Evidence of an isomeric pair in furan...HCl: Fourier transform infrared spectroscopy and ab initio calculations.

For the first time the coexistence of a σ- and a π-complex in the C₄H₄O:HCl system has been observed, in the same supersonic expansion of a molecular jet seeded with argon (or helium) or in a flow-cooled cell at 240 K. This is an exception to the third of the Legon–Miller rules which claims the σ-structure to be the only one to exist. On the grounds of energetic considerations and band contour simulations, two observed bands at 2787.7 and 2795.5 cm^-1 of the ν_s HCl stretching frequency are assigned to the two complexes, recorded as Fourier transform infrared spectra with a resolution between 0.2 and 0.5 cm^-1. Complementary calculations show that the use of the standard second-order Mo\ller–Plesset perturbation theory may be erroneous for such a complex, due of the overestimation of the dispersion contribution with respect to the electrostatic term. It is finally established that only a balanced version of the second-order Mo\ller–Plesset perturbation method, spin-component scaled-MP2, or a higher level of theory like a coupled-cluster approach, can provide a reliable energetic analysis for this complex. [ABSTRACT FROM AUTHOR]