Cyclobutadiene: The Antiaromatic Paradigm?

Double Group Transfer Reactions as Indicators of Aromatic Stabilization

“…The latter molecule, which has eight π electrons, was chosen as reference because it has the same number of conjugating σ bonds as benzene, and because it has been suggested that the ASE values exhibit a better correlation with nucleus-independent chemical shift (NICS) values when the latter reference system is chosen. [7] Using the same approach, we found for cyclobutadiene, a molecule which is considered as the archetypal antiaromatic molecule, [8] an ASE value of -31.9 kcal/mol with respect to all-trans-1,3,5-hexatriene, which agrees with the classification of cyclobutadiene as antiaromatic species. Therefore, our newly developed methodology provides a reliable scale of aromaticity which has also been successfully applied to estimate the aromatic strength of metallabenzenes, [9] systems where the geometric harmonic oscillator model of aromaticity (HOMA) method and magnetic NICS or magnetic susceptibilities are not applicable.…”

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

“…Ever since KekulØ first attempted the preparation of CBD in 1872, [1] this illusive molecule posed ac hallenge to experimental and theoretical chemists. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state.…”

“…Ever since KekulØ first attempted the preparation of CBD in 1872, [1] this illusive molecule posed ac hallenge to experimental and theoretical chemists. [2] Owing to the high reactivity of CBD,i tw as not until nearly ahundred years later that CBD derivatives could be synthesized and characterized. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state.…”

Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

“…According to the well-known Hückel rule, CBD is an antiaromatic molecule and is often regarded as an antiaromatic paradigm [1]. For a long time, many theoretical and experimental chemists have scrutinized CBD in comparison with benzene to reveal the nature of aromaticity/ antiaromaticity and localization/delocalization [2,3].…”