Calculated Ionization Energies Research Articles

A Vacuum Ultraviolet Photoionization Mass Spectrometry and Density Functional Calculation Study of Formic Acid-Water Clusters.

The interaction between formic acid (FA) and water (W) holds significant importance in various chemical processes. Our study combines vacuum-ultraviolet photoionization mass spectrometry with density functional calculations to investigate formic acid water clusters generated in supersonic molecular beams. The mass spectra obtained reveal the formation of protonated clusters as the major product. Enhanced intensities are observed in the mass spectra for a number of clusters holding the following composition, FA1W5H+, FA2W4H+, FA3W3H+, FA4W2H+, FA5W1H+ and FA6W2H+ compared to their neighbors with one less or one more water component. Our calculations shed light on these potentially stable structures, highlighting cyclic arrangements with molecules enclosed within the ring as the most stable structures, and demonstrate a decrease in the stability upon the addition of a water molecule. Comparing experimental appearance energies with calculated ionization energies suggests that fragmentation can occur from clusters of various sizes.

Ionization of small atmospheric acid–base clusters and its prospective role in seeding the growth of aqueous clusters

In the present study, we have examined the ionization of a set of small atmospheric acid–base clusters. The calculated ionization energy (IE) ranges from ∼660 kJ mol−1 to ∼1120 kJ mol−1, with clusters consisting of just the bases yielding the smallest IEs, while those of the acids yield the largest. For neutral clusters that form most readily, the IEs range from ∼790 kJ mol−1 to ∼880 kJ mol−1. These values are generally smaller than those of other chemicals with significant atmospheric presence. Thus, the radical cations of the acid–base clusters are not likely to ionize other atmospheric species. Charged atmospheric species have been hypothesized to facilitate the growth of large clusters. We have carried out a case study of the sequential hydration of an acid–base cluster and compared our results with previous studies of forming pure water clusters. The neutral acid–base cluster binds water more strongly than pure water clusters. Further strengthening of the binding can be seen for the radical cationic acid–base cluster. The resulting acid–base–water clusters may further represent a source of H3O+, which may proceed to facilitate cloud formation.

Elucidating the low temperature chemistry of o-xylene enhanced by the reaction pool of dimethyl ether

The branched aromatic hydrocarbon with short side chains, such as o-xylene, has weak low temperature oxidation reactivity, while it could proceed low temperature chain recycling reactions by blending a fuel component with higher oxidation reactivity. However, the study of low temperature oxidation kinetics of o-xylene is very scarce. Therefore, this work investigated the low temperature oxidation process of o-xylene in an atmospheric jet stirred reactor by adding a biofuel with higher low temperature oxidation reactivity, i.e. dimethyl ether (DME). A series of characteristic low temperature intermediates, such as phenyl hydroperoxides and ketohydroperoxide from the oxidation of o-xylene, were detected by using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The chemical structures of representative low temperature intermediates were determined by comparing the ionization thresholds from measured photoionization efficiency curves with calculated ionization energies. A kinetic model for o-xylene and DME mixture was developed and verified by literature and present experimental data. Rate of production (ROP) and sensitivity analyses were performed, which indicate that the low temperature oxidation chemistry of o-xylene, such as the negative temperature coefficient behavior, is enhanced by the reaction pool of DME. According to the classical low temperature oxidation mechanism of n-alkanes, the low temperature reactions of o-xylene were proposed. In particular, phenyl hydroperoxides and ketohydroperoxides were revealed to be key indicators for the occurrence of first and second O2 addition reactions during the oxidation of o-xylene, respectively. In addition, the sensitivity analysis uncovers the interaction kinetics between the reaction pools of o-xylene and DME, as well as their effects on fuel consumption and products formation. In summary, present evidence elucidates the low temperature chemistry of a representative branched aromatic, which could contribute to the development of future engines and control of combustion pollutants.

Experimental and Theoretical Analysis of Tricyclic Antidepressants by Ultraviolet Picosecond Laser Desorption Post-Ionization Mass Spectrometry.

