Dynamics of Clâ(H2O) + CH3I Substitution Reaction: The Influences of

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Dynamics of Cl(HO) + CHI Substitution Reaction: The Influences of Solvent and Nucleophile Meng Gu, Xu Liu, Li Yang, Shaozeng Sun, and Jiaxu Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00348 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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The Journal of Physical Chemistry

Dynamics of Cl-(H2O) + CH3I Substitution Reaction: The Influences of Solvent and Nucleophile

Meng Gu,# Xu Liu,# Li Yang,*,# Shaozeng Sun,*,§ and Jiaxu Zhang*,#

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MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, and §

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China

Author E-mail Address: [email protected], [email protected], and [email protected]

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ABSTRACT The study of microsolvation provides a deeper understanding of solvent effects on reaction dynamics. Here, the properties of the SN2 reaction of hydrated chloride with methyl iodide are investigated by direct dynamics simulations and how the solutesolvent interactions and the basicity of nucleophiles can profoundly affect the atomic level dynamics are discussed in detail. The results show that the direct-rebound mechanism dominates the substitution reaction, and the roundabout mechanism, which prevails in the indirect unsolvated counterpart reaction, still accounts for a high proportion of the indirect mechanisms. The involvement of a solvent water molecule does not significantly reduce the cross section and rate constant compared to the unhydrated reaction at high collision energy. By varying solvated Cl- to F-, the dominant mechanisms are totally different and in contrast, the dynamics of water does not show much difference and the departure of H2O tends to occur prior to the substitution reaction because of the facile breakage of hydrogen bond at high collision energy.

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I. INTRODUCTION Comprehensive studies have been performed for the gas phase X- + CH3Y → CH3X + Y- SN2 reactions in both experiments1-4 and computations4-12 because these reactions are vital in chemical and biochemical systems and organic synthesis. However, the dynamics may be more complicated and unusual in the liquid phase,13-15 especially for the X− + CH3Y SN2 reactions, and the reaction rates in the gas phase are usually orders of magnitude larger than those in solution.15,16 This is understood by the stronger stabilization of the reactants by solvent molecules than that for transition state [X--CH3---Y]− (TS), leading to the raised barrier height in solution.16,17 And the re-crossing effects of the non-equilibrium barrier can be probably significant for these SN2 reactions,18,19 and thus influence the reaction results. To study the effects of solutesolvent interactions on chemical reaction dynamics, the microsolvation provides an effective bottom-up approach. Increasing experimental and theoretical work has been carried out for SN2 reactions of type X−(H2O)n + CH3Y. Seeley et al.20 discovered that in addition to the classical Walden inversion, a new reaction mechanism, ligand switching has been found when solvent molecules are involved. Wester and co-workers21-23 has probed the dynamics of OH-(H2O)n=0-2 + CH3I reactions, by performing ion-imaging and scattering selected ion flow tube (SIFT) experiments. It was considered that the gradual hydration of OH- changes the reaction dynamics, and compared with the corresponding bare ion, 3



the kinetics were different. Subsequently, the solvent effects on the above reactions have been investigated by Hase and co-workers.24-28 They found that with the addition of one H2O molecule, the dominant stripping altered to rebound mechanism at high collision energy (Erel), and enhances the possibility of the indirect mechanism at low Erel. With the addition of second H2O molecule, the dynamics at all Erel are indirect and isotropic. Most recently, the microsolvation effects on the F-(H2O) + CH3I SN2 reaction have been investigated by direct dynamics simulation.29 A magnitude slower rate constant compared to the nonsolvated one has been observed in experiments. A hydrogen-bonded pre-reaction complex still has a great effect on the indirect events as its nonsolvated counterpart does. Most importantly, the phenomenon that the energetically favored hydrated X-(H2O) product is strongly suppressed relative to the unsolvated product15,30-34 has been clarified. It is supposed to attribute to the water molecule leaving before crossing the post-reaction region of the potential energy surface (PES). For the unsolvated reactions, by varying nucleophile ion from F- to Cl-, instead of forming a hydrogen-bonded complex, there is a "roundabout" mechanism for the Cl- + CH3I → I- + CH3Cl at high Erel.35-37 The initial translational energy efficiently transfers to rotation of CH3 around iodine and stretch of C-I bond, and after one or multiple CH3 rotation, the reaction overcomes the SN2 barrier. Some interesting issues are that does this "roundabout" mechanism exist for the microsolvated Cl- + CH3I system? What the 4


