Conformational Analysis and Vibrational Spectroscopy of Neutral and Cationic Monoethanolamine

We present a comprehensive investigation of the vibrational spectra and conformational distribution of neutral and cationic monoethanolamine (MEA) in the gas phase. Using infrared-vacuum ultraviolet non-resonant ionization fragmentation detected IR spectroscopy (NRIFD-IR), we obtained vibrational spectra in the 2500–3800 cm⁻¹ range for both neutral and cationic MEA. Density functional theory (DFT) calculations at the B3LYP-D3(BJ)/def2-TZVPP level were employed to elucidate the molecular structures and vibrational modes. Our analysis revealed twelve distinct conformers for neutral MEA, with N1-gʹGgʹ being the most stable, while cationic MEA exhibited four conformers, among which C1-gʹGt conformer was found to be the primary contributor to the observed spectra. The experimental spectra were interpreted through comparison with anharmonic calculations, allowing for detailed assignment of vibrational modes. Notably, we observed significant differences in the OH stretch region between neutral and cationic species, reflecting changes in intramolecular hydrogen bonding upon ionization. Furthermore, our study highlights the necessity for distinct scaling factors when calculating harmonic frequencies for neutral and cationic substances.