Molecular structure of tetrahydrothiophene-1-oxide and cis-2-methyltetrahydrothiophene-1-oxide from electron diffraction

Abstract

The molecular structures of tetrahydrothiophene-1-oxide (I) and cis-2-methyltetrahydrothiophene-1-oxide (II) have been studied by gas-phase electron diffraction. Both molecules have an asymmetric ring conformation. They can be regarded as frozen forms along the pseudorotational pathway with phase angles and amplitudes 145.2 ± 1.6° and 48.5 ± 0.4° for I(³T₂) and 154.9 ± 2.5° and 43.2 ± 0.8° for II(between ³T₂ and ³E); 0° phase angle corresponds to a half-chair ring with C₂ symmetry (⁴T₃). The SO bond takes a pseudoaxial orientation in both molecules, and the 2-methyl group is in a nearly equatorial position in II. The following bond distances (r_g/Å) and bond angles (°) were obtained (with estimated total errors): molecule I: S—C, 1.8277 ± 0.0037; (S)C—C, 1.5398 ± 0.0034; (C)C—C(C), 1.5408 ± 0.0034; SO, 1.4843 ± 0.0032; C—H, 1.1342 ± 0.0025; S—C—C, 105.3 ± 0.3 and 106.9 ± 0.3; (S)C—C—C, 102.2 ± 0.3 and 111.5 ± 0.3; C—S—C, 92.0 ± 0.3; C—S—O, 110.0 ± 0.4; molecule II: S—C, 1.8344 ± 0.0038; (S)C—C, 1.5432 ± 0.0032; (CH)C—C(CH₂), 1.5442 ± 0.0032; SO, 1.4852 ± 0.0031; C—C(H₃), 1.5438 ± 0.0032; C—H, 1.1243 ± 0.0026; S—C(CH₃)—C, 107.2 ± 0.1; C(CH₃)—C—C, 105.6 ± 0.6; (S)C(H₂)—C—C,108.1 ± 0.7; S—C(H₂)—C, 110.3 ± 0.7; C—S—C, 91.7 ±0.4; C—S—O, 105.3 ± 0.3; C—C(S)—C(H₃), 110.9 ± 1.0; S—C—C(H₃), 111.6 ± 0.3.