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 (CS-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.
- This article is part of the themed collection: 2017 PCCP HOT Articles