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Geometry and Conformation of Thietanium Ions from Diffraction Data and Ab Initio Calculations
Author(s) -
Destro Riccardo,
Ortoleva Emanuele,
Modena Giorgio,
Pasquato Lucia,
Lucchini Vittorio
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010418)84:4<860::aid-hlca860>3.0.co;2-1
Subject(s) - chemistry , ion , ring (chemistry) , crystallography , ab initio , ab initio quantum chemistry methods , conformational isomerism , crystal structure , computational chemistry , molecule , organic chemistry
Four‐membered ring thiosulfonium ions may be obtained quantitatively and under mild conditions by anionotropic rearrangement of C ‐( tert ‐butyl)‐substituted thiiranium ion precursors. Thus, t ‐4‐( tert ‐butyl)‐ r ‐1,2,2, c ‐3‐tetramethylthietanium tetrafluoroborate or hexachloroantimonate ( 2a or 2b , resp.) were formed from thiiranium ion 1 . The thietanium salts 2a and 2b were characterized by X‐ray crystal‐structure analysis. Their cation geometry was also 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 ‐methylthietanium ion 5 . Comparison of 2 , 3 , and 5 with 4 – the only other thietanium ion studied by XRD, where the C‐atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton – indicates that, in these systems, relief from substituent overcrowding is easily achieved by a folding of the four‐membered ring along the line connecting the two opposite C‐atoms. The corresponding 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 methylcyclobutane. For thietanium ion 2 , the frequencies of the two normal modes involving such ring deformation have calculated values of 61 and 85 cm −1 .