Packing phenyl unit

Fig. 7 a, b. Possible models of packing, for trans-planar chains of s-PS, for the limiting ordered modifications a) hexagonal (a") the relative heights of the centers of the phenyl rings are indicated in c/6 units [28] b) orthorhombic (P") the relative heights of the centers of the phenyl rings are indicated in c/4 units [7, 29]... 

A substitution of the methyl group in the para-position (Fig. 36) causes a larger expansion of the unit cell along c. The crystal packing of the resulting structure is still an efficient one, however the p-substituted methyl introduces already some steric hindrance into the structure the shortest distance of the methyl from its surroundings (adjacent phenyl walls ) is 3.56 A as compared to the normal van der Waals methyl — — phenyl distance of about 3.7 A49>. This steric misfit could be responsible for the preferential complexation of host 25 with m-cresol rather than with p-cresol. 

Various experimental techniques (dielectric relaxation, dynamic mechanical analysis, 1H, 2H and 13C solid-state NMR) have been used for investigating the secondary transitions of BPA-PC, and the block copolymers of BPA and TMBPA carbonates as well as compatible blends of BPA-PC and TMBPA-PC. They have provided lots of information on the motions of methyl, phenyl ring and carbonate units in bulk BPA-PC. The effect of intermolecular packing has also been clearly evidenced. 

A vast range of materials have been bonded to silica such as phenyl, cyano, nitro, amino, and diol functionalities. The variety of silica-based sorbents are listed in the United States Pharmacopeia (USP) [23] where it is noted that of the 33 column packings, 19 are silica based. The synthesis and characteristics of over 80 nonconventional bonded silicas have been reviewed [24]. In spite of the number of chromatographic packings, 50% of the chromatographers develop methods using a C18 sorbent [1]. Sorbents... 

---