Imipramine class tricyclic antidepressants have low ionization efficiencies that make them difficult to detect by using secondary ion mass spectrometry. Ultraviolet picosecond laser desorption postionization (ps-LDPI-MS) is examined here for the detection of four tricyclic antidepressants: imipramine, desipramine, amitriptyline, and clomipramine. About 30 ps laser pulses at either 213 nm (5.8 eV) or 355 nm (3.5 eV) are used for desorption of samples under vacuum, 7.9 eV (157 nm) fluorine laser pulses are used for post-ionization, and the ions so formed are detected by time-of-flight mass spectrometry. Detection of imipramine by 213 nm ps-LDPI-MS shows less fragmentation than either 355 nm ps-LDPI-MS or prior results from 800 nm fs-LDPI-MS. Ionization energies of imipramine, desipramine, amitriptyline, and clomipramine are predicted using density functional theory calculations and used to explain the corresponding ps-LDPI-MS data for these four compounds as resulting from single-photon ionization. The experimental observation of low-mass amine-containing fragments with calculated ionization energies below 7.9 eV is attributed mostly to dissociation during laser desorption, followed by single-photon ionization of the neutral fragments rather than the more traditional mechanism of unimolecular dissociation following single-photon ionization of the parent molecule.

Synthetic pathway of shallow n-type donor: Theoretical study of Li and B co-doped diamonds

To find potential n-type diamonds with shallow donor energy levels and low formation energies, the thermodynamic and electronic properties of Lithium-Boron (LiB) co-doped diamonds have been investigated using density functional theory (DFT) and hybrid density functional theory (HSE06). The structure, formation energy, electronic property and the potential synthesis pathway are analyzed. The results of the single Li defect is not a suitable candidate for a shallow n-type donor, as it distorts the diamond lattice and forms clusters, in agreement with previous work. The calculated ionization energy of Lii2-B defect (0.29 eV) and the impurities formation energy (0.196 eV) is much lower than that of Lii defect. It suggests that the introduction of B helps improve the solubility of Li in diamond. Further structural analysis has shown that the incorporation of B atoms inhibits the diffusion of Li atoms and simultaneously reduces the internal strain in the diamond, resulting in a significant reduction in the distance between the two lithium atoms from 2.061 Å to 1.629 Å. At last, a non-molecular synthesis pathway of Lii2-B co-doped diamond is proposed via prior generation of B-doped diamond at which Li atom is mobile which indicates there is a potential window-of-opportunity to prepare Lii2-B co-doped diamond to achieve n-type diamond.

Predicting the environmental fates of emerging contaminants: Synergistic effects in ozone reactions of nitrogen-containing alkenes.

While nitro and amino alkenes are common in pharmaceuticals, pesticides, and munitions, their environmental fates are not well known. Ozone is a ubiquitous atmospheric oxidant for alkenes, but the synergistic effects of nitrogen-containing groups on the reactions have not been measured. The kinetics and products of ozonolysis of a series of model compounds with different combinations of these functional groups have been measured in the condensed phase using stopped-flow and mass spectrometry methods. Rate constants span about six orders of magnitude with activation energies ranging from 4.3 to 28.2 kJ mol-1. Vinyl nitro groups substantially decrease the reactivity, while amino groups have the opposite effect. The site of the initial ozone attack is highly structure dependent, consistent with local ionization energy calculations. The reaction of the neonicotinoid pesticide nitenpyram, which forms toxic N-nitroso compounds, was consistent with model compounds, confirming the utility of model compounds for assessing environmental fates of these emerging contaminants.

A study of the valence photoelectron spectrum of uracil and mixed water-uracil clusters.

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures.

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation.

