Theoretical study of oxocarbons: structure and vibrational spectrum of the D 6h and C 2 forms of the rhodizonate ion Georgia M.A. Junqueira a,c , Willian R. Rocha b , Wagner B. De Almeida c , He ´lio F. Dos Santos a, * a NEQC: Nu ´cleo de Estudos em Quı ´mica Computacional, Departamento de Quı ´mica, ICE, Universidade Federal de Juiz de Fora, Campus Martelos, CEP 36036-330, Juiz de Fora, MG, Brazil b Departamento de Quı ´mica Fundamental, CCEN, Universidade Federal de Pernambuco, CEP 50670-901, Recife, PE, Brazil c LQC-MM: Laborato ´rio de Quı ´mica Computacional e Modelagem Molecular, Departamento de Quı ´mica, ICEx, Universidade Federal de Minas Gerais, Campus Pampulha, CEP 31270-901, Belo Horizonte, MG, Brazil Received 14 May 2004; revised 19 July 2004; accepted 21 July 2004 Available online 1 September 2004 Abstract The molecular properties of the rhodizonate ion in the D 6h planar and C 2 non-planar forms were calculated in gas phase at distinct ab initio levels of theory. The C 2 structure was found to be the global minimum, being only 0.50 kcal/mol lower in energy than the D 6h isomer at the MP4(SDQ)/6-311CG(2d)//MP2/6-311CG(2d) level. When the thermal correction and solvent effect were included in the relative Gibbs free energy, the C 2 isomer was found to be more favourable by w3 kcal mol K1 . The D 6h form is a stationary point with three imaginary frequencies. The infrared and Raman spectra were reported and band assignments made for both isomers, showing that these techniques are not conclusive to identify the form of the rhodizonate ion present in the medium. The effect of the counter ion (Li C ) on the structure and equilibrium position was also investigated, showing that the coordination with this metal stabilizes the non-planar C 2 form. q 2004 Elsevier B.V. All rights reserved. Keywords: Oxocarbons; Spectroscopic properties; Conformational analysis 1. Introduction The monocyclic oxocarbons dianions (C n O n ) 2K (nZ3, deltate; nZ4, squarate; nZ5, croconate; nZ6, rhodizonate) have been the subject of great interest since they were identified by West as a new class of aromatic systems [1] (Fig. 1). A wide range of spectroscopic and X-ray crystal- lographic data are available for squarate and croconate metal complexes [2], however, not much structural information have been obtained for deltate [3] and rhodizonate [4] ions, due to the relative instability of these two species. For over four decades, interest in the series of oxocarbons has focused on their aromaticity and electronic structure [5]. The structures of the deltate, squarate and croconate dianions are planar with D nh symmetry, as proposed from Raman and infrared (IR) spectroscopic analysis [6]. Furthermore, a recent theoretical study [7] concluded that the rhodizonate (C 6 O 6 ) 2K ground state has C 2 symmetry. In a recent experimental work [4a],a D 6h and C 2 structures were found for the rhodizonate dianion in the Rb 2 C 6 O 6 and K 2 C 6 O 6 complexes, respectively. In that study the C 2 structure was almost planar, distinct from the C 2 conformation reported by Schleyer and coworkers [7].A similar study was carried out for the croconate derivative [4b], where the D 5h and C 2v valence tautomers were identified in the solid state as a hydrogen bond complex with urea. The rhodizonate was discovered together with croconate in 1837 and since then many studies regarding their photochemistry [8] and coordination with transition metals [9] have appeared in the literature. Despite the extensive literature about oxocarbons [10], the theoretical studies are still sparse [6b,7,11–15]. In recent studies [12–15], we analyzed the structures and spectroscopic properties of croconate [12], deltate [14] and squarate [15] ions in the gas phase and aqueous solution using quantum mechanical 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.07.020 Journal of Molecular Structure (Theochem) 684 (2004) 141–147 www.elsevier.com/locate/theochem * Corresponding author. Tel.: C55-32-3229-3310; fax: C55-32-3229- 3314. E-mail address: helius@quimica.ufjf.br (H.F. Dos Santos).