Dynamics of the Fâ + CH3I â HF + CH2Iâ Proton Transfer Reaction

Subscriber access provided by University of Pennsylvania Libraries

Article

Dynamics of the F- + CH3I # HF + CH2I- Proton Transfer Reaction Jiaxu Zhang, Jing Xie, and William Louis Hase J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b08167 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Dynamics of the F- + CH3I → HF + CH2I- Proton Transfer Reaction

Jiaxu Zhang,a Jing Xie,b and William L. Hasec,*

a

Institute of Theoretical and Simulation Chemistry

Academy of Fundamental and Interdisciplinary Sciences Harbin Institute of Technology Harbin 150080, P. R. China b

Department of Chemistry University of Minnesota

Minneapolis, Minnesota 55455 USA c

Department of Chemistry and Biochemistry Texas Tech University Lubbock, Texas 79409 USA

1 ACS Paragon Plus Environment


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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

Abstract Direct chemical dynamics simulations, at collision energies Erel of 0.32 and 1.53 eV, were performed to obtain an atomistic understanding of the F- + CH3I reaction dynamics. There is only the F- + CH3I → CH3F + I- bimolecular nucleophilic substitution SN2 product channel at 0.32 eV. Increasing Erel to 1.53 eV opens up the endothermic F- + CH3I → HF + CH2I- proton transfer reaction, which is less competitive than the SN2 reaction. The simulations reveal proton transfer occurs by two direct atomic-level mechanisms, rebound and stripping, and indirect mechanisms, involving formation of the F----HCH2I complex and the roundabout. For the indirect trajectories all of the CH2I- is formed with zero-point energy (ZPE), while for the direct trajectories 50% form CH2I- without ZPE. Without a ZPE constraint for CH2I-, the reaction cross sections for the rebound, stripping, and indirect mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Å2, respectively. Discarding trajectories which do not form CH2I- with ZPE reduces the rebound and stripping cross sections to 0.1 ± 0.1 and 0.7 ± 0.5 Å2. The HF product is formed rotationally and vibrationally unexcited. The average value of J is 2.6 and with histogram binning n = 0. CH2I- is formed rotationally excited. The partitioning between CH2I- vibration and HF + CH2I- relative translation energy depends on the treatment of CH2I- ZPE. Without a CH2IZPE constraint the energy partitioning is primarily to relative translation with little to CH2Ivibration. With a ZPE constraint, energy partitioning to CH2I- rotation, CH2I- vibration, and relative translation are statistically the same. The overall F- + CH3I rate constant at Erel of both 0.32 and 1.53 eV is in good agreement with experiment and negligibly affected by the treatment of CH2I- ZPE, since the SN2 reaction is the major contributor to the total reaction rate constant. The potential energy surface and reaction dynamics for F- + CH3I proton transfer are compared with those reported previously (J. Phys. Chem. A 2013, 117, 7162-7178) for the isoelectronic OH- + CH3I reaction.


Page 3 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I. Introduction There have been extensive studies,1-6 both experimental7-14 and computational,10,12-18 of the atomistic dynamics for X- + CH3Y ion-molecule reactions. Most of these studies have focused on the SN2 mechanism forming CH3X + Y-, and have illustrated the importance of nonstatistical dynamics and non-traditional pathways for these reactions. In addition to this SN2 pathway, the X- + CH3Y reaction has a proton-transfer pathway forming HX + CH2Y-.19,20 Translational energy dependent cross sections for the proton transfer reactions of OH- with CH3Cl and CH3Br have been measured by mass spectrometry.19,20 The threshold energies for the proton transfer reactions were estimated.19 Though the SN2 pathway dominates at low collision energies, proton transfer is competitive at higher collision energies.20 The proton transfer pathway has been investigated in recent theoretical and computational studies.13,21-24 Barrier heights for the X- + CH3Y [X,Y = F, Cl, Br, I] have been calculated using high-level CCSD(T) theory.21 Using an analytic potential energy function, fit to high-level electronic structure calculations, chemical dynamics simulations were performed to investigate reaction pathways and atomistic mechanisms for the reactions of F- with CH3Cl and CH3F.22,23 Proton transfer is endothermic for these reactions and does not occur at low collision energies, but becomes important at high collision energies. The importance of proton transfer at low energies is illustrated by recent direct dynamics simulations and experimental studies of the OH- + CH3I reaction.13,24 The SN2 and proton transfer pathways are of nearly equal importance for temperatures of 210 – 500 K and low collision energies, with proton transfer dominating at high collision energies of 1.0 and 2.0 eV. In this article results of chemical dynamics simulations are reported for the F- + CH3I proton transfer pathway, to complement the reported SN2 dynamics for the F- + CH3I reaction at collision energies of 0.32 and 1.53 eV.12,25 The F- + CH3I reaction has a high barrier for proton transfer and it is not observed at the lower collision energy of 0.32 eV. The proton transfer atomistic mechanisms, cross section and rate constant, product energy partitioning, and scattering dynamics are reported for the high collision energy of 1.53 eV. These findings are compared with those reported previously for OH- + CH3I.13,24 The accuracy of the F- + CH3I simulations is also discussed.



