Crystal Structures and Density Functional Theory Calculations of o- and p‑Nitroaniline Derivatives: Combined Effect of Hydrogen Bonding and Aromatic Interactions on Dimerization Energy

The interplay of strong and weak hydrogen bonds, dipole–dipole interactions, and aromatic interactions of o- and p-nitroaniline derivatives was studied by combining crystal structure analysis and density functional theory (DFT) calculations. Crystal structures of four 2-nitroaniline derivatives, 2-((2-nitro­phenyl)­amino)­ethyl methanesulfonate (1A), 2-((2-nitro­phenyl)­amino)­ethyl 4-methylbenzenesulfonate (2A), N,N′-((1,3-phenyl­enebis­(oxy))­bis­(ethane-2,1-diyl))­bis­(2-nitro­aniline) (3A), and N-(2-chloro­ethyl)-2-nitroaniline (4A), and crystal structures of three 4-nitroaniline derivatives, 2-((4-nitro­phenyl)­amino)­ethyl methanesulfonate (1B), 2-((4-nitro­phenyl)­amino)­ethyl 4-methylbenzenesulfonate (2B), and tert-butyl-(2-((4-nitro­phenyl)­amino)­ethyl) carbonate (5B), were analyzed with regard to intra- and intermolecular hydrogen bonding and π–π interactions. The effect of o/p-substitution on π-electron distribution, aromaticity of the nitroaniline ring, and relative strength of intra- and intermolecular weak interactions were compared using HOMA indices, Hirshfeld surfaces, and DFT calculations. The 2-nitroaniline derivatives contain an intramolecular hydrogen bond between amino and nitro groups, creating a six-membered chelate ring, which formed pseudostacked structures with aromatic rings. van der Waals corrected DFT calculations showed that stacking of a phenyl and a chelate ring contributed significantly to the dimerization of 1A and 2A. Therefore, it can be concluded that the phenyl–chelate interactions are an important factor in stacking of the monomers in addition to C–H···O hydrogen bonding. In 4-nitroaniline derivatives, intermolecular N–H···O hydrogen bonds were in a significant role in packing, creating hydrogen-bonded dimers in 1B and 2B, and hydrogen-bonded chains in 5B.