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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 26, Number 8
BKCSDE 26(8)
August 20, 2005 

Structural and Conformational Studies of ortho-, meta-, and para-Methyl Red upon Proton Gain and Loss
Sun-Kyung Park, Choongkeun Lee, Kyung-Chul Min, Nam-Soo Lee*
Methyl red, Protonation, Conformation, Density functional theory
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. Monoprotonated species of MR are planar where the proton is attached to β-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 α-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.
1170 - 1176
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