The dipole moments and the structure of polyoxymethylene dimethyl ethers

The dipole moments of monoxymethylene dimethyl ether and dioxymethylene dimethyl ether were measured in hexane solution. From the dependence of the moments of these compounds on the number of oxygen atoms as well as from the temperature dependence of the moments of monoxymethylene dimethyl ether, it was concluded that these compounds are mixtures of rotational isomers, the predominant ones of which assume helical or almost helical structures arising from the gauche minima in the internal rotation about single bonds. The trans minimum, which is 1.74 kcal./mole higher than the gauche minima, is also significant and gives rise to rotational isomers having configurations deviating from the simple helical structure.