The anomalous solvent effect in the vibrational spectrum of 2,3-diphenyl-cycloprop-2-enone: an experimental and theoretical investigation

2,3-Diphenyl-cycloprop-2-enone (DPC) was investigated aiming to understand the origins of the anomalous solvatochromism in its vibrational spectrum. Its Raman and IR spectra in several solvents were obtained revealing that the nu(C=C) mode is much more sensitive to the solvent than the nu(C=O) mode. Hartree-Fock and density functional theory calculations were undertaken for obtaining the structure and the vibrational spectra of DPC and 2,3-dimethylcycloprop-2-enone (DMC), revealing that both in terms of structure as well as of vibrational spectrum, DPC and DMC are very similar. Such results indicated that DMC could be used as a model system to simulate the solvent effect in DPC, what was done using three different methods: the first takes into account the dielectric effect via the self-consistent reaction field (SCRF); the second is the supermolecular approach that considers explicity the formation of solute-solvent clusters and the third is a combined approach, SCRF + supermolecule. The anomalous solvatochromism in the vibrational spectrum of DPC can be understood on basis of the significant participation of the nu(C=O) mode in the one assigned to the nu(C=C) vibration, as well as by the presence of a Fermi resonance involving the former. In addition, the nu(C=O) mode involves a significant participation of the (C-C) mode (C-cyclopropene-C-phenyl) and the two interatomic distances, C=O and C-C, show opposite trends with increasing solvent polarity. Summing up, the anomalous solvatochromic effect-of DPC can be understood by the complex composition of its nu(C=O) mode that, in addition, is affected by the presence of a Fermi resonance. (C) 2002 Elsevier Science B.V. All-rights reserved.