Vibrational spectroscopy and ab initio MO study of the molecular structure and vibrational spectra of α- and γ-pyrones

The molecular structures and vibrational spectra of the monomeric forms of alpha- and gamma -pyrones were investigated by ab initio MO calculations, undertaken at the HF/6-31G* level of theory, and vibrational spectroscopy, including matrix-isolation FTIR spectroscopy. A complete assignment of the vibrational spectra of the studied compounds isolated in an argon matrix, at 8 K, or in the condensed phases, at room temperature, is presented and the vibrational data correlated with some important structural parameters. It is shown that the intermolecular interactions in the room temperature condensed phases do not affect the structure and vibrational properties of the studied molecules strongly, though the vibrational results clearly reveal minor changes induced in the carbonyl groups that indirectly affect the electron distribution in the whole pyrone rings, leading to an increase in their pi -electron delocalization. For the isolated monomers, both structural and vibrational results point to a relatively weak pi -electron delocalization in both alpha- and gamma -pyrone moieties. (C) 2001 Elsevier Science B.V. All rights reserved.