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solvated PES looks like? What is the impact of water molecule on SN2 reaction? And how the energy partitioning and scattering angle distribution varied compared to the unsolvation system? In order to solve the above problems, in this paper, we characterized the PES for Cl-(H2O) + CH3I reaction by performing a variety of electronic structure theories and an appropriate method was chosen to carry out chemical dynamics simulations. The simulation results are given and discussed in detail, and especially are compared with the unsolvated and F-(H2O) + CH3I systems to investigate the solvent effects and the influences of nucleophile on the underlying dynamics. II. COMPUTATIONAL METHOD A. Electronic Structure Calculations. Accurate PES is vital for interpreting the dynamics of SN2 reactions by chemical dynamics simulations. Herein, MP238,39 and DFT, with the B97-140 ， OPBE41, HCTH40742, B3LYP43, and BhandH44 functionals combined with ECP/d basis set, were used to determine the properties of the reactants, pre-reaction complexes, TSs, postreaction complexes, and products on the Cl-(H2O) + CH3I PES. These theories were recently evaluated for their accuracy in describing the energetics and dynamics of several SN2 reactions.5,6,8,9,24-37 A comparison between the results of these calculations with experiments and CCSD(T)/PP/t benchmark45 has been discussed in order to determine the most appropriate method for the subsequent direct dynamics simulations. 5



To ensure that the transition states link to the corresponding minima correctly, we calculated the intrinsic reaction coordinate (IRC)46 for both directions of the TS with various levels of theory. B. Direct Dynamics Simulations. The direct dynamics simulations were carried out at the B97-1/ECP/d level of theory for the Cl−(H2O) + CH3I reaction, which was chosen based on the above electronic structure calculations. The trajectories were computed using VENUS47,48 interfaced with the NWChem.49 For direct comparison with the unsolvated dynamics, the initial condition of trajectories for Cl−(H2O) + CH3I reaction were set to those for unsolvated system.36 The Erel is 1.9 eV and the temperatures for vibration (Tvib) and rotation (Trot) of reactants are set to 75 and 360 K, respectively. The direct dynamics trajectories were integrated by sixth-order symplectic algorithm50,51 with a 0.3 fs time step. The total integration time was selected to be 6 ps for the simulations and all the trajectories finished within the time range. The initial coordinates and momenta of the trajectories were determined using the quasiclassical sampling with the zero-point energy as described in detail in ref 52. III. RESULTS AND DISCUSSION 3.1. Potential Energy Surface. The potential energy curve and the corresponding stationary points optimized at B97-1/ECP/d level of theory are depicted in Figure 1. For this reaction, there is only an 6


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ion-dipole entrance channel, with an ion-dipole transition state (TS) of Walden inversion, connecting the pre-reaction Cl-(H2O)---CH3I (im1) and post-reaction CH3Cl--I-(H2O) (im2) ion-dipole complexes. Eventually, the im2 dissociates to three-body product P1 CH3Cl + I- + H2O, or products with hydrated iodine or methyl chloride, i.e., P2 CH3Cl + I-(H2O) or P3 CH3Cl(H2O) + I-. Similar to the products of F-(H2O) + CH3I reaction, P2 is energetically more preferred than P1 and P3, however P1 with free Ipowerfully dominates over the hydrated products for F-(H2O) + CH3I in experiment, which has been testified by the dynamic simulations.29,34 It is of interest that if the same phenomenon occurs for the current reaction and the following trajectory calculations could be helpful to reveal and understand it. As seen in Figure 1, when im1 overcomes the dynamical bottle neck (TS1), the water molecule migrates from Cl to I and the system goes to a potential well im2. As shown in Table 1, the energies of products P1 and P2 obtained by different methods are compared with the available experimental data. The reaction energetics with ZPE at 298 K are 0.1 and -10.1 kcal/mol for P1 and P2 from the experiments.53,54 Generally, DFT energies for P1 and P2 are in better agreement with the experimental values than MP2 ones, especially for HCTH407, by which P1 and P2 energies are only 0.2 and 0.4 kcal/mol different from the experimental data. Besides, the B3LYP and B97-1 functionals give the energies in good agreement with the experiment values, with the differences within 2 kcal/mol. 7



