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

Abstract

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.