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Theoretical modeling of the isopropylium cation: Exploring nonclassical carbonyl behavior

In the isopropylium cation, (CH3)2CHO+, the carbonyl moiety has been shown experimentally to behave nonclassically. This work examines a variety of theoretical methods that can be used to model the structure and bonding in the isopropylium ion. Comparisons between theoretical and experimental values are made with regard to C–O and C–C bond lengths, ν(CO), and the C–C–O bond angle. The Hartree-Fock and B3LYP (DFT) methods using the 3-21G basis set provide the most accurate overall description of the structure and bonding, as compared to the experimentally derived data. These theoretical models are then used to explore the reasons for the nonclassical behavior of the isopropylium cation. It appears that, upon complexation, the “s” character of the C–O bond increases, resulting in a stronger bond.