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙+) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙+ from 10-40 °C, giving the activation energy (Ea = 17.43 ± 1.64 kJ mol-1) and pre-exponential factor (A = (2.36 ± 0.05) × 1013 M-1 s-1). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [LnIII(TODGA)3(NO3)3] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙+, relative to the non-complexed ligand: k([LnIII(TODGA)3(NO3)3] + RH˙+) = (8.99 ± 0.93) × 1010, (2.88 ± 0.40) × 1010, and (1.53 ± 0.34) × 1010 M-1 s-1, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations-based on a model [LnIII(TOGDA)]3+ complex system-indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [LnIII(TEGDA)3(NO3)3], toward electrophilic attack is for the coordinated nitrate (NO3-) counter anions. Therefore, it is possible that radical reactions with the complexed NO3- counter anions dominate the differences in rates seen for the [LnIII(TODGA)3(NO3)3] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

Pathway exploration in low-temperature oxidation of a new-generation bio-hybrid fuel 1,3-dioxane

The joint and flexible utilization of renewable electricity, ligno-cellulosic biomass, and/or CO2 point sources to produce so-called bio-hybrid fuels is a promising solution to achieve carbon neutrality while still meeting the energy demand of the transportation sector. One of the new-generation bio-hybrid fuels is 1,3-dioxane. It has a special chemical structure with two oxygen atoms in a six-membered ring. In this work, the low-temperature oxidation of 1,3-dioxane was studied theoretically and experimentally. Potential energy surfaces of the products of the O2 recombination with the three radicals formed from the H-atom abstraction of 1,3-dioxane were calculated at the DLPNO-CCSD(T)/CBS//B2PLYP-D3/cc-pVTZ level. The reaction rate coefficients were calculated with the RRKM/master equation method (T = 500–2000 K, p = 0.01–100 atm). To validate the proposed pathways, low-temperature oxidation experiments of 1,3-dioxane were performed in a jet stirred reactor (JSR) coupled with a synchrotron photon ionization time of flight molecular beam mass spectrometer (T = 590 K, p = 1 bar). Key intermediates in the investigated pathways were captured and identified by the combination of measured photon ionization efficiency curves and calculated ionization energies. Compared to cyclohexane, which has no oxygen in the six-membered ring, 1,3-dioxane has much weaker C-H bonds for the carbon between the two oxygen atoms, thus enabling faster internal H-atom migration from ROO to QOOH. Furthermore, oxidation of 1,3-dioxane tends to favor cyclic ethers + OH (chain propagation) instead of alkenes + HO2 (chain termination), explaining its high reactivity in the low-temperature regime.

ZORA Gaussian basis sets for Fr, Ra, and Ac.

Segmented all-electron basis sets of double and triple zeta valence qualities plus polarization functions (DZP and TZP) for the elements Fr, Ra, and Ac to be used with the zeroth-order regular approximation (ZORA) were presented. These sets were constructed from the reoptimization of the contraction coefficients of the corresponding non-relativistic basis sets. In order to adequately describe electrons distant from the atomic nuclei, these sets were augmented with diffuse functions and were, respectively, designated as ADZP-ZORA and ATZP-ZORA. At the ZORA-B3LYP theory level, the relativistic sets were employed to calculate ionization energies of Fr, Ra, and Ac as well as bond lengths, dissociation energies, harmonic vibrational frequencies, and static mean dipole polarizabilities of some diatomics. Comparing with benchmark theoretical results and with experimental data available in the literature, it can be verified that our basis sets are able to produce reliable and accurate results. Evaluation of the performances of ZORA and second-order Douglas-Kroll-Hess Hamiltonians was performed.

Ionization Energies and Redox Potentials of Hydrated Transition Metal Ions: Evaluation of Domain-Based Local Pair Natural Orbital Coupled Cluster Approaches.

