Deciphering the conformational preference and ionization dynamics of tetrahydrofuran: Insights from advanced spectroscopic techniques.

Tetrahydrofuran (THF) has garnered significant attention due to its pivotal role in biological and chemical processes. The diverse array of conformations exhibited by THF profoundly impacts its reactivity and interactions with other molecules. Understanding these conformational preferences is crucial for comprehending its molecular behavior. In this study, we utilize infrared (IR) resonant vacuum ultraviolet photoionization/mass-analyzed threshold ionization (VUV-PI/MATI) mass spectroscopies to capture distinctive vibrational spectra of individual conformers, namely, "twisted" and "bent," within THF. Our conformer-specific vibrational spectra provide valuable insights into the relative populations of these two conformers. The analysis reveals that the twisted (C2) conformer is more stable than the bent (CS) conformer by 17 ± 15 cm-1. By precisely tuning the VUV photon energy to coincide with vibrational excitation via IR absorption, we selectively ionize specific conformers, yielding two-photon IR + VUV-PI/MATI spectra corresponding to the twisted and bent conformers. This investigation conclusively affirms that both the twisted and bent conformers coexist in the neutral state, while only the twisted conformer exists in the cationic state. These findings not only bridge gaps in existing knowledge but also provide profound insights into the behavior of this pivotal molecule in the realms of biology and medicine.

Effects of Rumen-Protected L-Tryptophan Supplementation on Productivity, Physiological Indicators, Blood Profiles, and Heat Shock Protein Gene Expression in Lactating Holstein Cows under Heat Stress Conditions.

In this study, we examined the effects of rumen-protected L-tryptophan supplementation on the productivity and physiological metabolic indicators in lactating Holstein cows under heat stress conditions. The study involved eight early lactating Holstein cows (days in milk = 40 ± 9 days; milk yield 30 ± 1.5 kg/day; parity 1.09 ± 0.05, p < 0.05), four cows per experiment, with environmentally controlled chambers. In each experiment, two distinct heat stress conditions were created: a low-temperature and low-humidity (LTLH) condition at 25 °C with 35-50% humidity and a high-temperature and high-humidity (HTHH) condition at 31 °C with 80-95% humidity. During the adaptation phase, the cows were subjected to LTLH and HTHH conditions for 3 days. This was followed by a 4-day heat stress phase and then by a 7-day phase of heat stress, which were complemented by supplementation with rumen-protected L-tryptophan (ACT). The findings revealed that supplementation with ACT increased dry matter intake as well as milk yield and protein and decreased water intake, heart rate, and rectal temperature in the HTHH group (p < 0.05). For plateletcrit (PCT, p = 0.0600), the eosinophil percentage (EOS, p = 0.0880) showed a tendency to be lower, while the monocyte (MONO) and large unstained cells (LUC) amounts were increased in both groups (p < 0.05). Albumin and glucose levels were lower in the HTHH group (p < 0.05). The gene expressions of heat shock proteins 70 and 90 in the peripheral blood mononuclear cells were higher in the ACT group (HTHH, p < 0.05). These results suggest that ACT supplementation improved productivity, physiological indicators, blood characteristics, and gene expression in the peripheral blood mononuclear cells of early lactating Holstein cows under heat-stress conditions. In particular, ACT supplementation objectively relieved stress in these animals, suggesting that L-tryptophan has potential as a viable solution for combating heat-stress-induced effects on the cattle in dairy farming.

Conformational diversity and environmental implications of trans-2-pentenal.

This study investigates the conformational intricacies of trans-2-pentenal (trans-2PA), a significant biogenic volatile organic compound. To unveil its potential implications in atmospheric chemistry and environmental pollution, we employ advanced infrared resonant vacuum ultraviolet mass-analysed threshold ionisation spectroscopy. Through this method, we identify the major conformers within trans-2PA, encompassing trans-s-trans (tt-) and trans-s-cis (tc-) structures with planar (cis) and non-planar (gauche) configurations introduced by a methyl group. In a pioneering spectroscopic examination, we analyze trans-2PA in both the neutral and cationic states. This approach allows us to gain a comprehensive understanding of its molecular behavior. Our conformer-specific vibrational spectra not only reveal the relative populations of the main conformers, notably tt-cis and tt-gauche conformers, but also shed light on atmospheric oxidation processes and lower tropospheric organic aerosol formation mechanisms. Our findings expand the understanding of the role of trans-2PA in environmental and biological contexts. Additionally, they contribute to a broader understanding of its influence on air quality, climate, and atmospheric dynamics. The collaboration between advanced experimental techniques and computational methods fortifies the scientific underpinning of this study, opening doors to further exploration in the realms of atmospheric chemistry and environmental science.

