Stacking state

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 r1L and 7js components of the spectra obtained are shown in Fig. 33. The vij of the T1L component is 114 kHz and is larger by 5 kHz than that of PBLG-fd2 at 19°C. This suggests that the amplitude of the libration in the stacked state is smaller than that in the free state. The Fq value of the 7js component is 109 kHz and is similar to that of PBLG-fd2. This is compatible with the assumption that the 7js component corresponds to the side chains in the free state. The fraction of the 7jL component is estimated to be about 60% from the integrated intensity of the spectrum at 19°C, which is comparable with the value estimated from T. ... 

The amplitude and correlation time tc of the libration in the stacked state were estimated by using the diffusion in a cone model. The semiangle of the cone 6c at 20° C were obtained to be 22° and 26° for the TiL component and PBG- d2 and PBLG- d2, respectively. The tc values were obtained to be about 10 12 s at 19°C in both states. The 6C of r1L component is 4° smaller than that of PBLG- d2, showing that the amplitude of the libration in the stacked state is smaller than that in the free state due to the steric hindrance between the adjacent side chains forming the stacking... 

The riL value of racemic PBG-ytL is several hundred ms and decreases gradually as temperature increases. These results show that the side chains participating in the stacking structure are not rigid. Tx at the y position below room temperature is governed by the rapid and small-amplitude libration as shown in Section 4.3. The libration is expected to exist in the stacked state as well as the free state. riL is considered to be governed by libration in the stacked state. In order to estimate the amplitude of the libration in the stacked state, we extracted the Tu component of the spectrum in the same way as that used for racemic PBG- d2. The spectra of the T1L and T, s components are shown in Fig. 37. The vq value of the r1L component is... 

The amplitude and correlation time rc of the libration in the stacked state are estimated by using the diffusion in a cone model. The semiangles of the cone 6C are obtained to be 17°, 18°, and 19° for the riL component at 20, 40, and 60°C, respectively, while 20°, 22°, and 22° for the PBLG-yd2 at 23, 40, and, 55°C, respectively. The 6C in the stacking structure is smaller by 3° to 4° than that in the free state at the y position. Comparing the 6C value at the y position with that at the position in the stacked state, the 6C value at the y position is 5° smaller than that at the position at 20° C. This is because the Cf 2H bond is located at a more terminal part of the side chain than is the Cy 2H bond. The rc values were obtained to be about 10 11 s for both stacked and free states in this temperature range. There is little difference in the correlation time between the stacked and free states. 

Later, Kondo and Danyluk (19) postulated the existence of two stacked conformations, I and II, I depicted as the regular right-handed helix (o), a)/330°,320 ), II as a "more loosely" base-stacked left-handed loop structure with u, (o/50, 80°. Shortly thereafter (20), II was described as to, u/80°,80, a linear form having u, o)/270, 90 was also postulated. Subsequently, the same authors (21) reversed the usual assignment of the 5 and 5" signals of the Xp- moiety. This reversal has now been recalled (24), concomitant with the introduction of a third stacked conformation stacked forms I, II and III supposedly in equilibrium with a single ( ) unstacked or extended form in solution. These three stacked states are supposed to be characterized by ... 

In the following the concentrations C, the mole fractions F and the physical property monitored a of the non-associated species carry the subscript F, those of the associated species the subscript A. Similarly, subscripts u and x denote unstacked and stacked states respectively. For the high-temperature limit 1 and always Oy (assumption b). 

Generally, for condensed phase flame retardancy, the morphology of the char residue may help to clarify the combustion mechanism. Figure 11.7 presents TEM and SEM microphotographs of PP/1.0 wt% Ceo nanocomposites after cone calorimetry. Interestingly, compared with the pristine Ceo crystallites with a size of around 100 nm, after combustion the sizes were much smaller, 30-50 nm, though well proportioned. An explanation was that the combustion of polymers at elevated temperatures would destroy or disorder the stack state of Ceo crystals, making the size of crystals tend to be the same. 

A possible explanation of the incremental upfield shift is based on the assumption that with rising numbers of single-strand nucleotides the stacking order of the preceding (5 -neighbouring) nucleotides are successively enhanced. This enhanced stacking order, which can be visualized by an increase of the residence probability in the ideally stacked state, also affects the terminal base pairs of the stem and the stability of the duplex as a whole. 

---