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Design of Bronsted Neutral Organic Bases and Superbases by Computational DFT Methods: Cyclic and Polycyclic Quinones and [3]Carbonylradialenes

The gas-phase proton affinities and basicities of a large number of extended polycyclic systems possessing a carbonyl oxygen head serving as a basic proton scavenger are explored by using DFT at the B3LYP/6-311+G(d, p)//B3LYP/6-31+G(d) level of theory. Some of these neutral organic superbases exhibit proton affinities in the range of 264-284 kcal mol-1. In constructing these systems it turned out that a =C(NMe2)2 fragment attached to a quinoid six-membered ring enhanced the basicity to a considerable extent. There is abundant and convincing evidence that protonation triggers strong aromatization of the quinoid rings. Moreover, sequential quinoid rings undergo the aromatic domino effect upon protonation if linearly aligned. Triadic analysis has revealed that highly pronounced basicity in some studied systems is a result of the synergistic action of Koopmans' frozen neutral base ionization energy contribution and subsequent relaxation of the radical cation.

proton affinity; basicity; aromaticity; cationic resonance; relaxation effects

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