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Superacidity of -P(OH)3 and -SO(OH)2 derivatives of cyclopentadiene and vinylcyclopentadiene in the gas phase: A computational DFT analysis

Strong Broensted acids were designed using a simple strategy of replacing double‐bonded oxygen atoms in mineral acids H3PO4 and H2SO4 with cyclopentadiene and vinylcyclopentadiene moieties, followed by a substitution with strong electron‐withdrawing cyano groups. Their intrinsic gas‐phase acidity was assessed by the B3LYP density functional theory calculations employing the 6-311++G(d, p) basis set, and many highly potent superacids were identified. The pronounced acidity of these conjugates is due to two predominant factors, namely, (i) an increase in the aromaticity of the five‐membered ring on deprotonation, as evaluated through the NICS parameters, and (ii) an extension of the conjugated pi‐network enabled by the cyano groups that efficiently distribute an excess negative charge over a large number of atoms in conjugate bases. The -SO(OH)2 derivatives are generally more acidic than the corresponding -P(OH)3 counterparts so that the ΔHacid values for the -SO(OH)2 and -P(OH)3 derivatives with CN substituents evaluated here are 260‐266 and 263‐272 kcal mol-1, respectively. Given the growing interest in highly acidic molecules together with related acidic catalysts and metal complexing agents, the results presented here should direct the attention toward utilizing proposed systems as improved organic materials, and their synthesis is highly recommended.

aromaticity ; cyclopentadiene derivatives ; mineral acids ; neutral organic acids ; superacidity

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