Conformational equilibria and interaction preference in the complex of isoprene–maleic anhydride

Abstract

The rotational spectrum of the isoprene–maleic anhydride complex has been investigated by pulsed jet Fourier transform microwave spectroscopy and interpreted with complementary quantum chemical calculations. Theoretical predictions have yielded four plausible isomers, all residing within an energy window of 12 kJ mol⁻¹. However, two distinct isomers characterized by a π–π stacked configuration have been experimentally observed in pulsed jets, which have differed in the orientation of isoprene over maleic anhydride. The relative population ratio of the two detected isomers has been estimated to be N_I/N_II ≈ 3/1 from rigorous measurements of the relative intensity on a set of μ_c-type transitions. Remarkably, this study underscores the pivotal role played by the interaction between the CC bonding orbital (π) of isoprene and the CC antibonding orbital (π*) of maleic anhydride in stabilizing the target complex.