Dimethyl ether, rotational barrier

The above analysis can also be used in connection with the problem of the methyl rotational barrier in double rotor molecules, e. g. dimethyl ether, relative to... 

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. 

Since oxygen is much smaller than a methylene group, the same kind of situation occurs in XVII as was discussed in the previous section. The barrier to methyl rotation in dimethyl ether is 2.7 kcal/mole >, only slightly lower than in propane, where the beirrier is 3.4 kcal/mole. Oxocane should therefore have the BC-1 conformation, as in methylenecyclooctaue rather than the BC-3 and BC-7 conformations. The presence of only a single process in the proton spectrum of XVII is immediately consistent with the BC-1 conformation, but requires rapid pseudorotation between the BC-3 and BC-7 forms at —170 °C if the latter two forms are the correct conformations. The pseudorotation barrier in XVII should be higher than in cyclooctane, and probably comparable to that in cyclooctanone (6.3 kcal/mole). Thus, pseudorotation of the BC-3 form should not be rapid at —170 °C, and further support for this h5q)othesis is provided by 1,3-dioxocane (see below). It is therefore probable that oxocane has the BC-1 conformation. 

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

The results for tetrahydrofuran are important since there is substantial evidence for pseudo-rotation from both far-i.r. and microwave spectroscopy. A series of absorptions between 20 and 100 cm can be fitted to equation (34) with mq% — (8.56 0.13) x 10 and (8.48 0.15) X 10 gem for the vibrational states y = 0 and y = 1, respectively. For a fit to the measured thermodynamic properties a value for mql of 8 X 10 g cm had previously been required, so the agreement is extremely satisfactory. Microwave spectroscopy has since afforded values for the first 15 energy levels for pseudo-rotation and shown that the barrier is very low. Additional levels have been estimated from these, their contributions to the thermodynamic properties calculated, and the total values of the latter obtained and shown to be in excellent agreement with the observed values. A calculation of the barrier to pseudo-rotation gave a value of 10.5 kJ mol . But it has been shown that if, for this calculation, the barrier height is transferred from dimethyl ether (11.4kJmol 0 rather than from methanol (4.48 kJmol ), then the... 

The comparison of the averaged estimates of the rotational barriers (Table 26.6) with experimental data (2.6 kcal/mol) for the dimethyl ether obtained from rotational spectra [44] outlines the excellent consensus, which is a trustworthy validation of the chosen theoretical approach. 

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