Original scientific paper

Heavy Atom Isotope Effects in Elimination Reactions. An ab initio Study

William H. Saunders Jr. ; Department of Chemistry, University of Rochester, Rochester, New York 14627, USA

Abstract

Heavy atom kinetic isotope effects (KIEs) in bimolecular elimination (E2) reactions, both anti and syn, were calculated for the leaving group (LG, 18F/19F, 35Cl/37Cl), the carbon attached to the leaving group (C-1, 12C/13C), and the carbon bearing the proton removed in the reaction (C-2, 12C/13C) by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The substrates examined were EtF, EtF solvated by one H₂O, and EtCl. The bases used were OH-, OH- solvated by one H₂O, LiOH, NaOH, F-, and Cl-. The KIEs found for each reaction were compared with bond orders calculated by the Pauling equation and changes in NPA charges from reactant to transition structure. Qualitative and semiquantitative but few truly quantitative correlations are found. Restricted subsets of the data (e.g, only anti eliminations of EtF) give best results, but even here the C-2 KIEs correlate poorly. LG KIEs correlate well with ΔQ(X) and n(CX), C-1 KIEs with n(CX) and n(CC) but with scatter, and C-2 KIEs with ΔQ(C-2) and n(CC) + n(CH) but with much scatter. The dependence of KIEs on transition structure appears too complex to be expressed well by simple relationships with bond orders and charge distributions.

Keywords

E2; KIE; transition structure; bond order; charge; BEBOVIB method

Hrčak ID:

131865

URI

https://hrcak.srce.hr/131865

Publication date:

1.8.2001.

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