Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1997-11, Vol.101 (46), p.8695-8700
• Main Authors: Fǎrcaşiu, Dan, Hâncu, Dan
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp9714680

Ab initio calculations with large basis sets and electron correlation were applied to the study of geometry of the 2-propyl cation in ion pairs with trihydro fluoroborate (A) or dihydrolithiate (B) as anion. The goal was to model the ion pair formed by ionization in a solvent with good anion-stabilizing properties, but of low dielectric constant, like trifluoroacetic acid (TFA). The effect of the anion, seen already at long distances, was that the preferred cation conformation changed from C1,C3 staggered as in the isolated carbocation (chiral 2-propyl cation, 1) to C1,C3 eclipsed (C s symmetry, 2). The optimized cation geometry was essentially the same at the MP2(FC)/6-31G*, MP2(FC)/6-31++G*, and MP2(FU)/6-311G** levels, but the position of the anion above the cation was somewhat more sensitive to the basis set. The preferred anion position was in the plane bisecting the C1−C2−C3 angle of 2 and in the region “inside” that angle. This “inside” displacement became more pronounced as the interionic distance, d, was made shorter; at the same time, the anion moved slightly off the bisecting plane. Elimination within the ion pair to form propene occurred at d < 2.5 Å. When the anion was allowed to “fall” freely, the reaction pathway was determined by the initial position of the anion:  elimination for a position “inside” the C1−C2−C3 angle and recombination to 2-fluoropropane (occurring at d = 1.5−1.7 Å) for a position “outside” the C1−C2−C3 angle. The equilibrium cation geometry did not change significantly in 2.B relative to 2.A, but the distortions toward elimination occurred at longer distances for the more basic anion B and elimination itself took place at d = 3.5 Å. The energy difference beween 1.A (optimized with the methyl groups held staggered) and 2.A at d = 3.4 Å was 2.39 and 3.25 kcal/mol at the MP2(FC)/6-31++G* and MP2(FU)/6-311G** levels, respectively. The equilibrium position of the anion paired with 1 was above the C1−C2 bond, close to the syn hydrogen at C1. Thus, methyl rotation along the lowest-energy pathway involves also a movement of the anion relative to the cation.