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

II. Potential Energy Surface Electronic structure calculations with DFT/B97-126,27 and MP228,29 theories were employed to investigate the stationary points of the reactants, intermediates, transition states, and products on the F- + CH3I → HF + CH2I- proton transfer potential energy surface (PES). These calculations utilize the aug-cc-pVDZ basis set30 for the C, H, and F atoms, an effective core potential (ECP) for the core electrons of iodine,31 and a 3s, 3p basis for its valence electrons. The latter was augmented by a d-polarization function with a 0.262 exponent and s, p, and d diffuse functions with exponents of 0.034, 0.039, and 0.0873, respectively.32 In previous work,33 a range of functionals were compared for the DFT calculations and B97-1 was found to give the best agreement with experimental energies. The NWChem computer program34,35 was used for the electronic structure calculations. Previous work33,36 indicated that for the SN2 reaction, DFT only gives a hydrogen-bonded entrance channel reaction path, with a hydrogen-bonded transition state (TS) [F--HCH2--I]connecting the hydrogen-bonded pre-reaction complex F----HCH2I and C3v post-reaction complex FCH3---I-. MP2 and CCSD(T) give these stationary points as well as the traditional C3v pre-reaction complex F----CH3I and central barrier [F--CH3--I]- TS.36 In contrast, for the work presented here, B97-1 and MP2 predict the same stationary points for the proton-transfer reaction. Figure 1 presents structures and relative energies without zero-point energy (ZPE) for the F+ CH3I proton transfer stationary points at the B97-1/ECP/d level of theory. This information, reported previously for OH- + CH3I proton transfer at the same level of theory, is also given in Figure 1 for comparison. For both the OH- + CH3I13,24 and F- + CH3I reactions, the first stationary point in proceeding from the reactants to products is the F----HCH2I hydrogen-bonded pre-reaction complex for the proton transfer and SN2 pathways. The energy of this X----HCH2I complex is very similar for both reactions and ~ -20 kcal mol-1 with respect to the reactants. With harmonic ZPEs included, the B97-1 F- + CH3I reaction energies for the proton-transfer and SN2 reactions are 16.9 and -45.1 kcal/mol, respectively, and in good agreement with experiment.33,37 Without ZPE added, these respective energies are 20.4 and -46.7 kcal/mol. The 298 K experimental reaction enthalpies for the proton transfer and SN2 reactions are 15.2 and -45.6 kcal mol-1, respectively.37 If the harmonic B97-1 frequencies are used to remove the thermal vibration enthalpies, along with the thermal rotation and translation enthalpies, the respective 0 K experimental reaction energies for the proton transfer and SN2 reactions are 14.5 and –45.4 4 ACS Paragon Plus Environment

Page 5 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kcal/mol. Both the experimental and B97-1 proton transfer reaction energies with ZPE included are in good agreement with the CCSD(T)-F12b/aug-cc-pVTZ(-PP) value of 15.2 kcal/mol for this energy.21 As shown in Figure 1, the DFT calculations for the proton transfer pathway predict the existence of a hydrogen-bonded pre-reaction complex F----HCH2I (A) and a transition state [CH2I--HF]- (TS1) connecting this complex with the post-reaction complex CH2I----HF (B). In this reaction path, A undergoes a concerted H-shift from the C- to F-atom and a migration of HF from the C- to I-atom in a clockwise direction forming B. Alternatively, HF can rotate counterclockwise and pre-reaction complex (A) isomerizes to complex FH---CH2I- (C) via TS2 [FH--CH2I]- followed by a HF shift (TS3) leading to the post-reaction complex D. The postreaction complexes B and D, and transition states TS1 and TS3 are trans-cis configurations through varying the dihedral angle of F-I-C-H. The trans-cis isomers B and D can interconvert easily to each other via TS4. For the OH- + CH3I reaction, both the proton transfer and SN2 channels are exothermic and the energies of all stationary points on both channels are lower than the reactants OH- + CH3I. Though the reaction energetics favor the SN2 pathway, the dynamics simulations and experiments identify the SN2, OH- + CH3I → CH3OH + I-, and proton transfer, OH- + CH3I → CH2I- + H2O, channels as having nearly equal importance.13,24 In contrast, for the F- + CH3I reaction, the proton transfer pathway is endothermic and the SN2 pathway is highly exothermic, and with harmonic ZPE included the B97-1/ECP/d reaction energies are 16.9 and -45.1 kcal/mol. The energies for the latter are submerged with respect to the energy of the reactants F- + CH3I, while for the former, most stationary points lie higher than the reactants except for the prereaction complex A. The F- + CH3I PES is rather flat in the post-reaction region, with the difference in the stationary point energies not more than 3.7 kcal/mol at the B97-1/ECP/d level of theory. As a result, the proton transfer pathway is expected to be less competitive than the SN2 pathway for the F- + CH3I reaction, as confirmed by the trajectory simulations shown below.

III. Direct Chemical Dynamics Simulation Procedure As described previously,12 direct chemical dynamics classical trajectory simulations38 were performed for the F- + CH3I reaction at collision energies of 0.32 and 1.53 eV. The simulations were performed at the DFT/B97-1/ECP/d level of electronic structure theory, which 5 ACS Paragon Plus Environment


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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

gives SN2 and proton transfer reaction energies in good agreement with experiment (see above discussion and reference 33). The 0.32 eV (7.4 kcal/mol) collision energy is less than the threshold energy for proton transfer and this reaction channel was not observed in the simulations. However, proton transfer was observed for the 1.53 eV collision energy and its reaction dynamics are discussed in the following Sections. The F- + CH3I simulations at 1.53 eV were performed with CH3I vibrational and rotational temperatures of Tv = 360 and Tr = 75 K, the conditions considered experimentally.12 Quasiclassical sampling39 was used to determine initial conditions for the trajectories, as described previously for the Cl- + CH3I10 and the F- + CH3I SN2 simulations.12,25 The simulations were performed using the VENUS chemical dynamics computer program40,41 interfaced to the NWChem electronic structure computer program.42,43 A total of 1600 trajectories were calculated for the 1.53 eV collision energy. Instead of sampling the impact parameter b randomly, the trajectories were calculated at fixed b of 1, 2, 3, 4, 5, 5.5, 5.75, and 6 Å. There were no SN2 or proton transfer reactions out of 200 trajectories at b = 6 Å.

IV. Simulation Results A. Reaction mechanisms, probabilities, and cross sections For the simulations at Erel = 1.53 eV the HF + CH2I- products are formed. As for the SN2 substitution reaction,12,25 this reaction also occurs via the direct rebound, direct stripping, and indirect mechanisms. Representative animations of these mechanisms are on the web portal hasegroup.ttu.edu. The probabilities for these mechanisms versus impact parameter are plotted in Figure 2, where it is seen that direct stripping is the most important mechanism. The reaction cross section σr was obtained by integrating the total Pr(b) over the impact parameter according to ∫Pr(b)2πbdb, and the resulting total σr is 2.1 ± 0.8 Å2, where the standard deviations in the Pr(b), shown in Figure 2, were used to determine the uncertainty in σr. The cross sections for the direct rebound, direct stripping, and indirect mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Å2, respectively, giving respective contributions to the total cross section of 10, 57, and 33%. Most of the indirect reaction occurs with only formation of the F----HCH2I complex, which is 27% of the total reaction. There are two other indirect mechanisms each contributing 3% of the total reaction. One is the roundabout mechanism and the other F----HCH2I complex formation followed by the roundabout. 6 ACS Paragon Plus Environment

