Phase transition temperatures compounds

Compounds undergoing solid-solid phase transitions have been used extensively for temperature calibrations in solid-state NMR, covered later in Section 4.1, but such compounds can also be used for calibrations in the liquid state. When solid, no resonances are observed, but sharp lines appear above the phase-transition temperature. Compounds that have been studied include p-dichlorobenzene (326.4 K), cyclododecane (333.8 K), 2 -hydroxy-4, 5 -dimethylacetophenone (344.2 K), hexamethylcyclohexanetrione-1,3,5 (353.3 K), p-dibromobenzene (360.7 K), p-diacetylbenzene (386.0 K), p-diiodobenzene (402.6 K), 1,2,4,5-tetrachlorobenzene (413.1 K) and hexa-methylbenzene (438.6 K). 

The compound in Fig. 3b exhibits two smectic phases (Sm and Sm ) in addition to nematic, whereas the compound in Fig. 3a exhibits only a nematic phase. The substitution of an alkoxy for an alkyl tail is known to shift phase transition temperatures considerably. In the cyano-biphenyls (Fig. 4), substitution of an alkoxy tail raises the melting point from 24 to 48 °C and T from 35 to 68 °C [22]. 

A mixture consisting of the step 5 product (0.025 mol), 6-bromohexyl acrylate (0.027 mol), K2CO3 (0.043 mol), KI (0.0037 mol), and 500 ml acetone were refluxed at 60°C for 24 hours and then purified as in the manner described in step 1 and 9.8 g of product isolated. The phase transition temperature from the crystal phase to the nematic phase was 105°C where the An of the compound using a laser beam having a wavelength of 589 nm at 80°C was 0.1305 (extrapolation value). 

The liquid crystalline properties of 43a-d were interesting as all derivatives showed very stable mesophases with phase widths of 57-109 K. It is remarkable that Schiff bases 43c,d are stable at temperatures above 300 °C. It also strikes that, in contrast to conventional calamitic liquid crystals, the Schiff bases 43c,d possess significantly higher phase transition temperatures compared to azo linked 43a,b. Additionally, the trans compounds tend to possess higher clearing temperatures due to the elongated shape of the molecules. 

The phase transition temperatures of compounds 7a and 7b are summarized in Table 2. Compound 7a shows a highly viscous smectic (denoted as Sml) phase, smectic (denoted as Sm2 and Sm3) phases with lower viscosity, and an SmA phase. On the other hand, compound 7b shows a highly viscous smectic (denoted as Sm4) phase and a nematic phase. 

It was generally found that double chain ammonium glutamic acid amphiphiles of the general formula 2Cn-Glu-CmN+ (15) typically form typical bilayer vesicles above their phase transition temperature.118 It is interesting to refer to the compound 2Ci2-Glu-CnN+ (15b, n = 12)111,119 in which the predominant superstructures were flexible filaments with... 

In many cases, high-temperature modifications of sulfidic compounds cannot be quenched for room temperature examination. Inversion twinnings, crystal morphology, or other crystallographic features may indicate the appearance of polymorphism. Under these circumstances differential thermal analysis (DTA) can be suitable for the determination of the exact phase transition temperatures. DTA determinations are practically valuable if used in conjunction with high-temperature X-ray diffraction methods. DTA apparatus can operate up to 1100 °C and can be specially designed for sulfides2-4) individual experimental techniques are included in these references. 

The occurrence of well defined sigmoidal variations of the e+ or o-Ps lifetimes with T may fail to be observed. This is the case in Agl, a compound well known to present a transition to an ionic superconducting phase above 423 K, due to the production of large amounts of cation vacancies. However, none of the PALS or DB parameters show evidence of a change at the phase transition temperatures [130], As it seems, the lifetime of the cation vacancy is too short to allow e+ trapping, due to the intense movement of the Ag+ ions from vacancy to vacancy through the lattice. 

TABLE 1. Phase transition temperatures for perfluorooalloyloxy compounds and corresponding optical anisotropy (An) and dielectric anisotropy (As). 

Figure 6.11 shows the activity of an artificial enzyme can be controlled based on the phase behavior of a lipid bilayer. The catalytic site for hydrolysis was supplied by a monoalkyl azobenzene compound with a histidine residue which was buried in the hydrophobic environment of a hpid bilayer matrix formed using a dialkyl ammonium salt. Azobenzene compound association depended on the state of the matrix bilayer. The azobenzene catalyst aggregated into clusters when the bilayer matrix was in a gel state. In contrast, the azobenzene derivative can be dispersed into the liquid crystalhne phase of the bilayer matrix above its phase transition temperature. This bilayer-type artificial enzyme catalyzed the hydrolysis of a Z-phenylalanine p-nitrophenyl ester. The activation energy for this reaction in the gel state is twice as large as that observed in the hquid crystalline state. The clustering of the catalysts upon phase separation suppress their catalytic activity, probably due to the disadvantageous electrostatic environment around the catalysts and the suppressed substrate diffusion. This activity control is unique to such molecular assembhes. 

---