Solvent effects on the infrared spectra of anilines. VII. 2-Substituted 4-nitroanilines

Measurements of N-H stretching frequencies of 4-nitroanilines in the presence of hydrogen bond acceptors show that the ease of forming a second intermolecular hydrogen bond in the presence of an ortho substituent decreases in the order hydrogen > methyl > bromo, methoxyl > nitro. This order demonstrates the importance of repulsions between lone pair orbitals on the ortho substituent and the acceptor molecule. Weak intramolecular hydrogen bonds are detected in 2-iodo- and 2-bromo-aniline, and such bonds can be strengthened by introduction of a 4-nitro substituent.