Intramolecular and intermolecular geometry of thiophenes with oxygen-containing substituents

The intramolecular and intermolecular geometries of six thiophenes carrying oxygen-containing substituents have been determined. Crystals of 2-methoxy thiophene and 3-methoxythiophene were grown in situ. on a diffractometer fr om liquid samples. The 2-methoxy group introduces significant distortions t o the thiophene nucleus and each molecule participates in four S ... O cont acts leading to an infinite bilayer. The extended structure of 3-methoxythi ophene comprises zigzag chains of molecules linked by S ... O contacts. Mol ecules of 2-acetyl-3-methoxythiophene are arranged in pairs about inversion centres, with ring centroids 3.835 Angstrom apart. 5-Cyano-3-hydroxythioph ene adopts the hydroxythiophene tautomeric form which allows conjugation be tween the S atom and the nitrile group: O-H ... N hydrogen bonding leads to chains which are crosslinked by S ... O contacts to give infinite two-dime nsional layers. 5-(Methylthio)thiophen-3(2H)-one exists exclusively as the thiophen-3(2H)-one form in the solid state. allowing maximum conjugative in teraction of both the ring heteroatom and the substituent with the carbonyl group: each molecule is linked to two of its neighbours through pairwise C -H ... O interactions, forming ribbons. Spiro[cyclohexane-1,2'-2',3'-dihydr othiophen]3'-one crystallizes with four independent molecules in the asymme tric unit with only minor differences between these: the five- and six-memb ered rings in each molecule are approximately orthogonal and C-H ... O hydr ogen bonding generates chains. The last two structures differ from the othe rs in that they lack a fully aromatic thiophene system.