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 Angstrom), 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 Angstrom), the preferred geometry had a C-3v conformation, and at d = 4.0 Angstrom 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 Angstrom, 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, 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 degrees against the second set, for two of the methyl groups of the tert-butyl cation in the crystal.