8040 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Bilde et al.
TABLE 2: Enthalpy Changes at 298 K for a Set of
Reactions of the Type RX + Cl f R + XCl Where R )
Methyl or a Halo-Substituted Methyl Group and X ) Cl,
Br, or Ia
Cl+halomethane reactants. Formation of CH2Cl+ICl from the
Cl+CH2ICl reaction is apparently exothermic (based upon the
rapid observed reaction); hence, the energetically most favorable
pathway for CH2ClI‚‚‚Cl decomposition is to CH2Cl+ICl, not
to CH2ICl+Cl.
RX
XCl
∆H
r
(298 K) (kJ mol-1)
CH
CH
CH
CH
CH
CH
CH
3
3
3
2
2
2
2
Cl
Br
I
Cl
BrCl
ICl
Cl
2
BrCl
ICl
BrCl
Cl
2
2
108 ( 1
76 ( 1
26 ( 1
96 ( 8
60 ( 17
2 ( 29
72 ( 8
78 ( 8
Since the Cl+CH2ICl reaction is very fast, its potential role
as an atmospheric degradation mechanism for CH2ICl warrants
consideration. The atmospheric photolysis rate of CH2ICl has
b
Cl
2
6
-5
a
recently been evaluated by Rattigan et al.; a value of 2 × 10
BrCl
ICl
a
b
-1
s
appears typical for the tropical and midlatitude marine
Br
2
boundary layer. Establishing chlorine atom concentrations in
the marine boundary layer is a topic of much current interest
within the atmospheric chemistry community. The best esti-
mates currently available suggest that typical marine boundary
CHCl
3
a
Heats of formation used to evaluate heats of reaction are taken
b
from ref 12 unless otherwise indicated. Heats of formation of CH
and CH ClI are estimates based on group additivity, taken from ref
2. Heat of formation of ICl is taken from ref 33.
2
ClBr
4
-3 29-31
2
layer levels of Cl atoms are around 10 /cm ,
although the
c
3
uncertainty in this estimate remains rather high. Using this
-
11
3
estimate in conjunction with the value k1 ) 8.5 × 10
cm
showing that reactions of atomic chlorine with CH3I, CH3Br,
CF3CH2I, and CD3CD2I proceed via two channels: adduct
formation and direct hydrogen abstraction. Adduct formation
rate constants are found to be much faster than the corresponding
hydrogen abstraction rate constants, but the dominant fate
of the weakly bound adducts in the experiments of Wine and
-1 -1
molecule
s
reported in this study gives a pseudo-first-order
-7 -1
rate constant of 8.5 × 10
s
for CH2ICl destruction via
reaction with Cl. Hence, reaction 1 will compete with photolysis
as an atmospheric destruction mechanism for CH2ICl only if
the average marine boundary layer concentration of Cl atoms
is near the high end of the range of possible values. There are
no kinetic data in the literature for the OH + CH2ICl reaction.
For this reaction to be important as an atmospheric destruction
mechanism for CH2ICl, its rate constant would have to be much
faster than the known rate constants for OH reactions with CH3I
2
4
co-workers was dissociation back to reactants.
Ab initio
calculations by McKee2 predict 298 K adduct bond strengths
4c
similar to those measured by Wine and co-workers (ranging
-
1
-1
from 24.5 kJ mol for CH3Br‚‚‚Cl to 59.l kJ mol for CD3-
2
5
CD2I‚‚‚Cl). Similar calculations by Lazarou et al. report
the existence of stable adducts of Cl atoms with HI, CH3I,
and CH3OCH2I with 298 K bond strengths of 31.1, 52.4, and
1.3 kJ mol , respectively. Recent studies of the reaction of
F atoms with CF2BrH, CH2BrCl, and CH3Br
-14
-14
3
-1 -1
and CF3I (7.2 × 10
and 3.1 × 10 cm molecule
s
at
11
2
98 K, respectively ).
-
1
5
Acknowledgment. Research at Georgia Tech was supported
26-28
have shown
by Grants NAGW-1001 and NAG5-3634 from the National
Aeronautics and Space Administration-Upper Atmosphere Re-
search Program.
that adduct formation is also important in the reaction of F
atoms with brominated methanes. It appears that the formation
of short-lived adducts is a common facet of the reactions of
F and Cl atoms with brominated and iodinated organic
compounds.
