Abstract
We measured the conformer-specific vibrational spectra of C4H6O isomers in neutral and cationic states using IR resonant vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy for the first time. Notably, the measured IR dip and hole-burn VUV-MATI spectra for each isomer represent the identifiable vibrational spectra of individual conformers in both states. Furthermore, we estimated the relative populations of individual conformers in crotonaldehyde (CA) and methyl vinyl ketone (MVK) isomers using the IR dip intensity, the corresponding Franck–Condon factor, and the IR absorption cross section. Our analysis revealed that the compositional ratio of s-trans to s-cis conformers in the CA isomer remained at 95.8 : 4.2 even under supersonic expansion, whereas that in the MVK isomer was determined as 90.6 : 9.4, which is consistent with previous research. These findings reveal that the conformational stability of each isomer depends on the position of the methyl group relative to the carbonyl group.
Highlights
Subsequent investigations have revealed that molecules with a sufficiently low conformer interconversion barrier exist in conformational equilibrium even at low temperatures,[7,8,9] whereas conformers with a high interconversion barrier retain their original composition during cooling via supersonic expansion.[10,11,12]
The two most intense peaks, at 77 861 and 78 638 cmÀ1 correspond to the 0–0 bands of methyl vinyl ketone (MVK) and CA, respectively, which is consistent with the respective adiabatic ionization energy (AIE) values of 867 Æ 4 and 640 Æ 3 cmÀ1 determined in previous studies by extrapolating to the zero- eld limit of the MVK and CA using vacuum ultraviolet (VUV)-MATI spectroscopy.[9,10]
Other than using FC simulations to determine the ionic transitions of the conformers expected in the S0 state, no method exists for identifying the vibrational peaks corresponding to individual conformers in the MATI spectrum of each isomer
Summary
The identi cation of individual conformers is typically performed using unique spectroscopic techniques capable of providing insights into the conformation-dependent reactivity of speci c chemical reactions.[1,2,3,4] Such techniques reduce conformational complexity by utilizing the adiabatic cooling of molecules that results from supersonic expansion in the absence of the non-equilibrium kinetic effect.[5,6] Subsequent investigations have revealed that molecules with a sufficiently low conformer interconversion barrier exist in conformational equilibrium even at low temperatures,[7,8,9] whereas conformers with a high interconversion barrier retain their original composition during cooling via supersonic expansion.[10,11,12] For the former case, identifying peaks that correspond to a speci c conformer in measured vibrational spectra requires additional exploration of the vibrational temperature-dependent conformational population, which, in turn, depends on the constructed potential energy surfaces associated with conformational interconversion.
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