The rate constant for OH−(H2O)2 + CH3I reaction was determined by selected ion flow tube (SIFT) experiments for temperatures in the range of 298–398 K. It is found to be an order of magnitude smaller than the collision capture rate constant, a result substantially different than found previously for the OH− + CH3I and OH−(H2O) + CH3I reactions. The rate constants for these reactions are only ∼25% and ∼two times smaller, respectively, than their collision capture rate constants. Only two product ions are observed experimentally, i.e. I− and I−(H2O), and their respective percentage yields are 90:10 and 83:17 at 298 and 348 K. The kinetics for the OH−(H2O)2 + CH3I reaction were also studied by direct dynamics simulations using the DFT/B97-1/ECP/d electronic structure theory, the same theory used in previous direct dynamics simulations of the OH− + CH3I and OH−(H2O) + CH3I reactions. Simulations for OH−(H2O)2 + CH3I at 387 K give respective percentage yields of 91:9 for I− and I−(H2O), in good agreement with the experimental results. For both the experiments and simulations, the microsolvated ion I−(H2O)2 is not formed and the formation of I− dominates I−(H2O). For the OH− + CH3I and OH−(H2O) + CH3I reactions the experimental and direct dynamics simulation rate constants agree. However, this is not the case for OH−(H2O)2 + CH3I, for which the simulation rate constant is 8–9 times larger than the experimental value. Comparisons of the experimental, simulation, and collision capture rate constants for the OH−(H2O)2 + CH3I reaction indicate the height of the submerged SN2 barrier for the reaction is an important feature of its potential energy surface. The actual barrier is expected to be higher than the value given by the DFT/B97-1 calculations. In future work it will be important to perform higher level electronic structure calculations and establish an accurate value for this barrier. Preliminary calculations reported here indicate the barrier height is sensitive to the electronic structure method.
Bibliographical noteFunding Information:
The research at Texas Tech University is based upon work supported by the Robert A. Welch Foundation under Grant No. D-0005 and the Air Force Office of Scientific Research under AFOSR Award No. FA9550-16-1-0133. The experimental research is supported by the Air Force Office of Scientific Research through the AFOSR-2303EP grant to A.A. Viggiano. The simulations were performed at the High Performance Computing Center (HPCC) at Texas Tech University (TTU), under the direction of Philip W. Smith, and on the Robinson computer cluster in the TTU Department of Chemistry and Biochemistry whose purchase was funded by the National Science Foundation CRIF-MU Grant CHE-0840493. The authors wish to acknowledge important discussions with David H. Bross and Branko Ruscic, Argonne National Laboratory, concerning proper electronic structure methods for calculating an accurate SN2 central barrier height for the OH?(H2O)2?+?CH3I reaction. J. Zhang wishes to thank the National Natural Science Foundation of China (No. 21573052) for support of his research. A. A. Viggiano wishes to thank Shaun Ard, Josh Melko, and Nick Shuman for checking the previous 348?K experimental measurement of the OH?(H2O)2?+?CH3I rate constant.
© 2016 Elsevier B.V.
Copyright 2017 Elsevier B.V., All rights reserved.
- Direct dynamics simulation
- Potential energy surface
- Rate constant
- Reaction kinetics