Page 7 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The SN2 reaction cross section at 1.53 eV is 8.6 ± 2.2 Å2,12 thus, proton transfer contributes ~ 20% of the reaction at this collision energy. The importance of proton transfer is expected to increase with collision energy and may surpass the SN2 reaction channel at high collision energy.13,21-24 B. Product Energy Partitioning With reactant and product harmonic ZPEs included, the B97-1 0 K reaction energy for F+ CH3I → HF + CH2I- proton transfer is 16.9 kcal/mol. The reactants collision energy Erel of 1.53 eV (35.3 kcal/mol) is significantly higher than the reaction endothermicity, and the energy available in the simulation for the HF + CH2I- products is 18.4 kcal/mol plus the small thermal vibrational and rotational energies of the CH3I reactant, whose combined average value is 0.6 kcal/mol. The average energy partitioning of the available energy to the product rotational, vibrational, and relative translational degrees of freedom is given in Table 1 for the total reaction and for reaction by the three atomic-level mechanisms. For each mechanism only a small fraction of the energy is transferred to HF internal energy. The energy partitioning is similar for the direct rebound and direct stripping mechanisms, whose fractions of energy transfer to relative translation are the largest and statistically the same. Energy transfer to CH2I- vibration is largest for the indirect mechanisms, occurring primarily at the expense of relative translation. In calculating the product energy partitioning values in Table 1, ZPE is subtracted from the vibrational energy of both HF and CH2I-. For CH2I- the harmonic B97-1 ZPE of 13.3 kcal/mol is subtracted and for HF the energy for the anharmonic n = 0 level of the B97-1 stretching potential is subtracted, which is 5.847 kcal/mol as compared to the harmonic value of 5.872 kcal/mol. For HF vibration, each of the fvib′ values is negative, since the average energy portioned to HF vibration is less than its ZPE. For CH2I- fvib′ is only negative for the direct rebound (DR) mechanism. Particularly interesting in the energy partitioning is the small amount of internal energy partitioned to HF vibration, the negative fvib′ values for HF, the large frot′ values for CH2I-, the large frel′ values for the DR and DS mechanisms, and the much smaller frel′ and larger fvib′ values for the indirect mechanisms. ZPE constraints for characterizing the trajectories are considered below in Section VI. The average HF vibrational and rotational quantum numbers n and J are -0.3 and 2.6, respectively. With histogram binning,44 all of the n population is in the n equal -0.5 to 0.5 7 ACS Paragon Plus Environment


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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

interval, representing the n = 0 ground state. The relative populations of J in the (J - 0.5) to (J + 0.5) intervals are 0.08, 0.30, 0.19, 0.21, 0.11, and 0.05 for J of 0, 1, 2, 3, 4, and 5, with a population of 0.06 for J > 5. There is an insufficient number of reactive trajectories to perform Gaussian binning45,46 for n and J. The negative average n value is expected, since as shown in Table 1 the average energy partitioned to HF is less than its ZPE. The OH- + CH3I → H2O + CH2I- product energy partitioning is compared with that for F+ CH3I in Table 1. The former simulations13 were performed at Erel of 1.0 and 2.0 eV in comparison to the F- + CH3I simulations at 1.53 eV. Proton transfer for OH- + CH3I has a B97-1 0 K exothermicity of -4.23 kcal/mol, so that for the Erel = 1.0 eV simulations the energy available to the products is ~ 27 kcal/mol in comparison to the 19 kcal/mol for the F- + CH3I simulations. The OH- + CH3I proton transfer product energy partitioning is similar to that found for F- + CH3I. The frel′ values are the same within statistical uncertainty, only a small amount of energy is transferred to HF and H2O rotation and vibration, and CH2I- rotation receives nearly 1/3 of the product energy. The only difference, and small, is that for OH- + CH3I less energy is partitioned to CH2I- vibration. The large CH2I- rotation is a kinematic affect. The abstracted H-atom of the HF product has a repulsive interaction with the C-atom of CH2I-, resulting in CH2 rotation about the heavy Iatom. Similar dynamics, leading to extensive product rotation, were found previously47 for CHCl2CH2Cl → HCl + CCl2CH2 dissociation, with HCl recoiling off the CH2-group which then rotates about the massive CCl2-group. C. Scattering dynamics The number of trajectories that form HF + CH2I- is too small to prepare a statistically meaningful histogram of the velocity scattering angles. However, the nature of the scattering could be identified and it is the same as described previously12 for the CH3F + I- SN2 reaction channel. The scattering is backward and forward, respectively, for the direct rebound and direct stripping mechanisms. For the indirect mechanism the scattering is isotropic and the overall scattering is also isotropic. V. F- + CH3I Reaction Rate Constant The rate constant has been measured for loss of F- by reaction with CH3I versus Erel and for a CH3I vibrational/rotational temperature Tvr of 297 K.48 Tvr is not expected to have a significant 8 ACS Paragon Plus Environment

Page 9 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effect on the F- + CH3I rate constant and, thus, though the simulations were not performed at Tvr = 297 K, the simulation rate constants may still be compared with experiment. This is illustrated by rate constants for OH- + CH3I.13,24 For Erel of 0.05 eV, which corresponds to a translational temperature of 387 K, and for CH3I vibrational and rotational temperatures of 330 and 130 K, simulations13 give a rate constant of (18.9 ± 1.5) x 10-10 cm3mol-1s-1, which is in excellent agreement with the experimental24 rate constant at 400 K of (19 ± 4.75) x 10-10 cm3mol-1s-1. The experimental F- + CH3I rate constant for Erel = 0.32 eV and Tvr = 297 K is approximately 17 x 10-10 cm3mol-1s-1.48 For the simulations12,25 at this collision energy only the SN2 products CH3F + I- are formed and the calculated cross section for their formation, with Tv = 360 K and Tr = 75 K, gives a rate constant k(Erel, Tv, Tr) = v(Erel)σ(Erel,Tv,Tr) = (21 ± 2) x 10-10 cm3mol-1s-1, which is in very good agreement with the experimental value. The rate constant has been measured for Erel from ~0.02 to 1.0 eV48 and, if a linear extrapolation of the constants for higher Erel is made, the experimental rate constant at Erel = 1.53 eV is approximated as ~3.0 x 10-10 cm3mol-1s-1. At this Erel, both the SN2 CH3F + I- and proton transfer HF + CH2I- products are formed in the simulations and the total cross section for their formation gives a rate constant of (4.4 ± 0.9) x 10-10 cm3mol-1s-1, which is in quite good agreement with the experimental value.