Determination of the highest occupied molecular orbital and conformational structures of morpholine based on its conformer-specific photoionization dynamics.

Morpholine, a heterocycle composed of an ether and amine, is commonly used as a precursor in many organic synthesis processes because of the nucleophilicity induced by the lone-pair electrons of the nitrogen atom within its ring. Herein, we investigated the conformer-specific photoionization dynamics of morpholine under molecular-beam conditions using high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) mass spectroscopy. Two-dimensional potential energy surfaces (2D PESs) associated with the conformational changes in the neutral (S0) and cationic (D0) ground states were constructed to identify the conformer(s) corresponding to the obtained VUV-MATI spectrum. The 2D PESs indicated that the chair and twisted boat forms with equatorial and axial NH conformations (four conformers with the following relative energies: Chair-Eq < Chair-Ax ≪ Twisted boat-Ax < Twisted boat-Eq) of morpholine lie on the global minimum of the S0 state. However, only the axial-like NH conformation in each form (stable Chair-Ax-like+˙ and Twisted boat-Ax-like+˙ conformers) exists in the D0 state. Accordingly, vibration assignment was performed based on Franck-Condon (FC) analyses of the adiabatic ionic transitions from each Chair-Eq and Chair-Ax conformer to the Chair-Ax-like+˙ conformer. The FC analyses revealed that only the Chair-Ax conformer contributes to the ionic transitions to the Chair-Ax-like+˙ conformer owing to the large FC factors, whose adiabatic ionization energy was determined to be 8.1003 ± 0.0005 eV. Consequently, adiabatic ionization arises because of electron removal from the highest occupied molecular orbital consisting of the nonbonding orbital of the N atom in the Chair-Ax conformer.

Valence molecular orbitals and cationic structures of 2-fluoropyridine by high-resolution ion spectroscopy and Franck-Condon fitting.

The alteration of the valence molecular orbitals' ordering of halopyridine molecules, by the introduction of a halogen atom(s) as substituent on the pyridine ring, has spurred an extensive interest for their investigation. Herein, the effect of a fluorine substituent on the two outermost orbitals of pyridine was elucidated by investigating the photoionization dynamics of 2-fluoropyridine (2-FP), considering that the geometrical changes with respect to the neutral geometry induced by adiabatic ionic transition affect the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectrum. The adiabatic ionization energy associated with the 0-0 band on the measured high-resolution VUV-MATI spectrum was determined to be 9.6702 ± 0.0004 eV (77 995 ± 3 cm-1), which differs considerably from the 9.401 eV by two-color ionization spectroscopy. Franck-Condon simulation of the MATI spectrum corresponded quantitatively with the experimental results. Interestingly, among the forbidden transitions under CS symmetry, an out-of-plane ring-bending mode resulting from the warped cationic structure of 2-FP with C1 symmetry was discovered. Rigorously, among the unassigned peaks, the first prominent peak at 78 532 cm-1 should rather be assigned as the origin of the excited electronic state (D1) of the 2-FP cation, in accordance with time-dependent density functional theory calculations. Natural bond orbital analysis led to the conclusion that such observations could be induced by electron removal from the highest occupied molecular orbital (HOMO) consisting of the π orbital of the pyridine ring and lone-pair orbital of the fluorine atom or from the HOMO-1 of the molecular non-bonding orbitals, to generate the two proximate electronic states of the cation.

