Methyl motion

Actually, the methyl and carbonyl dynamics within the BPA-PC repeat unit (Fig. 54) can be conveniently analysed by considering the methyl motion of DPP and the carbonyl motions of DPC. However, in the case of phenyl ring motions, neither DPP nor DPC correspond to the situation within BPA-PC. Indeed, in the latter the isopropylidene and the carbonate groups bonded to the phenyl rings are para to one another, with the Ci - Cp bonds lying almost on the same axis as the O - Cp bonds. 

Another important factor in these studies is steric constraints imposed on methyl motion by the molecular environment. In such cases, methyl rotation is not free, but rather is subject to a potential barrier greater than kT. The potential barrier, V7,, is calculated via a conventional expression of the type... 

Relaxation data for the methyl carbon could be measured only down to ca. -125°C below this temperature line broadening was too severe to obtain results. The near equal values observed for the Ti and Tip over much of the limited temperature interval is in accord with the methyl motion being on the high temperature side of the Ti-minimum. 

Methyl Motion and the Tensor. Photolysis of crystalline acetyl benzoyl peroxide with 0-18 in the peroxy positions gives methyl benzoate with most of the 0-18 bound to methyl rather than screuiibled between the carbonyl and ether positions. (11) In attempting to understand tliis discrimination we studied the epr spectrum of the methyl-benzoyloxy radical pair (M-B) trapped in a photolyzed crystal of the peroxide at low tenperature.( )... 

If we wished to be parsimonious about the amount of methyl motion in the crystal, we would choose the Z benzoyloxy radical with spin on the original carbonyl oxygen, although the direction of minimum spin-spin distance is more consistent with the II than with this Z radical. 

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