Charge Effect on the Diffusion Coefficient and the Bimolecular

Mar 20, 2009 - coefficients we obtained were larger in RTILs than the theoretical predictions by the Stokes-Einstein relation, whereas both values are...
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J. Phys. Chem. B 2009, 113, 5188–5193

Charge Effect on the Diffusion Coefficient and the Bimolecular Reaction Rate of Diiodide Anion Radical in Room Temperature Ionic Liquids Yoshio Nishiyama,* Masahide Terazima, and Yoshifumi Kimura* Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan ReceiVed: December 22, 2008; ReVised Manuscript ReceiVed: February 3, 2009

The diffusion coefficients of diiodide anion radical, I2-, in room temperature ionic liquids (RTILs) were determined by the transient grating (TG) method using the photochemical reaction of iodide. The diffusion coefficients we obtained were larger in RTILs than the theoretical predictions by the Stokes-Einstein relation, whereas both values are similar in conventional solvents. By comparison with the diffusion coefficients of neutral molecules, it was suggested that the Coulomb interaction between I2- and constituent ions of RTILs strongly affects the diffusion coefficients. The bimolecular reaction rates between I2- were calculated by the Debye-Smoluchowski equation using the experimentally determined diffusion coefficients. These calculated reaction rate were much smaller than the experimentally determined rates (Takahashi, K.; et al. J. Phys. Chem. B 2007, 111, 4807), indicating the charge screening effect of RTILs. 1. Introduction Transport properties of room temperature ionic liquids (RTILs) have attracted much attention of chemists,1 since they directly reflect the electric conductivity which is one of significant features of RTILs. Especially the translational diffusion coefficients of solvent ions2,3 and solute ions4,5 have been extensively measured by pulse-field-gradient-spin-echo (PGSE) NMR or electrochemical methods. For example, Watanabe et al. reported the diffusion coefficients of component ions of various RTILs,2 and discussed the relation between the diffusion coefficients and electric conductivities. Generally the electric conductivity of RTILs is smaller than that predicted by the Nernst-Einstein equation using the diffusion coefficients of the solvent ions. This smallness was interpreted as the result of ionic association between cations and anions. The ratio between the above two quantities is regarded as the estimate of the ionicity, that is, what extent cations and anions exist as independent ions not as ion pair. This property was found to be related with several thermodynamical properties of RTILs.2 It has been also reported that the effect of the solute charge on the diffusion is significant in RTILs. For example, Evans et al. measured the diffusion of N,N,N′,N′,-tetramethyl-p-phenylenediamine (TMPD) and its cation radical (TMPD+) in RTILs,4c and found that TMPD+ diffuses twice as slowly as TMPD in RTILs, while it diffuses about 1.15 times slower in acetonitrile. Katayama et al. also found a significant effect of the solute charge on the diffusion of several transition metal complexes in RTILs such as [Fe(CN)6]3- and [Fe(CN)6]4-.5 The solute diffusions in RTILs have also been discussed in terms of the bimolecular reaction rates. In the case of the diffusion-limited reactions, the reaction rates have been compared with theoretical rates (kdiff) estimated by the Smoluchowski equation,6

kdiff ) 4πRabDab

(1)

* Towhomcorrespondenceshouldbeaddressed.E-mail:yosi1166@kuchem. kyoto-u.ac.jp; [email protected].

SCHEME 1: Reaction Schemes after the Photoexcitation of I- in Solution

where Dab is the mutual diffusion coefficient of reactants and Rab is the reaction distance. In many cases, however, the diffusion coefficients of the reactant molecules are not available, because the reactants are often short-lived species. Hence, the diffusion coefficients are estimated by the Stokes-Einstein (SE) relation,

D)

kBT Cπηr

(2)

where kB, T, η, and r are the Boltzmann constant, the temperature, the solvent viscosity, and the radius of solute, respectively. The constant C is dependent on the boundary condition of the surface flow (four for the slip, six for the stick boundary condition). By using eq 2 with the stick boundary condition, eq 1 can be rewritten as

kdiff )

8000RT 3η

(3)

where R denotes the gas constant. The preceding studies on the diffusion-limited reactions in RTILs have thrown a deep doubt on the applicability of eq 3.7-10 For example, McLean et al. studied quenching reaction of the triplet state of benzophenone by naphthalene in imidazolium-based ionic liquids.8 They found that the reaction rates are nearly 10 times faster than the predictions by eq 3. The result suggests that the diffusion coefficient calculated by the SE relation is not necessarily correct. Takahashi et al. measured the bimolecular reaction rates between diiodide anion radical (I2-) in molecular liquids and in RTILs through the subsequent reaction after the photoexci-

10.1021/jp811306b CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009

Diffusion Coefficients of I2-, in RTILs

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tation of iodide (I-) (see Scheme 1).11 They found that the reaction rates were slightly larger in RTILs than the theoretical predictions by eq 2, whereas they were much smaller than the theoretical rates in conventional liquids. They interpreted the slow reaction rates in the molecular liquids by the effect of the Coulomb repulsion between the reactant molecules. The accelerated reaction rate in RTILs were considered to be due to the screening effect on the Coulomb repulsion by the solvent ions. However, since the SE equation does not always predict correct diffusion coefficients, the above discussions may not be correct. It is necessary to discuss the diffusion-limited reactions in RTILs on a firm basis of the experimentally determined diffusion coefficients of reactants. However, it is generally difficult to measure the diffusion coefficients of the reactant molecules for diffusion-controlled reactions, since they are often short-lived molecules and traditional methods for measuring the diffusion coefficients cannot be applied to such system. In this respect, it has been demonstrated that the transient grating (TG) method is powerful for the measurement of diffusion coefficients of short-lived species, e.g., various transient radicals or excitedstate species,12 and short-lived intermediate species of proteins.13 Recently, we also reported the measurement of diffusion coefficients of transient species in RTILs by this method.14 In the TG method, sinusoidal modulations of concentrations of reactant and product molecules are generated in the sample solution by two excitation laser pulses, and the diffusion of molecules across the grating is detected by a probe beam. Since the diffusion distance is quite short (ca. a few tens of µm), we can measure the diffusion coefficients of short lifetime molecules (ca. ms). In this paper, we applied the TG method to the diffusion of I2- in RTILs and in conventional liquids, and compared the bimolecular reaction rates determined by Takahashi et al.11 We found that the diffusion of I2- is affected strongly by the surrounding charges in RTILs, although the absolute values of the diffusion coefficient are faster than the SE prediction. Our result suggests a shielding effect of the Coulomb force between the reactants by the RTILs. 2. Experimental Section 2.1. Theoretical Background. The detail of the TG method is described elsewhere.12 In this section, we briefly mention the theoretical background and experimental foundation to determine the diffusion coefficient of I2- together with its potential problem. In the TG method the sample solution is irradiated by crossing of two laser beams simultaneously. This creates a light interference pattern in the sample solution, and iodide is photoexcited. Successive photochemical reactions (Scheme 1) are monitored by the diffraction of a probe beam. We assume that the intensity of the TG signal can be expressed as follows:12

ITG ) R(∆n)2 + β(∆k)2

(4)

where R and β are constants, ∆n is the peak-null difference of the refractive index, and ∆k is the peak-null difference of absorption coefficient. ∆n is generally expressed as a sum of contributions from the thermal (∆nth) and species (∆nspe) gratings. ∆k consists of only the species grating:

∆n ) ∆nth +

∑ ∆nspe,i

(5)

∆k )

∑ ∆kspe,i

(6)

where ∆nth is caused by the thermal energy, ∆nspe,i by the change of the molecular refractive index of species i, and ∆kspe,i by the change of the absorption coefficient, respectively. The thermal grating (∆nth) decays by thermal diffusion process,

∆nth(t) ) ∆nth(0) exp(-Dthq2t)