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

Pivaldehyde, which is an unwanted by-product released with engine exhaust, has received considerable research attention because of its hydrocarbon oxidations at atmospheric temperature. To gain insight into the conformer-specific reaction dynamics, we investigated the conformational structures of the pivaldehyde molecule in neutral (S0) and cationic (D0) states using the recently invented IR-resonant VUV-MATI mass spectroscopy. Additionally, we constructed the two-dimensional potential energy surfaces (2D PESs) associated with the conformational transformations in the S0 and D0 states to deduce the conformations corresponding to the measured vibrational spectra. The 2D PESs indicated the presence of only the eclipsed conformation in the global minima of both states, unlike those in propanal and isobutanal. However, comparing the IR-dip VUV-MATI spectra from two intense peaks in the VUV-MATI spectrum with the anharmonic IR simulations revealed the correspondence between the gauche conformer on the S0 state and the measured IR spectra. Furthermore, Franck-Condon analysis confirmed that most peaks in the VUV-MATI spectrum are attributed to the adiabatic ionic transitions between the neutral gauche and cationic eclipsed conformers in pivaldehyde. Consequently, electron removal from the highest occupied molecular orbital, consisting of the nonbonding orbital of the oxygen atom in pivaldehyde, promoted the formyl-relevant modes in the induced cationic eclipsed conformer.

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

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.

Development and Verification of Conformer-Specific Vibrational Spectroscopy.

Conformers have similar vibrational structures both in neutral (S0) and cationic (D0) 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 S0 state. We developed an inventive conformer-specific vibrational spectroscopic technique to measure identifiable vibrational spectra of individual conformers in both S0 and D0 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.

Probing the Photoionization Dynamics of 2-Cyclopenten-1-one via High-Resolution VUV-MATI Spectroscopy.

2-Cyclopenten-1-one (2CP), which is a cyclic enone, has been considered an important precursor because of its versatile functionality in the synthesis of natural products and materials for biofuels. Here, we report the adiabatic ionization energy (AIE) and cationic structure of 2CP in the ionic transition between the neutral S0 and the cationic D0 states probed by high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the 0-0 band position in the VUV-MATI spectrum supported by the VUV-photoionization efficiency curve, the AIE of 2CP was determined to be 9.3477 ± 0.0004 eV (75,395 ± 3 cm-1), which is in good agreement with the reference value but much more accurate. The measured MATI spectrum combined with the Franck-Condon fitting at the B3LYP/cc-pVTZ level revealed that the cationic structure of 2CP is twisted with the C1 symmetry, whereas the neutral 2CP has the CS symmetry. The results indicate that geometrical changes induced by ionization are mainly attributed to the electron removal from the highest occupied molecular orbital, which consists of nonbonding orbitals on the oxygen atom in the carbonyl group interacting with the σ orbitals in the molecular plane of 2CP. Consequently, lowering the C1 symmetry for cationic 2CP led to the promotions of the ring-bending and ring-twisting modes in the MATI spectrum, which correspond to the ring puckering and C═C twisting in the S0 state, respectively.

Innovative mass spectrometer for high-resolution ion spectroscopy.

Conventional ion spectroscopy is inapplicable for ions produced in low concentrations or with low spectral resolutions. Hence, we constructed a high-resolution vacuum ultraviolet mass-analyzed threshold ionization (HR VUV-MATI) spectrometer composed of a four-wave frequency mixing cell capable of generating long-lasting and intense VUV laser pulses of ∼1 × 1010 photons/pulse at wavelengths of 123.6-160.0 nm, a space-focused linear time-of-flight photoionization chamber with a new ion-source assembly, and a compact molecular beam chamber with a temperature-controlled pulsed nozzle for ion spectroscopy. The ion-source assembly and pulsing schemes enabled an ∼15-µs-delayed but extremely weak pulsed-field-ionization of the molecules in the zero-kinetic-energy (ZEKE) states and first-order space focusing of the generated MATI ions. These ZEKE states were effectively generated by a minute electric jitter from the high-lying Rydberg states, which were initially prepared via VUV photoexcitation. The spectral and mass resolutions (∼5 cm-1 and 2400, respectively) and the signal strength were simultaneously enhanced using this spectrometer. Moreover, it could be used to measure the fine vibrational spectrum from the zero-point level of the cation and the exact adiabatic ionization energy of the neutral molecule. Additionally, it could be used to measure the appearance energies of the photoproducts and elucidate the vibrational structures of the cationic isotopomers, utilizing other pulsing schemes. Furthermore, this spectrometer could be used to analyze the congested vibrational spectrum of a cation with multiple conformations. Thus, the HR VUV-MATI spectrometer-a potential alternative to photoelectron spectrometers-can be used to analyze the conformational structure-dependent reactivities.

