Ring Motions

Figure Bl.16.19. (a) CIDEP spectrum observed in die photolysis of xanthone (1.0 x 10 M) in cyclohexanol at room temperature. The stick spectra of the ketyl and cyclohexanol radicals with RPM polarization are presented, (b) CIDEP spectrum after the addition of hydrochloric acid (4.1 vol% HCl 0.50 M) to the solution above. The stick spectra of the ketyl and cyclohexanol radicals with absorptive TM polarization are presented. The bold lines of the stick spectra of the cyclohexanol radical show the broadened lines due to ring motion of the radical. Reprinted from [62].

The phenyl ring motion for peptides and various synthetic polymers have been studied with the solid state H NMR by many workers, undergoing a 180° flipping motion in a two-fold potential. The reported activation energies of the flipping motion reflect the degree of crystallinity, the crystal... 

R. M. Levy and R. P. Sheridan, Combined effect of restricted rotational diffusion plus jumps on nuclear magnetic resonance and fluorescence probes of aromatic ring motions in proteins, Biophys. J. 41, 217-221 (1983). 

Interestingly, the dumbbell component of a molecular shuttle exerts on the ring motion the same type of directional restriction as imposed by the protein track for linear biomolecular motors (an actin filament for myosin and a microtubule for kinesin and dynein).4 It should also be noted that interlocked molecular architectures are largely present in natural systems—for instance, DNA catenanes and rotaxanes... 

There is no evidence of protonation of simple aromatic rings in zeolites to form appreciable equilibrium concentrations of arenium ions. Figure 19 shows 13C MAS spectra of benzene in zeolite HY as a function of temperature (81). No ring protonation is reflected in the 13C shift, although temperature-dependent ring motion is observed. DFT calculations at the BLYP/... 

Figure 10 shows the evolution of T2m for the CH2 ring carbon C2 (the carbon code number is given in Fig. 7). The T2m minimum is observed around 20 °C, which indicates that at this temperature the correlation times of the ring motion are of the order of 5 x 10-6 s. Very similar results are observed for the CH2 ring carbon C3. The line of the C4 ring carbon seems to be less broadened than those of carbons C2 and C3. Moreover, the widths of the main-chain CH2 carbon and of the Ci ring carbon lines do not seem to be temperature-dependent over the range of temperature investigated. All these data indicate that the dynamic process involved is a ring motion that mainly affects the C2-H and C3-H internuclear vectors.

This example dealing with the precise identification of the ring motions involved in the y transition of poly(cyclohexyl methacrylate) clearly illustrates how convenient and powerful the MAS CP 13C NMR is for studying solid-state transitions of polymers. It is worth noting that, owing to the very low temperature at which the y transition occurs, the involved motions are very localised. 

As regards the phenyl ring motions, interesting features are obtained from measurements of 13C chemical shift anisotropy of protonated and unpro-tonated aromatic carbons. The chemical shift parameters are orthogonal to each other, with o and 072 in the phenyl plane and <733 bisecting the phenyl plane. 

The choice of chloral-PC is appropriate because (i) it shows a j3 transition at the same temperature as BPA-PC, and (ii) NMR measurements performed on CDCI3 solutions [31] lead to the same dynamics of segmental and phenyl ring motions as for BPA-PC. 

Without using any motional model, the temperature positions of T and Tip minima can be assigned an appropriate frequency 90 MHz at 120 °C from Ti and 43 kHz at - 34 °C from T r These two results fit quite well on the relaxation map of BPA-PC obtained from dynamic mechanical and dielectric relaxation. They support the fact that phenyl ring motions are involved in the /3 relaxation of BPA-PC. Furthermore, the Ti and T f> data can be simulated by considering the Williams-Watts fractional correlation function [33] ... 

In order to investigate the phenyl ring motions by 2H NMR, deuterated phenyl BPA-PC (BPA-d4-PC) has been prepared by deuteron substitution in ortho position to the carbonate link, and studied in the temperature range from - 110 to 120 °C [36]. 

As regards the activation energy associated with the phenyl ring motions, the two types of distributions lead to the same mean value of 37 kj mol-1. 

C NMR has been used to investigate the phenyl ring motions occurring at room temperature in BPA-PC, by using the dipolar rotational spin echo technique [40], Indeed, the reduction in dipolar coupling between carbons and directly attached protons arising from molecular motion (of frequency... 

The frequency heterogeneity of ring motions in BPA-PC is directly shown by the non-exponential decay of protonated aromatic 13C spin-lattice relaxation, either T or T p [39], as illustrated in Fig. 46 for T and T p. From T measurements at 15 and 50 MHz, the occurrence of phenyl ring motions in BPA-PC, at room temperature, with a frequency around 15 x 106 Hz is deduced. From T p measurements, ring motions around 3 x 105 Hz are also present, resulting in frequency heterogeneity over about 1.5 decades. Such... 

The temperature dependence of the phenyl ring motions has also been studied by 13C NMR at two different frequencies, 22.6 MHz [32] and 62.9 MHz [34], through the chemical shift anisotropy of the aromatic proto-nated carbons. 

In order to get a more precise description of the molecular motions occurring in BPA-PC, it is interesting to check whether, in addition to the ring motions about the 1,4 axis already described, main-chain reorientation takes place. 

For this purpose, analysis of the methyl group behaviour is appropriate, as mentioned for 2H NMR investigations (Sect. 5.3.2). Indeed, the ring motions about the 1,4 axis do not drag the methyl groups along, whereas main-chain motions do. 

Fig. 53 Dynamic mechanical loss spectrum of BPA-PC. The solid line is the result of simulation using the phenyl ring motion characteristics (from [34])...

In conclusion, it is unambiguous from the solid-state NMR investigations that phenyl ring motions are involved in the mechanical ft transition of BPA-PC. Additional support for this statement comes from the fact that the position and shape of the mechanical dynamic loss, G", can be well simulated by using the activation parameters and the Williams-Watts exponent deduced from the analysis of the phenyl ring motions [34], as shown in Fig. 53. 

From the various experimental investigations, it is clear that carbonate motions, as well as phenyl ring motions, are involved in the mechanical p transition of BPA-PC. The intermolecular contribution has been evidenced by several authors and the cooperative character of the motions has been pointed out. However, neither of the considered techniques can provide detailed information about the nature of intra- or inter-cooperativity occurring in the glassy state. 

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. 

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