The ionic nature of methylsulfur trichloride species Lucas S. Rodríguez Pirani, Mauricio F. Erben ⁎, Carlos O. Della Védova ⁎ CEQUINOR (UNLP-CONICET CCT La Plata), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115 (B1900AJL), C.C. 962, La Plata, Buenos Aires, Argentina abstract article info Article history: Received 7 August 2012 Accepted 30 September 2012 Available online 10 October 2012 Keywords: Sulfonium salts Raman spectroscopy Methyldichloro-sulfonium chloride Ionic/covalent The Raman spectrum of the methyldichloro-sulfonium(IV) chloride, CH 3 SCl 2 + Cl - , is reported. The unambiguous vibrational assignment for the CH 3 SCl 2 + cation is made on the basis of the solid state Raman spectra aided by high-level quantum chemical calculations and a normal mode analysis. Actually, the former reported methylsulfur trichloride, CH 3 SCl 3 , can be now described as CH 3 SCl 2 + Cl - . © 2012 Elsevier B.V. All rights reserved. Introduction. Halodimethylsulfonium halide chemistry started with the discovery of bromodimethylsulfonium bromide [1,2], a com- pound that offers considerable promise as a potent reagent in current organic chemistry [3,4]. Chlorodimethylsulfonium chloride was early synthesized [5] and its implications in the atmospheric chemistry were discussed recently [6]. However, less is known on the analogous monomethylated species, possible due to their inherent instability. “Methylsulfur trichloride”, CH 3 SCl 3 , was first synthesized by Brower and Douglas in 1951 [7] by chlorination of methyl disulfide in nearly quantitative yield. The crystalline compound is unstable and decom- poses on standing at room temperature mainly into chloromethane sulfenyl chloride and methanesulfenyl chloride [8]. As early stated, CH 3 SCl 3 is insoluble in non-polar solvents and it is hydrolyzed by water into methanesulfinic acid [7]. Moreover, chlorination of different types of organic sulfur compounds, such as thioesters and xanthates, also leads to the formation of alkylsufur trichlorides, which were chemically characterized [9,10]. Further studies concerning the mechanism of chlo- rination of sulfenyl chorides have been reported [11,12]. Although un- stable, these compounds are useful reagents for the synthesis of sulfur compounds. Thus, when organosulfur trichlorides, RSCl 3 , are treated with water, alcohols, or carboxylic acids, solvolysis occurs and the corre- sponding sulfinyl chlorides, RS(O)Cl, are smoothly produced in excellent yields [13]. Moreover, the reaction with optically active alcohols pro- duced alkyl chlorides with a high degree of inverted configuration [14]. The structure of CH 3 SCl 3 , whether ionic or covalent, has remained unresolved. As early stated by Douglass and coworkers in the series of very informative articles on CH 3 SCl 3 , “no evidence other than its sol- ubility characteristics has been found for an ionic structure for the compound” [7]. More recently, related methyldihalo-sulfonium species were synthesized and the vibrational assignments for CH 3 SCl 2 + SbCl 6 - [15–17] and CH 3 SCl 2 + AsF 6 - [18] were discussed. In this letter, we report the Raman spectrum of the solid product formed by chlorination of methyl methylxanthate [10]. The analysis of this spectrum together with the results from the quantum chemical calculations strongly suggests a CH 3 SCl 2 + Cl - ionic structure instead of covalence for this compound. Moreover, a definite assignment for the CH 3 SCl 2 + cation is given. Experimental. Methyl methylxanthate was dissolved in freshly dis- tilled hexane and chlorinated according to the method of Douglass and Osborne [10]. A gentle stream of chlorine was passed into the cooled solution until, after alternately shaking and settling, no more solid appeared to form. This orange liquor was filtered and the solid washed several times with chilly hexane. A 6 mm o.d. tube was filled with the solid and dried in a vacuum. The tube was flamed-sealed and maintained in liquid nitrogen until the measurement. The FT-Raman spectra were recorded in the region 4000– 100 cm -1 using a Bruker IFS 66v spectrometer equipped with Nd: YAG laser source operating at 1064 μm line with 7.5 mW power of spectral width 2 cm -1 . Higher laser power leads to decomposition of the sample, evidenced by the appearance of a red-yellow liquid. Quantum chemical calculations were performed with the GAUSS- IAN03 program package [19]. MP2 and B3LYP methods and gradient techniques were used for the geometry optimizations and calculation of the vibrational properties, together with standard basis sets up to the Pople-type 6-311++G**, which includes diffuse and polarization functions. The more extended aug-cc-pVTZ basis set was also applied with the B3LYP functional. Recommended frequency scale factor was applied [20] and the calculated Raman activities were converted to rela- tive Raman intensities following the usual methodology [21]. For the Inorganic Chemistry Communications 26 (2012) 66–68 ⁎ Corresponding authors. Tel./fax: +54 221 425 9485. E-mail addresses: erben@quimica.unlp.edu.ar (M.F. Erben), carlosdv@quimica. unlp.edu.ar (C.O. Della Védova). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.09.027 Contents lists available at SciVerse ScienceDirect Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche