On the protonated forms of alkyl-bonded polycyclic aromatic heterocycles: Structure prediction and characterization using density functional theory

When alkyl chains with electron-donating properties are bonded to heterocyclic rings in polycyclic aromatic hydrocarbons, a certain class of molecules is formed. These molecules become protonated in some industries as a result of their intimate contact with the acidic aqueous phase. However, little attention has been paid to the protonation of these molecules. The purpose of this article was to use computational methods to predict and characterize the final protonated states of these molecules. This study considered three distinct types of these molecules. DFT method was used to determine the most likely proton-accepting sites for the molecules, followed by optimization of their protonated states. The final stable forms were collected and compared based on their energy values, chemical hardness and dipole moments. Additionally, the geometrical properties and FTIR spectra of the most stable form of each type were determined. We found that π-bonding orbitals account for a significant portion of the proton-accepting region in all molecules, potentially allowing carbon atoms and heteroatoms to bond with proton. Not all of them, however, exhibited stable protonated states. This study has revealed the critical role of intramolecular non-bonding interactions in determining the proton-accepting ability of a probable site as well as the relative stability of protonated forms. Additionally, significant energy drop were demonstrated during protonation, as well as corresponding stability improvements. The final protonated molecular structures are critical for those studying the phenomena associated with these molecules specially designing agents for preventing protonation. •Both carbon atoms and heteroatoms can accept protons in an alkyl-bonded polycyclic aromatic heterocycle.•The effect of steric overcrowding is critical for the stability of protonated forms.•The primary change in intramolecular properties occurs in the vicinity of the proton-accepting site during protonation.•The protonated forms are identified by a shift in the FTIR spectrum.•The protonation leads to alteration in the dipole moment and the aromaticity of the molecule.