Water-Catalyzed Dehalogenation Reactions of Isobromoform and Its Reaction Products

Article abstract of DOI:10.1021/ja0390552

A combined experimental and theoretical study of the photochemistry of CHBr₃ in pure water and in acetonitrile/water mixed solvents is reported that elucidates the reactions and mechanisms responsible for the photochemical conversion of the halogen atoms in CHBr₃ into three bromide ions in water solution. Ultraviolet excitation at 240 nm of CHBr ₃ (9 × 10^-5 M) in water resulted in almost complete conversion into 3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-resolved resonance Raman (ps-TR ³) experiments and ab initio calculations indicate that the water-catalyzed O-H insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the production of the final products observed following ultraviolet excitation of CHBr₃ in water. These results have important implications for the phase-dependent behavior of polyhalomethane photochemistry and chemistry in water-solvated environments as compared to gas-phase reactions. The dissociation reaction of HBr into H⁺ and Br^- ions is the driving force for several O-H insertion and HBr elimination reactions and allows O-H and C-H bonds to be cleaved more easily than in the absence of water molecules. This water-catalysis by solvation of a leaving group and its dissociation into ions (e.g., H⁺ and Br^- in the examples investigated here) may occur for a wide range of chemical reactions taking place in water-solvated environments.

Full text of DOI:10.1021/ja0390552

Published on Web 02/24/2004
Water-Catalyzed Dehalogenation Reactions of Isobromoform
and Its Reaction Products
Wai Ming Kwok, Cunyuan Zhao, Yun-Liang Li, Xiangguo Guan, Dongqi Wang, and
David Lee Phillips*
Contribution from the Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road,
Hong Kong S.A.R., P. R. China
Received October 14, 2003; E-mail: phillips@hkucc.hku.hk
Abstract: A combined experimental and theoretical study of the photochemistry of CHBr3 in pure water
and in acetonitrile/water mixed solvents is reported that elucidates the reactions and mechanisms responsible
for the photochemical conversion of the halogen atoms in CHBr3 into three bromide ions in water solution.
Ultraviolet excitation at 240 nm of CHBr3 (9 × 10-5 M) in water resulted in almost complete conversion into
3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-
resolved resonance Raman (ps-TR3) experiments and ab initio calculations indicate that the water-catalyzed
O-H insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are
responsible for the production of the final products observed following ultraviolet excitation of CHBr3 in
water. These results have important implications for the phase-dependent behavior of polyhalomethane
photochemistry and chemistry in water-solvated environments as compared to gas-phase reactions. The
dissociation reaction of HBr into H+ and Br- ions is the driving force for several O-H insertion and HBr
elimination reactions and allows O-H and C-H bonds to be cleaved more easily than in the absence of
water molecules. This water-catalysis by solvation of a leaving group and its dissociation into ions (e.g.,
H+ and Br- in the examples investigated here) may occur for a wide range of chemical reactions taking
place in water-solvated environments.
Introduction
have been areas of intense recent interest. In addition, photo-
chemical processes of water treatment have used polyhalo-
methanes and other halogenated organic compounds to study
and better understand the dehalogenation and decomposition
of these types of compounds in water to design and improve
photocatalysts for water treatment.24,25
Polyhalomethanes such as CH2I2, CHBr3, CFCl3, and others
have been observed in the natural environment and are important
sources of reactive halogens in the atmosphere that have been
linked to ozone depletion in the troposphere and/or the
stratosphere.1-11 Halogen activation processes in the natural
environment12-22 and the role of polyhalomethanes such as
CH2I2 in the formation of iodine aerosols in the atmosphere23
Bromoform is the most abundant source of organic bromine
in the ocean and atmosphere, and this makes it an attractive
polyhalomethane to study.1,26 Ultraviolet excitation with 253.7
nm light (from a Hg lamp) of low concentrations (<10-6 M)
of CHBr3, CHBr2Cl, and CHCl2Br in aqueous solution led to
complete conversion of the halogens into halide ions (bromide
and/or chloride) with similar photoquantum yields of about
0.43.24 C-Br bond cleavage for the CHBr3 molecule and the
CHBr2 radical in the gas phase has been estimated to require
63.8 and 73.6 kcal/mol of energy, respectively.27 The 253.7 nm
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J. AM. CHEM. SOC. 2004, 126, 3119-3131
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A R T I C L E S
Kwok et al.
(Spectra-Physics, Spitfire) pumped by the second harmonic from a Nd:
YLF laser (Spectra-Physics, Evolution X) amplified the seed laser beam,
and the output from the regenerative amplifier (800 nm, 1ps, 1 kHz)
was frequency doubled and tripled by KDP crystals to generate the
probe (400 nm) and pump (267 nm) laser sources, respectively. The
time zero delay between the pump and probe laser beams in the TR3
experiments was found by using fluorescence depletion of trans-
stilbene. The time zero was ascertained by varying the optical delay
between the pump and probe beams to a position where the depletion
of the stilbene fluorescence was halfway to the maximum fluorescence
depletion by the probe laser. The accuracy of the time zero measurement
was estimated to be (0.5 ps, and a typical cross-correlation time
between the pump and probe pulses was also measured by the
fluorescence depletion method and determined to be about 1.5 ps
(fwhm). So as to employ the laser beams more effectively in the TR3
experiments and taking into account that the rotational reorientation
dynamics are much faster than the dynamics investigated in this study,
parallel polarization of the pump and probe laser beams was employed
rather than the magic angle polarization. The pump and probe pulses
were loosely focused onto a thin film stream (thickness ∼500 µm) of
the sample solution where typical pulse energies and spot sizes at the
sample for the pump beam were 15 µJ and 250 µm and for the probe
beam were 8 µJ and 150 µm. A backscattering geometry using an
ellipsoidal mirror with f/1.4 was employed to collect the Raman
scattered light and image the light onto the entrance slit of a 0.5 m
spectrograph. The 1200 groove/mm ruled grating blazed at 250 nm of
the spectrograph dispersed the Raman scattered light onto a liquid
nitrogen cooled CCD detector mounted on the exit port of the
spectrograph.
Each spectrum shown here was obtained from subtraction of scaled
probe-before-pump and scaled net solvent measurements from a pump-
probe spectrum to delete CHBr3 ground-state Raman bands and residual
solvent Raman bands, respectively. The known Raman shifts of the
solvent Raman bands were used to calibrate the spectra with an
estimated accuracy of (5 cm-1 in absolute frequency. Commercially
available 99% CHBr3 and spectroscopic grade acetonitrile solvent were
used without further purification to make half-liter volume CHBr3 (8
× 10-2 mol dm-3) samples prepared in acetonitrile and acetonitrile/
water (0.2%, 1%, 2%, 5%) mixed solvents. During the experimental
trials, the samples showed less than a few percent degradation as
determined from the UV absorption spectra obtained before and after
the TR3 measurement.
photon has about 113 kcal/mol of energy and is thus only likely
to break one carbon-halogen bond in the CHBr3, CHBr2Cl,
and CHCl2Br molecules following 253.7 nm excitation. How
does the 253.7 nm photolysis of low concentrations of CHBr3,
CHBr2Cl, and CHCl2Br in water lead to complete conversion
of the halogen atoms into bromide and/or chloride ion products,
and where does the energy come from to break all three carbon-
halogen bonds? Here, we present a combined experimental and
theoretical study of the photochemistry of CHBr3 in pure water
and in acetonitrile/water mixed solvents. This work elucidates
the reactions and mechanisms responsible for the photochemical
conversion of the halogen atoms in CHBr3 into three bromide
ions in water solution.
