Probing Aromaticity with Supersonic Jet Spectroscopy: A Case Study on Furan, Thiophene, and Selenophene.

Aromaticity is a century-old concept that is even introduced in high school textbooks. However, the determination of the order of aromaticity of molecules as simple as furan, thiophene, and selenophene is still challenging. This work describes how different theoretical and experimental methods posit different aromaticity orders. To benchmark the theoretical results and arrive at a conclusion, mass-selective electronic and vibrational spectroscopy of these five-membered heterocycles under isolated supersonic-jet-cooled conditions was necessary. Since the aromaticity order can be unveiled from the magnitude of the electron density in the ring, we used hydrogen bonding as a probe. The experimental results revealed that selenophene forms the strongest π-hydrogen bond, suggesting that selenophene is the most aromatic, followed by thiophene and furan. It is concluded that gauge-including magnetically induced currents (GIMIC) and relative ¹H and ¹³C NMR chemical shifts are better parameters to determine the aromaticity order in a similar class of molecules.