Theoretical Studies of Carbocations in Ion Pairs. Part 6:  The tert-Butyl Cation at Various Interionic Distances

Abstract

The structure of the tert-butyl cation (1) ion-paired with the trihydrofluoroborate anion (A), previously shown to be appropriate for such studies, was investigated by ab initio calculations, as a function of the interionic distance in the ion pair (d). At short distance (d ) 2.25 Å), the lowest energy cation conformation was a slightly distorted C s form. At intermediate distances (d ) 2.60, 2.95, 3.30, and 3.65 Å), the preferred geometry had a C 3V conformation, and at d ) 4.0 Å the cation adopted again a C s -type conformation, but the anion was no longer in the space above the carbon skeleton of the cation and it was tilted, interacting strongly with only one of the hydrogen atoms. Finally, at d ) 4.5 Å, the anion moved to the side of the cation, also interacting with only one hydrogen atom, whereas the cation adopted an asymmetric geometry, close to the C 3h form preferred by the isolated carbocation. Examination of spectral properties could indicate the average or most probable interionic distance in solution. Thus, the Raman spectrum had been interpreted as indicating a C 3V form of the cation, which suggests an interionic distance in the intermediate range. The insensitivity of cation structure to the nature of the anion for interionic distances beyond recombination range has been established by a comparison of anion A with the tetrafluoroborate anion (B). Optimization of the cation geometry in the triple ion A.1.A, held at the interionic distance found in the crystal, showed that the C s form with two hydrogens facing the farthest anion was the most stable, but the C s form with two hydrogens facing the nearest anion was almost equal in energy and the two C 3V forms (three hydrogens facing the farthest and the nearest anion, respectively) were only slightly less stable. The preferred geometry of the cation in the triple ion was the same as that found previously in a more complex aggregate, in which the cation and the two anions were allowed to move freely. This result shows that geometry optimizations at fixed interionic distances give reliable structures for carbocations in ion pairs or aggregates. The similar stability of the C s and C 3V conformations in the triple ion is in agreement with the identification of two sets of hydrogens, each twisted by 60°against the second set, for two of the methyl groups of the tert-butyl cation in the crystal.

Theoretical Studies of Carbocations in Ion Pairs. 5.1Structures and Interconversion of the 3-Methyl-2-butyl Cation and 2,3-Dimethyl-1-Protonated Cyclopropane

“…The electrophilic solvation of the anion was taken into account in our calculation by the use of a complex anion, to ensure that only a part of the negative charge would face directly the cation. 1a, It was important, however, not to use a very stable anion when trying to model the type of interaction occurring in ionization in solvolytic media which provide good anion stabilization but have both low dielectric constants and some nucleophilicity (e.g., TFA). The more extensive comparisons were made with the smaller 2-propyl cation, 25a,b and, to a more limited extent, tert -butyl cation . The very high energy required for heterolysis (perhaps 100 kcal/mol) is in good part compensated by the complexation energy, 44 kcal/mol for [F - HF] - ,31a 54kcal/mol for [H - LiH] - ,31b 64kcal/mol for [F - BH 3 ] - ,31c and 71kcal/mol for [F - BF 3 ] - ,31d bringing the difference (bond cleavage minus complexation) within the range of energy barriers for ionization in solution.…”

Influence of Carbocation Stability in the Gas Phase on Solvolytic Reactivity: Beyond Bridgehead Derivatives

“…It occurs in solvolysis reactions that lead to small, undelocalized carbocations. Initially, the effect was ascribed to a direct interaction of the solvent with the reacting center in an S N 2-type process and to Brønsted-type interactions of the solvent, in particular, with β-hydrogens . However, recent investigations show that NSP has a more general significance.…”

Theoretical Studies of Carbocations in Ion Pairs. 8. Search for Anchimeric Assistance in the Ionization of 2-Butyl Cation Precursors

“…The same approach allowed us to establish the aggregation of a hydronium salt in a nonpolar environment and predict correctly its NMR spectrum . We have looked in most cases at changes in the carbocation structure (2-propyl, 1-propyl,1d tert -butyl,1b 3-methyl-2-butyl 1c ) under the influence of an anion approaching from infinity to the tight ion pair distance. Later, we have also examined the generation of the carbocation in the ion pair in the ionization of 2-propyl fluoride upon interacting with a Lewis acid 1a…”