Hyperconjugation in β,β′‐silyl substituted vinyl cations – indications from IR spectroscopy

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Abstract
Alkyl substituted vinyl cations 1a–c have been synthesized in the form of their [B(C6F5)4]− salts and have been characterized by NMR and IR spectroscopy. The IR absorption bands detected for the νas(CC+) vibrations in vinyl cations are of great diagnostic value, due to their high intensity and relative isolated position in the IR spectra ($\tilde {\nu }$ = 1930–2020 cm−1). DFT calculations of the harmonic force constants at the B3LYP/cc‐pvtz level of theory allowed for an unequivocal assignment of the band after scaling of the theoretical values to account for the anharmonicity of the experimental vibrational potential. A study on the energy of the νas(CC+) of alkyl (1) and aryl substituted vinyl cations (2) revealed a systematic trend, which indicates a decreasing stabilization of the vinyl cations by β‐SiC hyperconjugation when the electron donating ability of the substituent R2 next to the positively charged carbon atom is increased. Copyright © 2010 John Wiley & Sons, Ltd.

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