One-photon VUV-MATI and two-photon IR+VUV-MATI spectroscopic determination of oxetane cation ring-puckering vibrations and conformation.

The conformational structures of heterocyclic compounds are of considerable interest to chemists and biochemists as they are often the constituents of natural products. Among saturated four-membered heterocycles, the conformational structure of oxetane is known to be slightly puckered in equilibrium because of a low interconversion barrier in its ring-puckering potential, unlike cyclobutane and thietane. We measured the one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) and two-photon IR+VUV-MATI spectra of oxetane for the first time to determine the ring-puckering potential of the oxetane cation and hence its conformational structure in the D0 (ground) state. Remarkably, negative anharmonicity and large amplitudes were observed for the ring-puckering vibrational mode progression in the low-frequency region of the observed MATI spectra. We were able to successfully analyze the progression in the MATI spectra through the Franck-Condon simulations, using modeled potential energy functions for the ring-puckering modes in the S0 and D0 states. Considering that the interconversion barrier and puckered angle for the ring-puckering potential on the S0 state were found to be 15.5 cm-1 and 14°, respectively, the cationic structure is expected to be planar with C2v symmetry. Our results revealed that the removal of an electron from the nonbonding orbitals on the oxygen atom in oxetane induced the straightening of the puckered ring in the cation owing to an increase in ring strain. Consequently, we conclude that this change in the conformational structure upon ionization generated the ring-puckering vibrational mode progression in the MATI spectra.

Determination of the highest occupied molecular orbital and cationic structure of 2-chloropyridine by one-photon VUV-MATI spectroscopy and Franck-Condon fitting.

2-Chloropyridine (2-CP) has received significant attention, owing to the effect of the substitution of a halogen in pyridine on the highest occupied molecular orbital (HOMO). To elucidate the substitution effect of the chlorine atom on the HOMO of pyridine, we obtained one-photon vacuum ultraviolet mass-analysed threshold ionization (VUV-MATI) spectra of 2-CP having 35Cl or 37Cl to analyse the isotope effect on the vibrational mode. Based on the 0-0 band in the MATI spectrum of 2-CP having 35Cl, the adiabatic ionization energy was determined to be 9.4743 ± 0.0005 eV (76 415 ± 4 cm-1) with a similar value for 37Cl, which is much lower but more accurate than the vertical value of 9.63 eV determined by photoelectron spectroscopy. Subsequently, the MATI spectrum, which was affected by the geometrical change with respect to the neutral geometry upon ionization, could be analysed by Franck-Condon fitting and spectral correlation between the two isotopomers. Notably, we observed the appearance of the out-of-plane ring bending modes resulting from ring distortion, unlike in pyridine. Furthermore, natural bond orbital analysis led to the conclusion that the warped structure with C1 symmetry of cationic 2-CP is induced by the electron removal from the HOMO consisting of the π orbital in the pyridine ring, which is stabilized by hyperconjugation with the lone-pair p orbitals of a nitrogen and chlorine atom.

Accurate conformational stability and cationic structure of piperidine determined by conformer-specific VUV-MATI spectroscopy.

