Infrared spectroscopy of the alcohol cations of ethanol, propanol, and butanol was performed to investigate their structures and hyperconjugation mechanisms. In the ethanol cation, the C-C bond hyperconjugates with the singly occupied molecular orbital (SOMO) at the oxygen atom, so that the C-C bond weakens and the bond length elongates. Multiple hyperconjugations among SOMO, the C-C bond, and the end C-H bond occur in the propanol cation and enhance the acidity of the C-H bond through the delocalization of its bonding σ electron. The butanol cation forms the oxonium-type structure through the proton transfer from the terminal CH bond.
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