VI. Consideration of Zero Point Energy Classical mechanics does not constrain ZPE flow49-51 and allows products of unimolecular52 and bimolecular53 reactions to be formed with vibrational energy less than their ZPE. For the current F- + CH3I → HF + CH2I- simulation, HF is formed with a vibrational quantum number n in the -0.5 to 0.5 interval which, with histogram binning,44 contributes to the ground vibrational state n = 0. There is some uncertainty in the approach to use for treating bimolecular reaction polyatomic products for bimolecular reactions, which have a vibrational energy less than their ZPE, as is the case here for some of the CH2I- product. A widely used approach is to discard trajectories which do not have ZPE in the polyatomic products.53-57 For the current simulation, 50% of the direct trajectories form CH2I- without ZPE, while for each of the indirect trajectories the CH2I- vibrational energy is greater than its ZPE. Of the direct trajectories, 67% of the direct rebound (DR) do not form CH2I- with ZPE, while this percentage is smaller and 47% for the direct stripping (DS). The vibrational energy of the HF product was not considered in discarding



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

trajectories, since as described above in Section IV.B histogram binning was used to determine the HF vibrational quantum number. Discarding the trajectories which do not form CH2I- with ZPE decreases the DR and DS cross sections and the F- + CH3I rate constant, and modifies the DR and DS product energy partitioning, as well as the total product energy partitioning. The DR and DS cross sections are changed from 0.2 ± 0.1 and 1.2 ± 0.4 Å2, respectively, to 0.1 ± 0.1 and 0.7 ± 0.5 Å2. The F- + CH3I rate constant, which is a sum of the rate constants for the SN2 and proton transfer reactions, is only lowered from (4.4 ± 0.9) x 10-10 to (4.2 ± 0.5) x 10-10 cm3mol-1s-1, and remains in quite good agreement with the experimental value of ~3.0 x 10-10 cm3mol-1s-1. The HF frot′ and fvib′, CH2I- frot′ and fvib′, and frel′ become 0.04, -0.11, 0.02, 0.38, and 0.67 for the DR mechanism and 0.03 ± 0.01, -0.19 ± 0.04, 0.29 ± 0.06, 0.39 ± 0.09, and 0.48 ± 0.09 for the DS mechanism. Compared to the energy partitioning fractions in Table 1, these fractions for trajectories discarded without ZPE in CH2I- are significantly larger and smaller, respectively, for fvib′ and frel′. For the total energy partitioning these fractions are 0.05 ± 0.02, -0.19 ± 0.03, 0.34 ± 0.05, 0.39 ± 0.06, and 0.41 ± 0.05. Compared to the values in Table 1, fvib′ is 0.19 units larger and frel′ is 0.20 smaller. It is noteworthy that no proton transfer was observed for the simulations at Erel of 0.32 eV (7.4 kcal/mol). Though this collision energy is less than the B97-1 threshold of 16.9 kcal/mol with harmonic ZPE in the reactants and products, as described above classical dynamics does not restrict the unphysical use of some of the 22.7 kcal/mol ZPE of CH3I to promote proton transfer and, thus, form the HF + CH2I- products without ZPE. This did not occur.

VII. Summary and Conclusions Detailed atomic level dynamics for the F- + CH3I reaction were investigated at 0.32 and 1.53 eV collision energies using DFT/B97-1/ECP/d direct dynamics simulations.12,25 Only the F+ CH3I → CH3F + I- SN2 substitution reaction is found at the 0.32 eV collision energy, but as the collision energy is further increased to 1.53 eV, the proton transfer channel F- + CH3I → HF + CH2I- becomes accessible after overcoming the H-abstraction barrier of 16.9 kcal/mol (0.73 eV). The dynamics for this proton transfer are analyzed here. This channel has a lower probability of reaction versus impact parameter, Pr(b), as compared to the SN2 channel, consistent with a barrier for proton transfer and a barrier less and exothermic SN2 reaction. The SN2 reaction cross 10 ACS Paragon Plus Environment

Page 11 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


section is 8.6 ± 2.2 Å2 and, if ZPE constraints are not considered, the proton transfer cross section is 2.1 ± 0.8 Å2. If trajectories are discarded, which do not form CH2I- with ZPE, the proton transfer cross section is lowered to 1.5 ± 0.4 Å2. At collision energies higher than the 1.53 eV considered here, proton transfer may become more important than the SN2 reaction.13,21-24 The proton transfer reaction is the combination of three mechanisms, also found for the SN2 reaction:12,25 i.e. direct rebound, direct stripping, and indirect; yielding backward, forward, and isotropic scattering, respectively. Without ZPE constraints, the respective cross sections for these mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Å2. Each of the trajectories for the indirect mechanism forms CH2I- with ZPE, but for the direct rebound and stripping mechanisms there are trajectories that form CH2I- without ZPE. If these trajectories are discarded, the respective cross sections for the rebound and stripping mechanisms are lowered to 0.1 ± 0.1, and 0.7 ± 0.5 Å2. Two striking features of the product energy partitioning, for each of the three mechanisms, is the low rotational and vibrational excitation of HF and the high rotational excitation of CH2I-. With histogram binning, HF is formed in the n = 0 vibrational state for all of the trajectories. The frot′ term for CH2I- is the largest for the indirect mechanism and 0.39 ± 0.08. For the indirect mechanism, the fractions frot′ and fvib′ for CH2I- and frel′ are statistically the same. However, for the direct mechanisms, with significant fractions of the trajectories forming CH2Iwithout ZPE, the product energy partitioning is primarily to relative translation with frel′ of 0.82 ± 0.12 and 0.74 ± 0.11 for rebound and stripping, respectively. However, if the trajectories are discarded, which do not form CH2I- with ZPE, the product energy partitioning fractions are statistically the same for the indirect and direct mechanisms. Without ZPE constraints the total energy partitioning fractions to frot′ and fvib′ for HF, frot′ and fvib′ for CH2I-, and frel′ are 0.05 ± 0.02, -0.17 ± 0.02, 0.31 ± 0.04, 0.20 ± 0.07, and 0.61 ± 0.07. These fractions become 0.05 ± 0.02, -0.19 ± 0.03, 0.34 ± 0.05, 0.39 ± 0.06, and 0.41 ± 0.05 with a constraint on the CH2I- ZPE. The F- + CH3I total SN2 and proton transfer rate constant, based on the cross sections from the simulations, agrees very well with the experimental48 value for both collision energies of 0.32 and 1.53 eV. At 0.32 eV the experimental rate constant is approximately 17 x 10-10 cm3mol-1s-1 in comparison to the simulation value of (21 ± 2) x 10-10 cm3mol-1s-1. At 1.53 eV the experimental rate constant is ~3.0 x 10-10 cm3mol-1s-1, while the simulation rate constant is (4.4 ± 0.9) and (4.2 ± 0.5) x 10-10 cm3mol-1s-1 without and with a CH2I- ZPE constraint, respectively. 11 ACS Paragon Plus Environment