Piperidine has received attention in pharmaceutical synthesis and biochemical degradation because of its conformational activity. We explored the conformational structures of piperidine in the neutral (S0) and cationic (D0) ground states by conformer-specific vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy, which provides high-resolution vibrational spectra for the corresponding cationic conformer. To identify conformers corresponding to the obtained VUV-MATI spectra, equilibrium structures of piperidine conformers in the S0 and D0 states were determined at various density functional theory levels, and potential energy surfaces associated with the conformational changes were constructed. Notably, the chair form interconverting between the equatorial NH and the axial NH conformers (Chair-Eq and Chair-Ax) in piperidine lies on the global minimum of the S0 state, but only the axial-like NH conformer (Chair-Ax-like) in chair form exists in the D0 state. The vibrational assignment of the observed spectra was accomplished through Franck-Condon (FC) analysis for adiabatic transitions between two Chair-Eq and Chair-Ax conformers and a cationic Chair-Ax-like conformer. Rigorous FC analysis revealed the precise structure of a cationic Chair-Ax-like conformer induced by removal of an electron from the lone-pair sp3 orbital of the nitrogen atom in piperidine. The adiabatic ionization energies of Chair-Eq and Chair-Ax conformers converting to a cationic state were determined to be 64 704 ± 4 cm-1 (8.0223 ± 0.0005 eV) and 64 473 ± 4 cm-1 (7.9936 ± 0.0005 eV), respectively. Consequently, the difference between their adiabatic ionization energies allowed the accurate determination of the conformational stability of Chair-Eq and Chair-Ax conformers in piperidine (231 ± 4 cm-1).

Conformer-specific VUV-MATI spectroscopy of methyl vinyl ketone: stabilities and cationic structures of the s-trans and s-cis conformers.

Methyl vinyl ketone (MVK), a volatile compound with photochemical activity, has received considerable attention in the fields of environmental chemistry and atmospheric chemistry. We explored the conformational stabilities of MVK in the neutral S0 and the cationic D0 states using conformer-specific vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy, which provided identifiable vibrational spectra for cationic MVK conformers. Based on the origin bands of the two individual conformers of MVK identified in the MATI spectra under different supersonic expansion conditions, the accurate adiabatic ionization energies of the s-trans and the s-cis conformers were determined to be 77 867 ± 4 (9.6543 ± 0.0005 eV) and 78 222 ± 4 cm-1 (9.6983 ± 0.0005 eV), respectively. The identifiable vibrational spectra of the two cationic conformers were further confirmed using vibrational assignments based on the Franck-Condon fit. Accordingly, precise cationic structures of the MVK conformers could be determined. The structural changes of the two conformers upon ionization could be attributed to the removal of an electron from the highest occupied molecular orbital of each conformer, which consists of nonbonding molecular orbitals on the oxygen atom in the carbonyl group interacting with the σ orbitals in the molecular plane. Consequently, the s-trans conformer was preferred by 48 ± 18 and 403 ± 18 cm-1 in the neutral ground S0 and the cationic D0 states, respectively, which was supported by density-corrected density functional theory calculations and natural bond orbital analysis.

Conformational potential energy surfaces and cationic structure of 3,4-dihydro-2H-pyran by VUV-MATI spectroscopy and Franck-Condon fitting.

Ring conformations of 3,4-dihydro-2H-pyran (34DHP) have attracted considerable interest owing to their structural similarity to cyclohexene, an important molecule in stereochemistry. In this study, we investigated the conformational interconversion of 34DHP in both the neutral (S0) and the cationic (D0) ground states. High-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy was utilized to obtain information regarding the adiabatic ionic transition between the S0 and the D0 states. Based on the 0-0 band in the VUV-MATI spectrum supported by the VUV-photoionization efficiency curve, the adiabatic ionization energy of 34DHP was accurately determined to be 8.3355 ± 0.0005 eV (67 230 ± 4 cm-1). To identify the conformer corresponding to this measured value, two-dimensional potential energy surfaces (2D PESs) associated with conformational interconversion in the S0 and the D0 states were constructed at the B3LYP/aug-cc-pVTZ level. It was revealed that in the S0 state, the twisted conformers undergo interconversion through the asymmetric bent conformation on the pseudorotational pathway, whereas in the D0 state, the half-bent conformers directly undergo interconversion via the planar conformation at the saddle point of 2D PES. The change in the conformational interconversion pathway upon ionization is attributed to electron removal from the highest occupied molecular orbital, which consists of a π orbital in the 2C-3C double bond interacting with a nonbonding orbital in the oxygen atom of 34DHP. Then, vibrational assignment of the observed spectrum could be achieved through Franck-Condon fitting for ionic transitions between the neutral twisted and the cationic half-bent conformers. The strong promotion of the ring bending and the 1O-2C-3C asymmetric stretching modes in the adiabatic ionic transitions confirmed the determined cationic structure of 34DHP.