Experimental and Computational Methods
Photochemistry and Product Analysis Experiments. Ultraviolet/
Visible Absorption and pH Measurements after Photolysis of CHBr3
in Water. Commercially available CHBr3 (99%) and deionized water
were used to prepare 500 mL sample solutions of about 9 × 10-5
M
CHBr3 in water. The sample solution was put in a 10 cm laser path-
length glass holder with quartz windows and was excited by 3 mJ of
240 nm unfocused laser beam from the first anti-Stokes hydrogen
Raman shifted laser line of the fourth harmonic of a nanosecond-Nd:
YAG laser in the photolysis experiments. The absorption spectra for
the photolyzed samples were acquired by employing a 1 cm UV grade
cell and a Perkin-Elmer Lambda 19 UV/vis spectrometer. The pH of
the photolyzed samples was measured by using a ThermoOrion 420A
pH meter equipped with a 8102BN combination pH electrode. The pH
electrode was calibrated with 7.00 pH and 4.01 pH buffer solutions.
13C NMR Measurements after 240 nm Photolysis of CHBr3 in
Water. Commercially available 13CHBr3 (Aldrich) and D2O (99.9%
D) solvent were used to prepare ∼20 mM sample solutions which were
placed in a UV-grade airtight NMR tube for the photolysis experiments.
13C NMR spectra were acquired using a Bruker Advance 400 DPX
spectrometer at room temperature. An initial 13C NMR spectrum was
obtained for the sample before photolysis, and additional spectra were
obtained after varying times following excitation by the 240 nm
unfocused laser beam until all of the 13CHBr3 sample was photolyzed
as determined by the decrease in its 13C NMR band at ∼12 ppm
referenced to the TMS band that was set as 0 ppm.
Ab Initio Calculations. The second-order Møller-Plesset perturba-
tion theory (MP2) was employed to examine the water-assisted O-H
insertion reactions of isobromoform (BrCHBr-Br) + nH2O f CHBr2-
(OH) + HBr + (n - 1)H2O (where n ) 1,2,3), and the decomposition
reactions of CHBr2(OH) + nH2O f HBrCO + nH2O + HBr (n )
0,1,2,3,4) and HBrCO + nH2O f CO + HBr + nH2O (n ) 0,1,2,...,4).
Both the geometry optimization and the frequency calculations were
done with the 6-31G* basis set for C, H, O, and Br atoms. MP2
calculations using a 6-311++G** basis set were also done to find the
optimized geometry, vibrational frequencies, and relative Raman
intensities for the CHBr2OH molecule. All of the calculations made
use of the Gaussian 98W program suite.28 Cartesian coordinates, total
energies, and selected output from the calculations for all of the
calculated structures shown in Figures 4-6 are given in the Supporting
Infrared (IR) Absorption Measurements after 240 nm Photolysis
of CHBr3 in Water. The evolution of gas was observed in both of the
photochemistry experiments described in parts 1 and 2 above. This
gas was collected and introduced into a 10 cm path-length IR gas cell
equipped with KBr windows. IR spectra of the gas obtained after
photolysis of CHBr3 in water were obtained using a Bio-Rad FTS 165
spectrometer.
We attempted to estimate the photoquantum yield for the photolysis
reaction of CHBr3 in water to produce 3Br- (e.g., 3HBr that dissociate
to 3H+ and 3Br- in water). The same setup as for the UV/vis absorption
measurements described above and a laser power meter to measure
the laser beam exciting the sample solution were used to make the
experimental measurements needed to estimate the photoquantum yield.
The equations given in the Supporting Information were used to find
the estimate of the photoquantum yield for production of the 3HBr
leaving groups.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
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Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
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M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
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Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98; Gaussian, Inc.: Pittsburgh, PA, 1998.
Picosecond Time-Resolved Resonance Raman (ps-TR3) Experi-
ments. A femtosecond mode-locked Ti:Sapphire laser (Spectra-Physics,
Tsunami) pumped by the second harmonic from a Nd:YVO4 laser
(Spectra-Physics, Millennia V) was employed as the seed beam for an
amplified laser system. A picosecond mode regenerative amplifier
(27) McGivern, W. S.; Sorkhabi, O.; Suits, A. G.; Derecskei-Kovacs, A.; North,
S. W. J. Phys. Chem. A 2000, 104, 10085-10091.
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Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
Figure 1. Absorption spectra obtained after 240 nm photolysis of 9 ×
10-5 M CHBr3 in water. The parent CHBr3 band at ∼223 nm decreases in
intensity, and a new band at ∼190 nm due to Br- grows in with an isosbestic
point at ∼215 nm with the parent band. The inset presents plots of ∆[Br-]
versus -∆[CHBr3] (O) and ∆[H+] (0), respectively, derived from the
absorption spectra and pH measurements acquired during the same
photochemistry experiments. The lines are linear best-fits to the data.
Information. IRC (intrinsic reaction coordinate) calculations were done
to confirm the transition state connected the appropriate reactants and
products.29
The stabilization energies from separated reactants to reactant
complexes may be overestimated noticeably due to basis set superposi-
tion error (BSSE) effects, and the counterpoise (CP) method is often
used to make a correction. We note that a study on water dimers found
estimates of the effects of BSSE by the CP method may not lead to
better results because they do not provide quantitative information about
the basis set deficiencies.30 In addition, the use of larger basis sets where
the CP correction is small does not guarantee accurate results, and when
smaller basis sets are employed, the inclusion of the CP correction
does not systematically improve the accuracy of the calculations.31 In
our present study, we are primarily interested in the relative energies
from the reactant complexes to their transition states which have the
same number of basis functions, and in this case the effect from BSSE
is expected to be small or canceled noticeably. This and the ambiguity
in the benefits of using BSSE corrections led us not to use the BSSE
corrections in the present study.
Figure 2. (A) 13C NMR spectra acquired before (i), during (ii), and after
complete photolysis (iii) of 13CHBr3 in D2O solvent. Photolysis converts
the parent 13CHBr3 band at 12 ppm into new photoproduct bands at 185
ppm and 166 ppm assigned to CO and HCOOH, respectively. The identities
of the photoproduct bands were confirmed by 13C NMR spectra of authentic
samples of CO (iv) and HCOOH (v). (B) Infrared spectrum of gas produced
during photolysis of CHBr3 in water.
Results
[Br-] versus the decrease in [CHBr3] has a linear relationship
with a slope of about 3. A plot of the changes in the [H+]
concentrations derived from the pH measurements was plotted
versus the changes in the [Br-] concentrations (inset of Figure
1), and this exhibits a linear relationship with a slope of about
1. These results for the UV/vis and pH photochemistry experi-
ments (in Figure 1) indicate that ultraviolet photolysis of CHBr3
in water at low concentrations (<10-4 M) releases 3H+ and
3Br- products (e.g., 3HBr leaving groups that dissociate to 3H+
and 3Br- in water solvent).
To determine the fate of the carbon atom from the CHBr3
parent molecule following 240 nm photolysis, a carbon-13
labeled sample of CHBr3 was used to repeat the 240 nm
photolysis experiments in water. 13C NMR spectra were obtained
before, during, and after complete photolysis of 13CHBr3 in D2O
solvent (shown in Figure 2A-i-iii) in an airtight NMR tube.
