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A thermochemical investigation of singlet dication (bipolaron) structures in a set of 3-methyl-thiophene oligo- mers (up to the length of 24-mers) was carried out. Equlibrium structures and heats of formation were calculated with the semiempirical quantum chemical method RHF/PM3 for the doubly oxidized forms. All-cisoid planar structural variants of the form An1Qn2An3 were considered, where A and Q are thiophene monomer units in aromatic and quinoidal chain segments, respectively. The types of segments were determined from their bond-length pattern. At this level of theory, di- verse structures combining both types of segments were found stable. The so-called "response equation based quantitative structure-property relationship" (REQ-QSPR) method, recently pro- posed by Fishtik et al. was used to perform and interpret regression data analysis. The REQ-based approach rationalizes mathematical treatment of stoichiometric equations. It provides a simple concept for breaking down a dataset for heat of formation into linear combination of terms that are easy to interpret in the framework of chemical groups. REQ-QSPR in- cludes arbitrary structural descriptors rather than being restricted to using only the number of chemical constituents. Ap- plied to our system, it is demonstrated that modelling nonlinear effects becomes possible as well. Supra-linear dependence of the energy with respect to the bipolaron localization length is indicated by REQ-QSPR, and detailed analysis suggests specific further calculations to carry out for improving the model.

Response Equation Based Thermochemical Analysis of Singlet Bipolaron Structures in Oligo(3-Methyl-Thiophenes)