CONFORMATION AND ELECTRONIC-STRUCTURE OF 3-METHYLCHALCOGENOTHIOPHENES

The conformation and the electronic structure of 3-methoxythiophene (T HOME) 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 a s a function of the X-Me torsion angle of THXME show that: (i) for bot h compounds the s-cis rotamer is an energy minimum lower in energy tha n the s-trans conformer; (ii) the s-trans rotamer is a local minimum f or THOME but a maximum for THSME; (iii) the gauche conformer is calcul ated to be the lowest minimum for THSME but is 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 gro up and the faced C-H group of the ring, the pi(ring) <-- X-lp charge transfer interaction, the repulsion between the charge in the C2=C3 do uble bond and the in-plane heteroatom lone pair, the pi(ring)-X-lp int eraction and the C2-C-Me electrostatic interaction. The possibility th at these compounds can give rise to highly conducting polymers is brie fly discussed. (C) 1997 Elsevier Science B.V.