The parent 13CHBr3 band at 12 ppm decreases in intensity, and
a new major photoproduct band appears at 185 nm accompanied
by a minor photoproduct band at 166 ppm (both relative to the
Observation of Efficient CHBr3 Dehalogenation after
Ultraviolet Photolysis in Water. Figure 1 shows ultraviolet/
visible (UV/vis) absorption spectra obtained after 240 nm
photolysis of CHBr3 (9 × 10-5 M) in water. The absorption
bands in the 220-230 nm region due to CHBr3 decrease in
intensity, while a new band at about 190 nm due to Br- increases
in intensity for increasing photolysis times. The clear isosbestic
point at ∼215 nm indicates Br- is directly produced from the
CHBr3 parent molecule. The pH of the sample solutions was
measured at the same time each UV/vis spectrum was acquired
during the photolysis experiments depicted in Figure 1. The inset
of Figure 1 shows a plot of ∆[Br-] versus -∆[CHBr3] derived
from the photolysis experiments and reveals the increase of
(29) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154-2161;
(b) J. Phys. Chem. 1990, 94, 5523-5527.
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9
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Kwok et al.
Figure 3. (A) Ps-TR3 spectra acquired after 267 nm photolysis of CHBr3 in acetonitrile/0.2% water mixed solvent at selected delay times between the pump
and probe pulses. The inset displays a plot of the intensities of the ν3′ isobromoform (2) and ν8 CHBr2OH (b) Raman bands as a function of delay time,
and the solid lines are single-exponential decay and growth fits of the data, respectively. (B) Ps-TR3 spectrum of CHBr2OH photoproduct (top) found by
subtracting an appropriately scaled 100 ps spectrum from the 500 ps spectrum shown in (A) (with a longer accumulation time) and comparison to MP2/
6-311++G** calculated Raman spectra for CHBr2OH. (C) Ps-TR3 spectra of the largest CHBr2OH Raman bands (ν7 and ν8) found as in (B) for selected
delay times in acetonitrile/trace amounts of water (a), acetonitrile/0.2% water (b), and acetonitrile/1% water (c) mixed solvents. The asterisks mark solvent
subtraction artifacts. (D) Plots of the isobromoform (solid points and lines) and CHBr2OH (open points and dashed lines) Raman band intensities as a
function of delay times in acetonitrile/trace amounts of water (circle), acetonitrile/0.2% water (triangle), acetonitrile/1% water (square) mixed solvents. Error
bars are indicated for the weaker Raman band intensities of CHBr2OH, while the error bars are much smaller and generally within the size of the data points
shown for the more intense isobromoform Raman bands.
TMS reference band). After complete photolysis, the parent
13CHBr3 band has disappeared, and only the photoproduct bands
at 185 and 166 ppm are left. Noticeable amounts of gas were
formed during the preceding photochemistry experiments. This
gas was collected, and an infrared spectrum was obtained (see
Figure 2B). This spectrum shows large bands at 2116 and 2174
cm-1 due to CO. 13C NMR spectra were obtained for authentic
samples of CO and HCOOH and compared to the spectra
obtained from the photochemistry experiments (Figure 2A-iv
and v). This comparison clearly shows the characteristic 185
ppm major band and the 166 ppm minor band in the 13C NMR
spectra for the photoproducts are due to CO and HCOOH
products, respectively.
M) of CHBr3 in water leads to the following overall reactions
for the major and minor photoproduct channels:
major products
CHBr3 + hν + nH2O f CO + 3HBr + (n - 1)H2O
(1)
minor products
CHBr3 + hν + nH2O f HCOOH + 3HBr + (n - 2)H2O
(2)
The experimental results shown in Figures 1 and 2 indicate that
the above reactions can convert almost all of the parent CHBr3
compound under low concentration conditions into CO and
3HBr major products and some HCOOH and 3HBr minor
Combining the preceding experimental photochemistry results
suggests that ultraviolet photolysis of low concentrations (<10-4
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Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
Figure 4. The optimized geometries for all of the reactants, reactant complexes, transition states, and product complexes obtained from the MP2/6-31G*
computations are shown for the isobromoform + nH2O f CHBr2OH + HBr + (n - 1)H2O, where n ) 1,2,3 (associated with structures (RC)1-3, (TS)1-3
,
and CHBr2OH + (n - 1)H2O), and CHBr2OH + nH2O f HBrCO + HBr + nH2O, where n ) 0,1,2 (associated with structures (RC)4-6, (TS)4-6, and
(PC)4-6), reactions.
products with no other discernible products. The photoquantum
yield for formation of 3HBr from reactions 1 and 2 was
determined to be about 0.46 ( 0.1 from the disappearance of
CHBr3 and formation of 3HBr (see Supporting Information for
more details). Thus, the overall photochemistry appears to be a
fairly efficient process. The present experimental results are
consistent with and in good agreement with previous studies
that showed 253.7 nm excitation of CHBr3 in water leads to
complete conversion into bromide ions with a photolysis
quantum yield of 0.43.24
CHBr3 in water can lead to efficient reactions to produce CO
and 3HBr major products accompanied by HCOOH and 3HBr
minor products? Picosecond time-resolved resonance Raman
(ps-TR3) experiments recently revealed that photolysis of CH2I2
in largely aqueous solutions leads to appreciable formation of
an isodiiodomethane (CH2I-I) photoproduct with its lifetime
becoming shorter in mixed aqueous solvents as the amount of
O-H bonds in the solvent system increases, suggesting that
isodiiodomethane could be reacting with the H2O solvent
molecules.32 Isodiiodomethane and other isopolyhalomethanes
have been shown both theoretically and experimentally to be
effective carbenoid species that can react with carbon double
Picosecond Time-Resolved Resonance Raman Spectra and
Results. What species produced by ultraviolet photolysis of
9
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A R T I C L E S
Kwok et al.
Figure 5. The optimized geometries for all of the reactants, reactant complexes, transition states, and product complexes obtained from the MP2/6-31G*
computations are shown for the CHBr2OH + nH2O f HBrCO + HBr + nH2O, where n ) 3 (associated with structures (RC)7, (TS)7, and (PC)7), and
HBrCO + nH2O f CO + HBr + nH2O, where n ) 0,1,2,3,4 (associated with structures (RC)8-12, (TS)8-12, and (PC)8-12), reactions.
bonds to produce cyclopropanated products33-40 in a manner
similar to the reaction of singlet methylene with carbon double
bonds.35,36,41,42 Carbenes such as singlet methylene and di-
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alcohols.51,52 Thus, one may expect that CH2I-I and other
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(44) Pople, J. A.; Raghavachari, K.; Frisch, M. J.; Blinkley, J. B.; Schleyer, P.
V. R. J. Am. Chem. Soc. 1983, 105, 6389-6398.
(37) Li, Y.-L.; Leung, K. H.; Phillips, D. L. J. Phys. Chem. A 2001, 105, 10621-
10625.
(45) Wesdemiotis, C.; Feng, R.; Danis, P. O.; Williams, E. R.; Lafferty, F. W.
J. Am. Chem. Soc. 1986, 108, 5847-5853.
(38) Fang, W.-H.; Phillips, D. L.; Wang, D.; Li, Y.-L. J. Org. Chem. 2002, 67,
154-160.
(46) Yates, B. F.; Bouma, W. J.; Radom, L. J. Am. Chem. Soc. 1987, 109, 2250-
2263.
