Abstract
Three isomers of the allyl cation C(3)H(5) (+) were obtained in salts with the carborane anion CHB(11)Cl(11) (-). Two of them, angular CH(3)-CH=CH(+) (I) and linear CH(3)-C(+)=CH(2) (II), were characterized by X-ray crystallography, and the third one, (CH(2)CHCH(2))(+) (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.