Theoretical study of the effects of substituents (F, Cl, Br, CH, and CN) on the aromaticity of borazine.

This paper presents a theoretical study of the effects of substituents (F, Cl, Br, CH, and CN) on the aromaticity of borazine (BNH), using density functional theory (DFT) and the Hartree-Fock (HF) method. The calculations to optimize the geometries, structural properties, and vibrational frequencies were performed using the same 6-311G(d,p) and 6-311++G(d,p) basis sets, comparing the methods with experimental results. In the analysis of the NICS values, it was found that that replacing the hydrogen atoms by halogen atoms (F, Cl, and Br) and CH reduced the aromaticity of the borazine molecule, while use of the CN group resulted in NICS values (0.9-2.0 Å) very close to those of borazine, presenting the following order of increasing aromaticity: BNH-(Br) < BNH-(Cl) < BNH-(F) < BNH-(CH) < BNH ~ BNH-(CN). All the spectra of the compounds showed only the presence of transition peaks distant from the UV region, reflecting the large energy difference between the HOMO and LUMO orbitals. After the substitution of the borazine ring, all the compounds presented an intensification of the spectrum, with a shift of the maximum absorbance toward red, indicative of a bathochromic effect. There was a direct inverse relation between the energy gap and the maximum wavelength of the compounds.