Structural and Conformational Studies of ortho-,meta-,and para-Methyl Red upon Proton Gain and Loss

摘要

The structures and conformations of ortho-,meta-,and para-methyl red (MR) upon proton gain and loss were studied by density functional calculations,and compared to methyl yellow for the effects of a carboxyl substitution.Internal hydrogen bonding causes the geometry of neutral o-MR planar,otherwise twist.Mono-protonated species of MR are planar where the proton is attached to beta-azo nitrogen.This loses its azo character a bit,and shows strong delocalization characterized as a quinonoid canonical structure.Di-protonated species of MR is proved to hold two protons at the amino and alpha-azo nitrogen atoms,and planar.It regains somewhat of its azo character,but still shows fairly delocalized property in terms of carbocationic canonical structures.The carboxyl substitution on 4-dimethylamino-trans-azobenzene structure has some delocalization effects on the geometry or conformation of MR derivatives whether neutral,mono-,di- or de-protonated.