Vibrational spectroscopy of neutral and cationic 2-Methoxyethanol

Hydrogen bonding interactions, such as the O-H...O interaction, play a crucial role in stabilizing conformations of both organic and biological molecules. In this study, we employed the infrared (IR)-vacuum-ultraviolet (VUV) non-resonant ionization detected IR spectroscopy (NRID-IR) method to investigate the molecular structure of neutral and cationic 2-methoxyethanol (CH3OCH2CH2OH, 2-ME). The stable structures and anharmonic IR spectra of neutral and cationic 2-ME were calculated using density functional theory (DFT) at the B3LYP-D3(BJ)/ def2-TZVPP level. Our results revealed that the two most stable conformers of neutral 2-ME exhibit a weak O-H...O intramolecular hydrogen bond, while the cationic 2-ME lacks the O-H...O interaction but contains a C-H...O intramolecular hydrogen bond. A comparison of the experimental and theoretical IR absorption spectra reveals that the most stable conformer (gauche-(anti-gauche)-trans) is the primary contributor to the observed IR spectra of neutral 2-ME in the 2700-7250 cm-1 range. Similarly, the second stable conformer is found to be the dominant contributor to the observed IR spectra of cationic 2-ME in the range of 2700-7100 cm-1. Additionally, the C-H fundamental stretching mode of cationic 2-ME is found to be blue-shifted by approximately 100 cm-1 compared to that of neutral 2-ME. Furthermore, the natural bond orbital (NBO) analysis suggests that in neutral 2-ME, the n(O)->sigma*(C alpha H) interactions result in significant negative hyperconjugation, leading to the weakening of C alpha H bonds and a subsequent reduction in vibrational energy.