Electronic Spectra of Chevreul's Salts

Journal of the Brazilian Chemical Society

Andrade, Jailson B. de¹  Universidade Federal da Bahia, Salvador, Brazil¹  Toma, Henrique E.¹  Universidade de São Paulo, São Paulo, Brazil¹  Silva, Luciana A. da¹ 

关键词: electronic spectra;    Chevreul's salt;    isomorphic salts;   

DOI  :  10.1590/S0103-50532002000500013

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The first known mixed valence sulfite complex, CuI2SO3.CuII SO3.2H2O, was prepared by M. Chevreul,1 in 1812. Its crystalline structure was described in terms of coordination polyhedra by Kierkegaard and Nyberg,2 consisting of [SO3] trigonal pyramids, [CuIO3S] tetrahedra and [CuIIO4(H2O)2 ] octahedra linked together, giving a three-dimensional network. Chevreul's salt derivatives have been prepared from the Cu(II) replacement by transition metal ions such as Mn(II), Fe(II) and Cd(II) ions.3-5 In such cases, the X-ray diffraction data were similar to those of Chevreul's salt, with minor changes in "d" spacings,3-5 forming an isomorphic series, i.e., Cu2SO3.CuSO3.2H2O, Cu2SO3.MnSO3.2H2O, Cu2SO3.FeSO3.2H2O and Cu2SO3.CdSO3.2H2O. As a consequence, a gradual substitution of the Cu(II) ions by the transition metal ions is also possible, giving rise to a variety of mixed compositions. Interestingly, the isomorphic species present distinct colors.4 The intense red color of the Chevreul's salt changes to yellowish-brown, after the substitution of Cu(II) by Fe(II). If the substitution is carried out with Mn(II) instead of Fe(II), a gradual transition to yellow color is observed. On the other hand, if Cu(II) is replaced by Cd(II), a faint yellow color is obtained. Although the interest for this kind of compounds dates from the beginning of the 19th century, only recently the spectroscopic properties of the Chevreul's salt have been investigated by Inoue et al.6 based on EPR measurements and diffuse reflectance absorption spectra. Up to the present time, there is a complete lack of information about the electronic interactions between the constituents of Chevreul's salt derivatives, especially concerning the types of electronic transitions responsible for their different colors. For this reason, this subject is focused in this paper, with emphasis on the theoretical evaluation of the charge-transfer bands in the Chevreul's salt.   Experimental Materials All chemicals used in the synthesis of the double sulfites were of analytical reagent grade. Copper, and mixtures of copper and manganese, copper and iron or copper and cadmium sulfate solutions (see compositions in Table 1) were saturated with sulfur dioxide gas at room temperature to give solutions with pH of approximately 1. These solutions were heated to 78 oC. The pH of the solutions was raised to 3.0-3.5 by dropwise addition of a 20% sodium carbonate solution, under magnetic stirring. The precipitation of the complex sulfites starts at about pH 3.0. The crystalline materials were immediately collected on a filter, washed with deionized water and rinsed with ethanol, followed by air drying. A more detailed account on their syntheses can be found elsewere.4,5   Measurements The electronic spectra were recorded on a Guided Wave spectrophotometer, model 260, equipped with a Wand bundle probe for in situ reflectance measurements. Total copper, manganese, iron, and cadmium contents were determined using an ICP/AES ARL, model 3410 instrument. Theoretical Calculations Molecular modeling calculations were carried out for the dimeric [CuI2(SO3) 2(SO3)2]6- center attached to two [CuII(H2O)2(SO 3)2]2- fragments, starting from the crystallographic bond lengths,2 in order to simulate the {CuI2(SO3) 2[CuII(H2O)2 (SO3)2]2} 6- repetitive group (Figure 1) in the tridimensional structure of the Chevrel's salt2 (SO32- = S bound sulfite, SO32- = O bound sulfite). Geometry optimization was carried out using the MM+ method within the HyperChem 6.0 program (Hypercube Inc. Gainesville, USA), and a gradient of 1 x 10-6 kcal (1 cal = 4.1840 J) as a convergence criterion in a conjugate gradient method. Spectral simulations were carried out in separate for the [CuI2(SO3) 2(SO3)2]6- center, but keeping the same previously optimized geometry for the {CuI2(SO3) 2[CuII(H2O)2 (SO3)2]2 }6- group (Figure 1). SCF molecular orbitals were obtained at the RHF level for the closed-shell Cu(I) ground-state species, using the ZINDO/S method,7-10 and the default parameters, for single CI excitations in an active space involving 20 frontier molecular orbitals (10 highest occupied and 10 lowest unoccupied MOs).   Results and Discussion The diffuse reflectance electronic spectrum of the Chevreul's salt reported by Inoue et al.6 in the 400-800 nm range, consists of two broad bands at 785 and 425 nm. The first one has been ascribed to a d-d transition of the octahedral Cu(II) ions.6 The second band (at 425 nm) has been tentatively assigned to an intervalence transition between the tetrahedral Cu(I) sites and the octahedral Cu(II) ones. This transition is responsible by the intense dark red color of the Chevreul's salt. In this work, the electronic spectrum of the Chevreul's salt has been extended to the near-infrared region, as illustrated in Figure 2. The spectra of the corresponding derivatives with Fe(II), Mn(II) and Cd(II) ions are also shown in Figure 2, for comparison purposes.   As can be seen in Figure 2, all the Chevreul's salt derivatives exhibit an absorption band around 425 nm and a less intense band in the visible region. This feature has also been recently reproduced from photoacoustic measurements.11 However, the absorption profile is markedly distinct for the Chevreul's salt, in which

【 授权许可】

Unknown