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Role of the hydrogen bonds in nitroanilines aggregation : charge density study of 2-methyl-5-nitroaniline.

Ellena, J. and Goeta, A. E. and Howard, J. A. K. and Punte, G. (2001) 'Role of the hydrogen bonds in nitroanilines aggregation : charge density study of 2-methyl-5-nitroaniline.', Journal of physical chemistry A., 105 (38). pp. 8696-8708.


The electron charge distribution of 2-Methyl-5-nitroaniline has been studied from high-resolution singlecrystal X-ray data at 100 K, and ab initio calculations which include X-ray structure factors computed from a superposition of ab initio molecular electron densities. Using the Hansen and Coppens' rigid pseudoatom multipolar model refinements were performed on both the experimental and the theoretical X-ray data sets from which, molecular atomic charges and dipolar moments were obtained. To understand the nature and the magnitude of the intermolecular interactions. the Atoms in Molecules theory was used to investigate the topology of the electron density of the in-crystal, both experimental interacting as well as theoretical noninteracting, and in-vacuum molecules. A meticulous analysis of the topological properties of the experimental charge density and of its Laplacian indicates, contrary to expectations, a two center character of the N-(HO)- O-. . . synthons that induce the known polar chain formation in nitroanilines and the presence of a C-methyl-H-O interaction further strengthening the chains. It also shows the attractive nature of the rather strong C-(HO)-O-. . . interactions that help the head-to-tail arrangement of the chains. They build two intermolecular six membered hydrogen bonded rings, embracing a N-(HO)-O-. . . interaction, that originate centrosymmetric dimers which impair the macroscopic second harmonic generation of the title compound. The authenticity of a previously proposed closed shell C-aryl-H(. . .)pi interaction between adjacent chains has been confirmed. The latter has not been observed in m-nitroaniline, 2-methyl-4-aniline or other related compounds with chains built from similar N-(HO)-O-. . . synthons and assembled head-to-head. Crystallization causes a molecular electric dipolar moment higher than that of the free molecule, the latter being coincident with the experimental value in solution, and with the one calculated from the refinement of the theoretical X-ray data. It also induces changes in the charge density distribution and its topology, and an enhancement of the intramolecular conjugation that can be related to a molecular aggregation mechanism ruled by the N- (HO)-O-. . . synthon. These findings strongly point to the existence of cooperative effects.

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Record Created:10 May 2007
Last Modified:08 Apr 2009 16:31

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