Conformation and electronic structure of 3-methylchalcogenothiophenes

The conformation and the electronic structure of 3-methoxythiophene (THOME) and 3-methylthiothiophene (THSME) have been determined by means of a multidisciplinary approach based on ultraviolet photoelectron and electron transmission spectroscopies and fully optimized ab initio 6-31G** and MP2/6-31G** calculations. The plots of the relative energy as a function of the X-Me torsion angle of THXME show that i) for both compounds the s-cis rotamer is an energy minimum lower in energy than the s-trans conformer; ii) the s-trans rotamer is a local minimum for THOME but a maximum for THSME; iii) the gauche conformer is calculated to be the lowest minimum for THSME but close to a maximum for THOME. These findings are rationalized in terms of a delicate balance of several effects such as the steric hindrance between the methyl group and the faced C-H group of the ring, the π*ring ← Xlp charge transfer interaction, the repulsion between the charge in the C2=C3 double bond and the in-plane heteroatom lone pair, the πring - Xlp interaction and C2-CMe electrostatic interaction. The possibility that these compounds can give rise to highly conducting polymers is briefly discussed