The mean and the standard deviation are used to evaluate the systematic errors of these calculations for all stationary points by comparison with CCSD(T) benchmark. The average values of MP2, OPBE, BhandH, B97-1, HCTH407, and B3LYP are 0.2, 1.2, -2.2, -1.3, -2.0, and -1.0 kcal/mol, respectively. The corresponding standard deviations are 0.3, 3.5, 3.3, 1.5, 1.9, and 2.0 kcal/mol. Overall, compared with CCSD(T) energies, MP2 gives the better performance than DFT functionals, and B97-1 gives a relatively smaller systematic error than the other DFT functionals. By considering the time consumption and by judging the mean, the standard deviation and the product relative energies, B97-1 functional is supposed to be the most reliable and practical method and thus is chosen to perform the chemical dynamics simulation. 3.2. Simulation Results 3.2.1. Reaction probabilities and cross sections Dynamics simulations are performed with the fixed impact parameter (b) of 0.5, 1, 1.5, 2, 2.5, 3 and 3.5 Å, respectively, at Erel = 1.9 eV. 500 trajectories were calculated at each b and no reactions occurred at the largest b of 3.5 Å. There are 20, 25, 16, 16, 6, 2 and 0 reactive trajectories for each of b value. Though there exist three product channels on the Cl-(H2O) + CH3I PES, almost all trajectories follow the three-body dissociation pathway leading to P1 CH3Cl + I- + H2O, which is the least favored product in energy. This is a general phenomenon for microsolvated SN2 reactions,27 and it is not unexpected, since at high collision energy, more energy is available for the dissociation 8


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of solvated products I-(H2O) and CH3Cl(H2O), leading to the loss of water. The following discussions about the dynamics of water will provide a deep understanding of this observation. The reaction probability Pr(b) versus impact parameter b is shown in Figure 2. Even if this reaction has no overall barrier, the simulation results bring out fairly low Pr(b), which is no more than 6% for each b, and decreases with the increasing 𝑏

b values. According to ∫0 𝑚𝑎𝑥 𝑃𝑟 (𝑏)2𝜋𝑏𝑑𝑏 , the total reaction cross section σr can be estimated, which is 0.71 ± 0.18 Å2 here. And the corresponding rate constant of k (Ecoll,Tvib,Trot) = v(Ecoll)σ(Ecoll,Tvib,Trot) is (2.18 ± 0.55) × 10-13 cm3mol-1s-1, slightly lower than that of Cl- + CH3I reaction as discussed below. 3.2.2. Reaction mechanisms Two direct mechanisms, direct rebound (DR) and stripping (DS), and an indirect (Ind) pathway for Ecoll = 1.9 eV are revealed and described in Figure 3 and 4. For both direct mechanisms, as reaction occurs, the water molecule keeps linking to Cl−, and then departs from the system before forming product CH3Cl. We have observed these two mechanisms in the previously calculated F− + CH3I6 and Cl− + CH3I35,36 SN2 reactions. Figure 5 depicts the ratio of various mechanisms in the column chart. As shown in Figure 5, the direct reaction mechanism is expected to dominate and a large fraction (∼74.6%) of the trajectories follows a rebound mechanism, suggesting its importance for Cl−(H2O) + CH3I SN2 reaction. In contrast, the stripping mechanism is contributed to 8.5%. 9



The atomic-level mechanisms of the indirect reactions are considerably complex and different from the direct ones. Six fundamental indirect mechanisms were identified as presented in Figure 5, i.e., roundabout (Ra) occurred after/before the water leaving the system (R0Ra/R1Ra), forming pre-reaction complex Cl−---CH3I (R0pre) or (H2O)Cl−---CH3I (R1pre) , and post-reaction complex CH3Cl---I− (R0post) or H2O--CH3Cl---I− (R1post), and barrier recrossing occurred after/before the water leaving the system (R0br/R1br). The remaining indirect mechanisms are coupled two or three types of the above six indirect mechanisms. As shown in Figure 5, the ratios for the direct and indirect mechanisms are 0.83:0.17. Of the total indirect mechanisms, ∼95% occurred involving Ra mechanism, indicating that at the high Ecoll of 1.9 eV, DR is the dominant mechanism and the roundabout is the most significant among the indirect mechanisms. Figure 4 shows the main mode for roundabout mechanism, where (H2O)Cl− first strikes the CH3 group, resulting in rotation around the iodine. Then, after methyl rotating once, Cl− attacks the back of the carbon while replacing the I− directly. It takes about 420 fs from the initial Cl-CH3 collision to the SN2 reaction. Ra usually occurs at the beginning of collision and easily couples with other indirect mechanisms, i.e., formation of complex and barrier recrossing mechanisms, which were also found in the dynamics of Cl- + CH3Cl55 and Cl- + CH3I35,36 previously. As shown in Figure 2, there is no stripping mechanism when the impact parameter is less than 1.5 Å, and in contrast, the DR mechanism mostly occurs at the relatively 10


