Structure and Vibrational Spectroscopy of 2-Methylallyl Alcohol

The intramolecular O−H···π hydrogen bond has garnered significant research interest in recent decades. In this work, we utilized the infrared (IR)-vacuum-ultraviolet (VUV) non-resonant ionization detected IR spectroscopy (NRID-IR) method to study the molecular structure of neutral and cationic 2-methylallyl alcohol (MAA, CH₂=C(CH₃)– CH₂–OH). Density functional theory calculations revealed five stable neutral and three stable cationic MAA conformers, respectively. Two neutral MAA conformers are expected to have an O−H···π intramolecular hydrogen bond interaction, based on the structural characterization that the OH group is directed toward the C=C double bond. The IR spectra of both neutral (2700–3700 cm⁻¹) and cationic MAA (2500–7200 cm⁻¹) were measured, and the anharmonic IR spectra were calculated at the B3LYP-D3(BJ)/def2-TZVPP level. The OH stretching vibration frequency of neutral MAA was observed at 3656 cm⁻¹, slightly lower than those of methanol and ethanol. In contrast, the OH stretching vibration of cationic MAA was red-shifted by about 140 cm⁻¹ compared to neutral MAA. The interaction region indicator and natural bond orbital analysis suggest that the O−H···π interaction in neutral MAA is weak, and may not play a major role in stabilizing the neutral MAA.