(7)

where Dth is the thermal diffusivity. q denotes the grating wavenumber vector given by 4π sin(θex/2)/λex using the crossing angle (θex) and the excitation wavelength (λex). The species grating decays by molecular diffusion and/or the disappearance due to the reaction,

∆nspe,i(t) ) ∆nspe,i(0) exp{-(Dspe,iq2 + kspe,i)t}

(8)

∆kspe,i(t) ) ∆kspe,i(0) exp{-(Dspe,iq2 + kspe,i)t}

(9)

where Dspe,i and kspe,i are the diffusion coefficient and the disappearance rate of the species i, respectively. In applying the TG method to determine the diffusion coefficient of I2- in Scheme 1, we have to adjust the experimental condition so as to observe the diffusion signal of I2efficiently. First of all, the bimolecular reaction of I2- should be suppressed during the diffusion time of I2-, i.e., Dspeq2 . kspe. The disappearance of I2-, or the production of I3-, obeys the second order rate law as, d[I3-] 1 d[I2 ] )) k2[I2-]2 dt 2 dt

(10)

If we use very small light intensity for excitation e.g., about 200 nJ mm-2, the concentration of I2- should be very small, about 30 nM for the sample solution having optical density of 1 at the excitation wavelength. Considering the values of k2 in literatures (listed in Table 2),11 the half-lifetime of I2- will be about several ten milliseconds in molecular liquids and several hundred milliseconds in RTILs. This is enough for the measurement of the diffusion as will be demonstrated. Next, we choose the probe wavelength so as to detect the signal due to I2- dominantly. According to eq 4, the intensity of the TG signal is proportional to the squares of the refractive index and absorption coefficient. Therefore if the probe wavelength is in resonant with a molecular absorption, the intensity of the TG signal due to the molecule will be enhanced. Here we chose the UV laser (363.8 nm) for the probe laser, since the wavelength is in resonant with the absorption band of I2(absorption coefficient is about 8800 M-1 for I2- at 365 nm15), whereas there is no absorption band for I- at this wavelength. 2.2. Materials. Potassium iodide (KI, 99.5%), methanol and ethanol (spectroscopic grade) were purchased from Nacalai Tesque and were used without further purification. RTILs used in this study were N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([N1,1,1,3][NTf2]) and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pp1,3][NTf2]), whose molecular structures are shown in Scheme 2. These RTILs were specially prepared by Kanto-Kagaku to satisfy the spectroscopic purity, and the water and halogen contents of original samples were less than 15 and 30 ppm,

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SCHEME 2: Molecular Structure of Constituent Ions of RTILs

respectively. The solvent was almost transparent at 240 nm (less than 0.02 in the optical density with 1 cm optical path length). 2.3. Apparatus. The experimental set up for TG method was almost the same as is described elsewhere,12-14 except for the excitation and probe laser sources. The excitation pulse (240nm) was generated by the second harmonic output of a dye laser (Continuum ND6000) operated at 480 nm pumped by the 355 nm third harmonic of a Nd:YAG laser (Continuum, Surelite II). The excitation pulse was split by a beam splitter and crossed into sample solution simultaneously with a certain angle. Typical laser power for the excitation was about 200 nJ mm-2. As a probe beam, the 363.8 nm line of an Ar-ion laser (Coherent Enterprise) was brought into the sample solution with an angle which satisfied the Bragg condition. The diffracted signal was detected by a photomultiplier tube (Hamatsu R928) and averaged by a digital oscilloscope (Tektronix TDS-5054). The repetition rate of the excitation was 0.02 Hz for molecular liquids, which was controlled by a mechanical shutter. In the case of RTILs, a single shot excitation was employed: after each measurement, the sample solution was stirred. Several TG signals were averaged to improve the S/N ratio. The value of q was determined by the decay rate of the thermal grating signal of a reference sample solution (nitrobenzene in ethanol)16 measured under the same experimental condition. In the measurement, we used a 1 cm quartz cuvette with a screw cap for the sample solution. The concentration of KI was adjusted between 0.5 and 3 mM. The solutions were filtered by a membrane filter to remove undissolved solute and/or dust before measurement. RTILs were used after evacuation for 2 h at 323 K to remove evaporable impurities such as water which may be contaminated during the solute dissolving procedure. In the cases of molecular liquids, the sample solution was used after O2 bubbling to quench solvated electron produced during the photoreaction. We did not use O2 bubbling for RTILs because small bubbles caused undesirable light scattering of the probe light. The temperature of the sample solution was controlled at 298 K by a temperature controller (FLASH 200 TemperatureControlled Cuvette Holder, QUANTUM NORTHWEST). After TG experiments, viscosities of sample solution were measured using a viscometer (Brookfield LVDV-II, cone plate CPE-40) at 298 K. 3. Results 3.1. TG Signal in Molecular Liquids. Figure 1a shows the TG signal after photoexcitation of I- in ethanol. Three phases in different time scales were observed (rise in hundreds of nanoseconds, decay in a few microseconds, and in tens of microseconds). As described before, the time profile of the TG signal reflects reaction kinetics (concentration changes of the reactants and products) and the diffusion process across the grating fringe. We can distinguish both processes by making

