Tails, chain

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

On the theoretical side, Marcelja [26] was first to account explicitly for flexible tail chains in nematic ordering, using the Maier-Saupe model potential (Eq. 1) for each segment of the molecule. More complex models were proposed by Samulski et al. [27] and Emsley et al. [28]. In these approaches alkyl chains are assumed to exist in a discrete set of conformers described by... 

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

Many of the mesogenic molecules are stericaUy asymmetric, which is determined by the fractures and bending of the molecular core as well as by the presence of the tail chains of different nature, including the branched, biforked or polyphilic moieties (Fig. 2c-f). In terms of the multipole model of molecular asymmetry introduced by Petrov and Derzhanski [34], we can speak about longitudinal or transverse steric dipoles or multipoles (Fig. 3). 

The combination of polar and steric dipoles and multipoles, together with the conformational flexibility of the tail chains, leads to the large variety of smectic phases [11-13]. In addition to the monolayer smectic Aj phase, where the molecules are oriented randomly up and down within each layer, there are... 

Fig. 64. Plausible model of the molecular packing of the cast multibilayer of 34 as produced by electron-density matching of the X-ray diffraction data. The plane of the benzene ring is arbitrarily set in the plane of the two alkyl chains. When the benzene plane is assumed to be in the perpendicular disposition, the tilt angles for the spacer and tail chains become slightly greater 50° for the spacer and 75° for the tail [445]...

Natta and co-workers had produced stereospecific polymers. For example, olefins like propylene have been polymerised in such a way as to yield long linear head to tail chains consisting of sequences of monomer units having the same steric structures. These polymers are called isotactic polymers and they crystallise easily, whereas those monomeric units of different steric arrangement phased at random do not crystallise well. These polymers are called atactic. Polymers of regular, alternating structure are called syndyotactic polymers. 

All the oxidants mentioned above possess a damaging effect on the cells. During oxidation of membrane lipids (especially those containing unsaturated fatty acid tails) chain reactions easily appeared, which result in irreversible violation of membrane integrity being inconsistent with viability of the cell. Protein and nucleic acids can be oxidized even earlier... 

Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes" while Qo doping appears to be variable" "". Doping with electron transport materials such as oxadiazoles give polymers with balanced properties for hole transport". The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important" as are head to head and tail to tail chain sequences in thiophene based polymers. Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV s are claimed to be good chemosensors and broad emission is observed from titania doped PPV . Electron rich dopants enhance the emission in the red region while electro and photoinduced infrared bands from PPV are similar . ... 

Iodobenzonitrile and 4-bromobenzonitrile form one-dimensional head-to-tail chains through C=N X interactions, Scheme 2.5.6 [47, 48]. 

Fiffire 2. (a) Stereoview of the crystal structure of urea. Note that the hydrogen-bonded head-to-tail chains run parallel to the c axis and each O atom forms four N-H - O acceptor hydrogen bonds, (b) The hydrogen-bond framework in crystalline urea viewed approximately along the c axis. (Data from ref. 25b). 

In crystalline compound [Fe(77 -CsMes)2][HC204] [H2C204]o.s, the hydrogen oxalate anions and the neutral oxalic acid molecules form linear chains (see Figure 5). The chains run parallel to the columns formed by the [Fe( 7 -CsMes)2] and [Co(77 -CsMes)2] cations, and contain deca atomic dimers formed by two hydrogen oxalate anions joined together via interactions. The anions are not arranged in the usual head-to-tail chain or... 

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