(39) Li, Y.-L.; Chen, D. M.; Wang, D.; Phillips, D. L. J. Org. Chem. 2002, 67,
4228-4235.
(47) Walch, S. P. J. Chem. Phys. 1993, 98, 3163-3178.
(48) Gonzalez, C.; Restrepo-Cossio, A.; Ma´rquez, M.; Wiberg, K. B. J. Am.
Chem. Soc. 1996, 118, 5408-5411.
(40) Li, Y.-L.; Wang, D.; Phillips, D. L. J. Chem. Phys. 2002, 117, 7931-
7941.
9
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Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
Figure 6. The optimized geometries for all of the reactants, reactant complexes, transition states, and product complexes obtained from the MP2/6-31G*
computations are shown for the HBrCO + nH2O f HBrCOH2 + (n - 1)H2O, where n ) 1,2 (associated with structures (RC)13-14, (TS)13-14, and (PC)13-14),
and HBrCOH2 + nH2O f HCOOH + HBr + nH2O, where n ) 0,1 (associated with structures (RC)15-16, (TS)15-16, and (PC)15-16), reactions.
isopolyhalomethanes could also react with water in a manner
similar to that of carbenes.
we report further ps-TR3 experiments to examine how the
isobromoform and CHBr2(OH) species are affected by the
concentration of water.
Preliminary ab initio work showed that CH2I-I was able to
react with water via an O-H insertion reaction to produce CH2I-
(OH) and HI products.53 This O-H insertion/HI elimination
reaction was also found to be catalyzed by the presence of a
second water molecule in a manner similar to the dichlorocar-
bene O-H insertion reactions with H2O and 2H2O.53 These
results for CH2I-I suggest that isopolyhalomethane molecules
are probably noticeably reactive with water and undergo O-H
insertion/HX elimination reactions to produce a halogenated
methanol product and HX leaving group. We recently directly
observed the O-H insertion reaction of isobromoform with
water to produce a CHBr2(OH) product in an acetonitrile/0.2%
water mixed solvent system using ps-TR3 spectroscopy.54 Here,
An overview of the ps-TR3 spectra obtained in acetonitrile
solvent with trace amounts of water, and acetonitrile with 0.2%,
1%, 2%, and 5% amounts of water, is given in Supporting
Information Figures S1-S5. These spectra are similar to those
observed previously in spectra obtained in acetonitrile/0.2%
water mixed solvent,54 and all show the isobromoform species
is produced within several picoseconds and then vibrationally
cooled on the 10-50 ps time-scale as has been found for
isobromoform and other isopolyhalomethanes in ps-TR3
experiments.32,54-57 The assignments of the isobromoform
Raman bands in the ps-TR3 spectra in Figures S1-S5 and Figure
3A are in good agreement with those previously observed in
ns-TR3 spectra of isobromoform in cyclohexane solvent,33 and
(49) Pliego, J. R., Jr.; De Almeida, W. B. J. Phys. Chem. 1996, 100, 12410-
12413.
(50) Pliego, J. R., Jr.; De Almeida, W. B. J. Phys. Chem. A 1999, 103, 3904-
(54) Kwok, W. M.; Zhao, C.; Li, Y.-L.; Guan, X.; Phillips, D. L. J. Chem.
Phys., in press.
3909.
(51) Moody, C. J.; Whitman, G. H. In ReactiVe Intermediates; Davies, S. G.,
Ed.; Oxford University Press: New York, 1992.
(52) Kirmse, W.; Meinert, T.; Moderelli, D. A.; Platz, M. S. J. Am. Chem. Soc.
1993, 115, 8918-8927.
(53) Li, Y.-L.; Zhao, C.; Kwok, W. M.; Guan, X.; Zuo, P.; Phillips, D. L. J.
Chem. Phys. 2003, 119, 4671-4681.
(55) Kwok, W. M.; Ma, C.; Parker, A. W.; Phillips, D.; Towrie, M.; Matousek,
P.; Phillips, D. L. J. Chem. Phys. 2000, 113, 7471-7478.
(56) Kwok, W. M.; Ma, C.; Parker, A. W.; Phillips, D.; Towrie, M.; Matousek,
P.; Zheng, X.; Phillips, D. L. J. Chem. Phys. 2001, 114, 7536-7543.
(57) Kwok, W. M.; Ma, C.; Parker, A. W.; Phillips, D.; Towrie, M.; Matousek,
P.; Phillips, D. L. Chem. Phys. Lett. 2001, 341, 292-298.
9
J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3125

A R T I C L E S
Kwok et al.
solvents. Figure 3C shows the 500-800 cm-1 region where the
larger ν8 and ν7 vibrational modes of CHBr2(OH) appear. Figure
3D presents plots of the Raman intensity of the CHBr2(OH)
product as a function of time-delay for the mixed solvent
systems with varying concentrations of water. The CHBr2(OH)
product bands in Figure 3C decay substantially faster at a high
concentration of water, and this indicates that the CHBr2(OH)
product undergoes further reaction with water to produce another
reaction product. The reactions of isobromoform and CHBr2-
(OH) with water molecules were explored by systematically
varying the number of water molecules in the reaction system
using ab initio calculations as described in the next section.
Building Water-Catalyzed O-H Insertion and HBr Elimi-
nation Reactions One Molecule at a Time. Ab initio calcula-
tions (MP2/6-31G*) were done to study the isobromoform +
nH2O f CHBr2OH + HBr + (n - 1)H2O where n ) 1,2,3,
CHBr2OH + nH2O f HBrCO + HBr + nH2O where n )
0,1,2,3, and HBrCO + nH2O f CO + HBr + nH2O where n
) 0,1,2,3,4 reactions. The optimized geometries for all of the
reactants, reactant complexes, transition states, and product
complexes obtained from the MP2 computations are shown in
Figures 4-6. IRC calculations were done to confirm the
transition states connected the appropriate reactants and prod-
ucts.29 The relative energy profiles (in kcal/mol) for the reactions
in Figures 4-6 are presented in Figure 7 and reveal that the
barriers to reaction (e.g., from the reactant complexes to their
respective transition state) become substantially smaller as the
number of H2O molecules in the reaction system increases. This
indicates water catalyzes these reactions. The reaction barrier
decreases from 10.8 kcal/mol for n ) 1 to 2.5 kcal/mol for n )
3 for the isobromoform + nH2O f CHBr2OH + HBr + (n -
1)H2O reaction, from 17.6 kcal/mol for n ) 1 to 2.25 kcal/mol
for n ) 3 for the CHBr2OH + nH2O f HBrCO + HBr +
nH2O reaction, and from 17.8 kcal/mol for n ) 1 to 8.6 kcal/
mol for n ) 3 for the HBrCO + nH2O f CO + HBr + nH2O
reaction.
