title: On the importance of electron correlation effects for the intramolecular stacking geometry of a bis-thiophene derivative.
abstract: The structure of dithienobicyclo[4.4.1]undeca-3,8-diene-11-one ethylene glycol ketal (database code RESVAN) was determined using the wave function theory (WFT) as well as density functional theory (DFT) methods combined with various Gaussian AO basis sets. The apparently most accurate procedure, employing the CCSD(T)/complete basis set (CBS), provides an S-S distance and an angle between the two thiophene rings which differ considerably from experimental values. The best agreement with the experimental data among all WFT methods was surprisingly obtained at the MP3/aug-cc-pVDZ and MP3/CBS(B) levels (the correction term to CBS was obtained by the aug-cc-pVDZ basis set). The very good results obtained by the CCSD(T)/6-31G* method are clearly a consequence of fortunate error compensation. MP2 calculations, even with a small basis set, overestimate the attraction between the thiophene rings, and the worst agreement with experimental data was found in full MP2/QZVP method optimizations (i.e., a strong distortion of the thiophene rings was observed). The SCS(MI)-MP2 and SCS-MP2 methods exhibit improvement over the MP2 procedure. All standard DFT approaches fail to predict reasonable S-S distances. The lack of intramolecular London dispersion energy results in too great distance between the thiophene rings. Much better agreement with experiment was obtained if advanced DFT methods, covering dispersion effects, were used. The best results were obtained at the TPSS-D/TZVP, M06-L/TZVP and B2PLYP-D/def2-TZVP levels. When a larger basis (LP in the case of TPSS functional) or more advanced versions of the new Truhlar functionals (M06-2X) was used, the agreement with experiment deteriorated. The accurate description of this molecule is highly functional/basis dependent and this dependence is hardly predictable. To estimate effects of neighboring molecules in the experimental crystal structure, an optimization in the electric field of the 26 closest RESVAN molecules was performed, which, however, leads to only moderate (<0.05 A) changes of the S-S distance.
authors.affiliation: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, 166 10 Prague, Czech Republic.
authors.forename: Kristýna
authors.initials: K
authors.lastname: Pluhácková
authors.forename: Stefan
authors.initials: S
authors.lastname: Grimme
authors.forename: Pavel
authors.initials: P
authors.lastname: Hobza