Geometry and conformation of thietanium ions from diffraction data and ab initio calculations

Four-membered ring thiosulfonium ions may be obtained quantitatively and un der mild conditions by anionotropic rearrangement of C-(tert-butyl)-substit uted thiiranium ion precursors. Thus, t-4-(tert-butyl)-r-1,2,2,c-3-tetramet hylthietanium tetrafluoroborate or hexachloroantimonate (2a or 2b, reap.) w ere formed from thiiranium ion 1. The thietanium salts 2a and 2b were chara cterized by X-ray crystal-structure analysis. Their cation geometry was als o optimized by ab initio calculations at the RHF/6-31G*//RHF/6-31G* level, as were those of its stereoisomer 3 and of the unsubstituted S-methylthieta nium ion 5. Comparison of 2, 3, and 5 with 4 - the only other thietanium io n studied by XRD, where the C-atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton - indicates that, in these systems, relief fro m substituent overcrowding is easily achieved by a folding of the four-memb ered ring along the line connecting the two opposite C-atoms. The correspon ding ring-deformation normal mode has a calculated frequency as low as 109 cm(-1) in ion 5, to be compared with a frequency of 138 cm(-1) in methylcyc lobutane. For thietanium ion 2, the frequencies of the two normal modes inv olving such ring deformation have calculated values of hi and 85 cm(-1).