Figure 1. (a) Typical TG signal after the photoexcitation of I- in ethanol. The horizontal axis is in a logarithmic scale. The solid line is the fitted line to eq 12. (b) The plot of τspe-1 in ethanol against q2. The solid line is the linear fit of τspe-1 to q2.

the TG measurements at several q conditions. If the kinetics of the signal is dependent on q value, this kinetics should be due to the diffusion.12,13 Since the rise at a few hundred nanoseconds was independent of the q value, it was assigned to the reaction kinetics. This signal was assigned to the production of I2- by the bimolecular reaction of I- and I, considering the time scale of the signal as follows. The production of I2- obeys the pseudofirst-order rate law,

d[I2-] d[I] )) k1[I][I-] ≈ k1′[I] dt dt

(11)

According to the value of k1 reported for the aqueous solution,17,18 the rate is close to the diffusion-limited reaction rate. Therefore, the rates in methanol and ethanol are expected to be close to that in water, whose viscosity is also similar (ca. 1010 M-1 s-1). Since we selected the concentration of I- in the order of mM, this reaction will occur in about one hundred nanoseconds, which matches the time constant of the observed kinetics. The subsequent two components were dependent on the value of q, and hence they were assigned to the diffusion processes. By comparing it with the signal of the reference sample, the fast decaying component (about one µs) was assigned to the thermal diffusion signal. The last one (several ten µs) was assigned to the diffusion process of I2- based on the discussion in section 2.1. The time profile of the TG signal after the production of I2was expressed as follows: 12

ITG(t) ) {∆nth exp(-Dthq2t) + ∆nspe exp(-t/τspe)}2 + {∆kspe exp(-t/τspe)}2

(12)

The decay rate of second and third component is given by

Diffusion Coefficients of I2-, in RTILs

τspe-1 ) Dspeq2 + kspe

J. Phys. Chem. B, Vol. 113, No. 15, 2009 5191

(13)