Hydrated transition metal ions are prototypical systems that can be used to model properties of transition metals in complex chemical environments. These seemingly simple systems present challenges for computational chemistry and are thus crucial in evaluations of quantum chemical methods for spin-state and redox energetics. In this work, we explore the applicability of the domain-based pair natural orbital implementation of coupled cluster (DLPNO-CC) theory to the calculation of ionization energies and redox potentials for hydrated ions of all first transition row (3d) metals in the 2+/3+ oxidation states, in connection with various solvation approaches. In terms of model definition, we investigate the construction of a minimally explicitly hydrated quantum cluster with a first and second hydration layer. We report on the convergence with respect to the coupled cluster expansion and the PNO space, as well as on the role of perturbative triple excitations. A recent implementation of the conductor-like polarizable continuum model (CPCM) for the DLPNO-CC approach is employed to determine self-consistent redox potentials at the coupled cluster level. Our results establish conditions for the convergence of DLPNO-CCSD(T) energetics and stress the absolute necessity to explicitly consider the second solvation sphere even when CPCM is used. The achievable accuracy for redox potentials of a practical DLPNO-based approach is, on average, 0.13 V. Furthermore, multilayer approaches that combine a higher-level DLPNO-CCSD(T) description of the first solvation sphere with a lower-level description of the second solvation layer are investigated. The present work establishes optimal and transferable methodological choices for employing DLPNO-based coupled cluster theory, the associated CPCM implementation, and cost-efficient multilayer derivatives of the approach for open-shell transition metal systems in complex environments.

Conformation-dependent low-temperature oxidation chemistry of methylcyclohexane: First oxygen addition and chain-branching

Conformational effect is an important feature of the low-temperature (low-T) oxidation chemistry of cycloalkanes. In this work, both theoretical calculation and conformational analysis were performed for the first oxygen addition in methylcyclohexane (MCH) oxidation to better understand the influence of the spatial orientation and the steric hindrance of substituted functional groups. Based on synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) measurements and adiabatic ionization energy calculations, reactive hydroperoxides, highly oxygenated molecules and stable products were identified in the jet-stirred reactor (JSR) oxidation of 1.0% and 0.5% MCH at 1.04 bar, 500–900 K and the equivalence ratio of 0.25. The abundant formation of 1-methylcyclohexene and the observation of alkenylhydroperoxides and their decomposition products verify the existence of conformation-dependent pathways in first and second oxygen addition in low-T MCH oxidation, respectively. A MCH oxidation model incorporated with conformation-dependent pathways was constructed to consider the conformational effects of cyclohexane ring and sidechain, and was validated against experimental data over a wide range of conditions in this work and literature. The consideration of conformational effects of cyclohexane ring and sidechain can generally better predict the measured species profiles and ignition delay time under low-T conditions. Based on modeling analysis, conformational effects of cyclohexane ring were found to play important roles in first oxygen addition and chain-branching in low-T MCH oxidation. Compared with the situation in low-T cyclohexane oxidation, the methyl substitution on cyclohexane ring reduces its conformational effects on chain-branching process.

Screening criteria for high-performance p-type transparent conducting materials and their applications

To accelerate discovery of promising p-type transparent conducting materials (TCMs), a screening strategy is formulated based on calculable selection metrics of thermodynamic stability, wide gap, low hole effective mass, and p-type dopability. Using this screening strategy, we identified three best-of class p-type TCM candidates (CaTe, BaTe and BeS) with combination properties of good transparency, high carrier mobility, and excellent hole dopability. Based on ionization energy calculations, we have established a concise and reliable criterion for assessing p-type performance. It is concluded that BeS is readily accessible an intrinsic p-type TCM, while CaTe and BaTe requiring extrinsic doping. Defect calculations are performed for the three compounds, providing strong support for our results. Although the defect calculations indicate a lack of shallow hole-producing defects in CaTe and BaTe, they both would be promising p-type TCMs when introducing impurities such as Na and K acceptor dopants. BeS contains an acceptor-type defect with a low formation energy (Be vacancies), which is responsible for its intrinsic p-type behavior. This physically formulated screening strategy would motivate the exploration and design of other advanced functional materials, and our established criterion could be another route to identify the p/n-type behavior of materials.

