Adiabatic Ionization Energies Research Articles

Interaction of Th with H0/-/+: Combined Experimental and Theoretical Thermodynamic Properties.

High-level electronic structure calculations of the low-lying energy electronic states for ThH, ThH-, and ThH+ are reported and compared to experimental measurements. The inclusion of spin-orbit coupling is critical to predict the ground-state ordering as inclusion of spin-orbit switches the coupled-cluster CCSD(T) ordering of the two lowest energy states for ThH and ThH+. At the multireference spin-orbit SO-CASPT2 level, the ground states of ThH, ThH-, and ThH+ are predicted to be the 2Δ3/2, 3Φ2, and 3Δ1 states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to be 0.832 eV in comparison to an experimental value of 0.87 ± 0.02 eV. The observed ThH- photoelectron spectrum has many transitions, which approximately correlate with excitations of Th+ and/or Th. The adiabatic ionization energy of ThH including spin-orbit corrections is calculated to be 6.181 eV. The natural bond orbital results are consistent with a significant contribution of the Th+H- ionic configuration to the bonding in ThH. The bond dissociation energies for ThH, ThH-, and ThH+ using the Feller-Peterson-Dixon approach were calculated to be similar for all three molecules and lie between 259 and 280 kJ/mol.

Threshold photoelectron spectroscopy of iminoborane, HBNH.

We report the mass-selected threshold photoelectron spectrum (ms-TPES) of iminoborane (HBNH), generated by pyrolysis of borazine. The adiabatic ionization energy (IE) of the X+ 2Π ← X 1Σ+ transition was determined to be 11.31 ± 0.02 eV and the wavenumber of the B-N stretching vibration in the cation was measured to be 1550 cm-1. The Renner-Teller splitting in the X+ 2Π state gives rise to two sets of vibrational progressions, separated by 70 meV.

Threshold photoelectron spectroscopy of 9-methyladenine: theory and experiment.

We present a combined experimental and theoretical study of single-photon ionization of 9-methyladenine (9MA) in the gas phase. In addition to tautomerism, several rotamers due to the rotation of the methyl group may exist. Computations show, however, that solely one rotamer contributes because of low population in the molecular beam and/or unfavorable Franck-Condon factors upon ionization. Experimentally, we used VUV radiation available at the DESIRS beamline of the synchrotron radiation facility SOLEIL to record the threshold photoelectron spectrum of this molecule between 8 and 11 eV. This spectrum consists of a well-resolved band assigned mainly to vibronic levels of the D0 cationic state, plus a contribution from the D1 state, and two large bands corresponding to the D1, D2 and D3 electronically excited states. The adiabatic ionization energy of 9MA is measured at 8.097 ± 0.005 eV in close agreement with the computed value using the explicitly correlated coupled cluster approach including core valence, scalar relativistic and zero-point vibrational energy corrections. This work sheds light on the complex pattern of the lowest doublet electronic states of 9MA+. The comparison to canonical adenine reveals that methylation induces further electronic structure complication that may be important to understand the effects of ionizing radiation and the charge distribution in these biological entities at different time scales.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate.

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C5H8O2) and methyl butyrate (MB, C5H10O2) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO2 loss as 66.6 ± 4.3 kJ mol-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at ΔfHo0K[CH2C(OH)OCH3+] = 502 ± 6 kJ mol-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH3 and CO2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

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 123.6-160.0nm, 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 (∼5cm-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-pointlevel 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.

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

We report the spectroscopic observation of the jet-cooled para-ethynylbenzyl (PEB) radical, a resonance-stabilized isomer of C9H7. The radical was produced in a discharge of p-ethynyltoluene diluted in argon and probed by resonant two-color two-photon ionization (R2C2PI) spectroscopy. The origin of the D0(2B1)-D1(2B1) transition of PEB appears at 19,506 cm-1. A resonant two-color ion-yield scan reveals an adiabatic ionization energy (AIE) of 7.177(1) eV, which is almost symmetrically bracketed by CBS-QB3 and B3LYP/6-311G++(d,p) calculations. The electronic spectrum exhibits pervasive Fermi resonances, in that most a1 fundamentals are accompanied by similarly intense overtones or combination bands of non-totally symmetric modes that would carry little intensity in the harmonic approximation. Under the same experimental conditions, the m/z = 115 R2C2PI spectrum of the p-ethynyltoluene discharge also exhibits contributions from the m-ethynylbenzyl and 1-phenylpropargyl radicals. The former, like PEB, is observed herein for the first time, and its identity is confirmed by measurement and calculation of its AIE and D0-D1 origin transition energy; the latter is identified by comparison with its known electronic spectrum (J. Am. Chem. Soc., 2008, 130, 3137-3142). Both species are found to co-exist with PEB at levels vastly greater than might be explained by any precursor sample impurity, implying that interconversion of ethynylbenzyl motifs is feasible in energetic environments such as plasmas and flames, wherein resonance-stabilized radicals are persistent.

Photoelectron Photoion Coincidence Spectroscopy of NCl3 and NCl2.

We investigate NCl3 and the NCl2 radical by photoelectron‐photoion coincidence spectroscopy using synchrotron radiation. The mass selected threshold photoelectron spectrum (ms‐TPES) of NCl3 is broad and unstructured due to the large geometry change. An ionization energy of 9.7±0.1 eV is estimated from the spectrum and supported by computations. NCl2 is generated by photolysis at 213 nm from NCl3 and its ms‐TPES shows an extended vibrational progression with a 90 meV spacing that is assigned to the symmetric N−Cl stretching mode in the cation. An adiabatic ionization energy of 9.94 ± 0.02 eV is determined.

Identification of the photoelectron spectra of HFCS via computing Franck–Condon factors

Some peaks present in the photoelectron spectrum of a mixture were proposed to be produced from thioformyl fluoride (HFCS) by Baker et al. (J. Phys. Chem. 99 (1995) 10147). This study aimed to identify whether the observed peaks were arisen from HFCS. We calculated the equilibrium structures and harmonic vibrational frequencies of HFCS and some ionic states of HFCS+ using the B3LYP, PBE0, M06-2X, and CCSD(T) approaches. We also simulated the photoelectron spectra of HFCS via computing Franck–Condon factors. The adiabatic ionization energies were computed using the CCSD(T) method combined with the complete basis set limit approach. A new ionic state of HFCS+ possessing a nonplanar structure was discovered. Both the simulated photoelectron spectrum and calculated adiabatic ionization energies of HFCS are in agreement with the experiments. Accordingly, we confirm that the reactive molecule HFCS can be identified using the photoelectron spectroscopic techniques.

A Photoionization Mass Spectrometry Investigation into Complex Organic Molecules Formed in Interstellar Analog Ices of Carbon Monoxide and Water Exposed to Ionizing Radiation

Abstract The formation of complex organic molecules by energetic electrons mimicking secondary electrons generated within trajectories of galactic cosmic rays was investigated in interstellar ice analog samples of carbon monoxide and water at 5 K. Simulating the transition from cold molecular clouds to star-forming regions, newly formed products sublimed during the temperature-programmed desorption and were detected utilizing isomer-specific photoionization reflectron time-of-flight mass spectrometry. Using isotopically labeled ices, tunable photoionization, and adiabatic ionization energies to discriminate between isomers, isomers up to C 2 H 4 O 2 and C 2 H 6 O 2 were identified, while non-isomer-specific findings confirmed complex organics with molecular formulas up to C 4 H 6 O 4 . The results provide important constraints on reaction pathways from simple inorganic precursors to complex organic molecules that have both astrochemical and astrobiological significance.

Structure and electronic properties of copper oxide clusters and the effect of reacting with water investigated using Monte Carlo simulations and DFT calculations

Copper oxide clusters are particularly effective catalysts for the formation of polychlorinated-dibenzo-p-dioxins and -dibenzofurans and other pollutants. In this study, ab initio Monte Carlo simulations and density functional calculations are performed to study the structures and electronic properties of copper oxide clusters, CuOn (n = 1–6), and their reactions with a single water molecule. The lowest-energy structures of CuOn and CuOn-water clusters for n = 1–6 are optimized using the aug-cc-pVDZ and LANL2DZ basis sets of the B3LYP functional. The lowest-energy structures of copper oxide clusters are found to be planar or nearly planar. Selected electronic properties including the binding energies, second differences of the energies, HOMO-LUMO gaps, adiabatic and vertical ionization energies, adiabatic electron affinities, reaction energies, and Bader charges are calculated and examined.

Electronic Spectroscopy of cis- and trans-meta-Vinylbenzyl Radicals.

