One-photon Mass-analyzed Threshold Ionization Research Articles

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.

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.

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.

Conformer-specific photoionization and conformational stabilities of isobutanal revealed by one-photon mass-analyzed threshold ionization (MATI) spectroscopy.

Isobutanal is an aliphatic aldehyde which has been extensively studied as an important intermediate in isomerization reactions as well as in astrochemically relevant models in the interstellar medium. Herein, we report on the conformer-specific photoionization and conformational stabilities of isobutanal utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet (VUV) pulses. The conformational population of isobutanal under different supersonic expansion conditions was explored to identify the conformers, from which their intrinsic photoionizations can be directly elucidated by measuring the VUV-MATI spectrum corresponding to each conformer. The observed MATI spectra could be analyzed through Franck-Condon simulations at the B3LYP/cc-pVTZ level for the isobutanal conformers, trans and gauche, for which the adiabatic ionization energies were precisely determined to be 78 133 ± 3 cm-1 (9.6873 ± 0.0004 eV) and 78 557 ± 3 cm-1 (9.7398 ± 0.0004 eV), respectively. Notably, only the gauche conformer undergoes a unique geometrical change upon ionization, resulting in the progression of the CHO torsional mode in the MATI spectra. Consequently, we determined the conformational stabilities of isobutanal by conformer-specific photoionization, given that the gauche is more stable than the trans by 162 ± 50 cm-1 in the neutral ground state, while the cationic gauche is less stable than the cationic trans by 262 ± 50 cm-1.

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

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.

Determination of the accurate ionization energy and cationic structure of cyanopyrazine by one-photon mass-analyzed threshold ionization spectroscopy

The ionization energy and cationic structure of cyanopyrazine have been determined for the first time by pulsed-field threshold ionization mass spectrometry. The ionization energy was accurately measured to be 80,305±6cm−1 (9.9566±0.0007eV) from the 0-0 band position in the mass-analyzed threshold ionization (MATI) spectrum obtained for the direct S0→D0 transition using a vacuum ultraviolet laser pulse generated by four-wave mixing in Kr. The highest occupied molecular orbital (HOMO) and singly occupied molecular orbital (SOMO) calculations indicated the extent of conjugation of nonbonding orbitals on nitrogen atoms with the π orbital in cyanopyrazine, and clarified the effect of substitution of H with CN in pyrazine on the ionization energy of cyanopyrazine. The observed MATI spectrum was interpreted using the Franck–Condon factors and vibrational frequencies calculated through adjustment of the geometrical parameters of the cyanopyrazine cation in the ground electronic state at the B3LYP/cc-pVTZ level. From rigorous analyses, it has been concluded that upon ionization, the cyanopyrazine cation in the D0 state should retain a planar structure with Cs symmetry, similar to the ground state molecule.

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of pyridine: determination of accurate ionization energy and cationic structure.

Ionization energies and cationic structures of pyridine were intensively investigated utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The present one-photon high-resolution MATI spectrum of pyridine demonstrated a much finer and richer vibrational structure than that of the previously reported two-photon MATI spectrum. From the MATI spectrum and photoionization efficiency curve, the accurate ionization energy of the ionic ground state of pyridine was confidently determined to be 73,570 ± 6 cm(-1) (9.1215 ± 0.0007 eV). The observed spectrum was almost completely assigned by utilizing Franck-Condon factors and vibrational frequencies calculated through adjustments of the geometrical parameters of cationic pyridine at the B3LYP/cc-pVTZ level. A unique feature unveiled through rigorous analysis was the prominent progression of the 10 vibrational mode, which corresponds to in-plane ring bending, and the combination of other totally symmetric fundamentals with the ring bending overtones, which contribute to the geometrical change upon ionization. Notably, the remaining peaks originate from the upper electronic state ((2)A2), as predicted by high-resolution photoelectron spectroscopy studies and symmetry-adapted cluster configuration interaction calculations. Based on the quantitatively good agreement between the experimental and calculated results, it was concluded that upon ionization the pyridine cation in the ground electronic state should have a planar structure of C(2v) symmetry through the C-N axis.