ZORA all-electron double zeta basis sets for the elements from H to Xe: application in atomic and molecular property calculations

From the segmented all-electron basis set of double zeta valence quality plus polarization functions (DZP) for the elements from H to Xe, the zeroth-order regular approximation (ZORA) is used to generate a DZP-ZORA basis set, i.e., the contraction coefficients of the DZP set are re-optimized using the minimum ZORA energy criterion. To properly describe electrons distant from the nuclei, a diffuse function is added to each atomic symmetry (s, p, d, and f). The later basis set is designated as DZP-ZORA augmented. To test the effectiveness of the basis sets developed in this work, calculations of ionization energies and mean dipole polarizabilities of some elements are performed using the ZORA-CCSD(T) method. At the same level of theory, bond lengths, dissociation energies, and harmonic vibrational frequencies of some diatoms are also reported. Comparison with experimental data and recommended values available in the literature is made. Except for polarizability, scalar relativistic effects are estimated for the other properties. The performances of the ZORA and second-order Douglas-Kroll-Hess Hamiltonians are evaluated.

Observation of low‐temperature chemistry products in laminar premixed low‐pressure flames by molecular‐beam mass spectrometry

AbstractThe formation of typical low‐temperature oxidation products is observed in laminar premixed low‐pressure flames investigated by photoionization molecular‐beam mass spectrometry at the Swiss Light Source. The C1–C4 alkyl hydroperoxides can be identified in n‐butane‐ and 2‐butene‐doped hydrogen flames by their photoionization efficiency spectra at m/z 48, 62, 76, and 90. C1–C3 alkyl hydroperoxides are also observed in a propane‐doped hydrogen flame and in a neat propane flame. In addition, threshold photoelectron spectra reveal the presence of the alkyl hydroperoxides. In the 2‐butene/H2 flame, the photoionization spectrum at m/z 88 also enables the identification of butenyl hydroperoxides by comparison with calculated ionization energies of the alkenyl hydroperoxides and a literature spectrum. The low‐temperature species are formed close to the burner surface with maximum mole fractions at 0.25–0.75 mm above the burner. At 0.5 mm, even the methylperoxy radical (CH3OO) is measured for the first time in a laminar premixed flame. The rate of production analyses show that consumption of the hydroperoxyalkyl radicals results in the formation of cyclic ethers. In the n‐butane/H2 flame, ethylene oxide, oxetane, and methyloxirane are identified. Besides expected small oxygenated species, for example, formaldehyde or acetaldehyde, the larger C4 oxygenates butanone (C2H5COCH3) and 2,3‐butanedione (C4H6O2) are formed in the two C4 hydrocarbon‐doped hydrogen flames. Quantification of alkyl hydroperoxides with estimated photoionization cross sections based on the corresponding alcohols, which have similar photoelectron structures to the alkyl hydroperoxides, shows that mole fractions are on the order of 10−5–10−6 in the n‐butane/H2 flame. Measurements are corroborated by simulations, which also predict the presence of some peroxides in detectable concentrations, that is, mole fractions larger than 10−7, under the investigated conditions. The observation of peroxide species and cyclic ethers in the investigated laminar premixed flames give new insights into the contribution of low‐temperature combustion chemistry in a flame.

A comprehensive spectroscopic study of urocanic acid: OVGF and EOM-CCSD approaches

The interest in the photobiology of the trans isomer of urocanic acid (UCA) is due to the important role of this molecule as a major absorber of UV light in the human skin. So, urocanic acid can act as a natural sunscreen. UV light induces photoisomerization of UCA into the cis- form. Herein, theoretical study of the electronic structure of various tautomeric forms of cis and trans isomers of urocanic acid is reported together with modeling of the photoelectron spectra of both isomers. The most stable conformers were used to calculate vertical valence ionization energies. Two high-level methods were used for calculating ionization energies and photoelectron spectra: Outer Valence Green’s Function (OVGF) and equation-of-motion couple cluster method (EOM-CCSD). Total electron energies were calculated at M06-2X/6-311++G(d,p) level. The orbital character associated with individual bands in the calculated photoelectron spectra was established using NBO method. This character was then used to assign calculated photoelectron spectra. The theoretical photoelectron spectra of cis and trans isomers were obtained and we found that orbitals localized on the CC and CO moieties are most responsible for the lowest energy ionizations. The ionization energies obtained assuming Koopmans’ approximation were also predicted for all tautomers and compared with OVGF and EOM-CCSD results. Also, we computed molecular electrostatic potential (MEP) surfaces for urocanic acid tautomers. The molecular structures of two bio-active conformers were based on X-ray diffraction data obtained for UCA bound to a receptor protein. These molecular structures were subsequently used for calculating corresponding ionization energies. Ligand-receptor interaction in the active site and HOMO orbitals for these two bioactive forms are also given.

