Thienothiophenes dipole moments

Dipole Moments of Isomeric Thienothiophenes (1-3) SeLENOPHENOSELENOPHENES (11-13) AND Selenophenothiophenes (14-16)... 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Whereas the dipole moments of thieno[3,2-b]thiophene (3) and selenolo[3,2-6]selenophene (4) are 0.00 D, the mixed system selenolo[3,2-6]thiophene (29) exhibits a dipole moment of 0.30 D. Perturbation of the symmetry of (3) by the introduction of an ethyl group in the molecule at C-2 generates a dipole moment of 0.30 D. A study of the other classical thienothiophenes, selenoloselenophenes and selenolothiophenes shows that the [2,3-6]-annelated systems exhibit a slightly higher dipole moment compared to the [3,4-6]- or [2,3-c]-annelated systems (76AHC(19)123). In the case of the nonclassical thiophenes (Id X = Y = S), (Id X = S, Y=NH) and (Id X = S, Y = 0) the predicted dipole moments are 0.00, 0.15 and 3.21 D respectively (74JA1817). Experimental verification is not possible since none of these compounds are known although it should be of interest to determine the dipole moments of the tetraphenyl derivatives (6) and (12) and the pentaphenyl compound (13). 

D. It was found that the [2,3-6]-annelated parent thienothiophenes, selenoloselenophenes, and selenolothiophenes exhibit higher dipole moment values than the [3,4-6]- or [2,3-c]-annelated systems <84CHEC-I(4)1037>. Other parent diheteropentalene dipole moments do not appear to have been measured. 

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