Conformational structures of the tetrahydrofuran cation determined using one-photon mass-analyzed threshold ionization spectroscopy

Abstract

One-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy was used to characterize the essential conformations of tetrahydrofuran (THF) and thus determine the stereochemistry of the furanose ring constituting the backbones of DNA and RNA. Since the VUV-MATI spectrum of THF exactly corresponds to the vibrational spectrum of the gas-phase THF cation, the above cation was detected using time-of-flight mass spectrometry featuring the delayed pulsed-field ionization of the target in high Rydberg states by scanning the wavelength of the VUV pulse across the region of the vibrational spectrum. The position of the 0-0 band in the recorded VUV-MATI spectrum was extrapolated to the zero-field limit, allowing the adiabatic ionization energy of THF to be accurately estimated to be 9.4256 ± 0.0004 eV. The above ionization was assigned to a transition between C₂-symmetric neutral (S₀) and cationic (D₀) ground states. The potential energy surfaces associated with molecular pseudorotation in the above states were constructed at the B3LYP/aug-cc-pVDZ level, being in good agreement with experimental observations. The twisted (C₂-symmetric) and bent (C_S-symmetric) conformers of the S₀ state were predicted to be separated by a small interconversion barrier, whereas the D₀ state exclusively existed in the C₂ conformation. Based on the above, the peaks in the MATI spectrum were successfully assigned based on the Franck–Condon factors and vibrational frequencies calculated by varying the geometrical parameters of the C₂ conformation, which determines the precise molecular structure of the THF cation.