It is instructive to compare the ab initio results to those
recently found for the dissociation of HBr in H2O complexes.58-61
The stabilization of the HBr(H2O)n clusters varied from 4.2 kcal/
mol for n ) 1 to 54.7 kcal/mol for n ) 5.60 This trend in the
stabilization energies as the number of H2O molecules increases
is similar to that previously found for X-(H2O)n clusters62,63
and the results presented here for isobromoform(H2O)n, CHBr2-
OH(H2O)n, and HBrCO(H2O)n reactant complexes and transition
states (see structures shown in Figures 4-6 and energy profiles
in Figure 7). The HBr elimination reactions of the [CHBr2OH]-
(H2O)n and [HBrCO](H2O)n species are similar to the dissocia-
tion of HBr in HBr(H2O)n clusters. The structures for the
[CHBr2OH](H2O)n where n ) 0,1,2,3 product complexes shows
that (PC)4 with no H2O and (PC)5 with one H2O (see structures
in Figure 4) have Br‚‚‚H distances of 1.442 and 1.473 Å that
are close to the values of 1.4-1.5 Å for a nondissociated HBr
molecule in the HBr(H2O)n complexes where n ) 0,1,2.60 The
Table 1. Comparison of Experimental and Calculated Vibrational
Frequencies (in cm-1) for the CHBr2OH Photoproduct Species
(See the Text for More Details on the Description of the MP2/
6-311++G** ab Initio Calculations)
Raman bands of
second species
in acet./0.2% of H O
2
500 ps spectrum
Ps-TR expt.
400 nm probe
CHBr2OH
MP2 calc.
6-311++G**
possible qualitative
description for the
vibrational modes
3
A′ ν1 3836 O-H stretch
ν2 3220
ν3 1419
ν4 1249
ν5 1219
6ν 1140
7ν 688
C-H stretch
O-H + C-H bend (asym.)
O-H + C-H bend (sym.) + C-O str.
C-H bend
1078
690
601
433
361
284
C-O stretch
Br-C-Br asymmetric stretch
Br-C-Br symmetric stretch
O-H bend
8ν 597
9ν 458
1ν0 373
1ν1 278
C-BrBrH umbrella
Br-C-Br stretch/bend
Br-C-Br in plane bend or scissor
ν
12 175
Table S1 provides details of the vibrational assignments. Figure
3A presents ps-TR3 spectra acquired at selected times after
photolysis of CHBr3 in acetonitrile/0.2% water solvent. The
Raman bands of isobromoform (the major band ν3′ is highlighted
in Figure 3A) decay, and new Raman bands assigned to a
CHBr2(OH) reaction product (the major band ν8 is highlighted
in Figure 3A) are formed. The formation of these new Raman
bands is directly correlated to the decay of the isobromoform
bands (see inset of Figure 3A that compares the decay of the
ν3′ isobromoform band intensity to that of the growth of the ν8
CHBr2(OH) band intensity) and indicates the new Raman bands
are a reaction product of isobromoform. The lifetime of
isobromoform strongly depends on the concentration of H2O.
The lifetimes vary between 283, 225, 133, 89, and 46 ps in
acetonitrile/trace amounts, 0.2%, 1%, 2%, and 5% of H2O mixed
solvents, respectively (see plots of the ν3′ isobromoform band
intensities in Figure 3D and ps-TR3 spectra in Figures S1-S5
of the Supporting Information). This indicates isobromoform
is reacting with H2O to make the reaction product observed
correlate with its decay. This is further supported by the
observation that, as the isobromoform Raman band intensity
decays faster as the water concentration increases, the second
species appearance grows in faster and directly correlates with
the reaction of isobromoform with water. Figure 3B compares
a ps-TR3 spectrum of the new reaction product found by
subtracting an appropriately scaled 100 ps spectrum (Raman
bands mainly due to isobromoform) from the 500 ps spectrum
(Raman bands due to both isobromoform and its reaction
product) of Figure 3A with the Raman spectrum computed from
ab initio calculations for the CHBr2(OH) molecule. Inspection
of Figure 3B shows there is good agreement between the
experimental ps-TR3 spectrum for the reaction product by itself
and the ab initio calculated Raman spectrum for the CHBr2-
(OH) molecule. This identifies CHBr2(OH) as the reaction
product species produced from the isobromoform O-H insertion
reaction with water (see Table 1 for details of the vibrational
band assignments).
(58) Conley, C.; Tao, F. M. Chem. Phys. Lett. 1999, 301, 29-36.
(59) Gertner, B. J.; Peslherbe, G. H.; Hynes, J. T. Isr. J. Chem. 1999, 39, 273-
281.
(60) Cabaleiro-Lago, E. M.; Hermida-Ramo´n, J. M.; Rodr´ıguez-Otero, J. J.
Chem. Phys. 2002, 117, 3160-3168.
Figure 3C presents selected ps-TR3 spectra for the CHBr2-
(OH) product found by subtracting an appropriately scaled 100
or 50 ps spectrum from spectra acquired at later times for CHBr3
photolysis in acetonitrile/trace amounts, 0.2%, and 1% mixed
(61) Hurley, S. M.; Dermota, T. E.; Hydutsky, D. P.; Castleman, A. W., Jr.
Science 2002, 298, 202-204.
(62) Lee, H. M.; Kim, K. S. J. Chem. Phys. 2001, 114, 4461-4471.
(63) Kowal, M.; Gora, R. W.; Roszak, S.; Lexzczynski, J. J. Chem. Phys. 2001,
115, 9260-9265.
9
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Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
Figure 7. Relative energy profiles (in kcal/mol) obtained from MP2/6-31G* ab initio calculations to study the (A) isobromoform + nH2O f CHBr2OH +
HBr + (n - 1)H2O, where n ) 1,2,3 with associated structures (RC)1-(RC)3, (TS)1-(TS)3, and (PC)1-(PC)3; (B) CHBr2OH + (n - 1)H2O f HBrCO +
HBr + (n - 1)H2O, where n ) 0,1,2,3 with associated structures (RC)4-(RC)7, (TS)4-(TS)7, and PC4-PC7; and (C) HBrCO + (n - 1)H2O f CO + HBr
+ (n - 1)H2O, where n ) 0,1,2,3,4 reactions with associated structures (RC)8-(RC)12, (TS)8-(TS)12, and (PC)8-(PC)12. Results are also shown from
similar calculations for the (D) HBrCO + (n - 1)H2O f HBrCOH2 + (n - 2)H2O, where n ) 1,2,3 with associated structures (RC)13, (RC)14, (TS)13,
(TS)14, (PC)13, and (PC)14, and HBrCOH2 + (n - 2)H2O f HCOOH + HBr + (n - 2)H2O, where n ) 0,1,2 with associated structures (RC)15, (RC)16,
(TS)15, (TS)16, (PC)15, and (PC)16, reactions. RC ) reactant complex, TS ) transition state, and PC ) product complex.
[CHBr2OH](H2O)n where n ) 2,3 product complexes (see
structure (PC)7 for n ) 3 in Figure 5 as an example) have Br‚
‚‚H distances of 1.920 and 1.986 Å, respectively, that are closer
to the values of ∼2.2 Å for a dissociated HBr molecule in the
HBr(H2O)n complexes60 where n ) 4,5 and suggest that HBr
is mostly dissociated in the [CHBr2OH](H2O)n with n ) 2,3
complexes. Similar behavior is found for the [HBrCO](H2O)n
product complexes where the H‚‚‚Br distances are 1.465 and
1.521 Å, respectively, for the n ) 0 and n ) 1 complexes (see
the structure for (PC)9 for n ) 1 as an example in Figure 5)
consistent with nondissociated HBr and change to 2.136 and
2.246 Å, respectively, for the n ) 2 and n ) 3 complexes (see
structure for (PC)11 for n ) 3 as an example in Figure 5)
consistent with an almost complete dissociation of HBr into
H3O+ and Br- species. These results indicate addition of more
water molecules results in greater stabilization energy, lower
reaction barriers from the reactant complexes to their respective
transition state, and greater dissociation of the HBr leaving group
in the product complexes.