Determination of the cationic conformational structure of tetrahydrothiophene by one-photon MATI spectroscopy and Franck-Condon fitting.

The conformers of tetrahydrothiophene (THT) in the neutral (S0) and cationic (D0) ground states have attracted significant attention in terms of the conformational interconversion through pseudorotation. Herein, these conformers were explored by utilising one-photon mass-analysed threshold ionization (MATI) spectroscopy using the coherently tunable vacuum ultraviolet laser pulse generated by four-wave difference-frequency mixing in Kr medium, which allowed the acquisition of the vibrational spectrum of the corresponding cation. To identify the conformer corresponding to the measured MATI spectrum, the potential energy surfaces associated with pseudorotation in the S0 and D0 states were constructed at the B3LYP/cc-pVTZ level, where the twisted conformer with C2 symmetry in both states lies at the global minimum, while the Cs and C2v conformations were located at the saddle points. Although most of the peaks observed in the spectrum could be assigned as the ionic transitions between the twisted conformers (C2 symmetry) in the S0 and D0 states, distinct nontotally symmetric modes could not be assigned to any allowed vibration. Hence, Franck-Condon fitting was applied for the vibrational assignments in the observed spectrum. This revealed that the cationic conformer had a bent-like twist conformation of C1 symmetry instead of C2 symmetry. Furthermore, the geometrical changes induced by the removal of an electron from the non-bonding orbital of the sulfur atom gave prominent overtones and combination bands of the ring out-of-plane modes associated with pseudorotation as well as the stretching of 2C-1S-3C in the ring.

Conformational preference and cationic structure of 2-methylpyrazine by VUV-MATI spectroscopy and natural bond orbital analysis.

Alkylpyrazines, which are well-known as aromatic substances and traditional medicines, are interesting molecular systems, and their methyl conformations result in unique structural and dynamical properties. We explored the conformational preference of the methyl group and the highest occupied molecular orbitals (HOMOs) of 2-methylpyrazine and its cation by utilizing high-resolution one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy and natural bond orbital analysis to understand the relevant molecular activities. The measured VUV-MATI spectrum of 2-methylpyrazine revealed its adiabatic ionization energy and the vibrational frequencies of its cation. From the 0-0 band in the MATI spectrum under the zero-field limit, the accurate adiabatic ionization energy was determined as 9.0439 ± 0.0006 eV (72 944 ± 5 cm-1), which is lower than that of pyrazine. The peaks observed in the spectrum were unambiguously assigned based on vibrational frequencies and Franck-Condon factors from quantum chemical calculations for individual totally symmetric transitions between the S0 and D0 states using the simple one-photon dipole selection rules. The most convincing molecular structure of the 2-methylpyrazine cation was determined by Franck-Condon fit spectral simulations. Upon removal of an electron from the non-bonding orbital (HOMO) on the para nitrogen atoms, a significant structural change takes place along the vibrational motion associated with ring distortion by contraction of the N-N distance, resulting in prominent overtones and combination bands. In addition, the methyl substitution of pyrazine lowered the adiabatic ionization energy and the methyl group preferred the anti-configuration with respect to the pyrazine moiety in the D0 state, resulting in a frozen internal rotation regardless of ionization.

Ring opening dissociation of cationic twisted conformer by conformer-specific cyclopentanone photoionization.

