Exploring Cyclopentadienone Antiaromaticity: Charge Density Studies of Various Tetracyclones Rumpa Pal, † Somnath Mukherjee, ‡ S. Chandrasekhar,* ,‡ and T. N. Guru Row †, * † Solid State and Structural Chemistry Unit and ‡ Organic Chemistry, Indian Institute of Science, Bangalore-560012, India * S Supporting Information ABSTRACT: A systematic study of six tetracyclones has been carried out using experimental and theoretical charge density analysis. A three pronged approach based on quantum theory of atoms in molecules (QTAIM), nucleus independent chemical shifts (NICS) criterion, and source function (SF) contributions has been performed to establish the degree of antiaromaticity of the central five-membered ring in all the derivatives. Electrostatic potentials mapped on the isodensity surface show that electron withdrawing substituents turn both C and O atoms of the carbonyl group more electropositive while retaining the direction of polarity. ■ INTRODUCTION The phenomena of aromaticity and antiaromaticity define two of the most fundamental concepts in chemistry. As these are virtual quantities, rather than physical observables, their quantification is a large and vigorously discussed field of research and different methods for their validation are still under development. The criteria upon which these concepts have been analyzed so far in the literature are (i) energies (aromatic stabilization and antiaromatic destabilization); 1 (ii) geometries (aromatic bond length equalization and antiar- omatic bond length alternation); and (iii) various magnetic effects including nucleus independent chemical shifts (NICS). 2 The quantitative relationships among the geometric, energetic, and magnetic criteria of aromaticity have been demonstrated for a wide range of five-membered heterocycles in which the cyclopentadienyl anion is the most aromatic, the singlet cyclopentadienyl cation is the most antiaromatic, and cyclo- pentadiene is nonaromatic. 3 These criteria have been applied to many other systems, e.g., homoaromatic carbocations 4 and aromatic pericyclic transition states. 5 Antiaromaticity is more challenging to define than aromaticity because molecules will adopt otherwise unfavorable geometric and electronic config- urations to minimize destabilization. We have been interested in tetraphenylcyclopentadienone (“tetracyclone”) and its derivatives particularly tetraarylcyclo- pentadienones. These molecules contain strongly absorbing chromophoric units with a low band gap (especially smaller than 1.5 eV) and are utilized in the fabrication of LEDs and photovoltaics. 6 Cyclopentadienones are also important because of their thermal [4 + 2] cycloaddition or Diels-Alder reactions with disubstituted acetylenes, furnishing polyphenylenes which are again used as photovoltaic materials. Though cyclo- pentadienone is extremely unstable and spontaneously under- goes Diels-Alder oligomerization even at very low temper- atures, tetracyclone, a deep purple colored solid (mp 218-220 °C), 7 and many of its derivatives are considerably stable compounds. The reactivity of cyclopentadienone has been attributed to the antiaromatic valence bond (VB) structure (Scheme 1) as one of the primary resonance forms. 8 Apparently, substitution of the cyclopentadienone ring with four aryl groups provides enough steric hindrance to Diels- Alder cycloaddition and makes these derivatives kinetically stable even at modestly high temperatures. However, at very high temperatures (>200 °C) in the presence of appropriate dienophiles it does undergo cycloaddition reactions. 9 Also, the expeditious synthesis of tetracyclone in high yield is particularly intriguing in light of its antiaromatic instability at the molecular level as tetracyclone readily precipitates out of solution; it raises the question whether crystal packing forces overrule molecular-level antiaromaticity in the lattice. This was Received: January 30, 2014 Revised: April 21, 2014 Published: April 22, 2014 Scheme 1. Unsubstituted Cyclopentadienone and Its Antiaromatic Valence Bond (VB) Structure Article pubs.acs.org/JPCA © 2014 American Chemical Society 3479 dx.doi.org/10.1021/jp5010924 | J. Phys. Chem. A 2014, 118, 3479-3489