where Dspe and kspe are and the diffusion coefficient and the disappearance rate of I2-, respectively. To determine the diffusion coefficient accurately, we measured the TG signal at different q-values and fit the signal in the longer time region by eq 12. Figure 1b shows the plot of τspe-1 against q2. The intercept of the plot is nearly equal to zero, which indicates that the disappearance rate of I2- is negligible as was expected in the previous section. Form the slope, the diffusion coefficient of I2- was determined. The TG signal in methanol was analyzed in a similar manner to the case of methanol. 3.2. TG signal in RTILs. Figure 2 shows the TG signal after photoexcitation of I- in [Pp1,3][NTf2]. The signal in [N1,1,1,3][NTf2] was similar to that in [Pp1,3][NTf2]. The decay time constants in the RTILs were nearly 2 orders of magnitude larger than those in methanol and ethanol at a similar q, because of their large viscosity. The time profile of the TG signal was analyzed by a similar way to the case of conventional liquids, although we had to include an additional term due to another species. The rise in the earlier time (ca. 10 µs) was assigned to the production of I2-, since the viscosity is larger by two orders magnitude than those of conventional liquids and this reaction will occur in about microseconds. The successive decay (ca. 100 µs) was assigned to the thermal diffusion. Then there was an apparent rise signal (ca. ms), which was absent in the case of conventional liquids. This signal was not dependent on the q value. The decay in the longer time region (ca. 100 ms) was dependent on the q-value, and hence it was assigned to the diffusion process. The signal after the production of I2- was well fit by the following equation,

Figure 2. (a) Typical TG signals after the photoexcitation of I- in [Pp1,3][NTf2]. The horizontal axis is shown in a logarithmic scale. The solid line is the fitted line to eq 14. (b) The plot of τspe-1 in [Pp1,3][NTf2] against q2. The solid line is the linear fit of τspe-1 to q2.

ITG(t) ) {∆nth exp(-Dthq2t) + ∆nX exp(-t/τX) + ∆nspe exp(-t/τspe)}2 + {∆kspe exp(-t/τspe)}2

(14)

Similar to the case of molecular liquids, the decay in the longest time scale was assigned to the diffusion of I2-. The component X denotes the disappearance of some unknown chemical species, since the time constant was not dependent on the q-value. The most probable origin of the signal is the unstable species produced by the reaction of the solvated electron with solvent, because we did not detect any TG signal without I- in solution to ensure that the TG signal derives from the photoreaction of I-. However, we do not discuss the identification of X any more in this paper, since the purpose of this paper is the determination of the diffusion coefficients of I2- and this byproduct does not affect the disproportionation reaction of I2.19 The diffusion coefficients of I2- in RTILs were determined by a similar plot as in the case of molecular liquids (Figure 2b). 4. Discussion 4.1. Diffusion Coefficients of I2-. The diffusion coefficients of I2- determined in this work are summarized in Table 1. In the Table, we also list the values of the diffusion coefficients estimated by using the SE relation with the stick boundary condition (eq. 2 with C ) 6). The value of radius of I2- is estimated to be 0.26 nm from the partial molar volume of I2-.20 As is shown in the Table 1, the diffusion coefficients of I2- in RTILs are two times larger than the predictions from the SE relation, whereas they are similar in molecular liquids. If the

Figure 3. Viscosity dependence of the diffusion coefficients of I2- in RTILs and conventional molecular liquids, together with those of molecules with similar sizes of I2-. Circles, triangles, diamonds and squares represent the diffusion coefficients of I2-, CO2, O2 and O2-, respectively. The data of CO2, O2 and O2- were taken from refs 23,22,21 and,4 respectively. The open and closed symbols represent the ones in molecular liquids and in RTILs, respectively. The solid line is the prediction from the SE relation using the approximate molecular radius of I2-.

SE relation predicts the diffusion coefficients of noncharged molecule correctly, this result may suggest that the charge effect accelerates the diffusion. However, this is not true. In Figure 3, we plot the diffusion coefficients of several small molecules (CO2, O2, and O2-) with similar size to I2- in RTILs and conventional liquid solvents taken from literatures4,21-24 against T/η. The molecular radii of these molecules are estimated to be 0.24 nm (CO2) and 0.22 nm (O2 and O2-) from their molar volume, respectively.25 The solid line in the figure shows the SE prediction for the case of I2-. This figure shows that the diffusion of small neutral molecule in RTILs is much faster than is predicted by the SE relation. Several studies suggested the possibility of large voids in RTILs causes the faster diffusion.7,22 Since the ions in RTILs are bulky and asymmetric,