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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

There are similarities and differences between the proton transfer product energy partitioning for F- + CH3I and that reported previously13,24 for the isoelectronic proton transfer OH- + CH3I → H2O + CH2I-. This latter reaction is exothermic13,24 and ZPE constraints are unimportant, in contrast to F- + CH3I proton transfer. The simulations for OH- + CH3I were performed at Erel of 1.0 and 2.0 eV (Table 1), energies lower and higher than the 1.53 eV simulation for F- + CH3I. For OH- + CH3I the product formed by proton transfer, i.e. H2O, receives very little rotational and vibrational excitation, as is the case for HF formed by F- + CH3I. Rotational excitation of CH2I- is important for OH- + CH3I at Erel of 1.0 eV, as found for F+ CH3I, but less important at 2.0 eV for OH- + CH3I. For OH- + CH3I proton transfer, the principal recipient of the product energy is frel′, which is also the case for F- + CH3I without ZPE constraints for CH2I-. However, with CH2I- ZPE constraints, energy partitioning is statistically the same to CH2I- frot′ and fvib′ and relative translation frel′ for F- + CH3I proton transfer. It is interesting that the double inversion SN2 mechanism, found for the F- + CH3Cl reaction,22 is not observed in either the 0.32 or 1.53 eV simulations for F- + CH3I, even though the barrier for this F- + CH3I mechanism is quite low and only 8.6 kcal/mol at the CCSD(T) level of theory.21 The double inversion TS was located for the DFT/B97-1/ECP/d level of theory, used for the current study, and it has a barrier of 9.4 kcal/mol, and both the TS structure and barrier are similar to the CCSD(T) results. The resulting TS structure and potential energy curve are shown in Figure 3. The double inversion potential energy curve in Figure 3(b) follows that given previously for the F- + CH3Cl reaction. However, it is important to note that this is not the curve given by an IRC calculation,58 which connects the double inversion transition state, TS1 in Figure 3(b), with F----HCH2I, the hydrogen-bonded pre-reaction complex A in Figure 1, and FH--CH2I-, complex C in Figure 1. In assessing this IRC curve and the potential energy curve in Figure 3(b), it is important to note that often the actual reaction pathway is not the IRC.13,59-61 In future work it will be important to establish the relationship between the IRC curve for F- + CH3I double inversion and the actual potential energy curve for this pathway. For F- + CH3Cl the double inversion mechanism has a cross section close to zero Å2 at 0.32 eV and only ~ 0.01 Å2 at 1.53 eV. If the cross sections for this mechanism are similarly small for the F- + CH3I reaction, the 1250 and 1600 trajectories calculated at 0.32 and 1.53 eV may be too small to see this event.12,25 That this mechanism is not observed as the collision energy is increased, even though it has a quite low barrier, indicates that the collision translation 12 ACS Paragon Plus Environment

Page 13 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energy does not promote reaction. This is known and understood for other polyatomic bimolecular reactions.62,63 For the Cl- + CH3Cl SN2 reaction, which has a central barrier higher in energy than that for the reactants, C-Cl stretch vibrational excitation of the CH3Cl reactant promotes a direct SN2 reaction without trapping in either the pre- or post-reaction Cl----CH3Cl complex.15,16 It will be of interest to determine what type of vibrational excitation promotes the double inversion SN2 mechanism for F- + CH3I. Finally, the reaction mechanisms revealed in this work would be beneficial for developing theoretical models to represent proton transfer reactions in other and more complex ion-molecule reactions.

Acknowledgements The research reported here is based upon work supported by the Robert A. Welch Foundation under grant No. D-0005. The simulations were performed at the High Performance Computing Center (HPCC) at Texas Tech University, under the direction of Philip W. Smith, and at Texas Advanced Computing Center (TACC) at the University of Texas at Austin. Bill Hase wishes to acknowledge important and enjoyable collaborations with the Al Viggiano and Roland Wester research groups. This work is also supported by the National Natural Science Foundation of China (nos. 21573052, 51536002), the Fundamental Research Funds for the Central Universities, China (AUGA5710012114), the SRF for ROCS, SEM, China, and the Open Project of Beijing National Laboratory for Molecular Sciences (no. 20140103).



