Protonation of S₄, S₆, and S₈Sulfur Cycles. A Quantitative Study

Abstract

We have determined the gas-phase basicities of S 6 and S 8 by means of Fourier transform ion cyclotron resonance spectroscopy (FT-ICR): 167.8 and 187.1 kcal/mol, respectively. An ab initio investigation of the local minima of the S 6 H + potential energy surface shows that the most stable protonated species corresponds to a distorted five-membered ring structure rather than to a six-membered ring species. When the final energy of the aforementioned protonated species is evaluated at the G2(SVP,MP2) level, the agreement with the experimental value is very good. A G2(MP2) survey of the S 4 and S 4 H + potential energy surfaces shows that the most stable neutral and protonated species present cis open-chain structures. The relative stabilities of the different species investigated are extremely sensitive to electron correlation effects. At the G2(MP2) level S 4 is predicted to be slightly more basic than S 6 in the gas phase. Some preliminary ab initio calculations on the most probable structures of S 8 H + are also reported. Experimental SectionMaterials. S 8 . A highly pure sample of cyclooctasulfur (99.99%, Aldrich) was used. lts purity was assessed by means of its mass spectrum. S 6 . Cyclohexasulfur was obtained, according to Schmidt and co-workers, 12 by treating bis(cyclopentadienyl)titanium pentasulfide with sulfur dichloride in CS 2 at 0°C in the dark. The product was further crystallized from cold CS 2 in the dark, dried, and immediately subject to the FT-ICR study. Again, purity was assessed by means of the mass spectrum.FT-ICR Spectrometer. The instrument is a modified Bruker CMS 47 FT-ICR mass spectrometer 13 used in previous studies. [4][5][6][7][8]14,15 A detailed description of the main features of this instrument are given in refs 4, 13, and 14. The substantial field strength (4.7 T) of its supraconducting magnet allows the monitoring of ion-molecule reactions for relatively long periods of time.