecular rate constant for reaction (5) was calculated to
Fig. 5. Transient absorption spectra at the end of 2 µs pulse.
(a) [CF2Br2] = 1.4 × 10−3 M, (b) [CF2Br2] = 1.4 × 10−3
M; [1,1-dimethylethanol] = 0.2 M. Inset: spectrum (b) af-
ter correction for the depletion of CF2Br2 and for the ab-
sorption due to 1,1-dimethylethanol radical; dose = 108
Gy.
be (1.20
0.13) × 1010 M−1 s−1. An attempt to crosscheck
were found to be quite different from each other, thus suggest-
ing the formation of two different species. A comparison re-
vealed that the transient absorption with λmax at ~350 nm re-
sembles that reported for Br2·− in the literature.29 Accordingly,
the above results can be explained by the following set of reac-
tions:19
·
H2O ——~~~→ eaq−, H·, ·OH, H2O2, HO2 ,
(7)
(5)
(8)
eaq− + CF2Br2 → CF2Br· + Br−,
H· + CF2Br2 → CF2Br· + Br− + H3O+,
−
eaq− + eaq → H2 + 2OH−, k9 = 5.5 × 109 M−1 s−1, (9)
H· + H· → H2, k10 = 5.0 × 109 M−1 s−1,
eaq− + ·OH → OH−, k11 = 3.0 × 1010 M−1 s−1,
H· + ·OH → H2O, k11 = 7.0 × 1010 M−1 s−1,
(10)
(11)
(12)
Br− + ·OH → Br· + OH−,
k13 = 1.1 × 1010 M−1 s−1,
(13)
(14)
Br· + Br− → Br2·−, k11 = 1.2 × 1010 M−1 s−1,
−
Fig. 6. Decay of eaq at 720 nm. in blank matrix (—) (bot-
tom x-axis) and in the presence of 2.6 × 10−4 M CF2Br2
(ꢁ) (top x-axis); 50 ns pulse; dose = 12 Gy. Inset: Depen-
dence of observed rate constant on CF2Br2 concentration.
and the transient absorption with λmax at 240 nm can be as-
signed to the CF2Br· radical. The growth rate at ~350 nm has

R. D. Saini et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1703
the value of k5 by observing the growth rate of CF2Br· at ~250
nm was unsuccessful, because the signal was weak and had in-
terfering optical absorption from the following: a) the parent
compound (bleaching), (b) eaq− and (c) the (CH3)2(·CH2)COH
radical in the presence of 1,1-dimethylethanol.
Molar Absorbance and Decay Kinetics of CF2Br· Radi-
cal. From the rate constants of competing reactions (5), (9),
and (11), it follows that > 97% of hydrated electron is con-
sumed by reaction (5) in the presence of 1.4 × 10−3 M CF2Br2.
Thus, G(CF2Br·) = 0.97 × G(eaq−), where the effects of reac-
tion (8), and any reaction of CF2Br· radical within this time pe-
riod, are neglected. Using the values of the radiolytic yield of
eaq− and the radiation dose absorbed (108 Gy), in a typical set
−
of experiments, we calculated the initial concentration of eaq
= 3.7 × 10−5 M and that of CF2Br· radical = 3.6 × 10−5 M,
which led to the molar absorbance of CF2Br· radical at λmax
,
245 nm = 710 M−1 cm−1. By considering the slopes of 1/∆A
vs time plot and ε at 260, 265, and 270 nm, where absorption
due to the parent compound is negligible, we obtained an aver-
age value of 1.4 0.4 × 109 M−1 s−1 (2k) for the rate constant
of the bimolecular decay of CF2Br· radical.
Reaction of CF2Br· Radical with O2. Using an aqueous
solution containing 3 × 10−3 M CF2Br2 and varying the con-
centration of oxygen from 10−5 to 10−4 M, the following reac-
tion was studied:
·
Fig. 7. Absorption spectrum of CF2BrO2 radical at 0.7 µs
after the 500 ns electron pulse. [CF2Br2] = 1.55 × 10−2 M;
[O2] = 2.7 × 10−4 M; dose = 56 Gy. Inset: Dependence of
observed rate constant of CF2Br· radical decay on the con-
centration of oxygen.
·
CF2Br· + O2 → CF2BrO2 .
(16)
The decay of the CF2Br· radical was monitored at 250 nm, and
sphere pressure of the gas as 5.78 × 10−2 M. This is much
higher than the reported solubilities of CF3Br and CFBr3, viz.,
2.16 × 10−3 and 1.47 × 10−3 M atm−1, respectively.31 How-
ever, such large differences in the solubilities/Henry’s law con-
stants are not uncommon for such halocarbons, e.g., the values
was found to follow a pseudo first-order kinetics, giving the
product CF2BrO2 radical within 30–8 µs for O2 concentrations
ranging over 25–125 µM. From a plot of the observed rate
constants against the concentration of oxygen (Fig. 7, Inset),
the bimolecular rate constant of reaction (16) was calculated to
be (1.9 0.5) × 109 M−1 s−1.
·
of Kh for CCl2F2 and CCl4 are 2.47 × 10−3 and 3.39 × 10−2
M
Absorption Spectrum of CF2BrO2· Radical.
The ab-
atm−1, respectively.31 It may be pointed out that Henry’s law
constant and the solubility reported here correspond to the spe-
cific conditions of our experimental setup, where a gas is equil-
ibrated with an aqueous solution over a flat surface. These re-
sults cannot be directly applied to the largely spherical or
curved surfaces of atmospheric aerosols, which lead to a low-
ering of the solubility due to the Kelvin effect.3 Further, the
boiling point of CF2Br2 (295 K) is very close to room tempera-
ture, and its specific gravity in the liquid phase is 2.271.
Therefore, an attempt to measure the solubility by shaking the
liquid CF2Br2 with water and then allowing the aqueous and
organic layers to separate out would result in a solubility high-
er than 5.78 × 10−2 M at room temperature, due to the fact that
both liquid–liquid and liquid–gas equilibria would be simulta-
neously operative; the former at the lower interface and the lat-
ter at the upper one.
·
sorption spectrum of the peroxyl radical, CF2BrO2 , is shown
in Fig. 7. This was obtained by the pulse radiolysis of an air-
equilibrated aqueous solution containing 1.55 × 10−2
M
CF2Br2, and 2.7 × 10−4 M oxygen,21 using a dose of 56 Gy.
Under these conditions, the following reaction is almost com-
pletely suppressed by reaction (5), thereby minimizing the in-
terference due to O2·− radical, which also absorbs in the 240–
280 nm region:
·−
eaq− + O2 → O2 , k14 = 1.9 × 1010 M−1 s−1.
(17)
Also, the interference from Br2·− radical is negligible because
of its slow formation, taking about 21 µs, at the dose em-
ployed. The spectrum (Fig. 7), obtained at 0.7 µs and correct-
ed for the bleaching of CF2Br2, has λmax at 265 nm, and it is
quite different from that of the CF2Br· radical (Fig. 5, Inset).
It is found that the absorption spectrum between 275 and
475 nm obtained after laser flash photolysis (Fig. 4) is very
similar to that obtained by pulse radiolysis (Fig. 5a), which is
assigned to the Br2·− radical. This could form by a direct two-
photon process, viz.,
·
Taking G(CF2BrO2 ) ꢀ G(CF2Br·), the molar absorbance for
·
CF2BrO2 radical was determined to be 593 M−1 cm−1 at 265
nm.
The aim of the present work was to measure Henry’s law
constant (Kh) of CF2Br2 and to investigate the photochemistry
of the latter in aqueous solution. Our measured value of Kh at
299 K indicates the solubility of CF2Br2 in water under 1 atmo-
·−
CF2Br2 + 2hν → CF2·+ + Br2
.
(18)

1704 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
Flash Photolysis and Pulse Radiolysis of CF2Br2
However, reaction (18) cannot explain the formation of Br− in
steady state photolysis, where the probability of its occurrence
would be practically zero. Also, a biphotonic process involv-
ing the photoionization of CF2Br2 molecule is not considered
for the same reason. A plausible mechanism can be as follows:
tion about the absorption spectrum and the reactivity of the pri-
mary product CF2Br· radical.
It has been estimated that on a per atom basis, bromine is
~50 times more efficient than chlorine in destroying atmo-
spheric ozone.32 This efficiency results from the fact that near-
ly 50% of the available bromine in the stratosphere exists in
the reactive form (Br· and BrO·), whereas only a percent or so
of stratospheric chlorine exists as ClO·. In view of this, it is
interesting to examine the atmospheric implications of our re-
sults. A higher value of its solubility compared to that of most
other halons and CFCs implies a reduced tropospheric lifetime
for CF2Br2 due to uptake by water in the oceans, rivers, lakes,
etc. However, detail information on the marine chemistry of
CF2Br2 is required to understand whether the oceans act as a
sink or reservoir for this molecule. In addition, an enhance-
ment in the solubility of CF2Br2 in the condensed aqueous
phase at the lower temperatures encountered in the strato-
sphere may be significant. This possibility coupled with the
fact that the UV photochemistry of CF2Br2 in the aqueous
phase is also different from that in the gas phase, partly leading
to the formation of Br− and H3O+ ions and hence to the miner-
alization of bromine, indicate that the findings of the present
work are likely to have an important impact on the atmospheric
modeling of CF2Br2 and on the depletion of ozone.
(CF2Br2)aq + hν → CF2Br· + Br·,
(19)
(20)
(CF2Br2)aq + hν → CF2BrOH + H3O+ + Br−,
where reactions (19) and (20) are two different primary photo-
chemical reactions which generate the reactants Br· and Br−
for reaction (14), and thus explain the intensity-square depen-
−
dence of the formation of Br2 . The photo-induced hydrolytic
reaction (20) accounts for the lowering of the pH as well as for
the formation of Br−. Further, reaction (19) represents a ho-
molytic cleavage (giving radicals), which is favored in the gas
phase as well as in non-polar solvents. Reaction (20) can be
considered to involve a heterolytic cleavage (giving cation and
anion), which is favored in polar solvents, like water, followed
by the reactions of ionic fragments with the solvent-molecules
available in the immediate vicinity, i.e.,
CF2Br2 + hν → CF2Br+ + Br−,
(20a)
(20b)
CF2Br+ + H2O → CF2BrOH + H+,
We thank Mr. Vijendra N. Rao and his colleagues for their
technical help in pulse radiolysis work.
H+ + nH2O → H3O+
Br− + nH2O → Br−
,
∆H = −11.3 eV (20c)
∆H = −3.6 eV (20d)
aq
,
aq
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