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The conformational equilibrium and vibrational properties of chalcone

The conformational preference (trans-s-cis vs. trans-s-trans) of chalcone in two solvents (tetrachloroethene and acetonitrile) and in the solid phase was studied experimentally by IR and Raman spectroscopy and theoretically within the polarizable continuum model (PCM) (at the B3LYP/6-31+G(d, p) level, in vacuo and in ten solvents ranging from εr= 1.4 to εr= 35.7). From the temperature dependent IR measurements of chalcone in tetrachloroethene (εr= 2.268) the conformational enthalpy difference was obtained ∆H0 = +(1.01 ± 0.03) kcal mol-1. The calculated value of 1.39 kcal mol-1 for tetrachloroethene solvent (εr= 2.268) is quite close. In acetonitrile the obtained enthalpy difference was negative, ∆H0 = −(1.4 ± 0.3) kcal mol-1 in contrast to the calculated positive value of 1.10 kcal mol-1. A thorough normal coordinate analysis was performed in terms of redundancy-free internal coordinates making full use of local symmetries. It is demonstrated that bands usually attributed either to carbonyl C=O or ethylenic C=C bond stretching are correctly described only as combinations of the two stretchings. This is important because explanations of the various experimental observations must accordingly be modified. For example, the solvent induced wavenumber shifts or intensity changes of the 1670 cm-1 band are not related exclusively to the properties of the carbonyl bond.

Chalcone ; Normal mode analysis ; Vibrational assignment ; Molecular conformation

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