X-ray, Fourier-transform infrared,1H and13C nuclear magnetic resonance, and PM3 studies of (N—H⋯N)+and (O—H⋯O)–intramolecular hydrogen bonds in a complex of 1,8-bis(dimethylamino)naphthalene with maleic acid

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Abstract
The crystal structure of the title compound has been determined by X-ray analysis. The intramolecular hydrogen bond lengths are 2.606(3)Å for the (NHN)+ bridge in protonated 1,8-bis(dimethylamino)naphthalene cation (DMAN+H) and 2.401(4)Å for the (OHO)– bridge in the hydrogen maleate anion (HM–). The H-bonds are asymmetrical and not strictly linear: NHN, 157(3)° and OHO, 170(5)°. The geometries of the N—H⋯N and O—H⋯C bridges of the investigated cation and anion are dominated by the spherical repulsions of their constituent atoms. The overlapping bands in the absorbance IR spectra of potassium hydrogen (deuteron) maleate are separated in the second-derivative spectra. The strong mixing of the in-plane modes with skeletal modes in the hydrogen maleate ion causes a larger separation (Δν≈ 115 cm–1) of the ν(CO) bands in comparison with those in other type A acid salts containing intermolecular hydrogen bonds (Δν≈ 20–35 cm–1). The observed lack of solvent effect on the IR absorption suggests that the hydrogen bonds in tetrabutylammonium hydrogen maleate and 1,8-bis(dimethylamino)naphthalene hydrogen maleate are not extremely polarizable. 1H and 13C NMR chemical shifts of the investigated compound were measured and identified in two-dimensional (2D) experiments. The 1H NMR spectra show two narrow signals at ca. 19.5 and 18.7 ppm due to the OHO and NHN protons, respectively. The structural parameters of the cation and anion were also determined by quantum-mechanical calculations with the semiempirical MNDO-PM3 method.

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