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.