An insight into the structure and vibrations of 4-nitroindole and 7-nitroindole by spectroscopic and DFT methods

The structure of 4-nitroindole (4NI) and 7-nitroindole (7NI) is optimised by B3LYP level using 6 - 31++G** and cc-pVTZ basis sets. The FT-IR and FT-Raman and NMR spectra of 4NI and 7NI are analysed in detail and compared. The observed H-1 and C-13 NMR chemical shifts are found good correlation with theoretical chemical shifts calculated by GIAO method. The vibrational frequencies of 4NI and 7NI have been precisely assigned and analysed. The electron density and electrostatic potential of 4NI is more than that 7NI molecule. The bond orbital analysis (NBO) is carried out. The dual Fukui descriptors (Delta f(k)), the dual local softness (Delta s(k)) and the multiphilicity descriptors (Delta omega(k)) have clearly reveals that the atoms N1, C2, C3, C4, C8 are more favourable for nucleophilic attack in both 4NI. In the case of 7NI molecule N1, C2, C3, C5, C7 are more prone to nucleophilic attack. The local reactivity descriptors favours electrophilic attack at the carbon atom C9 of 4NI and 7NI molecules. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study optimized the structures of 4-nitroindole and 7-nitroindole and analyzed their spectra, finding good correlation between theoretical and experimental results, as well as differences in electron density and local reactivity descriptors of the molecules.