Synthesis and characterization of the negative ion of non-Kekulé benzene

Abstract

The 2,4-dimethylenecyclobutane-1,3-diyl negative ion 2 has been generated in the gas phase from the reaction of atomic oxygen anion with 1,3-dimethylenecyclobutane 3. This negative ion is the necessary precursor for photoelectron spectroscopic measurements of the singlet–triplet splitting in the corresponding neutral biradical 1. Gas-phase ion–molecule reactions involving several different electrophiles, radical traps and Brønsted acids were used to identify the structure of ion 2, and to rule out the presence of other C₆H₆^˙– isomers. Ion 2 displays characteristic radical- and carbanion-type reactivity, including adduct formation with NO, COS and CO₂, S-atom abstraction from CS₂, and thiomethyl group abstraction from CH₃SSCH₃. The proton affinity of radical anion 2 was determined from acid–base bracketing experiments to be 383.3 ± 2.0 kcal mol^–1. The gas-phase acidity of hydrocarbon 3 was determined by bracketing to be 366.7 ± 3.0 kcal mol^–1, while the proton affinity of its conjugate base carbanion 7 was bracketed at 369.2 ± 2.0 kcal mol^–1. The 2.5 kcal mol^–1 difference is interpreted as evidence for protonation of the dienylic anion moiety in 7 at one of the exocyclic methylene groups to give 1-methyl-3-methylenecyclobutene as the lower energy C₆H₈ tautomer. The electron affinities of biradical 1 and the corresponding monoradical 2,4-dimethylenecyclobutyl 9 were measured by a kinetic method involving collision-induced dissociation of SO₂ adducts of ions 2 and 7. The biradical and monoradical were found to have identical electron affinities (EA), 26.3 ± 0.2 kcal mol^–1 (1.14 ± 0.01 eV). Density functional calculations of the structures and energies of 1, 2 and several related species were carried out at the B3LYP/6-31+G* level. Good agreement was achieved between the experimental thermochemistry and the predicted energetics based on isodesmic reactions. The experimental and theoretical thermochemistry reveal a dramatic deviation from CH bond energy additivity in forming triplet biradical 1 from hydrocarbon 3 by a hypothetical sequence of CH bond dissociations: the second ring CH bond strength is 16.6 ± 3.6 kcal mol^–1 stronger than the first due to electronic destabilization of the π system in 1 from antiaromaticity.