Development and Verification of Conformer-Specific Vibrational Spectroscopy

Abstract

Conformers have similar vibrational structures both in neutral (S 0 ) and cationic (D 0 ) states owing to the comparable force fields between their nuclei. Nevertheless, there is a continuous development of vibrational spectroscopic techniques to rigorously identify individual conformers in the designated molecule but only in the S 0 state. We developed an inventive conformer-specific vibrational spectroscopic technique to measure identifiable vibrational spectra of individual conformers in both S 0 and D 0 states. We measured isomer-specific vibrational spectra in both states for gas-phase acetone and oxetane isomers from a solution with azeotropic composition to verify the proposed techniques that are based on infrared (IR) resonant vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The measured IR dip VUV-MATI and IR hole-burn VUV-MATI spectra for each isomer, which correspond to isomer-specific vibrational spectra in both states, can be represented by IR-resonant VUV photoionization and one-photon VUV-MATI spectra of the binary mixture, respectively, under supersonic expansion conditions. The partial pressures of the individual isomers in the binary mixture with different mole fractions estimated according to the relative peak intensities in the measured spectra provide insights on solute−solvent interactions. We suggest that the verified IR-resonant VUV-MATI spectroscopy can form the basis of effective schemes toward conformational chemistry.

Conformational Structures of Neutral and Cationic Pivaldehyde Revealed by IR-Resonant VUV-MATI Mass Spectroscopy

“…In this study, we performed IR-resonant VUV-MATI spectroscopy using the home-built HR VUV-MATI mass spectrometer, and its details were provided in previous studies [ 23 , 24 , 31 , 39 ].…”

“…The subsequent efforts for analyzing the aliphatic molecules with extremely short lifetime in the excited electronic state motivated the development of the IR–VUV double-resonance techniques, despite the occasional emergence of uncontrollable fragmentations from excess VUV photon energy at 118 nm [ 20 , 21 , 22 ]. Extending the conventional laser double-resonance spectroscopic techniques, we recently developed a conformer-specific vibrational spectroscopy technique based on IR-resonant VUV-MATI mass spectroscopy, which can measure the identifiable vibrational spectra of individual conformers of a molecule in both S 0 and D 0 states [ 23 ]. As demonstrated, the newly invented techniques adopting the cold and collision-free environment of supersonic expansion can effectively elucidate the vibrational structures of individual conformers in both S 0 and D 0 states beyond the population redistribution [ 24 ].…”

Conformational diversity and environmental implications of trans-2-pentenal

“…Two types of IR spectra were measured using the IR resonant VUV-PI/MATI scheme in a supersonic expansion. 17,20 One IR spectrum encompassed the IR absorption spectrum, where ionisation occurred when the sum of the vibrational excitation (hn IR ) and VUV photon energies (hn VUV ) surpassed the molecular ionisation energy and the VUV energy was below this ionisation energy. The second IR spectrum involved conformer-specific IR spectroscopy.…”

“…The decrease in MATI ion signals due to the vibrational excitation-induced reduction in the initial conformer population led to distinctive VUV dip signals in the spectra. 17,20 To ensure that the measured spectra represented the vibrational spectra of the cationic tt-cis and tt-gauche conformers, the IR hole-burn VUV-MATI spectra of both conformers were combined by normalisation with respect to the 0-0 band of each conformer in the VUV-MATI spectrum of trans-2PA (Fig. 5(c) and (d)).…”

Unraveling the Conformational Preference of Morpholine: Insights from Infrared Resonant Vacuum Ultraviolet Photoionization Mass Spectroscopy

“…Previous techniques, such as ultraviolet (UV) and UV laser double resonance, focused primarily on hydrogen-bonded clusters or van der Waals complexes in chromophore-containing systems and were limited by the resonance-enhanced multiphoton ionization (REMPI) scheme. − Subsequent advancements in double resonance techniques involving IR and VUV that can circumvent the limitations of REMPI allowed for the analysis of aliphatic species without a chromophore group, albeit with some uncontrollable fragmentations. , In contrast, IR dip/hole-burn VUV-MATI spectroscopy offers a direct means to gather structural information on individual conformers in both the S 0 and D 0 states, providing precise experimental data on conformational populations. − Specifically, IR hole-burn VUV-MATI delivers spectra of individual conformers in the D 0 state, while IR dip VUV-MATI captures the distinct IR spectra of individual conformers in the S 0 state.…”