Eclipsed methyl group

The ROMP of neat 99 is remarkable in that it proceeds to 96% conversion in 18 h at 25 °C to yield an all-HT, 97% trans polymer. Dilution of the living polymer with benzene causes neither reversion to monomer nor backbiting to form cyclic oligomers. The driving force for polymerization in this case derives from the relief of torsional strain in the monomer caused by interaction between eclipsed methyl groups on the adjacent silicon atoms300. 

Polyhydric Alcohols.—Using a borate buffer at pH 9.2, Frahn and Mills have studied the zone-electrophoretic behavior of a number of diols (see Table VII). Thus 1,4-butanediol and 1,5-pentanediol were found not to react with borate ions, indicating that borate complexes involving 7- and 8-membered rings have little tendency to be formed. The more ready reaction of comparison with the erythro isomer, is not unexpected, since the 5-membered ring in the borate complex of the latter (but not the former) would possess eclipsed methyl groups at C2 and C3. The relative reactivity of the 2,3-butanediols toward borate... 

Probably the three protons of the eclipsing methyl group in — ( 113)3. 

Example Hthanc is stable in the staggered gauche) con formation. The transition state for rotating a methyl group in ethane has the eclipsed con form at ion, A gcom etry optim i/ation start in g from an eclipsed eon formation yields th e tran sition state. 

The most stable conformation of acetone has one of the hydrogens of each methyl group eclipsed with the carbonyl oxygen Construct a model of this conformation... 

The calculations of the optimum geometry show a slight lenghthening of the C—H bonds because of the electron release to the tc system. These calculations also reveal a barrier to rotation of the methyl group of about 1.5-2.0 kcal/mol. Interaction between the hydrogens and the n system favors the eclipsed conformation to fliis extent. Let us examine the... 

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

The origin of the preference for the eclipsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the eclipsed conformation. ... 

Examine the maximum energy conformations. What evidence is there for steric repulsion between methyl groups in the conformation that eclipses these groups ... 

Figure 3.9 A plot of potential energy versus rotation for the C2-C3 bond in butane. The energy maximum occurs when the two methyl groups eclipse each other, and the energy minimum occurs when the two methyl groups are 180° apart (anti. ...

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