Water Exchange Rates and Molecular
Mechanism around Aqueous Halide Ions
Harsha V. R. Annapureddy and Liem X. Dang*
AUTHOR ADDRESS: Physical Sciences Division,
Pacific Northwest National Laboratory,
Richland, WA 93352.
*[email protected]
(509) 375-2557
Activation volumes were evaluated using rate constants computed with different theories at
various pressures.
GH theory rate
Pressure,
M Pa
Chloride
Bromide
Iodide
k TST , ps !1
! GH
constant
RF method
! RF
k GH = ! GH k TST
0
1.07
0.043
100
1.20
0.027
200
1.32
0.018
0
1.22
0.034
100
1.33
0.025
200
1.46
0.020
0
1.52
0.035
100
1.64
0.028
200
1.78
0.020
k RF = ! GH k TST
0.10
0.046
0.04
0.033
0.02
0.024
0.09
0.042
0.05
0.033
0.04
0.030
0.09
0.053
0.05
0.047
0.04
0.036
rate constant
0.108
0.052
0.032
0.104
0.070
0.066
0.142
0.074
0.078
As discussed in the manuscript, we computed the activation volumes using the following equation:
Using the rate constants computed at different pressures, we plot
linear function. The slope obtained from this fitting gives
vs.
and fit it to a
.
The plots obtained for TST are provided below. All of the plots exhibit a positive slope, which
gives a negative activation volume (
).
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:(;&;+)
1/0//) 6/0//) 2/0//) 7/0//) 5//0//)
P
RT
Next, we use the same procedure with rate constants obtained from the RF method. Fits to the
rate constants obtained from the RF method are provided below. These plots have negative
slopes (with some statistical error), which give a positive activation volume (
).
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!"
!!
!
!!
4544'
4544'
345;4'
;4544' :4544' <4544' 84544' 644544'
;4544' :4544' <4544' 84544' 644544'
34564'
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345:4'
345<4'
1'2'34544<76;+'
'
345;4'
345=4'
34584'
36544'
4544'
4544'
P
RT
345:4'
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365;4'
34574'
365:4'
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4544'
4544' ;4544' :4544' <4544' 84544' 644544'
34564'
345;4'
345=4'
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*%$+&'())
34574'
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'
,$'&'()
345<4'
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Fits obtained using GH theory rate constants are provided below. Similar to the trend observed for
the RF method, all the plots yield negative slopes (with some statistical error), which give a positive
activation volume (
).
!"#$%&'()%*+,-%#"(+
P
RT
/0//+
/0//+ 8/0//+ 3/0//+ 6/0//+ 1/0//+ 9//0//+
&/09/+
!!
!
!!
&/02/+
!"
&/08/+
&/07/+
/0//+
/0//+ 8/0//+ 3/0//+ 6/0//+ 1/0//+ 9//0//+
(+.+&/0//12345+
+
&/09/+
!"#$%&'()
&/08/+
&/03/+
&/06/+
&/09/+
(+.+&/0//384:5+
+
*%$+&'()
&/02/+
&/08/+
(+.+&/0//37:25+
+
,$'&'())
&/02/+
&/03/+
&/04/+
&/01/+
/0//+
/0//+ 8/0//+ 3/0//+ 6/0//+ 1/0//+ 9//0//+
&/03/+
&/07/+
We followed the same process as reported in the literature by Kerisit and Rosso1 and Rustad and
Stack2 who also determined the activation volumes using rate constants from TST, the RF
method, and GH theory.
From our current studies, we observed that the trend in the variation of rate constants with
pressure for GH theory and the RF method is opposite to the trend noticed in TST for all the
halide ions.
In the case of solvent exchange around aqueous lithium studied previously by our group and also
by Rustad and Stack,2 the TST rate constants and transmission coefficients both increase as
pressure increases.
In their studies, Kerisit and Rosso1 observed that the TST rates and
transmission coefficients decrease as pressure increases.
Contrary to the two previous observations described above, we observed in this current study
that, for halides, TST rates and transmission coefficients have the opposite trend when pressure
increases. Because the computed transmission coefficients are very small and they also decrease
significantly as pressure increases, the effect on transmission coefficients overshadows the effect
of pressure on the activation barrier, thereby resulting in a decrease of rate constants (from GH
theory and RF method) as pressure increases.
As a result, we noted an opposite sign in
activation volumes. We also would like to point out that the rate constants and mechanism of
exchange are sensitive to several factors, such as choice of reaction coordinates, potential
models, etc.
Refrences
1. Kerisit S and KM Rosso. Transition path sampling of water exchange rates and mechanisms
around aqueous ions. Journal of Chemical Physics 2009, 131(11):114512.
2. Rustad JR and AG Stack. Molecular dynamics calculation of the activation volume for water
exchange on Li. Journal of the American Chemical Society 2006, 128 (46): 14778-14779.