We have done an NBO analysis to find the charge on the
leaving group Br atom for the reactant complexes, transition
states, and product complexes involved in the reactions inves-
tigated here in Figures 4-7. The NBO charges on the leaving
group Br atom in the product complexes (PC)4 and (PC)5 are
-0.239 and -0.309, respectively, while those of (PC)6 and (PC)7
are much higher with values of -0.723 and -0.747, respec-
tively. This is consistent with the (PC)4 with no H2O and (PC)5
with one H2O (see structures in Figure 4) having Br‚‚‚H
distances of 1.442 and 1.473 Å that are close to the values of
1.4-1.5 Å for a nondissociated HBr molecule in the HBr(H2O)n
complexes where n ) 0,1,2, while (PC)6 with two H2O
molecules and (PC)7 with three H2O molecules have Br‚‚‚H
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J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3127

A R T I C L E S
Kwok et al.
distances of 1.920 and 1.986 Å, respectively, that are closer to
the values of ∼2.2 Å for a dissociated HBr molecule in the
HBr(H2O)n complexes where n ) 4,5.60 A similar trend is
observed for the charge on the leaving group Br atom for the
[HBrCO](H2O)n product complexes (PC)8-(PC)12 for the
HBrCO + nH2O f CO + HBr + nH2O where n ) 0,1,2,3,4
HBr elimination reactions.
molecules increases in these reactions (see Figures 4-6). These
trends are consistent with water-catalysis of these reactions.
There are some interesting differences between the dissocia-
tion of HBr in HBr(H2O)n and the [CHBr2OH](H2O)n and
[HBrCO](H2O)n water-catalyzed HBr elimination reactions. For
example, the proton is transferred from the HBr molecule to
the O atom of a H2O molecule in the dissociation of HBr in the
HBr(H2O)n complexes, while the proton is transferred from the
OH group of the CHBr2OH molecule and the H-C moiety in
the [CHBr2OH](H2O)n and [HBrCO](H2O)n HBr water-cata-
lyzed elimination reaction, respectively. This leads to the proton
on the O-H group of CHBr2OH being shared with a H2O
molecule in the transition states (see structures for (TS)5 and
(TS)7 as examples in Figures 4 and 5) of the [CHBr2OH](H2O)n
HBr elimination reactions for n ) 1,2,3 with the O‚‚‚H-O-C
moiety having all of the O-H distances in a narrow range (from
1.03 to 1.3 Å). Similar behavior is observed in the transition
states (see structures for (TS)9 and (TS)11 as examples in Figure
5) for the [HBrCO](H2O)n water-catalyzed HBr elimination
reactions where the O-H distances in the O‚‚‚H‚‚‚C-O moiety
are similar to one another within a narrow range of 1.2-1.35
Å consistent with the proton being shared by a water molecule
and the CO group of HBrCO. The HBr elimination process in
the [isobromoform](H2O)n is coupled to the O-H insertion
reaction with water, and this results in the C-O bond formation
of the H2O molecule being coupled to a proton transfer to
another H2O molecule and the solvation of the terminal Br atom
(see structures (RC)3, (TS)3, and (PC)3 in Figure 4 for the
[isobromoform](H2O)3 reaction as an example). This indicates
that the water-catalyzed solvation and dissociation of a HBr or
similar leaving groups may be coupled to help drive other
reactions for other potential applications.
The terminal Br atom of isobromoform has NBO charges of
-0.58, -0.62, and -0.71 for (RC)1-(RC)3, respectively, for
the isobromoform + nH2O f CHBr2OH + HBr + (n - 1)-
H2O where n ) 1,2,3 reactions. These charges are consistent
with the isobromoform species having a CHBr2+‚‚‚Br- character
with more ionic character, longer Br-Br bond lengths, and
greater stabilization energy as the number of H2O molecules
increase. The charges on the terminal Br atom increase
significantly as the reactant complexes proceed to their respec-
tive transition states and have values of -0.806 for (TS)1,
-0.822 for (TS)2, and -0.760 for (TS)3. The charges initially
on the leaving group Br atom of the CHBr2(OH) molecule for
the CHBr2OH + nH2O f HBrCO + HBr + nH2O where n )
0,1,2,3 reactions are fairly small for their reactant complexes
with values of -0.013 for (RC)4, -0.061 for (RC)5, -0.085
for (RC)6, and -0.121 for (RC)7 [consistent with the CHBr2-
(OH) molecule having mostly covalent character] but increase
substantially in their respective transition states to -0.710 for
(TS)4, -0.596 for (TS)5, -0.412 for (TS)6, and -0.356 for
(TS)7. Similarly, the charges on the Br atom of the HBrCO
molecule in the HBrCO + nH2O f CO + HBr + nH2O where
n ) 0,1,2,3,4 reactions are moderate for their reactant complexes
with values of -0.075 for (RC)8, -0.157 for (RC)9, -0.214
for (RC)10, -0.200 for (RC)11, and -0.205 for (RC)12 and
increase significantly in their respective transition states to
-0.638 for (TS)8, -0.696 for (TS)9, -0.700 for (TS)10, -0.688
for (TS)11, and -0.678 for (TS)12. For all of these reactions,
the negative charge on the leaving Br atom increases signifi-
cantly from the reactant complexes to their respective transition
states, and as the number of H2O molecules increases there tends
to be less change in the negative charge of the leaving Br atom
as the reaction goes from its reactant complex to its transition
state. For example, the charges change by -0.226 from (RC)1
to (TS)1, -0.201 from (RC)2 to (TS)2, -0.053 from (RC)3 to
(TS)3 for the isobromoform + nH2O f CHBr2OH + HBr +
(n - 1)H2O where n ) 1,2,3 O-H insertion/HBr elimination
reactions. Similarly, the charges change by -0.696 from (RC)4
to (TS)4, -0.535 from (RC)5 to (TS)5, -0.327 from (RC)6 to
(TS)6, -0.235 from (RC)7 to (TS)7 for the CHBr2OH + nH2O
f HBrCO + HBr + nH2O where n ) 0,1,2,3 HBr elimination
reactions and by -0.563 from (RC)8 to (TS)8, -0.539 from
(RC)9 to (TS)9, -0.487 from (RC)10 to (TS)10, -0.489 from
(RC)11 to (TS)11, and -0.472 from (RC)12 to (TS)12 for the
HBrCO + nH2O f CO + HBr + nH2O where n ) 0,1,2,3,4
HBr elimination reactions. These smaller changes in the charge
distribution on the Br leaving atom as the number of water
molecules increases suggest less energy is needed for redistribu-
tion of the charge to the leaving group as the reaction proceeds
from the reactant complex to its corresponding transition state.
This also roughly correlates with generally smaller structural
changes taking place and lower reaction barrier heights (e.g.,
from reactant complex to transition state) as the number of H2O
Discussion
Reaction Mechanism for the CHBr3 + hν + n(H2O) f
CO + 3HBr + (n - 1)H2O Major Product Overall Reaction.
