Dimethyl ether internal rotation

L. Goodman and V. Pophristic, Where does the dimethyl ether internal rotation barrier come from , Chem Phys. Lett. 259, 287-295 1996. 

Microwave spectrum, structure, and internal rotation of dimethyl ether. 

For the DTO model we must have an estimate of the torsional vibration frequency and the barrier to internal rotation of the constituent monomers. The DTO model fits the experimental data for bulk polymer if H = 5.4 kcal/mole, vt — 1012 c.p.s., and Zt = 30 which are not unreasonable values. One would expect the barrier height to decrease upon dilution (if it changes at all) as the chain environment loosens up. Assuming that rotation about C—O—C bonds is predominate, we take the experimental values of H = 2.63 kcal/mole, vt = 7.26 x 1012 c.p.s. of Fateley and Miller (14) for dimethyl ether. Eq. (2.8) predicts rSJ° = 0.47 X 10-8 sec at 253° K with Zt = 30. We shall use this as our dilute solution result. [The methyl pendant in polypropylene) oxide will act to increase the barrier height due to steric effects, making this calculated relaxation time somewhat low for this choice of a monomer analog.] Tmax is seen to change only by a factor of 102—103 upon dilution in the DTO model. 

Calculated Rotational Potential Constants (kJ.mol ) Describing Internal Rotation About the C-O Bond" in Substituted Dimethyl Ethers, XCH2OCHJ... 

Figure 2 Internal rotation in dimethyl ether, methanol, acetone, acetaldehyde, and propene in descending order. In- keletal-plane and out-of-skeletal-plane hydrogen atoms are designated dark and light biue, respectively. The depiction is for simultaneous rotation to the SS conformers in the dimethyl compounds...

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