Ethylene bond rigidity

The shape of the ethylene molecule has been learned by a variety of types of experiments. Ethylene is a planar molecule—the four hydrogen and the two carbon atoms all lie in one plane. The implication of this experimental fact is that there is a rigidity of the double bond which prevents a twisting movement of one of the CHj groups relative to the other. Rotation of one CHt group relative to the other—with the C—C bond as an axis—must be energetically restricted or the molecule would not retain this flat form. 

The separation of formal charges in a polar limiting structure like 2b creates a dipole moment of ca. 20 D. Therefore, if such structures were of great importance, quite high dipole moments should be expected for push-pull ethylenes. Data for a reasonable number of mostly symmetrical and rather rigid compounds are known (Table 20). Several high dipole moments are observed, though not in the vicinity of those required for a complete transfer of the double-bond it... 

The previously mentioned ethylene molecule is planar that is, all 6 atoms lie in a single plane because the double bond is rigid. In Figure 6-13, the stiff double bond prevents the molecule from being twisted around the axis between the carbon atoms. If a reaction substitutes a different atom— like a bromine atom—for two of the hydrogens, the resulting compound can exist in either of two different structural configurations. 

Fumaric and itaconic acids are also used as the diacid component. Most reaction formulations involve a mixture of a saturated diacid (iso- and terephthalic, adipic) with the unsaturated diacid or anhydride in appropriate proportions to control the density of crosslinking (which depends on the carbon-carbon double-bond content of the prepolymer) for specific applications [Parker and Peffer, 1977 Selley, 1988], Propylene glycol, 1,4-butanediol, neopentyl glycol, diethylene glycol, and bisphenol A are also used in place of ethylene glycol as the diol component. Aromatic reactants are used in the formulation to improve the hardness, rigidity, and heat resistance of the crosslinked product. Halogenated reactants are used to impart flame resistance. 

MacGillivray s group has employed rigid bifunctional molecules [42], such as 1,3-dihydroxybenzene and 1,8-naphthalenedicarboxyHc acid [42a], as linear templates to organize reactants such as fra s-l,2-bis(4-pyridyl)ethylene via hydrogen bonds for single and multiple photoinduced [2+2] cycloadditions, for the template-controlled synthesis of a l,2,3,4-(4-pyridyl)cyclobutane] [42b] and also of paracyclophanes [42d]. Very recently, [ ]ladderanes ( =2,3) have been synthesized in the solid state by UV irradiation of 2(5-methoxyresorci-nol) 2(4-pyr-poly-m-ene) (m=2,3) [42e]. 

The effects of limited molecular flexibility and increasing deviation from coplanarity of the anthracene and ethylene ir-systems on the radiative properties have been assessed in a series of symmetrically 2,2-substituted l-(9-anthryl)ethylenes 87 [63]. As for structurally rigid 9-anthrylethylenes 88 and 89, for which rotation about the anthryl-ethylene single bond is not possible, and in which the ethylene double bond has been forced to be coplanar with the anthracene -system, their fluorescence quantum yields in cyclohexane are exceptionally high, i.e., 0.94 and 0.96, respectively, and the Stokes shifts are less than 200 cm 1 (see Figure 15). For nonplanar 9-vinylanthracene 87a and its dimethyl derivative 87b, whose ethylene double bond may be twisted out of the plane of the anthracene by about 60°, the quantum yield is 0.63, and the Stokes shifts are around 1000cm-1 (see Table 16). 

Polyester fibers, similar to polyamide fibers, represent another important family of fiber. Polyester fiber was discovered in England in 1941 and commercialized in 1950. Two common trade names of polyester are Dacron in the US and Terylene in the UK. The term polyester fiber represents a family of fibers made of polyethylene terephthalate. Dimethyl terephthalate is reacted with ethylene glycol in the presence of a catalyst, antimony oxide, to produce polyethylene terephthalate or polyester. The chain repeat structure of PET is given in Fig. 4.6. Although polyesters can be both thermosetting and thermoplastic, the term polyester has become synonymous with PET. Note that the PET chain structure is different from the simpler structure of nylon or polyethylene. In PET, the aromatic ring and its associated C-C bonds provide a rigidity to the structure. The polyester structure is also bulkier than that of nylon or polyethylene. These factors make polyester less flexible than nylon and polyethylene, and the crystallization rate of PET slower than that of nylon or polyethylene. Thus, when polyester is cooled from the melt, an appreciable amount of crystallization does not result. 

In the lowest ionized state of ethylene and acetylene, a n electron belonging to the double or triple C—C bond is removed. It is interesting to compare the calculations reported in Table 6 where all electrons were included in the SCF treatment and the results of non-empirical calculations limited to the n electron system. Approximating the interaction of the Jt electrons with the a core by a rigid GMS potential (see Sect. 5.1) and taking an effective nuclear charge equal to 3.18 for all the atomic... 

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