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Computational replication of the primary isotope dependence of secondary kinetic isotope effects in solution hydride transfer reactions : supporting the isotopically different tunneling ready state conformations

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Kashefolgheta,  Sadra
Ana Vila Verde, Theorie & Bio-Systeme, Max Planck Institute of Colloids and Interfaces, Max Planck Society;

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Citation

Derakhshani-Molayousefi, M., Kashefolgheta, S., Eilers, J. E., & Lu, Y. (2016). Computational replication of the primary isotope dependence of secondary kinetic isotope effects in solution hydride transfer reactions: supporting the isotopically different tunneling ready state conformations. The Journal of Physical Chemistry A, 120(25), 4277-4284. doi:10.1021/acs.jpca.6b03571.


Cite as: http://hdl.handle.net/11858/00-001M-0000-002A-D9DF-6
Abstract
We recently reported a study of the steric effect on the 1° isotope dependence of 2° KIEs for several hydride transfer reactions in solution (J. Am. Chem. Soc. 2015, 137, 6653). The unusual 2° KIEs decrease as the 1° isotope changes from H to D, and more in the sterically hindered systems. These were explained in terms of a more crowded tunneling ready state (TRS) conformation in D-tunneling, which has a shorter donor-acceptor distance (DAD), than in H-tunneling. In order to examine the isotopic DAD difference explanation, in this paper, following an activated motion-assisted H-tunneling model that requires a shorter DAD in a heavier isotope transfer process, we computed the 2° KIEs at various H/D positions at different DADs (2.9 Å to 3.5 Å) for the hydride transfer reactions from 2-propanol to the xanthylium and thioxanthylium ions (Xn+ and TXn+) and their 9-phenyl substituted derivatives (Ph(T)Xn+). The calculated 2° KIEs match the experiments and the calculated DAD effect on the 2° KIEs fits the observed 1° isotope effect on the 2° KIEs. These support the motion-assisted H-tunneling model and the isotopically different TRS conformations. Furthermore, it was found that the TRS of the sterically hindered Ph(T)Xn+ system does not possess a longer DAD than that of the (T)Xn+ system. This predicts a no larger 1° KIE in the former system than in the latter. The observed 1° KIE order is, however, contrary to the prediction. This implicates the stronger DAD-compression vibrations coupled to the bulky Ph(T)Xn+ reaction coordinate.