Reexamination of the C2H4F potential surface. the status of the classical 2-fluoroethyl cation: a local minimum or transition structure? |
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Authors: | George P Ford Krishan S Raghuveer |
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Institution: | George P. Ford*,Krishan S. Raghuveer |
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Abstract: | According to ab initio molecular orbital calculations carried out with full geometry optimization at the MP2/6–31G** level, the classical 2-fluoroethyl cation, FCH2CH2+, is a transition structure for H-scrambling in CH3CHF+. Single point MP4/6–31G** calculations at the optimized geometries predict the cyclic ethylene fluoronium ion to lie 24.2 kcal mol−1 above CH3CHF+ and 5.4 kcal mol−1 below the 2-fluoroethyl cation. ΔG‡ for ring opening of the cyclic fluoronium ion at -60° is estimated to be ca 15 kcal mol−1. This barrier is largely attributable to the powerful negative fluorine hyperconjugation in the transition state as described by Hoffmann and coworkers. When electron correlation effects are ignored a qualitatively different potential surface is obtained on which the 2-fluoroethyl cation is calculated to be a local minimum separated from the stable 1-fluoroethyl cation by an H-bridged transition state. |
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