Determination of the highest occupied molecular orbital and conformational structures of morpholine based on its conformer-specific photoionization dynamics

Abstract

Morpholine, a heterocycle composed of an ether and amine, is commonly used as a precursor in many organic synthesis processes because of the nucleophilicity induced by the lone-pair electrons of...

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

“…Previously, this technique was used to investigate the conformer-specific photoionization (PI) dynamics of morpholine. 26 The results revealed highly favorable Franck−Condon transitions for ionic transitions from the Chair-Ax conformer, while transitions from the Chair-Eq conformer were extremely unfavorable as a result of the existence of only the axial-like NH conformation in the cationic (D 0 ) state. Consequently, the observed ionic transitions were attributed solely to the Chair-Ax conformer with no information about the conformational preference in the neutral (S 0 ) state.…”

“…Furthermore, all of the distinct peaks observed in the IR dip spectrum of the Chair-Ax conformer (Figure a) matched the peaks observed in the IR absorption spectrum of morpholine (Figure b). However, owing to highly unfavorable FC transitions, we were unable to obtain the VUV-MATI spectrum of the Chair-Eq conformer . However, considering that the measured IR absorption spectrum of morpholine serves as a proxy for the vibrational spectra of both Chair-Eq and Chair-Ax conformers in equilibrium, the unique IR spectrum of the Chair-Eq conformer can be accurately derived by utilizing the IR absorption spectrum.…”

“…However, owing to highly unfavorable FC transitions, we were unable to obtain the VUV-MATI spectrum of the Chair-Eq conformer. 26 However, considering that the measured IR absorption spectrum of morpholine serves as a proxy for the vibrational spectra of both Chair-Eq and Chair-Ax conformers in equilibrium, the unique IR spectrum of the Chair-Eq conformer can be accurately derived by utilizing the IR absorption spectrum. To achieve this, we scaled the IR dip spectrum of the Chair-Ax conformer with respect to the relatively isolated peak intensity observed at 2963 cm −1 in the IR resonant VUV-PI spectrum of morpholine.…”

“…To address these challenges, we utilized high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy combined with two-dimensional potential energy surface analysis. − VUV-MATI is a powerful technique that can elucidate the precise conformational structures of molecules. Previously, this technique was used to investigate the conformer-specific photoionization (PI) dynamics of morpholine . The results revealed highly favorable Franck–Condon transitions for ionic transitions from the Chair-Ax conformer, while transitions from the Chair-Eq conformer were extremely unfavorable as a result of the existence of only the axial-like NH conformation in the cationic (D 0 ) state.…”

“…The IR absorption spectrum of morpholine in the S 0 state was obtained using IR resonant VUV-PI (Figure a). The VUV laser pulse had a frequency of 65 232 cm –1 , which was below the ionization thresholds of morpholine . The preceding IR laser pulse was scanned through the CH and NH stretching regions.…”