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

High Resolution Image Download MS PowerPoint Slide 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.