Transitions, gauche/trans

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

Molecular dynamics simulations [69] of citric acid in water at room temperature do show intermolecular hydrogen bonds to the solvent molecules along the entire trajectory [54]. To observe a gauche/trans-transition (T3) within a time-frame of a few 100 ps, the simulation temperature has to be elevated. Computational studies including a molecular environment capable of forming hydrogen bonds clearly are more relevant than gas-phase calculations in the search for biologically active conformations. 

Besides an intrinsic temperature dependence, gauche/trans transitions were discussed for the contrasting directions of shift. Other authors felt that the situation is even more complex and tried to fit the values for n-alkanes to a larger set of empirical increments. 

Fig. 23. Optimized geometries computed at MC-SCF/STO-3G for the gauche-trans transition state and for the pauc/ie-cyclobutane transition state. The values in brackets are the dihedral angles HCCC and H CCC. (Reprinted with permission from Bemardi et aL Copyright 1985...

It has been shown earlier [78] that an increase in the extended tmns-conformations fraction results in raising of the entanglements cluster network density Hence one may assume that increasing QLj, indicating a rise in gauche-trans transition probability, should also result in increasing In Figure 6.29 the dependence QL ... 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

Other Polysilylenes. The symmetrically substituted poly(di-n-tetra-decylsilylene) is reported to have a TGTG trans-gauche-trans-gauche ) conformation, and a bathochromic shift is observed in the UV spectrum at 54 °G with Xjnax shifting from 322 to 350 nm (35). The structures of many polysilylenes may be more complicated than what have been discussed thus far. As an example, the DSG data for PMHS are shown in Figure 24, The figure shows that Tg (glass transition temperature) is 220 K and that two... 

The temperature-dependent spectra were interpreted in terms of a two-site hop model, in which the deuterons undergo jumps through a dihedral angle of 103°. This type of motion is consistent with gauche-trans conformational transitions. At -88"C these motions appear static on the time scale of the deuterium NMR experiment, and at +85 °C the motions are in the fast exchange limit. The rate constants for these motions were obtained from the calculated spectra. An Arrhenius plot of these data show that the apparent activation energy is 5.8 kcal/mol. (Dynamic mechanical data (20 Hz) fall on the Arrhenius plot.) The transitions have an intermediate rate on the deuterium NMR time scale at 20 °C, with the correlation time for the motion being 7 x 10 6 s at this temperature. 

The IR rheooptical technique was applied to study the molecular processes that take place along the stress-strain curve of PET yams. The results indicate that the first modulus maximum marks the breakdown of the amorphous entanglement network and the start of molecular uncoiling by gauche-to-trans transitions. In addition, stress develops on the crystals and particularly on tie molecules with a short contour length in the amorphous domains. Ultimately, molecular fracture of taut-tie molecules (extended chains of polymers between crystals) causes the modulus to pass through a second... 

The y-transition is a broad relaxation in the temperature- or frequency-domain, interpreted as a localized crankshaft-like motion of the backbone of the chain. This interpretation of the DMA result [51] agrees well with the calorimetry. The calorimetric results were interpreted already in 1962 based on an energy estimation as a local relaxation of gauche conformations in the amorphous phase [53,54]. The broad increases in the heat capacity beyond the vibrational Cp of amorphous PE seen in Fig. 2.46 were interpreted as a gradual, local unfreezing of the gauche-trans equilibrium. 

---