Thiophene, molecule

Another group of conjugated thiophene molecules for future appHcations are those being developed as nonlinear optical (NLO) devices (75). Replacement of benzene rings with thiophene has an enormous effect on the molecular nonlinearity of such molecules. These NLO molecules are able to switch, route, and modulate light. Technology using such materials should become available by the turn of the twenty-first century. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Generation and trapping of tropothione 29 with dienophiles has been utilized in an approach to partially saturated fused thiophene molecules. For example, treatment of tropone with Lawesson s reagent in benzene, followed by introduction of jV-phenylmaleimide gave the adduct 30 in excellent yield <06TL9329>. 

A study of the geometric relationship between the thiophene molecule and the catalysts suggests that a two-point absorption reaction mechanism is involved and could be represented either as in A or B ... 

Fig. 1. Bond lengths (A) and angles of the thieno[3,2-A]thiophene molecule.

Thiophen, however, gives the complex [Cr(C4H4S)(CO)3] in which the thiophen molecule contributes four --electrons from the double bonds and two electrons from the sulfur atom (88). 

Fig. 17. STM image of thiophene on the triangular M0S2 nanoclusters at a sample temperature below 200 K. Thiophene molecules are evident in positions on top of the bright brim associated with an edge state (dark circles), and additionally thiophene decorates the perimeter of the cluster (gray striped circle). For clarity, the color scale in this image is circled twice to enhance contrast. At temperatures > 240 K, no indications of adsorbed thiophene were observed with STM.

Fig. 23. STM image of a M0S2 nanocluster first exposed to atomic hydrogen, then thiophene and subsequently imaged at 240 K in cold STM. The protrusion in the circle may represent a thiophene molecule or a hydrogenated derivative coordinated through the terminal sulfur atom to a sulfur vacancy. Reproduced from Reference (131) copyright (2004) with permission from Elsevier.

As an example, starting assumptions and the result of an HMO treatment of a model of the thiophene molecule are presented ... 

Perhaps the largest discrepancies in reported results are the relative values for the adsorption constants of H2S and thiophene molecules (THs, including thiophene, benzothiophene, and dibenzothiophene). The reported preference for adsorption on the direct desulfurization site ranges from H2S THs (122,123,125) to about the same (104) to H2S < c THs (125). 

Turning now to activation of the thiophene molecule, it has been shown that 2,5-dimethoxythiophene reacts with maleic anhydride to give the bis-adduct (293) in 42% yield (75JCS(Pl)2483). Initial addition of maleic anhydride followed by the usual sulfur extrusion leads to a diene, which then adds another molecule of maleic anhydride (Scheme 81). 

The calculations indicate that when Al-atoms approach thiophene rings, they preferentially interact with the carbon atoms located in the a-positions relative to the sulfur atom. In the case of the thiophene molecule interacting with two Al-atoms, each metal atom forms a single Al-C bond at these positions. 

The DFT-derived Fukui indexes as reactivity descriptors make it possible to predict the preferential sites of electrophilic attack on pyrrole, furan and thiophene molecules. Their relative reactivities depend on the local softness of the most reactive site in each system. The results obtained by this approach are in total agreement with experiment (05MI1). 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

The role of the catalyst from the perspective of the majority of thiophene molecules is not to activate thiophene. Rather, its function is to line up weakly bound (and, therefore, mobile) thiophene molecules for subsequent reaction with hydrogen atoms generated at the active site. It is possible that the main function of the catalyst is not to activate thiophene molecules but rather to dissociate dihydrogen molecules, thus generating mobile H atoms, the concentration of which... 

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