CONFORMATION AND CONFORMATIONAL ENERGIES OF DIMETHOXYMETHANE AND 1,1-DIMETHOXYETHANE

The 13C-1H NMR vicinal coupling constant 3JCH was measured for dimethoxymethane CH3OCH2OCH3 in the gas phase as well as in various solvents. The observed values tend to decrease slightly with temperature. The energy difference Eσ between the gg and tg forms was estimated to be -2.5 ± 0.2 kcal mol-1 in the gas phase. The coupling constant parameters determined concomitantly are as follows: (JT + JG-)/2 = 6.7 ± 0.1 and JG = 2.0 Hz. Values of Eσ observed in nonpolar solvents were found to fall in the same range. A somewhat smaller value (ca. -2.2 kcal mol-1) was derived from the measurements in (CD3)2SO. These values are reasonably consistent with the results of recent MO calculations by Wiberg and Murcko. A similar 13C NMR analysis of 1,1-dimethoxyethane CH3OCH(CH3)OCH3 indicates that the g-g and tg conformers exists in nearly equal amounts, i.e. Eτ= Eg-g - Etg ≅ 0 kcal mol-1. In the latter compound, the anomeric conformation ( g -) involves unfavorable steric interactions around the g- bond. © 1990.