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 the nitrogen atom within its ring. Herein, we investigated the conformer-specific photoionization dynamics of morpholine under molecular-beam conditions using high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) mass spectroscopy. Two-dimensional potential energy surfaces (2D PESs) associated with the conformational changes in the neutral (S₀) and cationic (D₀) ground states were constructed to identify the conformer(s) corresponding to the obtained VUV-MATI spectrum. The 2D PESs indicated that the chair and twisted boat forms with equatorial and axial NH conformations (four conformers with the following relative energies: Chair-Eq < Chair-Ax ≪ Twisted boat-Ax < Twisted boat-Eq) of morpholine lie on the global minimum of the S₀ state. However, only the axial-like NH conformation in each form (stable Chair-Ax-like⁺˙ and Twisted boat-Ax-like⁺˙ conformers) exists in the D₀ state. Accordingly, vibration assignment was performed based on Franck–Condon (FC) analyses of the adiabatic ionic transitions from each Chair-Eq and Chair-Ax conformer to the Chair-Ax-like⁺˙ conformer. The FC analyses revealed that only the Chair-Ax conformer contributes to the ionic transitions to the Chair-Ax-like⁺˙ conformer owing to the large FC factors, whose adiabatic ionization energy was determined to be 8.1003 ± 0.0005 eV. Consequently, adiabatic ionization arises because of electron removal from the highest occupied molecular orbital consisting of the nonbonding orbital of the N atom in the Chair-Ax conformer.