Rotational Spectroscopic Evidence for a π→π*(C═O) Interaction in a CO₂-Conjugated π System

The rotational spectrum of the isoprene-CO₂ complex was investigated using pulsed-jet Fourier transform microwave spectroscopy, complemented with quantum chemical calculations. Under supersonic jet expansion, only a single isomer was detected, in which CO₂ resides above the conjugated C═C-C═C plane of isoprene, engaging in a C···π contact with the delocalized π system. Complementary theoretical approaches─including the independent gradient model based on Hirshfeld partition, the extended transition state-natural orbitals for chemical valence, and sobEDAw energy decomposition analyses─were employed to elucidate the nature of the intermolecular interactions. These combined experimental and theoretical results provide the first direct spectroscopic evidence that the conjugated π system of isoprene donates electron density into the π*_(C═O) orbital of CO₂. This observation confirms the existence of a π→π* noncovalent interaction. The findings expand the understanding of CO₂-involved π···π* interactions and offer molecular-level insights relevant to the rational design of materials for CO₂ capture, activation, and conversion.