Microsolvated F–(H2O) + CH3I SN2 Reaction Dynamics. Insight into

Letter pubs.acs.org/JPCL

Microsolvated F−(H2O) + CH3I SN2 Reaction Dynamics. Insight into the Suppressed Formation of Solvated Products Jiaxu Zhang,† Li Yang,*,† Jing Xie,‡ and William L. Hase§ †

Institute of Theoretical and Simulation Chemistry, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China ‡ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States § Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States ABSTRACT: Microsolvation offers a bottom-up approach to investigate details of how solute−solvent interactions affect chemical reaction dynamics. The dynamics of the microsolvated SN2 reaction F−(H2O) + CH3I are uncovered in detail by using direct chemical dynamics simulations. Direct rebound and stripping and indirect atomic-level mechanisms are observed. The indirect events comprise ∼70% of the solvated reaction and occur predominantly via a hydrogen-bonded F−(H2O)···HCH2I prereaction complex. The reaction dynamics show propensity for the direct three-body dissociation channel F−(H2O) + CH3I → CH3F + I− + H2O after passing the reaction’s dynamical bottleneck. The water molecule leaves the reactive system before traversing the postreaction region of the PES, where water transfer toward the product species occurs. This provides an insight into the very interesting finding of strongly suppressed formation of energetically favored solvated products for almost all SN2 reactions under microsolvation.

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H2O molecule enhances the indirect reaction at low collision energy (Erel) and changes the reaction mechanism from primarily stripping to rebound at high Erel. With two H2O molecules, the dynamics is indirect and isotropic at all Erel. The trajectory simulation results are consistent with experiment and have provided an atomistic understanding of the role of the H2O molecule.19,20 The F− + CH3Y → CH3F + Y− (Y = Cl, Br, I) family of reactions is of particular interest because of their large reaction exothermicity, especially for F− + CH3I, and the possible effect on the features of the potential energy surface (PES)23 and the chemical reaction dynamics.4,8,24 The PES for the F− + CH3I reaction is different than the double-well potential energy profile that characterizes gas-phase SN2 reactions.12,23 In the prereaction region, there is a hydrogen-bonded F−···HCH2I complex in addition to the traditional F−···CH3I ion−dipole complex. In previous work,4 trajectory simulations reproduce the product energy partitioning and velocity scattering angle distributions measured in ion imaging experiments and reveal that an indirect mechanism with formation of the hydrogenbonded F−···HCH2I prereaction complex plays an important role in the chemical reaction dynamics, contributing up to ∼60% of the substitution reaction. The sequential question addressed here is whether the reaction dynamics under microsolvation are similar to those for the unsolvated F− + CH3I reactants.

ucleophilic substitution (SN2) reactions are of fundamental importance in chemistry and continue to be a significant model system in the field of chemical reaction dynamics. The SN2 reaction is sensitive to the conditions under which it is carried out, for example, whether it is performed in solution or in the gas phase. In the past decade, dynamics studies of gas-phase X− + CH3Y → CH3X + Y− SN2 reactions are fruitful in both experiments1−5 and computations.4−9 However, the dynamics in the liquid phase may be more complex and much different.10−12 With the stepwise addition of solvent molecules to the bare reactant anion, microsolvation offers a bottom-up approach to learn more about the transition of chemical reactions from the gas to liquid phase.5 The involvement of solvent molecules may alter the reaction dynamics from that for the unsolvated SN2 reaction.10,13−22 The trend for a decreasing reaction rate with increasing stages of ion solvation has been observed for SN2 reactions in experiments10 and theoretically explained by stronger stabilization of the reactants and products by the solvent than that for the transition state.17 Instead of the classical SN2 reaction path, a ligand switching mechanism to produce X−(CH3Y) followed by decomposition of the complex occurs for the X−(H2O)n + CH3Y reaction.21 Trajectory calculations found that the reaction dynamics are strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier, which leads to barrier recrossing and influences the reaction outcomes.22 The dynamics of OH−(H2O)n=1,2 + CH3I differ quite dramatically from those of the unsolvated system, as indicated by molecular beam ion imaging experiments.18 Adding one © 2016 American Chemical Society

Received: December 14, 2015 Accepted: January 28, 2016 Published: January 28, 2016 660

DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665

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The Journal of Physical Chemistry Letters