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smaller b. The indirect mechanisms occur at almost all b range. For the SN2 pathways that forming CH3Cl + I- + H2O, the resulting σr for DR, DS, and indirect mechanisms are 0.53 ± 0.13, 0.06 ± 0.04, and 0.13 ± 0.05 Å2, respectively. 3.2.3. Product energies and velocity scattering angle distributions For Cl-(H2O) + CH3I → CH3Cl + I- + H2O reaction, in order to further understand the essential mechanisms, the average fraction of the available product energy partitioned to internal energy (fint) are listed in Table 2, which are 0.45 ± 0.02, 0.60 ± 0.12, 0.78 ± 0.03 and 0.52 ± 0.02 for the DR, DS, Ind mechanisms and total reaction, respectively. Compared with the direct mechanisms, more energy is partitioned to internal degrees of freedom of the product for the indirect mechanisms at Erel=1.9 eV, and fint for direct stripping is larger than that for rebound mechanism. The velocity scattering angle distributions vs. cos(θ) for the Cl-(H2O) + CH3I → CH3Cl + I- + H2O SN2 reaction are shown in Figure 6. Relative to the CH3I initial velocity vector, when I- appears backward scattering along the opposite direction, cos(θ) is less than zero. Conversely, when CH3Cl scatters forward in the same direction, cos(θ) is greater than zero. We found that DR mechanism is mainly backward scattering with cos(θ) ranging from -1.0 to 0, and the average cos(θ) value is -0.56. For the DS mechanism, cos(θ) is between 0.0 and 0.8 with the average value of 0.34, primarily displaying a forward scattering. This indicates that all DS trajectories show forward scattering and the final relative velocity vector deviates from the initial one at least 30°. 11



For the indirect mechanism, the cos(θ) covers a broad range varying from -1.0 to 0.8, suggesting an overall isotropic distribution. In general, since direct rebound is the dominant mechanism, the total scattering distributions are generally backward scattering at Erel of 1.9 eV. 3.2.4. Dynamics of the H2O molecule In order to clarify the solvent effect on the SN2 reaction, understanding the role of water molecule is significant. A plot of the time of H2O leaving from the reactive system versus the time of the SN2 occurrence is given in Figure 7, which is helpful to reveal the behavior of water in different mechanisms. As shown in the figure, the symbol below diagonal line indicates that the water leaves prior to the substitution reaction, whereas the SN2 reaction occurs more rapidly for the trajectory above the diagonal. If the symbol is on the red diagonal line, it represents that the SN2 reaction and the water leaving occur simultaneously. For both direct mechanisms, DR and DS, as soon as the reactants Cl-(H2O) and CH3I collide with each other, the SN2 reaction crosses the central barrier and Walden inversion occurs. And at the same time enough energy is obtained for stretch mode of Cl···H(OH) to break the hydrogen bond, resulting in the leaving of water molecule. In contrast, the H2O molecule leaving precedes the SN2 reaction for most of the indirect reactions (57.9%), and they are almost roundabout events. There are also a significant proportion of reactions with H2O departure and substitution occurring simultaneously (31.6%), and few proceeding with H2O leaving after the SN2 12


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reaction (10.5%). Moreover, it can be seen that the closer to the coordinate origin, the faster the new products are generated. Generally speaking, for most of the trajectories, the collision and the departure of H2O occur almost at the same time, suggesting the water molecule prefers to leave from the system early and rapidly at high collision energy. This can explain why unsovlated products prevail for the microsolvated SN2 reactions. 3.3. Comparison with the Unsolvated Cl- + CH3I Reaction The PES and the stationary points for Cl-(H2O) + CH3I reaction are compared with those of the unsolvated Cl- + CH3I reaction.12,37 Overall, the PESs for both reactions are similar and present the "double well" features. Instead of the single SN2 product CH3Cl + I- for the Cl- + CH3I, the addition of H2O gives two new products with CH3Cl or Isolvated. The Cl- + CH3I → CH3Cl + I- reaction releases the heat of ~ -15.5 kcal/mol at the CCSD(T)/CBS level of theory. For the solvated system, solvating the reactants decreases the reaction exothermicity, with the values of the three product channels no more than -8.0 kcal/mol. The CCSD(T) energies relative to reactants Cl-(H2O) + CH3I was found to be 0.1 and -17.7 kcal/mol for the barrier and post-reaction complex (im2), which are 5.5 and 6.3 kcal/mol higher than the respective energies of the unsolvated analogs, whereas the energy of pre-reaction complex (im1) almost unchanged. And this brings out that the solvated central barrier of 11.4 kcal/mol from im1 to TS, is 5.3 kcal/mol larger than 6.1 kcal/mol of the unsolvated analog. The increased SN2 central 13



