Methyl group reorientations

It would be interesting to understand how methyl-group reorientation alone couples to sound waves. 

Two Ti minima were evident in the monomer. The low temperature minimum at —50 °C was attributed to methyl group reorientation. The high temperature minimum at 32 °C was said to be caused by molecular motion associated with the glass transition. 

In Fig. 9, the relaxation times are plotted versus reciprocal temperature for the 180 °C cure of DGEBA and MDA. The T, relaxation time minima at —70 °C as well as the Tle relaxation time minima at —130 °C were attributed to methyl group reorientation. 

The methyl group reorientation minima for the 54 °C cure was similar to the uncured resin whereas the 100 °C cure was fashioned after the 180 °C postcured resin. 

Relaxation studies on natural rubber (uncrosslinked and crosslinked)19) show that the experimental T2 curve is narrower near the T2 minimum than predicted by the BPP theory. Although the depth of the Tj minimum in some polymers, for example, the one due to the methyl group reorientation in uncured diglycidyl ether... 

The 51V NMR spectra of bisW-mcthylhydroxylaminc) complexes show signals for methyl groups in orientations assignable to the possible positions available on the nitrogen [7,8]. Unfortunately, detailed kinetics experiments have not been carried out. It would be of interest to know whether the end-for-end rotation is correlated with methyl group reorientation. 

The long-chain normal alkanes have a crystalline form in which the terminal methyl groups lie in a plane nearly normal to the chain axis.i At low temperatures the methyl group reorientation provides a rapidly relaxing site and it acts as a sink for the remainder of the proton spin system.2 Figure 2 shows T p data for a series of alkanes as a function of temperature. If spin diffusion were entirely rate controlling, the minimum would rise as the square of the number of methylene carbons, N if relaxation to the lattice were the bottle neck, the minimum should rise linearly with N. The data exhibits a rise of the minimum with the 1.6 or 1.7 power of N, a typically intermediate case. 

The interpretation of carbon T p data is complicated by the fact that spin-spin (cross-relaxation) processes as well as rotating frame spin-lattice processes may contribute to the relaxation (40). Only the latter process provides direct information on molecular motion. For the CH and CH2 carbons of PP, the Tip s do not change greatly over the temperature interval -110°C to ambient and, as opposed to the T behavior, the CH2 carbon has a shorter T p than the CH carbon. These results suggest that spin-spin processes dominate the Tip (46). However, below ca. -115°C, the Tip s for both carbons shorten and tend toward equality. McBrierty et al. (45) report a proton Ti minimum (which reflects methyl group reorientation at KHz frequencies) at -180°C. No clear minimum is observed in the data, perhaps due to an interplay of spin-spin and spin-lattice processes. Nonetheless, it is apparent that the methyl protons are responsible for the spin-lattice portion of the Tip relaxation for CH and CH2 carbons. 

The methyl groups reorient In the beginning, each carbon lone pair was pointing towards the palladium atom, while in the final getHnetry they point towards each other. 

For example, methyl group reorientation about the C3 axis. 

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