The Jahn-Teller effect in ${\rm CH}_{\rm 3} {\rm Cl}^{\rm + } ( {{\rm \tilde X}{}^{\rm 2}{\rm E}} )$ CH 3 Cl +(X̃E2): A combined high-resolution experimental measurement and ab initio theoretical study

The energy levels of CH(3)Cl(+)X̃(2)E showing strong spin-vibronic coupling effect (Jahn-Teller effect) have been measured up to 3500 cm(-1) above the ground vibrational state using one-photon zero-kinetic energy photoelectron and mass-analyzed threshold ionization spectroscopic method. Theoretical calculations have been also performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (PESs). In the theoretical calculations the diabatic potential energy surfaces are expanded by the Taylor expansions up to the fourth-order including the multimode vibronic interactions. The calculated spin-orbit energy splitting (224.6 cm(-1)) for the ground vibrational state is in good agreement with the experimental data (219 ± 3 cm(-1)), which indicates that the Jahn-Teller and the spin-orbit coupling have been properly described in the theoretical model near the zero-point energy level. Based on the assignments predicted by the theoretical calculations, the experimentally measured energy levels were fitted to those from the diabatic model by optimizing the main spectroscopic parameters. The PESs from the ab initio calculations at the level of CASPT2/vq(t)z were thus compared with those calculated from the experimentally determined spectroscopic parameters. The theoretical diagonal elements in the diabatic potential matrix are in good agreement with those determined using the experimental data, however, the theoretical off-diagonal elements appreciably deviate from those determined using the experimental data for geometric points far away from the conical intersections. It is also concluded that the JT effect in CH(3)Cl(+) mainly arises from the linear coupling and the mode coupling between the CH(3) deform (υ(5)) and CH(3) rock (υ(6)) vibrations. The mode couplings between the symmetric C-Cl stretching vibration υ(3) with υ(5) and υ(6) are also important to understand the spin-vibronic structure of the molecule.

Aromatic π–π interaction mediated by a metal atom: structure and ionization of the bis(η6-benzene)chromium–benzene cluster

Aromatic pi-pi interaction in the presence of a metal atom has been investigated experimentally and theoretically with the model system of bis(eta(6)-benzene)chromium-benzene cluster (Cr(Bz)(2)-Bz) in which a free solvating benzene is non-covalently attached to the benzene moiety of Cr(Bz)(2). One-photon mass-analyzed threshold ionization (MATI) spectroscopy and first principles calculations are employed to identify the structure of Cr(Bz)(2)-Bz which adopts the parallel-displaced configuration. The decrease in ionization potential for Cr(Bz)(2)-Bz compared with Cr(Bz)(2), resulting from the increase of the cation-pi stabilization energy upon ionization, is consistent with the parallel-displaced structure of the cluster. Theoretical calculations give the detailed cluster structures with associated energetics, thus revealing the nature of pi-pi-metal or pi-pi-cation interactions at the molecular level.

Rotational state selection of a CH3I+ ion beam using vacuum ultraviolet–mass-analyzed threshold ionization spectroscopy: Characterization using photodissociation spectroscopy

The A(2)A(1)<--X(2)E(3/2) transition of CH(3)I(+) was investigated by photodissociation (PD) of the cation generated by one-photon mass-analyzed threshold ionization (MATI). Compared to the PD spectrum obtained by excitation of the cation in the main 0-0 band in the MATI spectrum, those obtained by excitation of the cations in the satellite structures showed substantially simplified rotational structures for nondegenerate vibronic bands. Spectral simplification occurred because each satellite consisted mostly of cations with one K quantum number. Spectroscopic constants in the ground vibronic state and in the 2(1)3(5), 2(1)3(8), 3(9), and 3(13) nondegenerate vibrational states in A(2)A(1) were determined via spectral fitting. Also, those in the 2(1)3(n)6(1) (n=1?) degenerate state, which had been reported previously, was improved. The K quantum number in each satellite determined by the present high resolution study was compatible with the prediction by the symmetry selection rule for photoionization. That is, the K quantum number of the ion core in high Rydberg states accessed by one-photon excitation was found to be conserved upon pulsed field ionization. This work demonstrates generation of mass-selected, vibronically selected, and K-selected ion beam by one-photon MATI.