barrier is supposed to reduce the rate constant of the solvated reactions relative to the unsolvated ones as measured in experiment. It is significant to determine the variations of reaction probability, mechanism, cross section, and the distribution of product energy and velocity scattering angle by comparing Cl- and Cl-(H2O) reacting with CH3I in order to investigate the solvent effect. The simulation results show that the formations of CH3Cl + I- and CH3Cl + I- + H2O are the predominant pathways for the unsolvated and solvated reactions.36 As presented in Figure 2, the total reaction possibility of Cl-(H2O) was not dramatically reduced when compared to the unsolvated system at high collision energy range. Two factors could be responsible for this observation, i.e., the atomic-level mechanism and method of performing the simulations. For the dominant DR mechanism, the water departs from the system once the reactants are colliding with each other at high collision energy as discussed in Figure 7, and then the reaction proceeds in the manners of the process for unsolvated system without the hinder of solvent molecule, resulting in the similar reaction probabilities for Cl-(H2O)n=0,1 + CH3I reactions. On the other hand, the MP2 is employed to perform the calculations for the unsolvated system at high energy of 1.9 eV, which is supposed to give a relative low reaction probability compared to DFT method, but the properties of reaction mechanism, product energy and scattering angle distribution obtained by MP2 and DFT are in consistent.36

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Consequently, the total σr of Cl-(H2O) + CH3I is slightly smaller than that for Cl+ CH3I36, i.e., 0.71 ± 0.18 Å2 for Cl-(H2O) and 0.84 ± 0.32 Å2 for Cl-, respectively. Correspondingly, the total k of Cl-(H2O) is (2.18 ± 0.55) × 10-13 cm3mol-1s-1, around 0.90 × 10-13 cm3mol-1s-1 smaller than that for Cl-. It can be seen from the comparison that the effect of the solvation on the rate constant is not obvious. On the other hand, the reaction categories are also similar for the two reaction systems, including DR, DS, and Ind mechanisms, and their contributions are also alike. The fraction for the dominant DR mechanism is 0.74 and 0.58 for the solvated and unsolvated reactions, respectively, and the proportion of the dominant roundabout indirect mechanism is 0.15: 0.30. As shown in Table 2, for the total reaction mechanisms, more energy partitions to the internal energy of product for the solvated system with the value of 0.52 ±0.02 than does the unsolvated reaction (0.38 ±0.05), especially for direct DR and DS mechanisms. Since direct rebound mechanism prevails for both solvated and solvent-free reactions, the velocity scattering angle distributions are primarily backward scattering, which is in agreement with the experimental observation for Cl- + CH3I reaction.36 In general, the dynamics characters including reaction rates, mechanisms, and the distributions of product energy and velocity scattering angle do not present an obvious differences at high collision energy for Cl-(H2O)n=0,1 + CH3I reactions. This may arise from the fact that at high Erel, the water leaves the reaction system at early stage and the system 15



