Ethers rotational barriers

The aryl C—O—C linkage has a lower rotation barrier, lower excluded volume, and decreased van der Waals interaction forces compared to the C—C bond. Therefore, the backbone containing C—O—C linkage is highly flexible. In addition, the low barrier to rotation about the aromatic ether bond provides a mechanism for energy dispersion which is believed to be the principal reason for the toughness or impact resistance observed for these materials.15 17... 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

Experimentally, it is known that the cis isomer in 1-substituted propenes is more stable and has a lower rotational barrier. Some pertinent data are shown in Tables 13—14. In most cases, the experimental results agree with our predictions. An interesting trend obtains in the alkyl vinyl ether series. Specifically, two types of nonbonded attraction can obtain in these molecules ... 

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... 

Table 17. cis - trans energy differences and methyl rotational barriers for methyl-vinyl-ether... 

The structure and chiral properties of the enolate intermediate were then investigated. Treatment of 40 with KHMDS (1.1 equiv) in toluene-THF (4 1) at —78°C for 30 minutes followed by t-butyldimethylsilyl (TBS) triflate gave Z-enol silyl ether 54 and its -isomer 55 in respective isolated yields of 57% and 27%.30 In the lH NMR spectra of both 54 and 55, methylene protons of the MOM groups appeared as AB quartets, which indicates restricted rotation of the C(l)-N bonds. The rotational barrier of the C(l)-N bond of the major Z-isomer 54 was determined to be 16.8 kcal/mol at 92°C by variable-temperature NMR measurements in toluene- (400 MHz 1 H... 

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

Concerning the aggregation, it has been reported that allyllithium 23a in THF is predominantly monomeric possibly with some dimers27. Since 23a, on the other hand, is highly aggregated in diethyl ether 25,28), but the barriers are the same in both solvents 25), it is believed 23) that the state of aggregation of allyllithium 23a does not affect its rotational barrier. 

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. 

This study comprises a theoretical investigation of all possible stable conformers of two monomers and a dimer containing alkyl-bound ether fragments simulated with two quantum chemical methods. Basic structural parameters, characteristic vibrational frequencies and atomic electron density distribution are evaluated. Rotation barriers about the ether bond are estimated. The mean values of these quantities averaged over the entire conformer ensemble are supplemented to the parameters of the molecular mechanics force field AMBER and tested by several molecular dynamics simulations of one of the model systems - diethyl 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. 

---