| Abstract
| - The structures of carbocations formed in the ionization of 2-butyl precursors were investigated by high levelab initio MO calculations on the reaction of 2-butyl fluoride (1) with borane, which gives a C4H9+ cationpaired with trihydrofluoroborate (FBH3-, A). Two conformations of the “open,” secondary cation (2) in theion pair resulted from two conformations of 1, with F gauche and trans to C4 (2-g and 2-t, respectively). Noanchimeric assistance by hydrogen (in 1-g) or methyl (in 1-t) was evidenced. In fact, attempts at optimizingthe geometry of the H-bridged (3) and methyl-bridged (6) cations at short interionic distances (d) led to thecorresponding conformations of 2. Upon ion separation, proton transfer from 2 to the anion occurred atintermediate interionic distances, consonant with experimental observations in trifluoroacetic acid. Eliminationwas prevented by addition of a lithium cation to the ion pair, i.e., running computations on triple ions (2.A.Li+).Cation 6 became an energy minimum beyond d = 2.5 Å and 3 beyond 2.8 Å. Cation 2-g was still the moststable isomer at d = 3.2 Å, which was greater than the interionic distance in the crystals of the isomerictert-butyl cation salts (3−3.1 Å). Thus, spectral determinations of 2-butyl cations in the solid state should beinterpreted with 2-g as the main component of the ion mixture. When the ions became separated (d ≥ 4 Å),only the bridged ions were energy minima. In this process, bridging did not occur opposite to the leavinggroup to assist the ionization, but on the same side with it, being controlled by the electrostatic interactionwith the anion, as it departed from the vicinity of the cation. Such behavior was also noted in the ionizationof the 3-methyl-2-butyl homolog.
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