follows the process of the unsolvated reaction. However, it should be noted that the mechanisms of Cl- + CH3I are significantly dependent on collision energies,36 and it is expected that at low Erel range, the dynamics of the solvated reaction could be diverse from the unsolvated analogue. 3.4. Comparison with F-(H2O) + CH3I Reaction It is of great significance to compare the current reaction with F-(H2O) + CH3I11,34,56 to explore the influence of the nucleophile basicity on the reaction mechanism in microsolavted systems. Both the PES profile and dynamics exhibit differences for the two reactions. For F-(H2O) + CH3I reaction, a hydrogen-bonded complex F-(H2O)---HCH2I and a second close-lying ion-dipole complex F-(H2O)--CH3I are found in the entrance channel.56 However, due to the smaller electronegativity of Cl- compared to F-, only an ion-dipole complex Cl-(H2O)---CH3I is observed for Cl(H2O) + CH3I. F-(H2O)---CH3I overcomes the barrier of 5.5 kcal/mol at CCSD(T)/pp/t level of theory and leads to the formation of the post-reaction complex CH3F(H2O)--I-, with H2O connecting to F atom. Then, a H2O migration process occurs from fluorine to iodine ion side to form the CH3F---I-(H2O) easily.56 In contrast, after Cl-(H2O)--CH3I climbing up a double higher barrier of 11.4 kcal/mol with CCSD(T)/PP/t theory, the Walden inversion and the transportation of water from Cl- to I- simultaneously occurred for the substitution, and then the system goes down to the PES well to form post-reaction complex CH3Cl--- I-(H2O), for which the water has been transferred to I16


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side.56 The much higher central barrier for Cl-(H2O) + CH3I is expected to lead to much lower reaction probability compared to F-(H2O) + CH3I, which has been confirmed by the present work. Three products P1 CH3F + I- + H2O, P2 CH3F + I-(H2O) and P3 CH3F(H2O) + I- were found for F-(H2O) + CH3I34 reaction with their energies decreased in the order of P1> P3> P2, which are in line with the observation for Cl-(H2O) + CH3I reaction. The variation of F-(H2O) to Cl-(H2O) does not change the reaction exothermicities of the products for both reactions. More importantly, although the energy of P2 is lower than P1 and P3, the solvent free product P1 overwhelmingly dominant for both reactions, as observed in experiment and simulations.29,34 The dynamics simulation for F-(H2O) + CH3I has been carried out at high collision energy of Erel = 1.53 eV, which can be compared with the results of the current reaction at Erel = 1.9 eV. The maximum b value of 3.5 Å for Cl-(H2O) + CH3I is smaller than that for F-(H2O) + CH3I with bmax of 5.5 Å, and Pr(b) for the former one is nearly a half lower than the latter reaction. As a result, the obtained cross section of 0.71 ± 0.18 Å2 for Cl-(H2O) + CH3I SN2 reaction is much smaller than the value of 4.30 ± 0.90 Å2 for F-(H2O).34 And thus, the rate constant of (2.18 ±0.55) × 10-13 cm3mol-1s-1 for Cl-(H2O) + CH3I is orders of magnitude lower relative to that of (1.40 ±0.30) ×10-10 cm3mol-1s1

for F-(H2O) +CH3I. The contributions of diverse reaction mechanisms are totally

different for the two reactions. For the three-body decomposition reaction, the percentages of the DR, DS, and Ind mechanisms of the solvated Cl-(H2O) + CH3I 17



reaction are 75, 9, and 16%, respectively. And the corresponding respective fractions are 32, 11, and 57% for F−(H2O) + CH3I reaction.34 The results suggest that the variation of nucleophile from Cl-(H2O) to F-(H2O) results in the predominant mechanism changes from direct rebound to indirect mechanism. This is because that for F-(H2O) +CH3I reaction, there is a pre-reaction complex F−···HCH2I formed in the entrance channel due to the high basicity of fluorine, which plays a major role in the reaction mechanisms and product energy partitionings. In contrast, only a traditional ion-dipole complex is found for Cl-(H2O) +CH3I system. Moreover, the dominant indirect mechanisms are different, varying from the roundabout to the formation of pre-reaction complex when Cl-(H2O) is replaced by F-(H2O). The water molecule in the two reaction systems at high collision energy range did not behave very differently. For the direct trajectories of F-(H2O) + CH3I reaction, the elimination of H2O is simultaneous with SN2 reaction, which is in line with the solvated chloride system. For the dominant indirect events of F-(H2O) system, ~70% of them follow H2O leaving before SN2 substitution34, and this percentage is decreased somewhat to 57% for Cl-(H2O) indirect reactions. It can be seen that for both F-(H2O) and Cl-(H2O) system, water molecule tends to be away before or as soon as the substitution reaction occurs, which is because that the high collision energy is more likely to break the hydrogen bond of X-…H-OH.