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

References 1. Hase, W. L. Simulations of Gas-Phase Chemical Reactions: Applications to SN2 Nucleophilic Substitution. Science 1994, 266, 998-1002. 2. Chabinyc, M. L.; Craig, S. L.; Regan, C. K.; Brauman, J. I. Gas-phase Ionic Reactions: Dynamics and Mechanism of Nucleophilic Displacements. Science 1998, 279, 1882-1886. 3. Laerdahl, J. K.; Uggerud, E. Gas Phase Nucleophilic Substitution. Int. J. Mass Spectrom. 2002, 214, 277-314. 4. Mikosch, J.; Weidemüller, M.; Wester, R. On the Dynamics of Chemical Reactions of Negative Ions. Int. Rev. Phys. Chem. 2010, 29, 589-617. 5. Manikandan, P.; Zhang, J.; Hase, W. L. Chemical Dynamics Simulations of X- + CH3Y → XCH3+Y-Gas-Phase SN2 Nucleophilic Substitution Reactions. Nonstatistical Dynamics and Nontraditional Reaction Mechanisms. J. Phys. Chem. A 2012, 116, 3061-3080. 6. Barlow, S. E.; Van Doren, J. M.; Bierbaum, V. M. The Gas-Phase Displacement Reaction of Chloride Ion with Methyl Chloride as a Function of Kinetic Energy. J. Am. Chem. Soc. 1988, 110, 7240-7242. 7. Viggiano, A. A.; Morris, R. A.; Paschkewitz, J. S.; Paulson, J. F. Kinetics of the Gas-Phase Reactions of Cl- with CH3Br and CD3Br: Experimental Evidence for Nonstatistical Behavior. J. Am. Chem. Soc. 1992, 114, 10477–10482. 8. DeTuri, V. F.; Hintz, P. A.; Ervin, K. M. Translational Activation of the SN2 Nucleophilic Displacement Reactions Cl- + CH3Cl (CD3Cl) → ClCH3 (ClCD3) + Cl-: A Guided Ion Beam Study. J. Phys. Chem. A 1997, 101, 5969-5986. 9. Angel, L. A.; Ervin, K. M. Gas-Phase SN2 and Bromine Abstraction Reactions of Chloride Ion with Bromomethane: Reaction Cross Sections and Energy Disposal into Products. J. Am. Chem. Soc. 2003, 125, 1014-1027. 10. Mikosch, J.; Trippel, S.; Eichhorn, C.; Otto, R.; Lourderaj, U.; Zhang, J. X.; Hase, W. L. Weidemüller, M.; Wester, R. Imaging Nucleophilic Substitution. Science 2008, 319, 183-186. 11. Otto, R.; Brox, J.; Trippel, S.; Stei, M.; Best, T.; Wester, R. Single Solvent Molecules can Affect the Dynamics of Substitution Reactions. Nature Chem. 2012, 4, 534-538.


Page 15 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12. Mikosch, J.; Zhang, J.; Trippel, S.; Eichhorn, C.; Otto, R.; Sun, R.; de Jong, W. A.; Weidemüller, M.; Hase, W. L.; Wester, R. Indirect Dynamics in a Highly Exoergic Substitution Reaction. J. Am. Chem. Soc. 2013, 135, 4250-4259. 13. Xie, J.; Sun, R.; Siebert, M. R.; Otto, R.; Wester, R.; Hase, W. L. Direct Dynamics Simulations of the Product Channels and Atomistic Mechanisms for the OH- + CH3I Reaction. Comparison with Experiment. J. Phys. Chem. A 2013, 117, 7162-7178. 14. Xie, J.; Otto, R.; Mikosch, J.; Zhang, J.; Wester, R.; Hase, W. L. Identification of AtomicLevel Mechanisms for Gas-Phase SN2 X- + CH3Y Reactions by Combined Experiments and Simulations. Acc. Chem. Res. 2014, 47, 2960-2969. 15. Vande Linde, S. R.; Hase, W. L. A Direct Mechanism for SN2 Nucleophilic Substitution Enhanced by Mode Selective Vibrational Excitation, J. Am. Chem. Soc. 1989, 111, 2349-2351. 16. Hase, W. L.; Cho, Y. J. Trajectory Studies of SN2 Nucleophilic Substitution. III. Dynamical Stereochemistry and Energy Transfer pathways for the Cl- + CH3Cl Association and Direct Substitution Reactions. J. Chem. Phys. 1993, 98, 8626-8639. 17. Li, G.; Hase, W. L. Ab Initio Direct Dynamics Trajectory Study of the Cl- + CH3Cl SN2 Reaction at High Reagent Translational Energy. J. Am. Chem. Soc. 1999, 121, 7124-7129. 18. Wang, Y.; Hase, W. L.; Wang. H. Trajectory Studies of SN2 Nucleophilic Substitution. IX. Microscopic Reaction Pathways and Kinetics for Cl- + CH3Br. J. Chem. Phys. 2003, 118, 26882695. 19. Hierl, P. M.; Henchman, M. J.; Paulson, J. F. Threshold Energies for the Reactions OH- + CH3X → CH3OH + X- (X = Cl, Br) Measured by Tandem Mass Spectrometry: Deprotonation Energies (Acidities) of CH3Cl and CH3Br. Int. J. Mass Spectrom. Ion Processes 1992, 117, 475485. 20. Hierl, P. M.; Paulson, J. F.; Henchman, M. J. Translational Energy Dependence of Cross Sections for Reactions of OH-(H2O)n with CH3Cl and CH3Br. J. Phys. Chem. 1995, 99, 1565515661. 21. Szabó, I.; Czakó, G. Double-Inversion Mechanisms of the X- + CH3Y [X,Y = F, Cl, Br, I] SN2 Reactions. J. Phys. Chem. A 2015, 119, 3134-3140. 22. Szabó, I.; Czakó, G. Revealing a Double-Inversion Mechanism for the F- + CH3Cl SN2 Reaction. Nat. Commun. 2015, 6, 5972.



