Thiophene bonding

Figure 8 A comparison of benzo[b]thiophene bond lengths (A). 

The more appropiate axis choice for CpM(CO)2 is one in which the 2-axis is along the future Fe-thiophene bond. The mirror plane of this complex lies in theyz plane. The [CpFe(CO)2l fragment of 67, point group symmetry has at low energy three occupied metal-based orbitals (see left side of Figure 2) 17a (2, 3% yz, 18% —y, 52%), 18a ... 

Essentially following the procedural stages outlined in Section 5.8, the authors then computationally investigated the energetics of the conformations observed in the six polymorphic forms. Ab initio calculations at the RHF/6-3IG level were used to explore the conformational energy profile about the N-C (thiophene) bond. The... 

Chromene (95) undergoes a photochromic reaction at room temperature which is dependent on the thiophene bond structure <92CL2257>. A series of papers have reported photochemical ring closure of 3-chlorothiophenes to produce complex polycycles (Equation (2)) <92JHC1613,93JHC453,... 

As a rule, Eg decreases as the quinoid character of the polymer backbone increases. The low Eg of polyisothionaphthene, a non-classical polythiophene derivative, results from the presence of the benzene ring which is built on the thiophene along the thiophene bond and stabilizes some quinoid contribution into the ground-state geometry. 

Reduction of Steric Clashes Between Side Chains and the Polythiophene Backbone. Each side chain has steric contacts with the lone pairs on the sulfur atom in the backbone (Figure 1). These steric interactions induce rotation about the thiophene-thiophene bonds and decrease k orbital overlap along the backbone. Head-to-head couplings involve two side chain-sulfur lone pair interactions and result in greater steric strain.(30-32,58) There are three main methods for decreasing the steric interactions between the side chains and the polymer backbone. The first involves eliminating head-to-head couplings within the polymer. The second approach is to use side chains that are sterically less bulky than alkyls (e.g. alkoxy). 

Figure 5.4 Structure of thiophene (bond lengths in pm, bond angles in degrees).

Trifluoromethylpyranopyrazole 121 reacted at C(6) atom with excess of methyl mercaptoacetate in the presence of triethylamine to form a derivative 122 of trifluo-romethyl thiophene bonded with a pyrazole fragment. The reaction took place via pyran ring opening and intramolecular aldol-type condensation [71],... 

Figure 7. The three principal thiophene-thiophene bond forming reactions used in the synthesis of unsubstituted thiophene oligomers. The third reaction employs a palladium complex as a catalyst.

T system, the PPP a—-tt method (133). The -tt and a net charges and bond orders of thiophene and thiazole are compared in Table 1-5. Whatever the method considered the variation of the indices occurs in the same sense when passing from thiophene to thiazole the replacement in the 3-position of a carbon atom by a nitrogen induces... 

Summarizing, the introduction of nitrogen at the place of C-3 in thiophene does not deeply disturb the electronic environment of the sulfur atom, but it induces in the rest of the molecule some alternating modification of the electronic density (Figs. 1-3 and 1-4). The perturbations induced by the nitrogen in the tt bond order of thiophene are... 

Fig. 1-6). The structure obtained for thiazoie is surprisingly close to an average of the structures of thiophene (169) and 1,3,4-thiadiazole (170) (Fig. 1-7). From a comparison of the molecular structures of thiazoie, thiophene, thiadiazole. and pyridine (171), it appears that around C(4) the bond angles of thiazoie C(4)-H with both adjacent C(4)-N and C(4)-C(5) bonds show a difference of 5.4° that, compared to a difference in C(2)-H of pyridine of 4.2°, is interpreted by L. Nygaard (159) as resulting from an attraction of H(4) by the electron lone pair of nitrogen. 

Rg. 1-7. Molecular structures of thiophene and 1,3,4-thiadiazole bond lengths in A (left), bond angles in degrees (right). 

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

In valence bond terms the mesomers indicated by (1—7) reflect the ground-state position of thiophene. Mesomer (1) is the principal contributor to the ring stmcture (2) and (3) are significant (4—7) contribute in a minor way to the stmcture. 

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