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IV. CONCLUSIONS. In this study, the properties of stationary points on the microslovated Cl-(H2O) + CH3I PES are investigated by MP2 and the various DFT functionals. When compared with the higher level CCSD(T)/PP/t//MP2/ECP/d results, it has been found that B97-1 functional gives a relatively smaller systematic error than do the other DFT functionals. The followed direct dynamics simulations are therefore carried out at B97-1/ECP/d level of theory for Cl-(H2O) + CH3I reaction. Compared to the unsolvated reaction of Cl- + CH3I, the addition of water slightly lowers the reaction possibility and the rate constant is correspondingly reduced. To some extent, this reduction is mainly due to the higher central barrier between the prereaction complex and TS for the solvated system than its nonsolvated counterpart. The steric effect of the water molecule barely prevents the reaction proceeding for the solvated reaction since the water is inclined to detach from the reactive system before the SN2 substitution at high collision energy. Both reactions are dominated by the direct rebound mechanism, and most importantly, the round-about mechanism still prevails in the indirect reactions for the hydrated reaction as behaved in the unsolvated counterpart. In addition, it is found that the solvated products preferred in energy are suppressed for current reaction, which has been supported by experiments and dynamics simulations. For the variation of nucleophile from Cl-(H2O) to F-(H2O), some significant distinctions were observed, which could have important implications. The ion-dipole 19



complexes were found for reaction involving Cl-(H2O), and in contrast, both hydrogenbonded and ion-dipole types of pre- and post-reaction complexes were formed before and after crossing the SN2 barrier for F-(H2O) + CH3I. That is originated from the much stronger electron-withdrawing ability of F- than Cl-, which highly facilitates the formation of hydrogen bond. Consequently, going from Cl-(H2O) to F-(H2O), the dominant mechanism changes from direct rebound to the indirect mechanism. In addition, compared to F-(H2O), the dynamics of Cl-(H2O) give the relatively lower cross section and rate constant.

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(7) Szabo, I.; Czako, G. Revealing a Double-inversion Mechanism for the F- + CH3Cl SN2 Reaction. Nat. Commun. 2015, 6, 5972. (8) Sun, L. P.; Song, K. Y.; Hase, W. L. A SN2 Reaction that Avoids its Deep Potential Energy Minimum. Science 2002, 296, 875-878. (9) Zhang, J.; Hase, W. L. Electronic Structure Theory Study of the F- + CH3I → FCH3 + I- Potential Energy Surface. J. Phys. Chem. A. 2010, 114, 9635-9643. (10) Szabo, I.; Czako, G. Dynamics and Novel Mechanisms of SN2 Reactions on ab Initio Analytical Potential Energy Surfaces. J. Phys. Chem. A 2017, 121, 9005-9019. (11) Olasz, B.; Czako, G. High-Level-Optimized Stationary Points for the F−(H2O) + CH3I System: Proposing a New Water-Induced Double-Inversion Pathway. J. Phys. Chem. A 2019, 123, 454−462. (12) Szabo, I.; Czako, G. Benchmark ab Initio Characterization of the Complex Potential Energy Surface of the Cl− + CH3I Reaction. J. Phys. Chem. A 2017, 121, 5748−5757. (13) Voehringer-Martinez, E.; Hernandez-Soto, H. Water Catalysis of a RadicalMolecule Gas-Phase Reaction. Science 2007, 316, 691-691. (14) Laerdahl, J. K.; Uggerud, E. Gas Phase Nucleophilic Substitution. Int. J. Mass Spectrom. 2002, 214, 277-314.

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ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (nos. 21573052, and 51536002), the Natural Science Foundation of Heilongjiang Province of China (no. B2017003), the Open Project of Beijing National Laboratory for Molecular Sciences (20150158), and the Fundamental Research Funds for the Central Universities, China (AUGA5710012114, 5710012014). Support is also provided by the High Performance Computing Center (HPCC) at Texas Tech University.

Supporting Information Available: Stationary point geometries and frequencies calculated by MP2 and different DFT functionals. This information is available free of charge via the Internet at http://pubs.acs.org.