On the basis of the present experimental and theoretical results
as well as other work already available in the literature, the
following reaction mechanism is proposed for the CHBr3 + hν
+ n(H2O) f CO + 3HBr + (n - 1)H2O major product overall
reaction observed upon photolysis of low concentrations of
CHBr3 in aqueous solutions:
photolysis of CHBr3 to form CHBr2 and Br fragments
step 1 CHBr3 + hν f CHBr2 + Br
solvent-induced geminate recombination of the CHBr2
and Br fragments to form isobromoform
step 2 CHBr2 + Br f isobromoform (BrCHBr-Br)
water-catalyzed O-H insertion/HBr elimination
reaction of isobromoform with nH2O
step 3 isobromoform + n(H2O) f
CHBr2OH + HBr + (n - 1)H2O
water-catalyzed HBr elimination reaction of CHBr2OH
with nH2O solvent
step 4 CHBr2OH + n(H2O) f HBrCO + HBr + n(H2O)
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3128 J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004

Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
chloromethanol species has been observed in both low-temper-
ature matrix isolation experiments69 and gas-phase experi-
ments67,70 and in all cases was found to decompose in the dark
to H2CO and HCl products and to be accelerated by heteroge-
neous processes in the gas phase.67 These experimental results
are consistent with step 4 of the major channel proposed reaction
mechanism and the current ab initio results for the HBr
elimination reaction of CHBr2OH insofar as CHBr2OH behaves
like CHCl2OH and CH2ClOH. Both HBrCO and HClCO were
observed to decompose to CO + HX (X ) Br or Cl) in the
dark with the decomposition accelerated by heterogeneous
processes.68,71 In addition, the decomposition of the closely
related HClCO molecule has been experimentally observed in
aqueous solutions68 and found to produce CO + HCl major
products as well but at a much faster rate than in the gas-phase
reaction.71 This is consistent with step 5 of the proposed reaction
mechanism and our present ab initio results for the HBrCO +
n(H2O) reactions. The decomposition of HClCO in aqueous
solutions also formed HCOOH + HCl minor products68 similar
to the HCOOH + HBr we observe as minor products after 240
nm photolysis of CHBr3 in water. This suggests that decomposi-
tion of HBrCO has two pathways with the major one leading
to CO + HBr products in step 5 of the major reaction pathway
and a minor pathway leading to HCOOH + HBr products.
Further ab initio calculations were done for the HBrCO +
n(H2O) f HBrC(OH)2 + (n - 1)H2O where n ) 1,2,3 and the
HBrC(OH)2 + n(H2O) f HCOOH + HBr + n(H2O) where n
) 0,1 reactions (see structures and energy profiles in Figures 6
and 7, respectively). The barrier for the HBrCO + n(H2O) f
HBrC(OH)2 + (n - 1)H2O where n ) 1,2,3 reaction remains
high (21.6 kcal/mol for two water molecules involved) because
it is an O-H insertion reaction without an HX leaving group
and is thus not significantly water-catalyzed by additional H2O
molecules beyond n ) 2. However, the decomposition of the
HBrC(OH)2 + n(H2O) f HCOOH + HBr + n(H2O) where n
) 0,1,2 reaction proceeds easily (1.4 kcal/mol for one water
molecule involved) as do the other HBr elimination water-
catalyzed reactions of halomethanols. The high barrier for the
HBrCO + nH2O f HBrC(OH)2 + (n - 1)H2O where n ) 1,2,3
O-H insertion reaction (>20 kcal/mol) as compared to the low
barrier for the HBrCO + n(H2O) f CO + HBr + nH2O where
n ) 0,1,2,3 HBr elimination reaction (about 8-9 kcal/mol)
together with the proposed reaction mechanism explains why
the major products are CO and 3HBr and the minor products
are HCOOH and 3HBr following 240 nm photolysis of CHBr3
in water.
water-catalyzed HBr elimination of HBrCO in
nH2O solvent
step 5 HBrCO + n(H2O) f CO + HBr + n(H2O)
add steps 1-5 to obtain the major product overall reaction
CHBr3 + hν + n(H2O) f CO + 3HBr + (n - 1)H2O
Ultraviolet excitation at wavelengths longer than 250 nm for
many bromine- and/or iodine-containing polyhalomethanes in
gas and solution phases results in direct cleavage of a carbon-
halogen bond to produce haloalkyl radical and halogen atom
fragments. A study of bromoform photodissociation in the gas
phase indicated only primary C-Br bond cleavage will take
place at wavelengths longer than 218 nm because there is not
enough energy for secondary dissociation of the CHBr2 radical
to occur.27 This indicates step 1 in the above reaction mechanism
is the primary photochemical start of the bromoform photodis-
sociation reaction after 240 nm excitation. Ps-TR3 experiments
(see Figures S1-S5 in the Supporting Information) clearly
showed that some of the initially produced CHBr2 radical and
Br atom fragments undergo solvent-induced geminate recom-
bination to form an isobromoform species within a few
picoseconds in room-temperature solutions as has been found
for photolysis of other polyhalomethanes in room-temperature
solutions.32,54-57,64-66 This establishes that step 2 of the proposed
reaction mechanism occurs to an appreciable degree probably
on the order of 50-70%. We note that at low concentrations
(<10-4 M) most of the cage escape photofragments appear to
be scavenged by the other fragment from the initially dissociated
molecule which gives no appreciable Br2, Br2-, or Br3
-
products. However, at higher concentrations in the millimolar
and higher range, the cage escape photofragments have ap-
preciable scavenging by fragments from other dissociated and/
or reacting molecules, and absorption bands become noticeable
-
for Br2, Br2-, and Br3 final products in the photochemistry
experiments (not shown). The present ps-TR3 experiments found
the isobromoform species reacts with water to directly produce
the CHBr2OH product species (see Figure 3), and this provides
direct vibrational spectroscopic evidence that isobromoform
undergoes O-H insertion reactions with H2O molecules con-
sistent with step 3 of the proposed reaction mechanism.
The ab initio results given here indicate that CHBr2OH
undergoes water-catalyzed HBr elimination to produce HBrCO
and HBr products. This is consistent with step 4 in the proposed
reaction mechanism and the ps-TR3 experiments here that show
accelerated decomposition of CHBr2OH with increasing water
concentration (see Figure 3C and D). Not much is known about
CHBr2OH because the ps-TR3 experimental spectra are the only
experimental observation of this compound to our knowledge,
but the very closely related CHCl2OH molecule was found to
decay into HClCO + HCl in the gas phase and decayed much
faster in aqueous solvent (<20 µs).67,68 The closely related
Implications for Decomposition of Polyhalomethanes,
Halomethanols, and Haloformaldehydes in Aqueous Envi-
ronments. The ultraviolet photolysis for wavelengths >250 nm
of most polyhalomethanes in the gas phase leads predominantly
to a direct carbon-halogen bond cleavage and formation of
haloalkyl radical and halogen atom fragments with a near unity
photon quantum yield. However, ultraviolet photolysis of CHBr3
at low concentrations in aqueous solution leads to conversion
of the parent molecule into 3HBr and CO (major) and HCOOH
(64) Tarnovsky, A. N.; Alvarez, J.-L.; Yartsev, A. P.; Su¨ndstrom, V.; Åkesson,
E. Chem. Phys. Lett. 1999, 312, 121-130.
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Åkesson, E. J. Phys. Chem. A 2002, 106, 5999-6005.
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J.; Nielsen, O. J. Chem. Phys. Lett. 2000, 322, 97-102.
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11288-11292.
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4777.
9
J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3129

A R T I C L E S
Kwok et al.
(minor) stable products with about a 0.46 photon quantum yield.
