Rotational investigation of Biarylic Thienyl Pyridines and their Monohydrates: The role of the S···N intramolecular interaction

• Rotational-computational investigation of biarylic thienyl pyridines in the gas phase • Chirped-pulse broadband microwave spectroscopy in the 2-8 GHz cm-wave region • Observation of two Z and E conformations for 2-(2-thienyl)pyridine and 2-(2-pyridyl)benzothiophene • Observation of the Z -oriented monohydrates, stabilized by a O-H···N hydrogen bond • DFT and ab initio calculations of the monomers and monohydrates We used jet-cooled broadband rotational spectroscopy and quantum mechanical calculations to study the potential energy surface, molecular structure and intra- and intermolecular interactions of the biarylic thienyl pyridines of 2-(2-thienyl)pyridine and 2-(2-pyridyl)benzothiophene and their monohydrates. Two isomers of the bare molecules were identified in the gas phase, characterized by planar structures and zusammen ( Z ) or entgegen ( E ) orientations around the ring junction. A single Z -isomer was observed for both monohydrates, primary stabilized by a hydroxyl-to-nitrogen (O-H···N) hydrogen bond and secondary C-H···O interactions. The computational study included D3 dispersion-corrected hybrid (B3LYP) and double hybrid (B2PLYP) density functional methods, with additional calculations at the RI-MP2 and DLPNO-CCSD(T) levels. NBO calculations examined the donor-acceptor hyperconjugative effects involving the nitrogen and sulfur atoms, suggesting that their participation in the larger stability of the Z form is not decisive and may involve other intramolecular interactions. In particular, examination of the electronic density shifts (EDS) further suggests that non-covalent N···S chalcogen interactions partially contribute to the preference for the Z conformation.