Unraveling the Conformational Preference of Morpholine: Insights from Infrared Resonant Vacuum Ultraviolet Photoionization Mass Spectroscopy

Abstract

The preference for different conformations in morpholine has a notable effect on its behavior and reactivity in organic synthesis. Herein, we explored the intricate conformational properties of morpholines through a combination of advanced mass spectrometric techniques and theoretical calculations. Notably, we employed infrared (IR) resonant vacuum ultraviolet (VUV) mass-analyzed threshold ionization spectroscopy to measure the unique vibrational spectra of the distinct conformers (Chair-Eq and Chair-Ax) in morpholine for the first time. Through precise VUV photon energy adjustments to coincide with the vibrational excitation via IR absorption, we effectively pinpointed the adiabatic ionization thresholds corresponding to the Chair-Eq (65 442 ± 4 cm −1 ) and Chair-Ax (65 333 ± 4 cm −1 ) conformers. This allowed us to accurately determine the conformational stability between the two conformers (109 ± 4 cm −1 ). By shedding light on the conformational properties of morpholine, this study brings far-reaching implications to the fields of organic synthesis and pharmaceutical research.