Abstract
AbstractWe demonstrate that Raman spectroscopy is able to discriminate between cross‐conjugated (CC) and pseudo‐cross‐conjugated mesomeric betaines (PCCMBs) by means of key bands sensitive to conjugation. As targets, we have selected the PCCMB 2,5 dimethyl‐1‐phenylpyrazolium‐3‐carboxylate and its isomer the CCMB 1,3‐dimethyl‐2‐phenylpyrazolium‐4 carboxylate. Structurally, they differ only in the position of the carboxylate group bonded to the pyrazolium ring. However, this feature provides different conjugational character. We provide evidences for the fact that this structural difference has a measurable impact in the vibrational wavenumbers as Raman spectroscopy can directly account for it. We also prove that the observed Raman features are not only originated by the kinetic effect caused by the change of the carboxylate position, but this atomic re‐organization also modifies the potential energy of the molecule. This energy directly depends on the electronic structure, which determines the pseudo‐cross‐conjugation character. Copyright © 2009 John Wiley & Sons, Ltd.
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