K selection in one-photon mass-analyzed threshold ionization of CH3I and CD3I to the X̃E3∕22 state cations

One-photon mass-analyzed threshold ionization (MATI) spectra for the X (2)E(3/2) states of CH(3)I(+) and CD(3)I(+) were measured using vacuum ultraviolet radiation generated by four-wave mixing in Kr. Spin-orbit density functional theory calculations at the B3LYP/aug-cc-pVTZ level and spin-orbit/Jahn-Teller calculations were made to aid vibrational assignment. Each vibrational band consisted of several peaks due to different DeltaK transitions, which could be assigned by using molecular parameters determined in the previous high resolution photodissociation spectroscopic study. Possibility of generating mass-selected, vibronically selected and K-selected ion beam with decent intensity by one-photon MATI was demonstrated. The ionization energies to the X (2)E(3/2) states of CH(3)I(+) and CD(3)I(+) corrected for the rotational contribution were 9.5386+/-0.0006 and 9.5415+/-0.0006 eV, respectively.

One-photon ionization spectroscopy of jet-cooled oxazole and thiazole: the role of oxygen and sulfur in the π-conjugation of heterocyclic compounds

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of jet-cooled oxazole and thiazole has been carried out to give the precise adiabatic ionization energies of 9.5959+/-0.0006 and 9.3633+/-0.0009 eV, respectively. The structural change upon ionization has been revealed in the vibrationally resolved one-photon MATI spectra. Simulations based on the Franck-Condon analysis using the molecular structures calculated by the density functional theory reproduce the experiment very well for both molecules. The ionization-driven structural change of thiazole is quite different from that of oxazole in terms of the detailed geometrical shape, ascribed to the difference in the pi-conjugation nature of two molecules. The role of oxygen and sulfur in the stabilization of heterocyclic systems is discussed through the inspection of the calculated molecular orbitals involved in the photoionization.

Vibrational Spectroscopy and State-Specific Dissociation Dynamics for Vinyl Chloride Cation in the B̃ State

C(2)H(3)(35)Cl+ in the ground vibronic state was generated by one-photon mass-analyzed threshold ionization spectrometry, and its photodissociation in the 461-406 nm range was investigated. Ionization energy to the ground state of C(2)H(3)(35)Cl+ was 10.0062 +/- 0.0006 eV while its B state onset was higher by 2.7456 +/- 0.0003 eV. A vibrational spectrum of the cation in the B state obtained by recording the product ion yield as a function of wavelength was analyzed by referring to the quantum chemical results at the TDDFT/B3LYP/6-311++(df,pd) level. Analysis of product time-of-flight profiles recorded with different laser polarization angles showed that the dissociation pathway for the cation in the B state changed with the vibrational energy, from internal conversion to X and statistical dissociation therein to curve crossing to C and repulsive dissociation therein. B --> C curve crossing seemed to occur along a direction close to the C-Cl bond stretch.

One-photon mass-analyzed threshold ionization spectroscopy (MATI) of cis-dichloroethylene ( cis-C 2H 2Cl 2): Cation structure determination via Franck–Condon fit

A high-quality one-photon mass-analyzed threshold ionization (MATI) spectrum of cis-C 2H 2Cl 2 was obtained by using vacuum ultraviolet radiation generated by four-wave mixing in Kr. The ionization energy determined from the position of the 0–0 band in the spectrum was 9.6578 ± 0.0006 eV. Ten vibrational fundamentals for the cation were identified. Most of the overtones and combinations could be assigned properly by comparing with the quantum chemical calculation results. The equilibrium geometry of the cation was determined through Franck–Condon fit.

