Isomers of the Allyl Carbocation C₃H₅ ⁺ in Solid Salts: Infrared Spectra and Structures

Three isomers of the allyl cation C₃H₅ ⁺ were obtained in salts with the carborane anion CHB₁₁Cl₁₁ ^-. Two of them, angular CH₃-CH=CH⁺ (I) and linear CH₃-C⁺=CH₂ (II), were characterized by X-ray crystallography, and the third one, (CH₂CHCH₂)⁺ (III), is formed in an amorphous salt. The stretch vibration of the charged double bond C=C⁺ of I and II is decreased by 162 cm^-1 (I) or 76 cm^-1 (II) as compared to that of neutral propene. This result contradicts the prediction of DFT and MP2 calculations with the 6-311G++(d,p) basis set that the appearance of the positive charge on the C=C bond should increase its stretch vibration by 200 cm^-1 (I) or 210 cm^-1 (II). According to infrared spectra, the CC bonds in isomer III have one-and-a-half bond status. Isomers I and II in the crystal lattice are stabilized due to uniform ionic interactions with neighboring anions with partial transfer of a positive charge to them. Additional stabilization of II is provided by a weak hyperconjugation effect. Isomer III is stabilized in the amorphous phase due to ion paring with a counterion and a strong intramolecular hyperconjugation effect.