Electronic and geometrical structure of methylthiophenes and selected dimethyl-2,2'-bithiophenes

The ionization energy and the attachment energy values of 4,4?-, 3,4?-, and 3,3?-dimethyl-2,2?-bithiophenes have been recorded by means of photoelectron He I and electron transmission spectroscopy, respectively. The data obtained, together with those previously reported for thiophene, 2- and 3-methylthiophene, and 2,2?-bithiophene, are discussed with the help of ab initio 3-21G* and semiempirical calculations. Ab initio data closely reproduce ionization energy trends, while experimental values for virtual molecular orbitals (MO's) compare better with MINDO/3 results. At the ab inito 3-21G* level, gaseous 2,2?-bithiophene and its 4,4? and 3,4? derivatives have two stable rotamers (anti, ? = 140-150°; syn, ? = 40-50°), while in 3,3?-dimethyl-2,2?-bithiophene the two rings are nearly orthogonal to each other. Conformational changes cause variations in the electronic structure clearly visible in the ionization and attachment energy spectra.