Kinetic data for reactions of Cl atoms with a number of
chloro- and bromo-substituted methanes have been reported in
the literature. Room-temperature rate constants for Cl reactions
with CH3Cl, CH2Cl2, CHCl3, CH3Br, CH2Br2, and CH2ClBr are
References and Notes
(
1) Davis, D.; Crawford, J.; Liu, S.; McKeen, S.; Bandy, A.; Thornton,
D.; Rowland, F.; Blake, D., J. Geophys. Res. 1996, 101, D1, 2135.
2) Chameides, W. L., Davis, D. D., J. Geophys. Res. 1980, 85, C12,
383.
3) Solomon, S., Rolando, R. G., Ravishankara, A. R., J. Geophys.
(
7
(
-
13
3
-1 -1
all within the range (1-5) × 10
hydrogen abstraction being the dominant reaction pathway;
activation energies for these reactions lie in the range 6.7-
cm molecule
s
with
Res. 1994, 99, D10, 20, 491.
1
2
(4) Moore R. M., Tokarczyk, J. Geophys. Res. 1992, 19, 1779.
(5) Class, T. H.; Ballschmiter, K. J. Atmos. Chem. 1988, 6, 35.
-1 11
(6) Rattigan, O. V.; Shallcross, D. E.; Cox, R. A., J. Chem. Soc.,
Faraday Trans. 1997, 93, 2839.
1
1.4 kJ mol . Formation of a weakly bound adduct has been
24
observed for the Cl+CH3Br reaction, and such adducts
presumably can form in other Cl+haloalkane reactions as well;
however, in most cases adduct formation appears to be rapidly
reversible. The room-temperature rate constant for hydrogen
abstraction from CH3I by Cl is 8 × 10 cm molecule s .
The CH3I‚‚‚Cl adduct is more strongly bound than are Cl
adducts with chloro- or bromo methanes, but in the absence of
(7) Roehl, C. M.; Burkholder, J. B.; Moortgat, G. K.; Ravishankara,
A. R.; Crutzen, P. J. J. Geophys. Res. 1997, 102, 12819.
(8) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 9, 399.
(9) Nicovich, J. M.; Wang, S.; McKee, M. L.; Wine, P. H. J. Phys.
Chem. 1996, 100, 680 and references therein.
-
13
3
-1 -1 24
(10) Stated minimum purity of liquid phase in high-pressure cylinder.
(11) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437.
(12) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
2
4
scavengers, its primary fate is dissociation back to Cl+CH3I.
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J., Evaluation No. 12, Jet Propulsion Laboratory Publication 97-4; Jet
Propulsion Laboratory: Pasadena, CA, 1997 and references therein.
The room-temperature rate constant reported in this study for
nonreversible channels of the Cl+CH2ClI reaction is 100-800
times faster than those of the Cl+haloalkane reactions discussed
above, and the dominant reaction pathway is halogen transfer
as opposed to hydrogen transfer for the reactions discussed
above. The heats of reaction for halogen transfer channels in
Cl reactions with the halomethanes discussed above are listed
in Table 2. The atypical kinetic behavior observed for the
Cl+CH2ICl reaction can be rationalized on thermochemical
grounds. Production of dihalogen products from Cl reactions
with CH3Cl, CH2Cl2, CHCl3, CH3Br, CH2Br2, CH2ClBr, and
CH3I is endothermic in all cases; hence, the energetically most
favorable pathway for adduct decomposition is back to
(13) Busch, G. E.; Mahoney, R. T.; Morse, R. I.; Wilson, K. R. J. Chem.
Phys. 1969, 51, 449.
(
(
14) Park, J.; Lee, Y.; Flynn, G. W. Chem. Phys. Lett. 1991, 186, 441.
15) Tyndall, G. S.; Orlando, J. J.; Kegley-Owen, C. S., J. Chem. Soc.,
Faraday Trans. 1995, 91, 3055.
(16) Fletcher, I. S.; Husain, D. Chem. Phys. Lett. 1977, 49, 516.
(17) Clark, R. H.; Husain, D. J. Photochem. 1983, 21, 93.
(
18) Chichinin, A. I.; Krasnoperov, L. N. Chem. Phys. Lett. 1989, 160,
48.
19) Seetula, J. A.; Gutman, D.; Lightfoot, P. D.; Rayez, M. T.; Senkan,
S. M. J. Phys. Chem. 1991, 95, 10688.
20) Braun, W.; Herron, J. T.; Kahaner, D. K. Int. J. Chem. Kinet. 1988,
20, 51.
4
(
(