The geometric structures and conformational properties of 1,1-dimethoxyethene (1,1-DME), (Z)-1,2-dimethoxyethene (Z-1,2-DME), and tetramethoxyethene (TME) have been determined by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G, HF/6-31G*, and MP2/6-31G*). Additional theoretical calculations have been performed for (E)-1,2-dimethoxyethene and trimethoxyethene. The calculations predict three or more possible conformations for these compounds in the energy range below about 2 kcal mol-1. For 1,1-DME, the GED experiment results in a mixture of two conformers. The main form (61(7)%) possesses C 2 v symmetry, with both methoxy groups synperiplanar to the CC double bond (φ1,2(CCOC) = 0°). In the second conformer, one methoxy group is oriented synperiplanar and the other one anticlinal (φ2 = 131(7)°). For Z-1,2-DME and TME, only one conformation was observed in the GED analyses. Z-1,2-DME possesses a structure with syn- and antiperiplanar (φ2 = 152(6)°) methoxy groups (C 1 symmetry). In TME, all groups are oriented anticlinal, alternatingly above−below−above−below the molecular plane (D 2 symmetry). This compound is twisted around the CC bond by 14(3)°. The MP2 calculations reproduce the conformational properties of these methoxyethenes perfectly, whereas the HF/3-21G approximation leads to incorrect results. The MP2/6-31G* method predicts for the preferred conformation of (E)-1,2-dimethoxyethene a structure with C 2 h symmetry and both methoxy groups synperiplanar to the CC bond. For trimethoxyethene, eight minima were detected on the energy surface. In the ground-state structure, two groups are oriented anticlinal and one synperiplanar.
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