An analysis of FT-Raman spectra of quinidine (C20H24N2O2) aqueous solutions with varying pH (which was regarded as an external perturbation) was performed using the 2D correlation method. The main course of changes in the quinidine solution appears to be: protonation changes of the quinuclidine nitrogen N1, followed by protonation changes of nitrogen N13 in the quinoline, leading to the appearance of cross-peaks in the synchronous and asynchronous correlation maps. The intensity changes of peaks at 1369 cm-1 for the unprotonated quinidine molecule, and characteristic peaks at 1387 cm-1 and 1389 cm-1 for protonated quinuclidine and double protonated quinidine, respectively, along with the decrease in pH, confirmed that the change in the pH of the quinidine solution has an influence on the protonation process of the Cinchona alkaloid. The negative synchronous and asynchronous cross-peaks at (1385, 823) cm-1 and (1387, 822) cm-1, respectively, indicate the importance of remodeling the quinoline fragment, during the process of a double protonation of the quinidine molecule. Bands correlating with 2809 cm-1 confirmed the importance of the methoxy group in the process of quinidine protonation. The creation of hydrogen bonds after double protonation of the Cinchona alkaloids, assisted by the CH3-O group, give an interesting insight into the changes in the studied compound occurring along with a decrease in pH.
Keywords: 2D correlation method; Cinchona alkaloid; FT- Raman spectroscopy; Quinidine.
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