These results indicate the photochemistry of CHBr3 exhibits
significant phase dependence with very different reactions taking
place in a solvated water environment as compared to the gas
phase. The water-catalyzed reactions of isobromoform with
water and the subsequent HBr elimination reactions of CHBr2-
OH and HBrCO with water elucidated here may be noticeable
sources of halogens and/or acid formation in the natural
environment. To our knowledge, the water-catalyzed reactions
of isopolyhalomethanes such as isobromoform have not been
considered for the water-solvated photochemistry of polyha-
lomethanes (such as CH2I2, CH2BrI, CH2Br2, CHBr3, CCl4,
CCl3F, and others) that have been observed in the natural
environment from natural and/or man-made sources.1-11
reactions that are acid catalyzed in heterogeneous and/or
multiphase environments.
Using the Dissociation Reaction of HX into H+ and X-
To Drive Other Reactions. The dissociation reaction of the
HBr leaving group by water into H+ and Br- ions helps drive
several O-H insertion and HBr elimination reactions (see
reaction steps 3-5 in the reaction mechanism section) that lead
to formation of three Br- ions following ultraviolet photolysis
of CHBr3 in water (this work and ref 24). The water solvation
of the HBr leaving group enables this HX dissociation reaction
into ions to be coupled to other reactions and to facilitate
cleavage of C-X, O-H, and C-H bonds. For example, the
water solvation of HBr helps cleave the Br-Br bond of
isobromoform and an O-H bond of water during the O-H
insertion reaction to make a CHBr2OH product with H3O+ and
Br- leaving groups (e.g., the water-solvated dissociated form
of HBr). Similarly, the water solvation of HBr helps break the
C-Br and O-H bonds of CHBr2OH to make a HBrCO product
with H3O+ and Br- leaving groups. The water solvation of HBr
enables the C-H and C-Br bonds of HBrCO to be broken
easily to form CO and HBr major products. This water-catalysis
by solvation of a leaving group to enable its dissociation into
ions (e.g., H+ and Br- in the example studied here) appears to
be a fairly general phenomena for the dehalogenation of
halogenated species in aqueous solutions. This may be utilized
for improving the design and efficiency of photocatalysts and/
or catalysts for decomposition of halogenated compounds in
water treatment processes. The water-catalysis by solvation of
a leaving group to enable its dissociation into ions helps explain
where the energy comes from to break all three C-Br bonds in
CHBr3 to form 3Br- ions. Even though excitation by one photon
of ultraviolet light (240 or 253.7 nm) has only enough energy
to break one C-Br bond in CHBr3, this allows appreciable
formation of the very reactive isobromoform species that can
then undergo the water-catalyzed O-H insertion and HBr
elimination reactions discussed earlier. This type of reaction
can likely be even more general insofar as water solvation of a
molecular leaving group leads to its dissociation into ions and
can be coupled to drive other reactions. This may occur for a
wide range of chemical reactions in water environments.
The present study suggests that ultraviolet photolysis of
polyhalomethanes in solvated aqueous environments would
release the halogens as HX leaving groups instead of X atoms
as are typically found in the gas phase. This suggests that the
pH in the solvated aqueous environment around the parent
polyhalomethane becomes more acidic and may significantly
influence reactions associated with the activation of halogens
in aqueous sea-salt particles because many reaction schemes
presented depend on pH.12-22 For halogen activation on aqueous
sea salt particles, reactions such as HOX + H+ + X- f X2 +
H2O (where X ) Cl and/or Br) and/or HOX- + H+ f X +
H2O have been proposed as the key activation step where H+
and X- help activate the halogen atom.12-22 The work described
here indicates photolysis of CHBr3 at low concentrations in a
water-solvated environment releases three HBr groups. We
speculate that if this happens in aqueous sea-salt particles the
HBr released may cause analogous reactions to activate halogens
such as by the HOX + H+ + Br- f XBr + H2O reaction.
Ultraviolet photolysis of several polyhalomethanes in aqueous
solutions has been observed to release strong acids via their
isopolyhalomethane chemistry (this work and ref 53). We note
that a wide range of isopolyhalomethanes have been experi-
mentally observed in condensed phase environments following
ultraviolet photolysis of polyhalomethanes that contain Cl, Br,
and/or I atoms in organic solvents.32-34,37,39,40,55-57,64-66,72-78
This and the similar observation of isopolyhalomethanes formed
in largely aqueous solutions32 suggest that ultraviolet photolysis
of many polyhalomethanes in water-solvated environments
probably produces isopolyhalomethanes via solvent-induced
geminate recombination of the initially formed photofragments.
Thus, the formation of isopolyhalomethanes and their associated
chemistry may be fairly common for polyhalomethane photo-
chemistry in water-solvated environments. We think it would
be worthwhile for a number of research groups with differing
areas of expertise to pursue further research to better understand
how this water-solvated chemistry may actually influence or
affect the chemistry of the natural environment particularly for
Conclusions
An experimental and theoretical study of the photochemistry
of CHBr3 in pure water and in acetonitrile/water mixed solvents
was presented. Ultraviolet excitation of CHBr3 in water gave
almost complete conversion into 3HBr leaving groups and CO
(major product) and HCOOH (minor product) molecules. Ps-
TR3 experiments in conjunction with results from ab initio
calculations indicate that the water-catalyzed O-H insertion/
HBr elimination reaction of isobromoform and subsequent
reactions of its products lead to the production of the final
products. These results have important ramifications for the
phase-dependent behavior of polyhalomethane photochemistry
and chemistry in water-solvated environments relative to gas-
phase reactions. Gas-phase photolysis with ultraviolet light
(>230 nm) of polyhalomethanes typically results primarily in
cleavage of one carbon-halogen bond to produce a haloalkyl
radical and halogen atom fragments. In contrast, ultraviolet
photolysis (>230 nm) of polyhalomethanes such as CHBr3 at
low concentrations in water-solvated environments appears to
lead to cleavage of multiple carbon-halogen bonds and releases
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Water-Catalyzed Dehalogenation Reactions
A R T I C L E S
halide ions as part of strong acid leaving groups. The solvation
of the HBr leaving group by water enables the dissociation
reaction into H+ and Br- ions to be used to catalyze several
O-H insertion and HBr elimination reactions of isobromoform
and its reaction products. This enables O-H and C-H bonds
to be cleaved more easily in the presence of water molecules
than in the absence of water molecules.
Supporting Information Available: Details for finding the
experimental estimate of the photoquantum yield for formation
of 3Br- from ultraviolet photolysis of CHBr3 in water, Figures
S1-S5 showing an overview of the ps-TR3 spectra obtained at
varying time delays following 267 nm photolysis of CHBr3 in
acetonitrile/trace amounts, 0.2%, 1%, 2%, and 5% water mixed
solvents, and Table S1 comparing the isobromoform Raman
band vibrational frequencies obtained in this work to those
previously obtained in cyclohexane solution and from density
functional theory calculations of ref 31. Cartesian coordinates,
total energies, and zero-point energies obtained from the ab initio
calculations (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. This work was done in the Ultrafast Laser
Facility at the University of Hong Kong and was supported by
grants from the Hong Kong Research Grants Council (HKU/
7087/01P and HKU 1/01C) to D.L.P. W.M.K. thanks the
University of Hong Kong for a Postdoctoral Fellowship. We
would like to thank Dr. P. Matousek, Dr. M. Towrie, Dr. A.
W. Parker, and Professor D. Phillips for their helpful discussions
and advice on setting up our ultrafast laser system.
JA0390552
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