The structural and electronic properties of the 3,7-dinitrodibenzobromolium cation and chloride were studied
using first-principle methods, based on the density functional theory (DFT). Different forms of exchange and
correlation functions and basis sets were considered, and their suitability in studying such large molecules
were assessed by comparing the calculated results with available X-ray diffraction data. The DFT results
were also compared with those obtained previously using the Hartree−Fock (HF) method. The DFT methods
improved on the calculated IR and Raman spectra, confirming the superiority of the DFT methods over the
HF method in predicting harmonic frequencies, particularly for organic nitro compounds. The geometric
parameters obtained using the hybrid DFT methods, such as B3LYP, B3P86, and B3PW91, are compatible
with the results of the HF method. The structure of 3,7-dinitrodibenzobromolium chloride was optimized
with and without symmetry constraints (such and C
s and C
2v) separately. The C1 structure obtained from
optimization without any symmetry constraints and the C
s structure were proven to be identical, using more-stringent convergence criteria in the structure optimization. Intrinsic reaction coordinate (IRC) calculation
was performed, following the frequency calculation for the optimized C
2v structure. The results confirmed
that the C
2v structure is the transition structure connecting the two energy minimum stuctures in C
s symmetry.
A strong ionic bond is formed between the chloride anion and the 3,7-dinitrodibenzobromolium cation, with
a Br−Cl bond length of 2.606 Å. Further studies were conducted to obtain the electronic density, electrostatic
potential, and charge distribution of the chloride and the cation in its planar form and with the rotation of the
nitro group. The charge distribution of other halides are also investigated and discussed. Knowledge of the
electron properties is useful for understanding the bonding and biological activities of this molecule.