Head-to-tail chains

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]. 

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. 

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... 

Automated solid phase synthesis, which has enabled synthesis libraries of peptides (15) and nucleic acids (16) and their use in drug discovery, is emerging as a useful tool for carbohydrate production (17). This technology has been difficult to develop largely due to the aforementioned inherent complexity of carbohydrates (18). Both nucleic acids and proteins can be assembled only in head-to-tail chains, whereas a chain of carbohydrates has the potential to elongate at any one of five different points on the sugar and these linkages can... 

The propagation mechanism for the free-radical polsrmerization of methacrylic monomers involves the head-to-tail chain addition of monomer imits to grow the polymer chain. Chain termination can occur by either radical combination or disproportionation (41). 

Chemical defects, e.g., head-to-head linking in an otherwise head-to-tail chain. A typical case is the real structure of the chain of polyvinylfluoride (CH2-CF2), which contains a sizeable fraction of undesired head-to-head defects [89]. Once the chemistry of a given polymer is approximately known, other kinds of defect structures can be envisaged. A typical case is often found for isotopically substituted chains when substitution is not ideally complete. 

This is substantially but not entirely correct for several reasons. Firstly, these bipolarons and other charge carriers are not truly mobile, coasting along from one end of an infinite polymer chain to another, as in the idealized 1-d conduction model. Rather, they are localized or confined by features such as defects or discontinuities in the extended conjugation (for example an sp defect in the idealized sp extended conjugation of P(Ac) or a cross link or ortho branch in the idealized head-to-tail chain of P(ANi)), and by attraction to dopant counterions which pin them down. 

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