Figure 1. Schematic energy profile for the F−(H2O) + CH3I reaction at the B3LYP/ECP/d level of theory. The energies in kcal/mol are classical energies without zero-point energy (ZPE). The 0 K calculated (red) and experimental (blue) reaction exothermicities with ZPE included are in parentheses and square brackets, respectively. The 0 K experimental values are calculated from standard molar enthalpies of formation in refs 26 and 27, with the harmonic B3LYP frequencies used to remove the thermal vibration enthalpies, along with the thermal rotation and translation enthalpies.

likelihood of a “roaming”28 F−(H2O) finding the correct geometry to attack the carbon atom. With Walden inversion, the system passes TS2 and goes to a postreaction complex FCH3(H2O)···I− with the water molecule hydrogen-bonded to both the fluorine and iodine atoms. Energetically, the water molecule can easily shift to reach a new minimum corresponding to the second postreaction complex FCH3··· I−(H2O). This is accomplished with a barrier of only 0.2 kcal/ mol, in line with a previous prediction for Cl−(H2O) + CH3Br.14 It should be noted that the dynamics as detailed below do not follow the above reaction profile and intrinsic reaction coordinate (IRC)29 PES, with the system trapped in the post-TS complexes. The B3LYP/ECP/d calculations show that three exothermic channels are open for the F−(H2O) + CH3I SN2 reaction, that is

Bierbaum et al. investigated the microsolvated reaction F (H2O) + CH3I using the tandem flowing afterglow selected ion flow tube (FA-SIFT) technique.13 The measured rate constant is (8.64 ± 0.09) × 10−10 cm3 s−1 at ∼300 K, which is ∼2 times smaller than that of the nonsolvated reaction. One remarkable finding, as is the case for almost all X−(H2O) + CH3Y reactions,12,15,17−20 is that formation of the solvated I−(H2O) product is strongly suppressed with respect to its unsolvated counterpart I−, despite the larger exothermicity for the former, which makes it the statistically favored product channel. The branching ratio of the free I− pathway versus the solvated I−(H2O) pathway is ∼0.9:0.1 (±50%). This is an interesting feature for SN2 reactions with microsolvation,19,20 and the detailed dynamical effects caused by individual solvent molecules are needed to understand this observation. To assist in understanding these experimental results and elucidate the atomistic-level reaction mechanism, a detailed direct dynamics simulation for the monosolvated F−(H2O) + CH3I SN2 reaction is therefore desirable. Chemical dynamics simulations require an accurate PES, which governs the motion of the atoms for the chemical reaction. Here, we report a PES profile for the F−(H2O) + CH3I reaction at the DFT/B3LYP/ ECP/d4,25 level of theory. As Figure 1 shows, B3LYP/ECP/d gives reaction energies for different product channels in good agreement with experiment.26,27 More detailed electronic structure theory calculations for the F−(H2O) + CH3I PES will be given in a forthcoming publication. The reaction profile begins by hydrogen bonding between F−(H2O) and CH3I, with H2O connected to F− via another hydrogen bond. Transformation from the hydrogen-bonded complex F−(H2O)···HCH2I to the ion−dipole complex F−(H2O)···CH3I results in only a small change of ∼0.8 kcal/ mol in the potential energy. The barrier from the latter to TS2 is also modest and ∼1.4 kcal/mol. The flatness of the PES in this region facilitates reorientation of H2O, which increases the −

F−(H 2O) + CH3I → CH3F + I− + H 2O ΔE = −19.4 kcal/mol

(1)

F−(H 2O) + CH3I → CH3F + I−(H 2O) ΔE = −29.6 kcal/mol

(2)

F−(H 2O) + CH3I → (H 2O)FCH3 + I− ΔE = −22.8 kcal/mol

(3)

The products separate from each other in channel 1, and I− and CH3F are solvated by the water molecule in the other two channels. The B3LYP reaction energies for channels 1, 2, and 3 are −19.4, −29.6, and −22.8 kcal/mol, respectively. Thus, channel 2 with I−(H2O) formation is the most energetically favored product channel due to the larger solvation energy for I−(H2O) as compared to that for (H2O)FCH3. 661

DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665

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The Journal of Physical Chemistry Letters The reaction pathways predicted by the stationary points is only a model for the reaction, and a deeper understanding of the actual atomistic mechanisms is obtained by chemical dynamics simulations. The simulations reported here, for the solvated F−(H2O) + CH3I SN2 reaction, are performed by direct dynamics4,8 on the full dimensional B3LYP/ECP/d PES, using the VENUS chemical dynamics computer program30−32 interfaced to the NWChem electronic structure computer program.33 Results are reported for a 0.32 eV collision energy Erel and the reactants’ rotational and vibrational temperatures of 75 and 360 K, respectively, which are the experimental and simulation conditions for the unsolvated F− + CH3I reaction.4 With these initial conditions, a direct comparison may be made with the unsolvated dynamics. Three product channels 1, 2, and 3, as shown above, are observed from the trajectory calculations, which amount to more than 90% of the substitution reaction. The present study mainly focuses on these three major channels, and their reaction probabilities versus impact parameter b, Pr(b), are plotted in Figure 2. Weak coupling between the intermolecular

system as CH3F is formed. Stripping occurs when F−(H2O) approaches CH3I from the side and directly strips away the CH3 group along with H2O detaching from F−, with the CH3F product scattering forward. As seen for the F− + CH3I reaction, the rebound mechanism occurs for small impact parameters and results in backward scattering, whereas the stripping mechanism is a larger impact parameter event and gives forward scattering. Approximately 90% of the indirect reaction occurs by forming the hydrogen-bonded F−(H2O)···HCH2I prereaction complex. Considering the low barrier of only 1.0 kcal/mol between the hydrogen-bonded complex F−(H2O)··· HCH2I and ion−dipole complex F−(H2O)···CH3I as shown in Figure 1, the latter is also a possible intermediate for this indirect mechanism. However, it has a very short lifetime and contributes less to the reaction dynamics. The atomistic dynamics for the hydrogen-bonded indirect mechanism are depicted in Figure 3, where F−(H2O) interacts

Figure 2. Reaction probability Pr(b) for the F−(H2O) + CH3I reaction versus impact parameter b at a 0.32 eV collision energy. The lines are for channel 1 (black dashed line with open square), channel 2 (red solid line with star), and channel 3 (green dotted line with open triangle). The Pr(b) for the F− + CH3I → CH3F + I− reaction is also included (black solid line with solid square) for comparison, which was presented previously.4

Figure 3. Atomistic dynamics of a typical trajectory for the dominant indirect mechanism for channel 1 of F−(H2O) + CH3I → CH3F + I− + H2O. The reaction proceeds via formation of the hydrogen-bonded F−(H2O)···HCH2I prereaction complex.

attractively with the H atom of CH3I and the system becomes temporarily trapped in the prereaction potential energy well (Figure 1). Then F−(H2O) attacks the C atom backside and displaces I− with H2O detached. The hydrogen-bonded complex formation mechanism was observed previously in the dynamics for the nonsolvated X− + CH3I (X = F, OH−) system4,5,8 and in recent simulations of the monosolvated OH−(H2O) + CH3I SN2 reaction.19 The lifetime of the F−(H2O)···HCH2I complex ranges from ∼130 fs to 1 ps, which is shorter compared to the F−···HCH2I complex lifetime of 150 fs to 3 ps for the F− + CH3I reaction but longer than the HO−···HCH2I complex lifetime (125−650 fs) for the OH− + CH3I reaction. It is worth noting that although the postreaction complexes FCH3(H2O)···I− and FCH3···I−(H2O) are potential minima on the SN2 reaction pathway, the majority of the reactive trajectories (more than 90%) calculated here for Erel of 0.32 eV either pass through or pass by these minima without forming a complex. Channel 3 occurs by mechanisms similar to the above three for channel 1, except that the water molecule remains attached to CH3F via hydrogen bonding when the system overcomes

and intramolecular modes of F−(H2O)···CH3I results in inefficient IVR. Accordingly, even though the reaction has no overall barrier, the simulations still predict a very low reaction probability, that is, less than 20% for each impact parameter for the different channels, which generally decreases as the impact parameter increases. Channel 1, a three-body dissociation, occurs over a wide b range of 0−9.5 Å. It comprises more than 80% of all of the reactive trajectories and dominates the reaction at a collision energy of 0.32 eV. Channel 2 is observed at the relatively small b range of 0−5 Å, and channel 3 is observed for the broad b range of 0−9 Å. Both channels have a smaller percentage than does channel 1. Similar to what has been observed for the unsolvated F− + CH3I reaction,4,8 channel 1 occurs by two direct mechanisms, that is, rebound and stripping, and an indirect mechanism that dominates. For the rebound mechanism, F− attacks the backside of CH3I, directly displaces I−, and reverses its direction with CH3 attached. The water molecule remains attached to F− as reaction occurs and then dissociates from the 662

DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665

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The Journal of Physical Chemistry Letters TS2 and then I− leaves. For channel 2, direct rebound and an indirect mechanism predominated by F−(H2O)···HCH2I complex formation occur, leading to the solvated I−(H2O) product. For the rebound mechanism, F−(H2O) attacks CH3I backside, directly displaces I−, and scatters backward. Then, the water molecule passes over the methyl group to attach to the leaving I− ion by hydrogen bonding, and the hydrated I−(H2O) ion is formed. The indirect mechanism is identical to the direct rebound, except that the system becomes transiently trapped in the prereaction potential energy well and forms a F−(H2O)··· HCH2I hydrogen-bonded intermediate. The principal mode for this indirect mechanism is depicted in Figure 4. Channel 2 is a

section of channel 1 is almost an order of magnitude larger than those of channels 2 and 3, consistent with the much higher Pr(b) for the former. The experimental reaction rate constant for F−(H2O) + CH3I is ∼(8.6 ± 0.1) × 10−10 cm3 mol−1 s−1 at 300 K, corresponding to a collision energy Erel of ∼0.04 eV, and it is about 2 times slower than that for the unsolvated reaction.13 For the Erel = 0.32 eV studied here, the calculated total cross section including all three channels gives a rate constant of k(Erel,Tv,Tr) = v(Erel)σ(Erel,Tv,Tr) = (3.6 ± 0.6) × 10−10 cm3 mol−1 s−1, where Tv and Tr equal 360 and 75 K, respectively. The calculated rate constant is ∼1/2 of the experimental value, and the difference may be mainly caused by the different collision energy in view that the reaction rate for the unsolvated system is weakly dependent on the vibrational and rotational temperatures Tvr.2 The rate constants versus collision energy for OH−(H2O) reacting with CH3I were calculated at 0.05 and 0.5 eV,19 and the values are (8.6 ± 0.5) and (6.6 ± 0.5) × 10−10 cm3 mol−1 s−1. The rate constant at 0.5 eV is ∼3/4 of that at 0.05 eV, a trend in agreement with those of the F−(H2O) + CH3I reaction at 0.32 and 0.04 eV (room temperature). Trajectory calculations for F−(H2O) + CH3I over a wide range of collision energies are now in progress. In agreement with experiment,13 the simulations find that the reaction of F−(H2O) with CH3I shows a preference for formation of free I− ion product instead of the energetically favored solvated I−(H2O). The branching ratio of I−/I−(H2O) has been estimated experimentally to be 0.9/0.1 at ∼300 K, which is consistent with the simulations giving 0.92/0.08 at a collision energy of 0.32 eV. The simulations indicate that ∼90% of the I− ion is formed via channel 1 with minor contributions from channel 3. The dynamics of the OH−(H2O) + CH3I reaction were studied by Wester and co-workers using crossed beam ion imaging.34 Formation of the unsolvated I− products was strongly preferred over the solvated species I−(H2O), consistent with the current results for the F−(H2O) + CH3I reaction. Analyzing the product velocity distributions for the OH−(H2O) + CH3I reaction indicates that a direct rebound mechanism dominates the I− pathway. Similar isotropic scattering with low product translational energies was found in Wester’s experiments for all of the solvated reaction products and part of the unsolvated products, which was suggested34 to result from trapping in the postreaction complex (H2O)CH3OH···I− in the reaction exit channel. In contrast, trajectory calculations reveal that trapping in this complex is negligible and indirect mechanisms involving the hydrogen-bonded (H2O)HO−···HCH2I prereaction complex contribute to the isotropic scattering observed in experiments.19 Similar dynamics, without postreaction complex formation, are also found for the present F−(H2O) + CH3I simulations. The simulations provide insight for understanding this recurrent phenomenon of suppressed formation of solvated products for microsolvated SN2 reactions. The calculations reported here indicate that the dynamics of the trajectories for the F−(H2O) + CH3I reaction do not follow the IRC path, with the system trapped in the postreaction complexes. Instead, the reaction has the propensity for the direct three-body dissociation channel F−(H2O) + CH3I → CH3F + I− + H2O (channel 1), after crossing the central barrier TS2. This type of dynamics is reminiscent of those for the F− + CH3OOH → HF + CH2O + OH−7 and OH−(H2O) + CH3I19 reactions, which avoid the deep potential energy minimum for their postreaction complex. The release of potential energy to the asymmetric F−

