Molecular structure and non-covalent interaction of 2-thiophenecarboxaldehyde and its monohydrated complex.

The rotational spectrum of 2-thiophenecarboxaldehyde was investigated by using supersonic jet Fourier transform microwave spectroscopy. The measurements were extended to the S, S, C, and O isotopologs for the cis conformer, as well as to the S and C isotopologs for the trans conformer, leading to an accurately structural determination of the two observed conformers. The unchanged experimental P values upon isotopic substitution indicate effective planar geometries of the two conformers. The ring structures of thiophene are slightly different between the cis and trans conformers. Two isomers of the monohydrated complex of 2-thiophenecarboxaldehyde, formed between a cis or trans monomer with water stabilized by an O-H⋯O hydrogen bond (HB) and an additional (C=O)CH⋯O(HO) or (C)CH⋯O(HO) HB, respectively, were observed in jet expansion. The noncovalent interactions attributed to the stabilization of the monomer and the monohydrated complex are revealed by quantum chemical methods. The interaction energy for trans-w-1 is 4 kJ mol larger than that of cis-w-1, attributed to the relative stronger CH⋯O HB. The relative abundance of the two conformers of the 2-thiophenecarboxaldehyde monomer and the two isomers of the complex was estimated in the jet.