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TABLE 1: Diffusion Coefficients of I2- in Various Solvents, Together with the Viscosity of the Solution. The Unit of the Diffusion Coefficients Is 10-11 m2 s-1

U(r) )

DI2solvent

experimentala

SEb

η/cP

methanol ethanol [N1,1,1,3][NTf2] [Pp1,3][NTf2]

120 ( 15 79 ( 6 2.50 ( 0.10 1.17 ( 0.11

152 83 1.20 0.62

0.55c 1.01c 70a 135a

a

kdiff ) 2πDaa

This work. Calculation by eq 2 using the stick boundary condition and r ) 0.26 nm. c The values for pure solvent taken from ref 16.

(18)

r0 ) e2 /4πε0εrkBT

(19)

In Table 2, the bimolecular reaction rates calculated from eqs 15 and 18, together with the experimental values are listed. In the calculation, the reaction distance Raa is assumed to be twice the bond length of I2-, 0.63nm.27 Since the dielectric constants of RTILs used here have not been reported until now, we use the values of other ionic liquids with similar structure; the value of N,N,N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide is used for [N1,1,1,3][NTf2], and the value of N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide is used for [Pp1,3][NTf2].28 As is shown in the table, the reaction rates in molecular liquids are close to the values predicted by eq 18, whereas the reaction rates in RTILs are much larger than predicted by eq 18 and rather close to the values by eq 15. This result suggests that the Coulomb interaction between I2- molecules are more shielded than that is expected from the static dielectric constant. Since the static dielectric constant is correlated with the dipole moment density, it is considered that the enhanced charge shielding effect of RTILs is contributed largely by net charge of solvent molecules. In the case of dilute electrolyte solution, the shielding effect by surrounding ions of Coulomb potential energy is described with Debye length, rD as follows.26

(15)

where Daa is equal to twice of diffusion coefficient of I2- in the present case. For the bimolecular reaction of charged species, the following Debye-Smoluchowski equation, which includes Coulomb potential between reactants, has been often used instead of eq 15:26

∫R∞ exp{U(r)/kBT}/r2dr]-1

r0 exp(r0 /Raa) - 1

where

RTILs may have large voids in comparison with molecular liquids, and solute may feel smaller friction by moving through the voids. From Figure 3, it is clear that there is a significant effect of the solute charge on the diffusion in RTILs. The diffusion of I2- is several times slower than that of CO2 in the molecular liquids whereas it is over ten times slower in RTILs. The similar trend is recognized for the diffusions of O2 and O2-. The slowness of ion diffusion is considered to be mainly due to the charge-charge interaction between ion and solvent ions. It is also reasonable that for the larger size of the solute molecule the difference between charged and noncharged species becomes small. As is mentioned in Introduction, for the cases of TMPD and its cation radical (TMPD+) in RTILs TMPD+ is only twice slower than TMPD.4c Since in the larger molecule the charge is delocalized over the molecule, the interaction with RTILs is weakened compared with the cases of smaller ions. 4.2. Bimolecular Reaction Rate between I2-. Here we discuss the bimolecular reaction rate of I2- on the basis of the diffusion coefficients experimentally determined. In the case of bimolecular reaction between the same species, a factor of 1/2 is required to eq 1 in order to avoid the double counting as

kdiff ) 2πDaa[

(17)

where ε0 and εr are permittivity in the vacuum and the dielectric constant of solvent, respectively. Then, kdiff is expressed as follows:

b

kdiff ) 2πDaaRaa

e2 4πε0εrr

U(r) )

ZaZbe2 exp(-r/rD) 4πε0εrr

(20)

where Za and Zb are charge of reactant a and b, respectively. rD represents the distance over which the electric field of a charged molecule is shielded by surrounding ions. Several MD simulations estimated the shielding length for charge of ions in RTILs. Del Po´polo and Voth indicated that the screening length between solvent ions in 1-ethyl-3-methylimidazolium nitroxide ([EMIm][NO3]) is about 0.7 nm.29 Shim et al. calculated the charge distribution around the ion pair in [EMIm][Cl] and

