The anomalous solvent effect in the vibrational spectrum of 2,3-diphenyl-cycloprop-2-enone: an experimental and theoretical investigation L.C.J. Almeida 1 , P.S. Santos * Instituto de Quı ´mica da Universidade de Sa ˜o Paulo, Sao Paulo, CEP, 05508-900, SP, Brazil Received 4 February 2002; accepted 21 February 2002 Abstract 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 n (C Ä/C) mode is much more sensitive to the solvent than the n (C Ä/O) mode. Hartree-Fock and density functional theory calculations were undertaken for obtaining the structure and the vibrational spectra of DPC and 2,3-dimethyl- cycloprop-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 n (C Ä/O) mode in the one assigned to the n (C Ä/C) vibration, as well as by the presence of a Fermi resonance involving the former. In addition, the n (C Ä/O) mode involves a significant participation of the (C à /C) mode (C cyclopropene à /C phenyl ) and the two interatomic distances, C Ä/O and C à /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 n (C Ä/O) mode that, in addition, is affected by the presence of a Fermi resonance. # 2002 Elsevier Science B.V. All rights reserved. Keywords: 2,3-Diphenyl-cycloprop-2-enone; Vibrational spectrum; Solvent effect; Hydrogen bonding; Dielectric effects 1. Introduction The effect of the solvent on the spectral proper- ties of molecules, generally referred as solvato- chromism, has been investigated for many years, generating a copious literature [1]. Particularly in the case of the vibrational spectrum, it is known * Corresponding author. Tel.:  /55-11-3818-3839; fax:  /55- 11-3031-2858 E-mail address: diretor@quim.iq.usp.br (P.S. Santos). 1 In memoriam. Spectrochimica Acta Part A 58 (2002) 3139  /3148 www.elsevier.com/locate/saa 1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S1386-1425(02)00103-8