Methoxyethenes:  Structures and Conformations

Abstract

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 CC double bond (φ1,2(CCOC) = 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 CC 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 CC bond. For trimethoxyethene, eight minima were detected on the energy surface. In the ground-state structure, two groups are oriented anticlinal and one synperiplanar.

Ligand Close Packing, Molecular Compactness, the Methyl Tilt, Molecular Conformations, and a New Model for the Anomeric Effect

“…3,4 230(2), 280(2) 236, 280, 311, 324 232(4) H' 2 CA C H T U N G T R E N N U N G (OMe) 2 1,3 H 1 ···H 2 180 181 180 H 3 ···H 4,5 179 (6) 179 (6) 179 ( Dimethoxyethene has been studied both theoretically and experimentally and it has been found that the cis,cis conformer is the lowest energy experimentally observed conformer. [24] The 1,3-and 1,4-distances for both molecules are given in Table 10. The 1,3-distances show that the O 1 , O 2 , and O 3 atoms are close-packed around C 1 in both molecules The relative compactness of the three conformations of both molecules is determined by the 1,4 C 3 ···O 1 , C 2 ···O 2 , distances which show that the compactness of these molecules increases in the order tt < tg < gg in agreement with the calculated relative energies (Table 12).…”

“…Extensive ab intio calculations are available for fluoromethanol [24] and methoxymethylfluoride. [25] In these cases the gauche conformation has been shown to be more stable than the trans conformation.…”