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

23. Szabó, I.; Telekes, H.; Czakó, G. Accurate Ab Initio Potential Energy Surface, Thermochemistry, and Dynamics of the F- + CH3F SN2 and Proton-Abstraction Reactions. J. Chem. Phys. 2015, 142, 244301. 24. Xie, J.; Kohale, S. C..; Hase, W. L.; Ard, S. G.; Melko, J. J.; Shuman, N. S.; Viggiano, A. A. Temperature Dependence of the OH- + CH3I Reaction Kinetics. Experimental and Simulation Studies and Atomic-Level Dynamics. J. Phys. Chem. A 2013, 117, 14019-14027. 25. Zhang, J.; Mikosch, J.; Trippel, S.; Otto, R.; Weidemüller, M.; Wester, R.; Hase, W. L. F- + CH3I→ FCH3 + I- ReactionDynamics.Nontraditional Atomistic Mechanisms and Formation of a Hydrogen-Bonded Complex. J. Phys. Chem. Lett. 2010, 1, 2747-2752. 26. Becke, A. D. Density-Functional Thermochemistry. V. Systematic Optimization of Exchange-Correlation Functionals. J. Chem. Phys. 1997, 107, 8554−8560. 27. Hamprecht, F. A.; Cohen, A. J.; Tozer, D. J.; Handy, N. C. Development and Assessment of New Exchange-Correlation Functionals. J. Chem. Phys. 1998, 109, 6264−6272. 28. Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. 29. Adams, G. F.; Bent, G. D.; Bartlett, R. J.; Purvis, G. D. In Potential Energy Surfaces and Dynamics Calculations; Truhlar, D. G., Ed.; Plenum: New York, 1981; p 133. 30. (a) Dunning, Jr., T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007-1023. (b) Woon, D. E.; Dunning, Jr., T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. III. The Atoms Aluminum through Argon. J. Chem. Phys. 1993, 98, 1358-1371. 31. Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82, 284-298. 32. Hu, W. P.; Truhlar, D. G. Structural Distortion of CH3I in an Ion-Dipole Precursor Complex. J. Phys. Chem. 1994, 98, 1049-1052. 33. Zhang, J.; Hase, W. L. Electronic Structure Theory Study of the F- + CH3I → FCH3 + IPotential Energy Surface. J. Phys. Chem. A. 2010, 114, 9635-9643. 34. Bylaska, E. J.; de Jong, W. A.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Valiev, M.; Wang, D.; Apra, E.; Windus, T. L.; Hammond, J.; et al. NWChem, A Computational Chemistry Package for Parallel Computers, version 5.1; Pacific Northwest National Laboratory: Richland, WA, 2007. 16 ACS Paragon Plus Environment

Page 17 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35. Kendall, R. A.; Apra, E.; Bernholdt, D. E.; Bylaska, E. J.; Dupuis, M.; Fann, G. I.; Harrison, R. J.; Ju, J.; Nichols, J. A.; Nieplocha, J. High Performance Computational Chemistry: An overview of NWChem a Distributed Parallel Application. Comput. Phys. Commun. 2000, 128, 260−283. 36. Sun, R.; Xie, J.; Zhang, J.; Hase, W. L. The F- + CH3I → FCH3 + I- Entrance Channel Potential Energy Surface: Comparison of Electronic Structure Methods. Int. J. Mass Spectrom. 2015, 377, 222-227. 37. Ruscic, B. Active Thermochemical Tables (ATcT); available at ATcT.anl.gov. 38. Sun, L., Hase, W. L. Born-Oppenheimer Direct Dynamics Classical Trajectory Simulations. Rev. Comput. Chem. 2003, 19, 79-146. 39. Peslherbe, G. H.; Wang, H.; Hase, W. L. Monte Carlo Sampling for Classical Trajectory Simulations. Adv. Chem. Phys. 1999, 105, 171-201. 40. Hase, W. L.; Duchovic, R. J.; Hu, X.; Komornicki, A.; Lim, K. F.; Lu, D. H.; Peslherbe, G. H.; Swamy, K. N.; Vande Linde, S. R.; Varandas, A.; Wang, H.; Wolf, R. J.; Hase, W. L. Quantum Chemistry Exchange (QCPE) Bulletin 1996, 16, 671. 41. Hu, X.; Hase, W. L.; Pirraglia, T. Vectorization of the General Monte Carlo Classical Trajectory Program VENUS. J. Comput. Chem. 1991, 12(8), 1014-1024. 42. Bylaska, E. J.; de Jong, W. A.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Valiev, M.; Wang, D.; Apra, E.; Windus, T. L.; Hammond, J. et al. "NWChem, A Computational Chemistry Package for Parallel Computers, Version 5.1" (2007), Pacific Northwest National Laboratory, Richland, Washington 99352-0999, USA. 43. Kendall, R. A.; Apra, E.; Bernholdt, D. E.; Bylaska, E. J.; Dupuis, M.; Fann, G. I.; Harrison, R. J.; Ju, J.; Nichols, J. A.; Nieplocha, J. et al., High Performance Computational Chemistry: An Overview of NWChem a Distributed Parallel Application. Comput. Phys. Commun. 2000, 128(12), 260-283. 44. Truhlar, D. G.; Muckerman, J. T. In Atom-Molecule Collision Theory: A Guide for the Experimentalist; Bernstein, R. B., Ed.; Plenum: New York, 1979; p 505. 45. Bonnet, L; Rayez, J. C. Quasiclassical Trajectory Method for Molecular Scattering Processes: Necessity of a Weighted Binning Approach. Chem. Phys. Lett. 1997, 277, 183-190. 46. Bonnet, L.; Rayez, J. C. Gaussian Weighting in the Quasiclassical Trajectory Method. Chem. Phys. Lett. 2004, 397, 106-109. 17 ACS Paragon Plus Environment


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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

47. Sun, L.; Park, K.; Song, K.; Setser, D. W.; Hase, W. L. Use of a Single Trajectory to Study Product Energy Partitioning in Unimolecular Dissociation: Mass Effects for Halogenated Alkanes. J. Chem. Phys. 2006, 124, 06413. 48. Su, T.; Morris, R. A.; Viggiano, A. A.; Paulson, J. F. Kinetic Energy and Temperature Dependences for the Reactions of F- with Halogenated Methanes: Experiment and Theory. J. Phys. Chem. 1990, 94, 8426-8430. 49. Swamy, K. N.; Hase, W. L. A Quasiclassical Trajectory Calculation of the H + C2H4 → C2H5 Bimolecular Rate Constant. J. Phys. Chem. 1983, 87, 4715-4720. 50. Schatz, G. C. The Origin of Cross Section Thresholds in H + H2: Why Quantum Dynamics Appears to be More Vibrationally Adiabatic than Classical Mechanics. J. Chem. Phys. 1983, 79, 5386-5391. 51. Lu, D.-h.; Hase, W. L. Classical Mechanics of Intramolecular Vibrational Energy Flow in Benzene. IV. Models with Reduced Dimensionality. J. Chem. Phys. 1988, 89, 6723-6736. 52. Hase, W. L.; Buckowski, D. G. Dynamics of Ethyl Radical Decomposition. II. Applicability of Classical Mechanics to Large Molecule Unimolecular Reaction Dynamics. J. Comput. Chem. 1982, 3, 335-343. 53. Gray, J. C.; Truhlar, D. G.; Clemens, L.; Duff, J. W.; Chapman, Jr., F. M.; Morrell, G. O.; Hayes, E. F. Quasiclassical Trajectory Calculations Compared to Quantum Mechanical Reaction Probabilities, Rate Constants, and Activation Energies for Two Different Potential Surfaces for the Collinear Reaction H2 + I → H + HI, Including Dependence on Initial Vibrational State. J. Chem. Phys. 1978, 69, 240-252. 54. Varandas, A. J. C. Excitation Function for H + O2 Reaction: A Study of Zero-Point Energy Effects and Rotational Distributions in Trajectory Calculations. J. Chem. Phys. 1993, 99, 10761085. 55. Varandas, A. J. C. Trajectory Binning Scheme and Non-Active Treatment of Zero-Point Energy Leakage in Quasi-Classical Dynamics. Chem. Phys. Lett. 2007, 439, 386-392. 56. Czakó, G. Accurate Ab Initio Potential Energy Surface, Thermochemistry, and Dynamics of the Br(2P, 2P3/2) + CH4 → HBr + CH3 Reaction. J. Chem. Phys. 2013, 138, 134301. 57. Czakó, G.; Liu, R.; Yang, M.; Bowman, J. M.; Guo, H. Quasiclassical Trajectory Studies of the O(3P) + CX4(vk = 0,1) → OX(v) + CX3(n1n2n3n4) [ X = H and D] Reactions on an Ab Initio Potential Energy Surface. J. Phys. Chem. A 2013, 117, 6409-6420. 18 ACS Paragon Plus Environment