Figure 4. Atomistic dynamics of a typical trajectory for the indirect mechanism for channel 2 of F−(H2O) + CH3I → CH3F + I−(H2O). The reaction proceeds via formation of the hydrogen-bonded F−(H2O)···HCH2I complex.

small impact parameter process and is observed only at b less than 5 Å, which is a sign that small b favors transfer of H2O from F− to I−, with efficient IVR for the water molecule to shift to the departing I−. At the termination of the trajectories, ∼12% of the I−(H2O) product complexes have internal energy larger than the threshold energy for dissociation and are assumed to ultimately dissociate, losing H2O. In all three reaction channels 1−3, only a small fraction of the trajectories actually follow the direct reaction path, and the majority (∼64%) of the trajectories follow an indirect F−(H2O)···HCH2I complex formation path, illustrating the importance of this hydrogen-bonded complex for the solvated F−(H2O) + CH3I SN2 reaction. The fractional contributions of channels 1, 2, and 3 to the direct rebound and stripping mechanisms are 0.14, 0.32, 0.10, and 0.15, 0.0, 0.25, respectively. This clearly shows the direct rebound and direct stripping contribute equally to channel 1, with rebound more important in channel 2 and stripping more important in channel 3. The reaction cross section σr is obtained by integrating Pr(b) over the impact parameter, that is, σr = ∫ Pr(b)2πb db, and the resulting values are 21.9 ± 3.4, 1.1 ± 0.3, and 2.2 ± 1.5 Å2 for reaction channels 1, 2, and 3, respectively. The reactive cross 663

DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665

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The Journal of Physical Chemistry Letters C···I− stretch motion of the reaction coordinate, as the system moves off of the central barrier, tends to propel I− from (H2O)FCH3. In the language of IVR, there is very weak coupling between (H2O)FCH3 + I− relative translation and O···F−C bending and other vibrational degrees of freedom of the reactive system. The motion of the water molecule along the reaction path is driven by the H-bond between the water molecule and the fluorine ion. For most reactive trajectories, the F···H stretch mode gets enough energy to break the hydrogen bonding as the substitution reaction occurs, leading to the SN2 product CH3F; that is, H2O departure from the reactive system is simultaneous with F− displacement of I− for the SN2 reaction. As the system moves off the central barrier, it moves directly to the product CH3F + I− + H2O without the water molecule attached to CH3F or transferred to the leaving I−. The dynamics of the solvating H2O molecule was studied for the OH−(H2O) + CH3I simulations.19 At a collision energy of 0.5 eV, which is close to 0.32 eV reported here, the indirect reaction is more important, and for approximately 44% of the indirect trajectories, H2O departure and the SN2 product CH3OH formation are nearly simultaneous. The probability is enhanced for H2O to depart before CH3OH is formed for the OH−(H2O) + CH3I, compared to the F−(H2O) + CH3I reaction. Inefficient formation of the solvated I−(H2O) product also arises from rapid separation of the CH3F + I− + H2O species in comparison to the longer time scale for O···F−C bending for the H2O shift to I−. Such a hierarchy of time scales for intramolecular motions and inefficient structural transitions has been observed in the OH− + CH3F reaction6 and may be important in enzyme catalysis, where motions associated with the reaction center may be much faster than those associated with conformational changes of the enzyme.35 The SN2 reaction probabilities versus impact parameter, Pr(b), for the solvated and unsolvated reactions are compared in Figure 2 for the collision energy of 0.32 eV. As discussed above, the three-body dissociation channel 1 dominates the F−(H2O) + CH3I reaction with much larger Pr(b) than that for channels 2 and 3. The total Pr(b) extends to large b for both solvated (9.5 Å) and unsolvated (8.75 Å) reactions, but the solvated Pr(b) is approximately a factor of 3 smaller than the unsolvated one. As a result, the solvated SN2 reaction cross section is appreciably lower than the unsolvated value and is 25.1 ± 3.9 Å2, as compared to 108.7 ± 9.7 Å2 for the unsolvated reaction.4 This difference in cross sections probably arises from multiple factors. One is the steric effects by the water molecule that prevent reactive collision for the solvated dynamics.15 The lower reaction cross section also gives a smaller rate constant for the solvated reaction. For Erel = 0.32 eV and Tv/Tr = 360/75 K, the solvated rate constant is (3.6 ± 0.6) × 10−10 cm3 mol−1 s−1 and approximately 5 times less than the unsolvated value. Experimentally, the unsolvated rate constant is ∼2 times that of the solvated one at ∼300 K (0.04 eV).13 This indicates that the ratio of kunsolvated versus ksolvated decreases with lower Erel. The solvated and unsolvated reaction dynamics have similar direct rebound, direct stripping, and indirect atomistic mechanisms. For the solvated F−(H2O) + CH3I reaction, the rebound, stripping, and indirect percentages of the reaction are 14, 15, and 71% for channel 1, 32, 0, and 68% for channel 2, and 10, 25, and 65% for channel 3, respectively. The corresponding respective fractions for the unsolvated F− + CH3I → CH3F + I− SN2 reaction are 18, 23, and 59%.4 The participation of the water molecule leads to more indirect reaction mechanisms at 65−71% for the solvated reaction. The