We investigated the dissociation processes of a cationic conformer, induced by conformer-specific photoionization of cyclopentanone (CP) using a one-photon vacuum ultraviolet (VUV) laser pulse of energy in the range 9.24-9.92 eV for a few nanoseconds, generated by four-wave difference frequency mixing in a Kr cell. The adiabatic ionization energy of the CP was accurately determined to be 9.2697 ± 0.0009 eV, based on the VUV photoionization efficiency curve obtained using high-resolution VUV-photoionization time-of-flight (TOF) mass spectroscopy. The constructed potential energy contours, associated with the twisting and out-of-plane motions in the S0 and D0 states, revealed that the ionization energy value corresponded to a twisted conformer with C2 symmetry at the global minimum. Subsequently, the low photon energy above the ionization onset of the twisted conformer in the CP led to C2H4 elimination, producing a C3H4O+ fragment directly prior to CO elimination for the C4H8 + fragment. The appearance energies for the C3H4O+ and C4H8 + were determined to be 9.7068 ± 0.0017 eV and 9.7483 ± 0.0017 eV, respectively, by measuring the fragmentation yield curves for two fragments analyzed in the TOF mass spectra. The formation enthalpy for each fragment ion at 0 K, evaluated using the measured and thermochemical data, enabled the realization of plausible structures for the produced fragment ions. Consequently, based on the results of the quantum chemical calculation on the dissociation processes of the twisted CP cation (t-CP+), we suggest that the fragmentation processes to C3H4O+ and C4H8 + correspond to the methylketene and (E)-2-butene cations, respectively.

Formyl torsion and cationic structure of gauche conformer in isobutanal by conformer-specific VUV-MATI spectroscopy and Franck-Condon fitting.

We report conformational and vibrational assignments of vacuum ultraviolet mass-analyzed threshold ionization spectrum of the isolated gauche conformer, based on previously determined conformer-specific photoionization and conformational stabilities of isobutanal. The vibrational spectrum of the pure cationic gauche conformer was acquired by removing the trans conformer via conformationally effective cooling with Ar carrier gas. The peaks in the spectrum were assigned by Franck-Condon (FC) fitting by adjusting the cationic geometrical parameters of the gauche conformer at the CAM-B3LYP/cc-pVTZ level. Based on the good agreement between the experimental and calculated results, we were able to determine the precise structure of the cationic gauche conformer of isobutanal with C1 symmetry. Notably, the unveiled vibrational structure was mainly attributed to a geometrical change along the vibrational motions associated with the formyl torsion and CC stretching upon ionization, resulting in their prominent spectral overtones and combination bands with other fundamental vibrations. On the potential energy curve for the formyl torsion of the cationic gauche conformer determined by FC fitting, the transition barrier at 313 cm-1 preserved the hindered formyl torsion in the case of a harmonic potential well, which was confirmed by the progression of formyl torsion, namely, 331, 332, and 333 observed at 60, 120, and 180 cm-1, respectively.

Identification and composition of conformational isomers and their cations in crotonaldehyde by VUV-MATI spectroscopy.

Crotonaldehyde is a simple α,ß-unsaturated aldehyde that reacts stereochemically with nucleophilic reagents according to its conformational structure. Identifiable vibrational spectra of the cationic crotonaldehyde conformers were measured using one-photon vacuum ultraviolet mass-analysed threshold ionization (VUV-MATI) spectroscopy. From the 0-0 bands for the individual conformers confirmed by Franck-Condon (FC) simulations, the precise adiabatic ionization energies were determined to be 9.7501 ± 0.0004 eV (78 640 ± 3 cm-1), 9.7620 ± 0.0004 eV (78 736 ± 3 cm-1), 9.7122 ± 0.0004 eV (78 334 ± 3 cm-1), and 9.6480 ± 0.0004 eV (77 816 ± 3 cm-1) for the trans-s-trans (tt)-, trans-s-cis (tc)-, cis-s-trans (ct)-, and cis-s-cis (cc)-crotonaldehyde, respectively. The complete vibrational assignments were accomplished for the peaks observed in the VUV-MATI spectrum from the calculated vibrational frequencies and the FC factors according to the dipole selection rules for one-photon absorption. In addition, the composition at ambient temperature was determined to be 1.000 (93.0%): 0.037 (3.4%): 0.036 (3.4%): 0.002 (0.2%) for the tt-/tc-/ct-/cc-conformers from the relative intensities of the 0-0 bands in the MATI spectrum normalized with the calculated dipole transition probabilities as proven by the NMR data for the trans- and cis-stereoisomers. This study ascertained the existence of the s-cis conformers of crotonaldehyde for the first time.