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 30 of 41

Table 1. The Relative Energies for Cl¯(H2O) + CH3I Stationary Points.a,b Stationary points

MP2

OPBE

BhandH

B97-1

HCTH407

B3LYP

CCSD(T) c

Cl¯(H2O)---CH3I (im1)

-11.7

-7.3

-13.8

-10.4

-10.0

-9.4

-11.3 (-10.6)

0.2

0.2

-4.4

-3.7

-3.2

-3.4

0.1 (-0.9)

CH3Cl---I¯(H2O) (im2)

-17.4

-16.1

-22.8

-17.5

-18.1

-16.3

-17.7 (-16.4)

CH3Cl + I¯+ H2O (P1)

4.4

-2.4

0.9

1.9

-0.1

1.0

4.0 (-3.6)

(0.1)

CH3Cl + I¯(H2O) (P2)

-7.4

-10.9

-9.1

-9.0

-10.5

-9.1

-8.0 (-7.3)

(-10.1)

CH3Cl(H2O) + I¯(P3)

0.1

-3.7

-5.7

-1.7

-3.2

-1.8

0.0 (-1.0)

[H2O---Cl---CH3---I]¯(TS)

Exptd

a

Energies (without ZPE) at 0 K are in kcal/mol with respect to the Cl¯(H2O) + CH3I reactants.

b

ECP/d basis set was used for MP2 and DFT calculations and the CCSD(T)/PP/t energies were calculated based on the geometries at

MP2/ECP/d level of theory. c

Values in parentheses include ZPE.

d

The reaction enthalpies of reaction (with ZPE) at 0 K calculated based on the data in refs 53 and 54.

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Table 2.Internal energy fractions of product energy partitioning for Cl-(H2O) + CH3I → CH3Cl + I- +H2O reactive collision a and comparison with unsolvated reaction at 1.9 eVb.

a

DR

DS

Ind

Total

Cl-(H2O) + CH3I

0.45 ±0.02

0.60 ±0.12

0.78 ±0.03

0.52 ±0.02

Cl- + CH3I

0.29 ±0.03

0.33

0.80 ±0.04

0.38 ±0.05

Results are reported for the individual DR, DS, and Ind mechanism and the total

reaction. b

Ref. 36.

31



Page 32 of 41

Figure Captions Figure 1. The optimized structures of stationary points as well as the potential energy curve for the Cl-(H2O) + CH3I SN2 reaction optimized at the B97-1/ECP/d level of theory.

The

energies

are

in

kcal/mol

without

ZPE.

The

values

of

CCSD(T)/PP/t//MP2/ECP/d are in parentheses and experimental data (ref 53, 54) are in square brackets, respectively. Figure 2. Schematic of reaction probability Pr(b) versus impact parameter b for the Cl−(H2O) + CH3I reaction at Erel = 1.9 eV and comparison with unsolvation reaction at Erel = 1.9 eV. The green, blue, and pink dashed line are for DR, DS, and Ind mechanisms, respectively. The red and black solid line represent the total reaction probability for the solvated and unsolvated system. Figure 3. Atomistic dynamics of a typical trajectory for the direct mechanism of Cl−(H2O) + CH3I → CH3Cl + I− + H2O. The left pannel shows the direct rebound mechanism and the right one is characterized by the direct stripping mechanism. Figure 4. Atomistic dynamics of a typical trajectory for the roundabout indirect mechanism of Cl−(H2O) + CH3I → CH3Cl + I− + H2O. Figure 5. Probability of the individual reaction mechanisms calculated at the B971/ECP/d level of theory. DR, DS, R0Ra/R1Ra, roundabout occurs after/before the water leaves the system; R0pre, Cl−---CH3I; R0post, CH3Cl---I−; R1pre, (H2O)Cl−---CH3I pre-reaction complex; R1post, H2O---CH3Cl---I− post-reaction complex; R0br/R1br,

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barrier recrossing occurs after/before the water leaves the system. The trajectory results are properly weighted by b and Pr(b) the reaction probability versus b. Figure 6. Distribution of velocity scattering angles for different atomistic mechanisms of the Cl−(H2O) + CH3I → CH3Cl + I− + H2O SN2 reaction at Erel=1.9 eV and comparision with unsolvation reaction at Erel = 1.9 eV. Distributions are given for the DR (green), DS (blue), Ind (pink) mechanisms, Total reaction (red) and Total of unsolvated reaction (black). Figure 7. Scatter plot of H2O-leaving time versus CH3Cl-formation time for the Cl−(H2O) + CH3I → CH3Cl + I− + H2O SN2 reaction at Erel=1.9 eV. The green, blue, and pink circle represents the DR, DS, and Ind mechanisms, respectively.

33



Figure1.

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Figure 2.

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Figure 3.

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Figure 4.

37



Figure 5.

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Figure 6.

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Figure 7.

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TOC Graphic

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Dynamics of Clâ(H2O) + CH3I Substitution Reaction: The Influences of

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