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

Three isomers of the allyl cation CH were obtained in salts with the carborane anion CHBCl . Two of them, angular CH-CH=CH () and linear CH-C=CH (), were characterized by X-ray crystallography, and the third one, (CHCHCH) (), is formed in an amorphous salt. The stretch vibration of the charged double bond C=C of and is decreased by 162 cm () or 76 cm () 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 () or 210 cm (). According to infrared spectra, the CC bonds in isomer have one-and-a-half bond status. Isomers and 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 is provided by a weak hyperconjugation effect. Isomer is stabilized in the amorphous phase due to ion paring with a counterion and a strong intramolecular hyperconjugation effect.