180° Flip motion

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... 

Use of Proton and 13C NMR at temperatures from 27 to 400 °C provide very detailed information as to the nature of these motions [30], Thus, it has been shown that even at 300 °C the phenylene ring displays a rapid 180° flipping motion. Above the transition temperature of 350 °C the ester unit also begins to rotate in the form of 180° flips as a result of lattice expansion (see Fig. 7). Furthermore, the entire repeat unit participates in a synchronous motion. This should be interpreted as a jumping motion rather than free or random rotation. 

Fig. 7.2.2 [Hanl] Solid-echo wideline spectra of the ring deuterons of bisphenyl-a polycarbonate-d at 253 K. The phenyl rings undergo a 180° flip motion with a wide distribution of motional correlation times, (a) Spectrum with signals from fast and slow flipping rings.

Fig. 3.12. NMR spectra simulated for the phenylene ring that undergoes the 180° flip motion with the reduced frequency of k (=kAq ).

Fig. 14.4. (a) 2D solid-state NMR spectra of a bundle of oriented, industrial PET fibers. The diagonal peaks are labeled according to Fig. 14.5. The 180° phenylene motion about the para-axis is reflected in the sharp exchange peak between positions 2 and 3. The high intensity along the diagonal is due to carbon atoms which have not changed their position and, therefore, their frequency during the mixing time r , 1 s. (b) Contour plot of the spectrum shown in Fig. 14.4(a). The 20 linearly spaced lines between 1.5 and 17% of the maximum height of the spectrum indicate clearly the 180° flip motion. In addition, a cut through the spectrum at peak 2 is shown.

Fig. 14.5. Sketch of repeat unit of PET. A 180° flip motion of the phenylene unit changes the orientation between the C—H bond of a protonated aromatic carbon atom and the applied magnetic field. Bo (labeled as sites 2 and 3). Note the difference in orientation of approximately 18° between the fiber axis and the para-axis of the aromatic ring.

The temperature dependenee and the exact geometry of slow molecular reorientations in imidazolium methyl sulfonate have been investigated using modem ID MAS exehange speetroscopy. Earlier high-temperature studies showed a fast 180° flip motion of the imidazole ring, which was shown to slow... 

The temperature dependent T data are shown in Fig. 9. 7j values decrease from 28 ms at 21°C with increasing temperature, and show a minimum of 6.4 ms at 80° C. These results indicate the presence of the motion with a Larmor frequency of 30 MHz at this temperature. This minimum was found to be attributed to the flipping motion of a phenyl ring from the result of our other experiments discussed in later section.13 The jump rates of the flipping motion were estimated with a two-site jump model that a C-2H bond jumps between two equivalent sites separated by 180°, and that the angle made by the C-2H bond and the rotational axis is 60°. The quadrupole coupling constant of 180 kHz and the asymmetry parameter approximated to zero were used in the calculation. The calculated values for fitting with the... 

The line shapes were calculated for the flipping motion with the two-site jump model described above, and the calculated spectra are shown in Fig. 11 for the higher temperature region. The experimental line shapes at 20 and 30° C are well reproduced showing the motional mode and rates obtained by T analysis are reasonable at least around these temperatures. Above 40°C the calculated line shapes are invariable and remain in the powder pattern undergoing a rapid flipping motion, while the experimental ones... 

The temperature dependent T results of racemic PBG-d5 are shown in Fig. 29. As shown in Section 4, T of PBLG-d5 achieves a broad minimum at 80° C, showing that the rate of the flipping motion is as high as the Larmor frequency of 30 MHz at this temperature. The Tx value of racemic PBG-d5 decreases more slowly than that of PBLG-d5 as temperature is raised, and drops steeply around 60°C. The T value of racemic PBG-d5 was found to be same as that for PBLG-ds above 80°C. There is little difference in the T values of both samples even below the transition temperature of 90°C. These differences in... 

T reveal that the reorientation of the phenyl ring is restricted, but that most of the phenyl rings undergo the flipping motion even in the stacked state. This agrees with the result of the temperature dependence of the line shapes and 13C CP/MAS measurements.76... 

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

The ft transition clearly originates from motions of the hydroxypropyl ether sequence, but the crosslink points are also involved. Whereas the motions involved in the low-temperature part of the ft transition are quite isolated motions of HPE units, when temperature increases a cooperativity appears directly with the mobility of the crosslinks and, indirectly, with the jr-flip motions of the DGEBA phenyl rings. 

As the 2D 2H spectra are insensitive to the 77--flip motion itself, the observed exchange intensity can only originate from the rotations around the local chain axis. Indeed, the 2D 13C NMR spectrum of 13C carboxyl (Sect. 8.1.4.1) has shown that n-flips of the ester groups are accompanied by rotations around the chain axis by about 20°. 

In the (T - Ta) range from - 160 to - 90 °C, the larger increase in toughness observed for MT0.5I0.5 than for MI does not agree with the difference in ve (it would go in the opposite direction). Furthermore, it cannot be explained from the yield stress values since they are identical in this temperature range, as shown in Fig. 111. The effect of the intramolecular cooperativity of the n-flip motions of the fere-phthalic rings, already invoked for the results in the low temperature range, appears to be the most likely reason these cooperative... 

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