(16)

where U(r) denotes the Coulomb potential energy between reactant molecules. Since I2- reacts in a dielectric medium, U(r) may be given by

TABLE 2: The Bimolecular Reaction Rate of I2- Experimentally Determined Together with the Theoretical Estimation Based on the Smoluchowski Equation (eq 15) and the Debye-Smoluchowski Equation (eq 18). The Unit of the Rate Is 107 M-1 s-1

a

solvent

experimentala

Smoluchowski

Debye-Smoluchowski

εr

methanol ethanol [N1,1,1,3][NTf2] [Pp1,3][NTf2]

77 53 11 6.6

550 380 12 5.6

100 35 0.036 0.017

33 25 12.3b 11.5c

Referenc 11. b The value of N,N,N-trimethyl-N-butylammonium N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide; ref 28.

bis(trifluoromethanesulfonyl)imide;

ref

28.

c

The

value

of

Diffusion Coefficients of I2-, in RTILs [EMIm][PF6], and estimated the screening length as around 1 nm.30 Lynden-Bell calculated the potential filed around a positive charged atom and found that the screening length is around 1 nm.31 Taking into consideration these studies, our result leads to the idea that the charge of I2- in RTILs may be almost screened in the vicinity of it, probably by cations which are expected to surround I2-. 5. Concluding Remarks In this work, we determined the diffusion coefficients of the transient intermediate I2-, which is produced after the photoexcitation of I- in solution by the transient grating method. The diffusion coefficients in RTILs were much smaller than those of neutral molecules of similar sizes, indicating that the Coulomb force from solvent ions significantly affects the diffusion process. Using the diffusion coefficients determined experimentally, we have succeeded in comparing the bimolecular reaction rate with the theoretical predictions using the Smoluchowski model and the Debye-Smoluchowski model. The bimolecular reaction rate was not faster than the diffusion limited reaction rate. However, the bimolecular reaction rate of I2- in RTILs was confirmed to be much faster than the theoretical prediction from the Debye-Smoluchowski model, indicating that the Coulomb repulsive between reactants are screened by the ionic field of the solvent molecules. It is an interesting issue how the shielding effect from the solvent molecules changes with approaching the reactant molecules. These points may be clarified by using MD simulations. Experimentally, the dissociation dynamics may be another approach to investigate the dynamical effect of the screening and is now under investigation. Acknowledgment. This work is supported by the Grant-inAid for Scientific Research (No. 17073012) from the Ministry of Education, Culture, Sports, Science, and Technology. We are grateful for Dr. K. Takahashi (Kanazawa University) for the useful discussion. Y.N. is supported by the research fellowship of Global COE program, International Center for Integrated Research and Advanced Education in Material Science, Kyoto University, Japan. References and Notes (1) Ionic Liquids in Synthesis; Welton, T., Ed.; Wiley-VCH-Verlag, Weinheim, Germany, 2003. (2) (a) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603–4610. (b) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593–16600. (c) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103–6110. (d) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2005, 109, 16474–16481. (e) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833–2839. (f) Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 19593–19600. (3) (a) Kawano, R.; Watanabe, M. Chem. Commun. 2003, 330–331. (b) Kawano, R.; Watanabe, M. Chem. Commun. 2005, 2107–2109. (c) Zistler, M.; Wachter, P.; Wasserscheid, P.; Gerhard, D.; Hinsch, A.; Sastrawan, R.; Gores, H. J. Electrochim. Acta 2006, 52, 161–169. (4) (a) Buzzeo, M. C.; Klymenko, O. V.; Wadhawan, J. D.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Phys. Chem. A 2003, 107, 8872– 8878. (b) Evans, R. G.; Klymenko, O. V.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Electroanal. Chem. 2003, 556, 179–188. (c) Evans, R. G.; Klymenko, O. V.; Price, P. D.; Davies, S. G.; Hardacre, C.; Compton, R. G.

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