Theoretical Multipolar Atom Model Transfer in Nitro-Derivatives of N‑Methylaniline

The nitroanilines are an example of compounds in which the coexistence of electron-rich and electron-deficient substituents, connected through a conjugated π-electronic system, makes their molecular second-order hyperpolarizability and second-harmonic generation efficiency particularly high. This property makes them extremely interesting from the point of view of charge density distribution analysis. The electron density of three isomeric molecules, i.e., N-methyl-2-nitroaniline, N-methyl-3-nitroaniline, and N-methyl-4-nitroaniline, was calculated theoretically through the multipolar atom model transfer. Two types of refinement models, i.e., multipolar atom model (MAM) and independent atom model (IAM), have been applied for analysis of model improvement concerning the electron-density parameters transfer. It results in a more precise molecular structure in terms of geometry and thermal displacement parameters along with a reduction of statistical refinement factors and residual electron densities. The proposed approach enables the extraction of relevant electron density-derived information, where the intrinsic quality of X-ray data does not allow a “true” multipolar refinement. The effect of ortho-, meta-, and para-substitution on π-electron distribution and aromaticity of the nitroaniline ring was compared using harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shift (NICS) indexes. In the paper, the electronic effects from the charge density parameters have been examined along with the study of intermolecular interactions using two different approaches: one, based on the Hirschfeld surfaces analysis, and the second, based on the dissociation energy estimation from topological analysis.