Charge-transfer Character in the Intramolecular Hydrogen Bond: Electronic Structures and Spectra of Hydrogen Maleate Anion and Related Molecules

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Abstract
Ultraviolet absorption spectra were measured with aqueous solutions of maleic acid (H2M) at various pH values and in concentrated sulfuric acid solutions. With the aid of the theoretical calculations of electronic structures by the modified CNDO-CI method, relatively strong bands observed at 193.5 nm for H2M, at 210.0 nm for the hydrogen maleate anion (HM−), and at 202.8 and ≈230 nm for the dinegative ion (M2−) are assigned to the π–π* transition bands. Special attention was paid to the charge-transfer (CT) character (pertinent to the strong intramolecular hydrogen bond) in the excited states of HM−. Configuration analysis of the wave functions revealed us that the first π–π* band of HM− covered by the much stronger 210 nm band is rich in the CT character pertinent to the hydrogen bond.

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