Theoretical approach to the out-of-plane deformation of 1,3-disubstituted azulenes |
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Authors: | Verónica A Jiménez |
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Institution: | (1) Departamento de Química Orgánica y Grupo de Química Computacional, Universidad de Concepción, Casilla 160-C, Concepción, Chile |
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Abstract: | Computational calculations at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels were employed to analyze the structure and
conformation of 1,3-bis(4-bromophenyl)azulene (1), 1,3-bis(2-thienyl)azulene (2), and 1,3-bis(2-pyrrollyl)azulene (3) in order to rationalize the out-of-plane deformation found in the azulene cores of 1 and 2 in the crystalline state, whereas compound 3 shows a totally planar azulene moiety. Our results indicate that 1,3-disubstituted azulenes possess two almost equally stable
and easily convertible minimum energy conformers, which differ in the relative orientation of the substituent groups and in
the planarity degree of the azulene core. An absolute planarity index (P) is introduced to quantify the out-of-plane distortion found in the azulenes under study. The aromaticity of minimum energy
conformers was evaluated by means of geometric (HOMA), magnetic (NICS), and energetic (the frequency of the lowest out-of-plane
vibration, νmin) aromaticity indicators, which suggest that compound 3 possesses the most aromatic azulene core within the group. Calculated molecular dipole moments suggest that the conformation
of 1,3-disubstituted azulenes in the crystalline state can be explained in terms of electrostatic intermolecular interactions
rather than relative stability of planar and non-planar conformers. |
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Keywords: | 1 3-Disubstituted azulenes Out-of-plane distortion Planarity |
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