Many MO calculations have been carried out on thiophene and the nonclassical thienofuran, thienopyrrole and thienothiophene. Triplet ground states were predicted for these heterocycles. The theoretical controversies over sulfur d- orbital participation in these systems provided an added impetus to the experimental chemists and these nonclassical heterocycles became the objects of chemical investigation since 1967 (75ACR139). [Pg.1058]

Quantum calculations were also used to estimate reactivities151-187 and to obtain theoretical electronic spectra15 -162 of thienothiophene isomers and dithienothiophenes.123,162 [Pg.168]

In 1949 Cox and co-workers ° from X-ray structural analysis and quantum-mechanical calculations suggested that thieno[3,2-b]thiophene (2) possesses a ground state intermediate form between the extremes with completely localized double bonds and that with complete delocalization of TT-electrons. The discrepancy between the theoretical and experimental (Fig. 1) values (0.05 A) for the central C,—Cg bond was noted, and the thienothiophene (2) molecule was shown to be planar and to have a center of symmetry. [Pg.165]

The reactivities of isomeric thienothiophenes calculated in n -electron approximation by the PPP method, and those calculated considering all valence electrons, show reasonable agreement. It should be noted, however, that the choice of parameters in PPP calculations is somewhat arbitrary, especially for heavy atoms (e.g., sulfur). This may lead to a discrepancy between theoretical (in 7r-electron approximation) and experimental estimation of reactivities. For example, Clark applied the semiempirical method PPP SCF MO to calculate the reactivities of different positions in thienothiophenes 1—3, thiophene, and naphthalene from the localization energy values and found the following order of decreasing reactivity for electrophilic substitution thieno[3,4-b]-thiophene (3) > thieno[2,3-Z>]thiophene (I) > thieno [3,2-b]thiophene [Pg.187]

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