An electron transfer mechanism of O 4 formation Antonio Carlos Pav~ ao a , Jos  e Carlos de F. Paula a , Rog erio Custodio b , Carton A. Taft c, * a Departamento de Qu  ımica Fundamental, Universidade Federal de Pernambuco, 50.670-901 Recife, Pernambuco, Brazil b Instituto de Qu  ımica, Universidade Estadual de Campinas, 13083-970 Campinas, S~ ao Paulo, Brazil c Centro Brasileiro de Pesquisas F  ısicas – CBPF, R. Dr. Xavier Sigaud, 150 URCA, Rio de Janeiro, RJ 22290-180, Brazil Received 3 June 2002; in final form 17 January 2003 Abstract A resonating valence bond electron transfer mechanism of combining two O 2 molecules to form an O 4 molecule is presented. The predicted molecular states of the reaction path D 1h ! C 2v ! D 2h are supported by the present ab initio molecular orbital calculations. The CASPT2 BSSE calculations yield a stable diamagnetic D 2h O 4 molecule with a very weak chemical bond between the monomers, in good agreement with experiments. A low activation barrier energy of 26 cal/mol for the O 4 formation is found. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Assuming that two O 2 molecules can combine to form an O 4 molecule, chemically and magneti- cally saturated, Lewis [1] could explain the ap- parent anomaly in the magnetic properties of liquid oxygen which do not obeys CurieÕs law. By analysis of the magnetic susceptibility variation against O 2 concentration in a N 2 mixture at three different temperatures, he found that the dimer formed from two triplet state molecules could gain stability from electron pairing in order to form a weak chemical bond, with an emission of 128 cal/ mol of O 4 formed. The O 4 molecule had already been proposed by Dolezalek in 1910 [2]. In 1928, WulfÕs theory of the atmospheric ozone suggested that the absorption of light by the O 4 molecule in the EarthÕs atmosphere could account for the observed failure of radiation in the region of 2200– 2000  A [3,4]. Long and EwingÕs spectroscopic investigations in gaseous oxygen at temperatures around 90 K showed the presence of a ðO 2 Þ 2 van der Waals molecule and accordingly there is no need to suggest a Lewis pairing of the electrons [5]. They obtained dimer dissociation energy of 530  70 cal/mol. Their visible and infrared spec- tra suggest that in some ðO 2 Þ 2 molecules each O 2 is undergoing hindered rotation against the weak van der Waals bond [5]. However, the polarised high resolution O 4 spectra in solid neon host at 4.2 K is consistent with D 2h structure, and less likely with D 2d structure [6,7]. Adamantides et al. [8,9] Chemical Physics Letters 370 (2003) 789–794 www.elsevier.com/locate/cplett * Corresponding author. Fax: +55-21-586-7400. E-mail address: taft@cbpf.br (C.A. Taft). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00163-5