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

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 (S) and cationic (D) 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 S state. However, only the axial-like NH conformation in each form (stable Chair-Ax-like˙ and Twisted boat-Ax-like˙ conformers) exists in the D 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.