Computational Investigation of Electronic Effects on Reactivity in Aromatic Heterocycles

Abstract

Density functional theory (DFT) was used to study how heteroatom identity affects the electronic properties of pyrrole, furan, thiophene, and pyridine. The structures were optimized and their frontier molecular orbital energies were calculated and compared. Pyrrole had the highest HOMO energy (−5.1 eV) and the smallest HOMO–LUMO gap (4.3 eV), while pyridine had the lowest HOMO energy (−6.4 eV). Furan and thiophene fell between these two compounds. Additional conceptual DFT descriptors were estimated from the frontier orbital energies. Pyrrole had the lowest approximate ionization potential (5.10 eV), while pyridine had the highest approximate ionization potential (6.40 eV) and electronegativity (4.15 eV). The overall difference between pyrrole and pyridine agreed with their known behavior toward electrophiles. However, the calculated HOMO energies of furan and thiophene did not reproduce their established electrophilic substitution ordering. These results show that frontier orbital energies are useful for comparing the electronic properties of heterocycles, but they do not fully predict chemical reactivity on their own.