Page 19 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


58. Fukui, K.; Kato, S.; Fujimoto, H. Constituent Analysis of the Potential Gradient Along a Reaction Coordinate. Method and an Application to CH4 + T Reaction. J. Am. Chem. Soc. 1975, 97, 1-7. 59. Sun, L.; Song, K.; Hase, W. L. A SN2 Reaction that Avoids its Deep Potential Energy Minimum, Science 2002, 296, 875-878. 60. López, J. G.; Vayner, G.; Lourderaj, U.; Addepalli, S. V.; Kato, S.; de Jong, W. A.; Windus, T. L.; Hase, W. L. A Direct Dynamics Trajectory Study of F- + CH3OOH Reactive Collisions Reveals a Major Non-IRC Reaction Path. J. Am. Chem. Soc. 2007, 129, 9976-9985. 61. Xie, J.; Otto, R.; Wester, R.; Hase, W. L. Chemical Dynamics Simulations of the Monohydrated OH-(H2O) + CH3I Reaction. Atomic-Level Mechanisms and Comparison with Experiment. J. Chem. Phys. 2015, 142, 244308. 62. Guo, H; Jiang, B. The Sudden Vector Projection Model for Reactivity: Mode Specificity and Bond Selectivity Made Simple. Acc. Chem. Res. 2014, 47, 3679−3685. 63. Jiang, B.; Guo, H. Relative Efficacy of Vibrational vs. Translational Excitation in Promoting Atom-Diatom Reactivity: Rigorous Examination of Polanyi’s Rules and Proposition of Sudden Vector Projection (SVP) Model. J. Chem. Phys. 2013, 138, 234104.



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

Table 1. Simulation Average Fractions of Product Energy Partitioninga F- + CH3I → HF + CH2ICH2I-

HF frot′

fvib′

frot′

fvib′

frel′

DR

0.03±0.01

-0.05±0.05

0.23±0.08

-0.03±0.14

0.82±0.12

DS

0.04±0.02

-0.17±0.04

0.26±0.06

0.13±0.10

0.74±0.11

Ind

0.08±0.04

-0.21±0.02

0.39±0.08

0.43±0.10

0.31±0.05

Total

0.05±0.02

-0.17±0.02

0.31±0.04

0.20±0.07

0.61±0.07

OH- + CH3I → H2O + CH2ICH2I-

H2O frot′

fvib′

frot′

fvib′

frel′

0.05±0.01

0.71±0.03

Erel = 2.0 eV Total

0.06±0.01

0.04±0.02

0.14±0.01

Exp

0.60±0.04 Erel = 1.0 eV

Total

0.06±0.01

0.01±0.01

0.28±0.01

Exp

0.11±0.01

0.54±0.02 0.47±0.09

a

The f’s are fractions of energy partitioning for rotational, vibrational, and relative translational. DR, DS, and Ind denote energy partitioning for the direct rebound, direct stripping, and indirect mechanisms, respectively. Total is the combined energy partitioning for these mechanisms. The OH- + CH3I results are from reference 13; Exp represents experiment. The negative values for fvib′ result from an average energy in vibration less than the ZPE. The uncertainties are standard deviations of the mean. 20 ACS Paragon Plus Environment

Page 21 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. Potential energy curve and stationary points for the F- + CH3I → HF + CH2I- and OH+ CH3I → H2O + CH2I- proton-transfer reactions at the B97-1/ECP/d level of theory. The energies shown are in kcal/mol and are relative to the reactants. Zero-point energies are not included. The OH- + CH3I results were presented previously.24 Figure 2. Simulation probabilities of the F- + CH3I → HF + CH2I- reaction, versus impact parameter, for the three atomic-level reaction mechanisms and the total reaction: ─■─, direct rebound; ---○---, direct stripping; ····∆····, indirect; -·-·-×-·-·-, total. The uncertainties in the Pr(b) are standard deviations. Figure 3. The SN2 double inversion TS structure (a) and potential energy curve (b) for the B971/ECP/d theory used for the current simulations. For the TS, the F, elongated H, C, and I atoms are in a single plane.



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

F- + CH3I → HF + CH2I-

OH- + CH3I → H2O + CH2I-

Figure 1.


Page 23 of 25

0.15

0.10 Pr(b)

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05

0.00

0

1

2

3

4

5

6

b (Angstrom)

Figure 2.



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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

(a)

(b)

Figure 3.


Page 25 of 25

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC GRAPHIC

Potential energy curve for the F- + CH3I → HF + CH2I- proton-transfer reaction


Dynamics of the Fâ + CH3I â HF + CH2Iâ Proton Transfer Reaction

Recommend Documents

Dynamics of the Fâ + CH3I â HF + CH2Iâ Proton Transfer Reaction