A computational study of substituted flavylium salts and their quinonoidal conjugate-bases: S0 -&gt; S1 electronic transition, absolute pKa and reduction potential calculations by DFT and semiempirical methods

Adilson A. Freitas,Frank H. Quina,Luís G. Dias,K. Shimizu

doi:10.1590/s0103-50532007000800014

Abstract

The electronic transitions for flavylium cations and quinonoidal bases of 17 substituted flavylium salts have been studied at semiempirical and DFT (density functional theory) levels. Solvent effect on electronic spectra was included by Polarizable Continuum Model, PCM. We assigned longest-wavelength absorption maxima to HOMO ® LUMO transition. Both levels of theory gave good results for electronic transitions of flavylium cations whereas only TDDFT-PCM calculations could be used for electronic transitions of their quinonoidal bases. We also performed absolute pKa calculations of nine flavylium salts at DFT level. The pKa calculated values by our PCM parameterization gave excellent results with mean absolute deviation less than a half of one pKa unit. One-electron reduction potentials were carried out for 5 flavylium cations at DFT level. The theoretical results found were in good agreement with experimental values after adjustment for a systematic deviation.

Highlights

Anthocyanins constitute the major red and purple pigments in plants and can be found in fruits, flowers and leaves.[1,2] Interest in the anthocyanins stems from the fact that they are omnipresent in our diet, exhibit unusual chemical and photochemical properties,[3,4,5,6,7,8,9] and have potential for application as food dyes[1] and antioxidant additives.[10,11] The basic chromophore of anthocyanins isA Computational Study of Substituted Flavylium SaltsJ
We present the results of a systematic quantum chemical study of the cationic form (AH+) and the neutral quinonoidal base form (A) of a series of anthocyanins analogues at the ab initio level
The absolute reduction potential of the normal hydrogen electrode (NHE) were calculated by using the same expressions presented above and experimental values tabulated in the NIST Chemistry Webbook[53] (ΔG0H+ = 263.98 kcal mmfooor llt--11h;aenΔNdGHΔH0 2GE(H0gw2)(eg=r)e3=e5m–91.p4.l4okkycceaadllmtmooomll-1-1i;n)Δ.iGmEH0xi2zpe(egrp)imo=sesn–ibt9al.el3v2earklruocerassl from the calculation

Summary

Introduction

Anthocyanins constitute the major red and purple pigments in plants and can be found in fruits, flowers and leaves.[1,2] Interest in the anthocyanins stems from the fact that they are omnipresent in our diet, exhibit unusual chemical and photochemical properties,[3,4,5,6,7,8,9] and have potential for application as food dyes[1] and antioxidant additives.[10,11] The basic chromophore of anthocyanins is

A Computational Study of Substituted Flavylium Salts

Computational Methodology

Results and Discussion

Conclusions