Electrochemical Aziridination of Tetrasubstituted Alkenes with Ammonia

Ammonia (NH3) is an ideal nitrogen source in terms of availability, reactivity, safety, atom economy, environmental compatibility, and ease of isolation. However, its utility for amine synthesis is...

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N-Ar)X Complexes.

Relativistic zero order regular approximation (ZORA) density functional theory computations, coupled with the conductor-like screening model for solvation effects, are used to investigate the redox properties of a series of biscyclopentadienyl pentavalent uranium(V) complexes Cp2U(═N-Ar)X (Ar = 2,6-Me2-C6H3; X = OTf, C6F5, SPh, C═CPh, NPh2, Ph, Me, OPh, N(TMS)2, N═CPh2). Regarding the UV/UIV and UVI/UV couple systems, a linear correlation (R2 ∼ 0.99) is obtained at the ZORA/BP86/TZP level, between the calculated ionization energies and the measured experimental E1/2 half-wave oxidation potentials (UVI/UV) and between the electron affinities and the reduction potentials E1/2 (UV/UIV). The study brings to light the importance of solvation effects that are needed in order to achieve a good agreement between the theory and experiment. Introducing spin-orbit coupling corrections slightly improves this agreement. Both the singly occupied molecular orbital and the lowest unoccupied molecular orbital of the neutral UV complexes exhibit a majority 5f orbital character. The frontier molecular orbitals show a substantial ancillary ligand X σ and/or π character that drives the redox properties. Moreover, our investigations allow estimating the redox potentials of the X = Ph, X = C6F5, and N(TMS)2 UV complexes for which no experimental electrochemical data exist.

Structural and electrical characteristics of rare-earth modified bismuth layer structured compounds

Polycrystalline ceramics Bi3ZrVO9, Bi2LaZrVO9, Bi2NdZrVO9, and Bi2SmZrVO9 of the Aurivillius family were prepared through the mixed oxide reaction method. The grain size in the samples decreases with decreasing bond dissociation energy of the elements with oxygen. The grain boundary contribution in the samples activates at temperature ≥350 °C, resulting in a very large dielectric constant albeit at a lower frequency. The complex dielectric plot confirms the Cole-Cole model of relaxation in the compounds. All these samples exhibit ferroelectric nature. The substitution of the rare earth elements seems to substantially decrease the oxygen vacancy and thus the loss-tangent. The contribution of the Debye part and the DC conductivity to the loss tangent is resolved. The calculated ionization energy for both the relaxation and the conduction processes indicate the involvement of doubly ionized oxygen vacancy. The correlated barrier hopping (CBH) model seems to be the dominant conduction mechanism in the samples.

Characterisation of the first electronically excited state of protonated acetylene C2H3 + by coincident imaging photoelectron spectroscopy

ABSTRACT We present a combined experimental and theoretical study of the threshold photoelectron spectroscopy of the vinyl radical encompassing the first triplet excited state of the vinyl cation. The radicals were produced in a flow-tube reactor by hydrogen abstraction of C2H4 and CH4 using fluorine atoms generated in a microwave discharge. Vinyl was ionised with synchrotron vacuum ultraviolet radiation. A double imaging coincidence setup was used to record the threshold photoelectron spectrum. The experimental and simulated spectra show a marked adiabatic transition to the 3A′′ state with a short vibrational progression dominated by the C = C stretching mode. The adiabatic ionisation energy to this state is measured precisely at 10.747 ± 0.008 eV. In combination with the adiabatic ionisation energy to the 1A1 state from the Active Thermochemical Tables (ATcT), we find a singlet–triplet gap of 2.27 ± 0.01 eV (219 ± 1 kJ mol−1). Calculated ionisation energies and Franck–Condon factors for the singlet 1A′′ excited state are also given.