Intramolecular hydrogen bonds in fluorinated, methoxylated, or unsubstituted 2-(anilino)-1,4-naphthoquinones. A theoretical study

Density functional calculations at the BP86/6-311G(d,p) level of theory were realized to analyze the existence of intramolecular hydrogen bonds in a series of neutral 2-anilinonaphthoquinones. The ortho position of the aniline ring (C2a) was unsubstituted in two of them, substituted by a fluorine atom in another two compounds, and substituted by a methoxy group in an additional pair of compounds. The characteristic features of hydrogen bond formation (elongation of OC1, NH, FC2a and OC2a bonds, increase in the out-of-plane NH bending frequency, decrease in the NH stretching frequency) suggest the formation of a regular (two-center) intramolecular hydrogen bond in 2-anilinonaphtoquinones, involving nitrogen as donor and an oxygen in the naphtoquinone fragment as acceptor. The geometry and frequency changes also suggest the formation of a second and weaker hydrogen bond, resembling a bifurcated (three-center) intramolecular hydrogen bond, in fluorine- or methoxy-ortho-substituted 2-anilinonaphtoquinones.

Graphical abstractThe atomic centers involved in the two kinds of hydrogen bonds that may appear in the studied molecules: regular or two-center hydrogen bonds and bifurcated or three-center hydrogen bonds.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► DFT provides evidence of intramolecular hydrogen bonds in 2-anilinonaphthoquinones. ► Key changes in interatomic distances and vibration frequencies signal hydrogen bonds. ► Bifurcated, OH(N)-F or OH(N)-O, and regular, OH(N), intramolecular hydrogen bonds.