Abstract
This study presents a comprehensive density functional theory (DFT) investigation into the stepwise
reduction of nitrobenzene to aniline via nitroso and hydroxylamine intermediates. Two mechanistic pathways
were explored: a non-catalyzed route in both gas and ethanol phases and a surface-catalyzed route facilitated
by a palladium (Pd) catalyst. Geometry optimizations, thermodynamic parameters, frontier molecular orbital
(FMO) analysis, and vibrational frequency (IR) simulations were carried out for all key species involved:
nitrobenzene (Ph-NO₂), nitrosobenzene (Ph-NO), phenylhydroxylamine (Ph-NHOH), and aniline (Ph-NH₂).
A comparison of gas-phase and solvent-phase geometries revealed solvent-induced elongation of N–O
and N–H bonds, particularly in polar intermediates, consistent with ethanol’s stabilizing effect. FMO analysis
showed a notable decrease in HOMO–LUMO energy gaps in the solvent phase, indicating enhanced reactivity.
IR spectra further supported these findings, with observable shifts in characteristic stretching frequencies
upon solvation.
In the Pd-catalyzed pathway, adsorption of reactants and intermediates on the Pd surface significantly
altered molecular geometries and lowered reaction energy barriers. Calculated adsorption energies and bond
elongations suggest strong Pd–O and Pd–N interactions that facilitate bond activation. The overall energy
profile indicates a smoother and more favorable reduction pathway on the Pd surface compared to the noncatalyzed routes.
These results provide mechanistic insight into the catalytic role of Pd in hydrogenation reactions and
highlight the importance of solvent effects in modulating electronic and structural properties. This
comparative approach enhances our understanding of nitroarene reductions and offers valuable guidance for
catalyst design and process optimization.