Conformational structure of cationic tetrahydropyran by one-photon vacuum ultraviolet mass-analyzed threshold ionization spectroscopy

Abstract

Isolating and identifying the conformational forms of molecules are imperative processes to investigate the chemical reaction pathways of individual conformers. Herein, we explored the conformational structures of tetrahydropyran in the neutral (S₀) and cationic (D₀) states by varying the supersonic expansion conditions using one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The constructed 2D potential energy surfaces associated with conformational interconversion between the chair and boat forms in the S₀ and D₀ states revealed that the ionic transitions observed in the MATI spectra correspond to the most stable chair conformer. Accordingly, based on the 0–0 band in the VUV-MATI spectrum supported by the VUV photoionization efficiency curve, the adiabatic ionization energy for the conversion of the chair conformer to a cationic state was determined to be 74 687 ± 4 cm⁻¹ (9.2600 ± 0.0005 eV). Definitive vibrational assignment of the measured MATI spectra using Franck–Condon fitting revealed the cationic structure of the twisted chair conformer. The geometrical change upon ionization promoted the vibrational modes associated with ring inversion and deformation motions in the cationic state. This behavior, which was attributed to the effect of electron removal from the highest occupied molecular orbital (HOMO) consisting of the nonbonding orbital of the oxygen atom, reveals the role of electrons in the HOMO.