unsolvated system has a hydrogen-bonded F−···HCH2I prereaction complex, and ∼91% of the indirect reactions occur via this complex.4,8 It plays an important role in the dynamics including the product energy partitioning and velocity scattering angle distributions. It is quite interesting that there is a hydrated counterpart F−(H2O)···HCH2I, with a similar structure, for the solvated system. Here, forming F−(H2O)··· HCH2I comprises ∼90% of the indirect reaction, similar to the unsolvated simulations, and its effect on the dynamics will require further work. In this work, the atomic-level dynamics for the microsolvated F−(H2O) + CH3I SN2 reaction are studied by direct dynamics simulations at a 0.32 eV collision energy to compare with the unsolvated dynamics. Solvating the reactant species opens up new SN2 pathways leading to both bare and solvated products, and three major exothermic products CH3F + I− + H2O (channel 1), CH3F + I−(H2O) (channel 2), and (H2O)FCH3 + I− (channel 3) are observed. Overall, the H2O molecule leaves the reactive system as the substitution reaction occurs, before traversing the postreaction region of the PES for water shift (Figure 1). The atomistic motions tend to take the reactive system directly to the three-body dissociation products CH3F + I− + H2O after overcoming the SN2 central barrier. The dominant events in which H2O departs the reactive system as the SN2 product CH3OH is formed or before CH3OH formation are also observed for the OH−(H2O) + CH3I reaction.19 This type of dynamics for the solvating water molecule may be a general component for microsolvated SN2 reactions, which might be important for understanding the prevalent finding for these reactions that the formation of solvated products is suppressed, although they are energetically favored.12,13,15,17−20 We plan to perform direct dynamics simulations for other microsolvated systems to see if this is a general phenomenon. Under the influence of microsolvation, and only one water molecule, the total reaction cross section decreases remarkably for the solvated system. As a result, the solvated rate constant is ∼5 times smaller than the unsolvated one at a 0.32 eV collision energy. The similar direct rebound, direct stripping, and indirect atomistic mechanisms are observed for both unsolvated and solvated SN2 reactions, but the indirect mechanism is more important for the solvated dynamics. The indirect mechanism dominates the solvated reaction and occurs by forming a hydrogen-bonded F−(H2O)··· HCH2I prereaction complex. The hydrogen-bonded complex F−···HCH2I has been found to play a significant role for the unsolvated F− + CH3I dynamics, and now, the hydrated F−(H2O)···HCH2I is identified in the microsolvated SN2 reaction. It will be of interest to determine its importance for the F−(H2O) + CH3I SN2 reaction at different collision energies as well as for other and more complex microsolvated SN2 reactions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21573052, 21403047, 51536002), the Fundamental Research Funds for the Central 664

DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665

Letter

The Journal of Physical Chemistry Letters

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Universities, China (AUGA5710012114, 5710012014), the SRF for ROCS, SEM, China, and the Open Project of Beijing National Laboratory for Molecular Sciences (Grant 20140103). The research reported here by W.L.H. is supported by the Robert A. Welch Foundation under Grant No. D-0005. Support is also provided by the High Performance Computing Center (HPCC) at Texas Tech University, under the direction of Philip W. Smith.

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DOI: 10.1021/acs.jpclett.5b02780 J. Phys. Chem. Lett. 2016, 7, 660−665