AB-INITIO AND DENSITY-FUNCTIONAL STUDY OF THE RESONANCE RAMAN-SPECTRAOF METHYL RED, ETHYL RED AND THEIR PROTONATED DERIVATIVES

Resonance Raman (RR) spectra of the acid-base indicators Methyl Red an d Ethyl Red and their monoprotonated derivatives were recorded using e xcitation in the region of their lowest energy electronic absorption b ands. The molecular geometries and vibrational spectra of Methyl Red a nd its protonated derivative were calculated using ab initio (HF/3-21G ) and density functional (B3-LYP/3-21G) methods. Force constants and p otential energy distributions were determined from the calculated spec tra, after appropriate scaling. Comparison of the RR spectra with the calculated Raman band positions enabled precise vibrational band assig nments to be made. The RR spectra provide clear evidence for the struc tural and electronic changes that accompany protonation. (C) 1998 John Whey & Sons, Ltd.