The D0(2A″)-D1(2A″) electronic transition of resonance-stabilized radical C9H9 isomers cis- and trans-meta-vinylbenzyl (MVB) has been investigated using resonant two-color two-photon ionization (R2C2PI) and laser-induced fluorescence. The radicals were produced in a discharge of m-vinyltoluene diluted in Ar and probed under jet-cooled conditions. The origin bands of the cis and trans conformers are at 19 037 and 18 939 cm-1, respectively. Adiabatic ionization energies near 7.17 eV were determined for both conformers from two-color ion-yield scans. Dispersed fluorescence (DF) was used to conclusively identify the cis-conformer: ground-state cis-MVB eigenvalues calculated for a Fourier series fit of a computed vinyl torsion potential are in excellent agreement with torsional transitions in the 19 037 cm-1 DF spectrum. R2C2PI features arising from cis- or trans-MVB were distinguished by optical-optical hole-burning spectroscopy and vibronic assignments were made with guidance from density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. There is a notable absence of mirror symmetry between excitation and emission spectra for several totally symmetric modes, whereby modes that are conspicuous in emission are nearly absent in excitation, and vice versa. This effect is largely ascribed to interference between Franck-Condon and Herzberg-Teller contributions to the electronic transition moment, and its pervasiveness a consequence of the low symmetry (Cs) of the molecule, which permits intensity borrowing from several relatively bright electronic states of A″ symmetry.

The Ionization Energies of Dust-Forming Metal Oxide Clusters

Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigated the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They included clusters of magnesia (MgO)n, silicon monoxide (SiO)n, alumina (Al2O3)n, and titania (TiO2)n with stoichiometric sizes of n = 1–8. The magnesia, alumina, and titania clusters showed relatively little variation in their ionization energies with respect to the cluster size n: 7.1–8.2 eV for (MgO)n, from 8.9–10.0 eV for (Al2O3)n, and 9.3–10.5 eV for (TiO2)n. In contrast, the (SiO)n ionization energies decrease with size n, starting from 11.5 eV for n = 1, and decreasing to 6.6 eV for n = 8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral–neutral reactions.

Threshold Photoelectron Spectroscopy of the CH2I, CHI, and CI Radicals.

VUV photoionization of the CHnI radicals (with n = 0, 1, and 2) is investigated by means of synchrotron radiation coupled with a double imaging photoion-photoelectron coincidence spectrometer. Photoionization efficiencies and threshold photoelectron spectra (TPES) for photon energies ranging between 9.2 and 12.0 eV are reported. An adiabatic ionization energy (AIE) of 8.334 ± 0.005 eV is obtained for CH2I, which is in good agreement with previous results [8.333 ± 0.015 eV, SztárayJ. Chem. Phys.2017, 147, 01394428688391], while for CI an AIE of 8.374 ± 0.005 eV is measured for the first time and a value of ∼8.8 eV is estimated for CHI. Ab initio calculations have been carried out for the ground state of the CH2I radical and for the ground state and excited states of the radical cation CH2I+, including potential energy curves along the C–I coordinate. Franck–Condon factors are calculated for transitions from the CH2I(X̃2B1) ground state of the neutral radical to the ground state and excited states of the radical cation. The TPES measured for the CH2I radical shows several structures that correspond to the photoionization into excited states of the radical cation and are fully assigned on the basis of the calculations. The TPES obtained for the CHI is characterized by a broad structure peaking at 9.335 eV, which could be due to the photoionization from both the singlet and the triplet states and into one or more electronic states of the cation. A vibrational progression is clearly observed in the TPES for the CI radical and a frequency for the C–I stretching mode of 760 ± 60 cm–1 characterizing the CI+ electronic ground state has been extracted.

Threshold photoelectron spectroscopy and density functional theory studies on the CF2Cl2 ionization energies towards the B2B1 and C2A1 ionic states

Threshold photoelectron spectroscopy and density functional theory calculations were performed on dichlorodifluoromethane (CF2Cl2) in the photon energy range of 11.70–13.90 eV. Using optimized geometries and vibrational frequencies of the CF2Cl2 neutral and the corresponding cation in the B2B1 and C2A1 states at the ωB97XD/aug-cc-pVTZ level of theory, the Franck-Condon factor simulation was conducted for these two bands. According to the great agreement between experimental and simulated spectra, the observed vibrational progressions of the B2B1 band was reliably assigned, where the 0–0 transition band indicated a precise adiabatic ionization energy (AIE) towards the B2B1 ionic state of 13.150 ± 0.005 eV. The three symmetric vibrational frequencies of B2B1 were determined to be ν1+ = 1170 cm−1, ν2+ = 645 cm−1, and ν4+ = 242 cm−1, respectively. Similarly, the AIE towards the C2A1 ionic state was suggested to be 13.340 eV, based on its plausible Franck-Condon simulation.

The Vertical and Adiabatic Ionization Energies of Silicon Carbide Clusters, (SiC)n, With n = 1–12

The Vertical and Adiabatic Ionization Energies of Silicon Carbide Clusters, (SiC)n, With n = 1–12

Ionization Energies and Dyson Orbitals of the Iso-electronic SO2, O3, and S3 Molecules from Electron Propagator Calculations.