Study of the upper spin–orbit state of the chloroiodomethane cation by mass-analyzed threshold ionization

The vibrational spectrum of CH 2ClI + in the upper spin–orbit state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectrometry. The accurate ionization energy of the upper spin–orbit state was determined to be 10.3045 ± 0.0006 eV. The energy gap between the upper and lower spin–orbit states was 0.5539 ± 0.0012 eV. The frequency of the Cl–C–I bending mode was found to be most affected by the spin–orbit interaction. This suggests that the avoided crossing between the lower and upper spin–orbit states occurs along a direction close to the Cl–C–I bending coordinate.

Mass-Analyzed Threshold Ionization Study of Vinyl Chloride Cation in the First Excited Electronic State Using Vacuum Ultraviolet Radiation in the 107−102.8 nm Range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.

One-Photon Mass-Analyzed Threshold Ionization Spectroscopy (MATI) of trans-Dichloroethylene (trans-C2H2Cl2): Cation Structure Determination via Franck−Condon Fit

One-photon mass-analyzed threshold ionization (MATI) spectrum of trans-C(2)H(2)Cl(2) was obtained by using vacuum ultraviolet radiation generated by four-wave mixing in Kr. The ionization energy determined from the position of the 0-0 band in the spectrum was 9.6306 +/- 0.0006 eV. Ten vibrational fundamentals for the cation were identified. The spectrum also displayed abundant overtones and combinations, most of which could be assigned adequately by comparing with the quantum chemical results. It was found that channel interaction was not important for this system. The equilibrium geometry of the cation was estimated through the Franck-Condon fit.

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.

Preparing transition-metal clusters in known structural forms: The mass-analyzed threshold ionization spectrum of V3

The first results are presented of a new experiment designed both to generate and characterize spectroscopically individual isomers of transition-metal cluster cations. As a proof of concept the one-photon mass-analyzed threshold ionization (MATI) spectrum of V3 has been recorded in the region of 44,000-45,000 cm-1. This study extends the range of a previous zero-kinetic-energy (ZEKE) photoelectron study of Yang et al. [Chem. Phys. Lett. 231, 177 (1994)] with which the current results are compared. The MATI spectra reported here exhibit surprisingly high resolution (0.2 cm-1) for this technique despite the use of large discrimination and extraction fields. Analysis of the rotational profile of the origin band allows assignment of the V3 ground state as and the V3+ ground state as , both with D3h geometry, in agreement with the density-functional theory study of the V3 ZEKE spectrum by Calaminici et al. [J. Chem. Phys. 114, 4036 (2001)]. There is also some evidence in the spectrum of transitions to the low-lying excited state of the ion. The vibrational structure observed in the MATI spectrum is, however, significantly different to and less extensive than that predicted in the density-functional theory study. Possible reasons for the discrepancies are discussed and an alternative assignment is proposed which results in revised values for the vibrational wave numbers of both the neutral and ionic states. These studies demonstrate the efficient generation of cluster ions in known structural (isomeric) forms and pave the way for the study of cluster reactivity as a function of geometrical structure.

One-photon mass-analyzed threshold ionization spectroscopy of 2-chloropropene (2-C3H5Cl) and its vibrational assignment based on the density-functional theory calculations

A high-quality mass-analyzed threshold ionization (MATI) spectrum of 2-chloropropene, 2-C3H5Cl, is reported. Its ionization energy determined for the first time from the 0-0 band position was 9.5395+/-0.0006 eV. Almost all the peaks in the MATI spectrum could be vibrationally assigned utilizing the frequencies calculated at the B3LYP6-311++G(3df,3pd) level and the Franck-Condon factors calculated with the molecular parameters obtained at the same level. In particular, the observed methyl torsional progression could be reproduced very well through quantum-mechanical calculations using the molecular parameters obtained at this level. Dramatic lowering of the torsional barrier inferred from the experimental data was entirely compatible with the B3LYP6-311++G(3df,3pd) results. The torsional barrier and the internal rotational constant determined by fits to six torsional peaks were 53.6 and 5.20 cm(-1), respectively. A brief discussion at the level of molecular orbital is presented to account for the dramatic lowering of the torsional barrier upon ionization.