Adiabatic and vertical ionization energies corresponding to the X̃ , à , and B̃ final states of SO2+, O3+, and S3+ have been calculated with a variety of electron-propagator and coupled-cluster methods. The BD-T1 electron-propagator method for vertical ionization energies and coupled-cluster adiabatic and zero-point corrections yield agreement with experiment to within 0.1 eV in all cases but one. The remaining discrepancies for the à state of SO2+ indicate a need for higher levels of theory in determining cationic minima and their accompanying vibrational frequencies. Predictions for the still unobserved à and B̃ final states of S3+ are included. To account for increased biradical character in O3 and S3, highly correlated reference states are required to produce the correct order of final states. Electron correlation plays a subtle role in determining the contours of the Dyson orbitals obtained with BD-T1 and NR2 electron-propagator calculations.

Ionization energy and thermochemistry of CF2Cl2 determined from threshold photoelectron spectroscopy

In this work, we performed a joint study of threshold photoelectron spectroscopy and density functional calculations on photoionization of CF2Cl2. Using the optimized geometries and vibrational frequencies of the CF2Cl2 neutral and cation in ground state at the ωB97XD/cc-pVTZ level, the Franck-Condon simulation was performed for the X2B2 band. Based on the great agreements between the experimental and simulated spectra, the observed vibrational progressions of the CF2Cl2(X1A1) → CF2Cl2+(X2B2) transition are well assigned, leading to a corrected adiabatic ionization energy of 11.565 ± 0.010 eV. The enthalpies of formation and C-Cl bond energies of CF2Cl2 neutral and cation are then calculated, respectively.

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate.

We studied the valence photoionization of vanillin by photoelectron photoion coincidence spectroscopy in the 8.20-19.80 eV photon energy range. Vertical ionization energies by EOM-IP-CCSD calculations reproduce the photoelectron spectral features. Composite method calculations and Franck-Condon simulation of the weak, ground-state band yield the adiabatic ionization energy of the most stable vanillin conformer as 8.306(20) eV. The lowest energy dissociative photoionization channels correspond to hydrogen atom, carbon monoxide, and methyl losses, which form the dominant C8H7O3+ (m/z 151) and the less intense C7H8O2+ (m/z 124) and C7H5O3+ (m/z 137) fragment ions in parallel dissociation channels at modeled 0 K appearance energies of 10.13(1), 10.40(3), and 10.58(10) eV, respectively. On the basis of the breakdown diagram, we explore the energetics of sequential methyl and carbon monoxide loss channels, which dominate the fragmentation mechanism at higher photon energies. The 0 K appearance energy for sequential CO loss from the m/z 151 fragment to C7H7O2+ (m/z 123) is 12.99(10) eV, and for sequential CH3 loss from the m/z 123 fragment to C6H4O2+ (m/z 108), it is 15.40(20) eV based on the model. Finally, we review the thermochemistry of the bi- and trifunctionalized benzene derivatives guaiacol, hydroxybenzaldehyde, anisaldehyde, and vanillin. On the basis of isodesmic functional group exchange reactions, we propose new enthalpies of formations, among them ΔfH°298K(vanillin, g) = -383.5 ± 2.9 kJ mol-1. These mechanistic insights and ab initio thermochemistry results will support analytical works to study lignin conversion involving vanillin.

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.

Distinction of photoelectron spectroscopy of cis- and trans-acrolein explored by theoretical computation

• The photoelectron spectra for the two lowest cationic states of cis - and trans -acrolein have been simulated. • Trans -acrolein is identified as the primary isomer responsible for the experimental photoelectron spectra. • The cis -acrolein anion is calculated to be more stable than the trans form, contrary to acrolein molecules. • The barrier for trans - cis isomerization of acrolein anion is high, and which isomer of acrolein anion will be produced is unknown We calculated the equilibrium structures and harmonic vibrational frequencies of acrolein (including molecule, cation and anion) using the B3LYP, PBE0, and M06-2X approaches. We also computed the Franck–Condon factors and simulated the photoelectron spectra of acrolein molecules and anions. The adiabatic ionization energy (AIE) and electron affinity were computed using the CCSD(T) approach via extrapolation to the complete basis set limit. Trans -acrolein is identified as the primary isomer responsible for the photoelectron spectra of acrolein being observed. The calculated AIEs were consistent with the experiment. The cis -acrolein anion is predicted to be more stable than its trans- counterpart. However, the potential barrier might prohibit the isomerization of the acrolein anion. Accordingly, the type of isomer of acrolein anion that will be produced from electron-attaching to the more stable trans -acrolein molecule remains unknown. The simulated photoelectron spectra of the acrolein anion can be used to identify the isomer being produced.