Experimental Vertical Ionization Energies Research Articles

Improving convergence of experimental and computed vertical ionization energies using the ionization potential equation‐of‐motion coupled‐cluster with singles and doubles method

AbstractComparison of statistically evaluated experimental vertical ionization energies (IEs) for 53 medium‐sized molecules (6‐34 atoms) with ionization potential equation‐of‐motion coupled‐cluster with singles and doubles (IP‐EOMCCSD) computations shows that discrepancies between computed and experimental results can be accounted for with a combination of experimental and theoretical contributions. Discrepancies can be minimized by extrapolating computations to the complete basis set limit and correcting for vibrational zero‐point energy (ZPE) while comparing with experimental IEs calculated as the intensity‐weighted mean band position to account for band asymmetries. This procedure reduced the average discrepancy for ethylene, (E)‐2‐butene, 2,5‐dihydrofuran, and pyrrole from 0.25 to 0.05 eV. Agreement between reported vertical IEs and computations without either making adjustments as described in this paper or using complete simulation of the ionization spectrum should be considered fortuitous. The comparisons made in this work show that estimates of vertical and adiabatic IE made using IP‐EOMCCSD extrapolated to the complete basis set limit and corrected for vibrational ZPE can be used with reasonable confidence when experimental values are not available.

A first principles investigation on the electron donor ability of synthetic melatonin derivatives: implications for their antioxidant activity

The electron donor ability of 37 melatonin (MLT) derivatives was investigated at the LC-ωPBE/6–311+G(d) level of theory, which was chosen based on a benchmark study using the experimental vertical ionization energy of MLT as the reference value. Twenty-three of these derivatives had been already synthesized, while 14 are proposed here for the first time. Those with better electron donor ability were identified using vertical ionization energies and the full electron donor acceptor map, both in aqueous solution. They are expected to have the best antioxidant activity provided that the main reaction mechanism ruling such activity is the electron transfer reaction from the MLT derivatives to free radicals. The drug-likeness of the studied compounds was analyzed using the Lipinski and Ghose rules, as well as the Veber criteria. Their synthetic availability and toxicity were also estimated. Considering the calculated data, altogether, two of the already synthesized compounds are proposed as the best prospects for being tested as oral drugs, with therapeutic uses as antioxidants. In addition, two of the derivatives designed here are proposed as the best candidates to be synthesized and tested for antioxidant activity, with potential to be used as oral drugs. These results might motivate the synthesis of these compounds; thus, their potential role as protectors against oxidative stress—and the associated health issues—could be experimentally tested.

Piecewise linearity in theGWapproximation for accurate quasiparticle energy predictions

We identify the deviation from the straight line error (DSLE) -- i.e., the spurious non-linearity of the total energy as a function of fractional particle number -- as the main source for the discrepancy between experimental vertical ionization energies and theoretical quasiparticle energies, as obtained from the $GW$ and $GW$+SOSEX approximations to many-body perturbation theory (MBPT). For self-consistent calculations, we show that $GW$ suffers from a small DSLE. Conversely, for perturbative $G_0W_0$ and $G_0W_0$+SOSEX calculations the DSLE depends on the starting point. We exploit this starting-point dependence to reduce (or completely eliminate) the DSLE. We find that the agreement with experiment increases as the DSLE reduces. DSLE-minimized schemes, thus, emerge as promising avenues for future developments in MBPT.

Photoelectron spectra and electronic structure of nitrogen analogues of boron β-diketonates

The electronic structure of the valence levels of seven nitrogen-containing boron complexes was investigated using methods of ultraviolet photoelectron spectroscopy and density functional theory. The ionization energies of π- and σ-levels were obtained from photoelectron spectra. The electronic structure of nitrogen-containing compounds was compared with the electronic structure of β-diketonates. It was shown the influence of various substituents on carbon and nitrogen atoms of six-membered ring on the electronic structure of complexes. The changes in the electronic structure after the substitution of atoms in condensed cycles have been identified.In order to compare the experimental vertical ionization energies IEi with Kohn–Sham orbital energies εi we used the analogue of Koopmans theorem and average amendment to the orbital energy of the electrons (δ¯i). For 26 electronic levels of seven studied complexes, the calculated values are in good accordance with experimental energy intervals between electron levels.

Electronic and optical properties of pure and modified diamondoids studied by many-body perturbation theory and time-dependent density functional theory.

Diamondoids are small diamond nanoparticles (NPs) that are built up from diamond cages. Unlike usual semiconductor NPs, their atomic structure is exactly known, thus they are ideal test-beds for benchmarking quantum chemical calculations. Their usage in spintronics and bioimaging applications requires a detailed knowledge of their electronic structure and optical properties. In this paper, we apply density functional theory (DFT) based methods to understand the electronic and optical properties of a few selected pure and modified diamondoids for which accurate experimental data exist. In particular, we use many-body perturbation theory methods, in the G0W0 and G0W0+BSE approximations, and time-dependent DFT in the adiabatic local density approximation. We find large quasiparticle gap corrections that can exceed thrice the DFT gap. The electron-hole binding energy can be as large as 4 eV but it is considerably smaller than the GW corrections and thus G0W0+BSE optical gaps are about 50% larger than the Kohn-Sham (KS) DFT gaps. We find significant differences between KS time-dependent DFT and GW+BSE optical spectra on the selected diamondoids. The calculated G0W0 quasiparticle levels agree well with the corresponding experimental vertical ionization energies. We show that nuclei dynamics in the ionization process can be significant and its contribution may reach about 0.5 eV in the adiabatic ionization energies.

Photoemission spectra and density functional theory calculations of 3d transition metal-aqua complexes (Ti-Cu) in aqueous solution.

Photoelectron spectroscopy measurements and density functional calculations are combined to determine the lowest electron binding energies of first-row transition-metal aqua ions, titanium through copper, with 3d(1) through 3d(9) electronic configurations, in their most common oxidation states. Vertical ionization energies are found to oscillate considerably between 6.76 and 9.65 eV for the dications and between 7.05 and 10.28 eV for the respective trivalent cations. The metal cations are modeled as [M(H2O)n](q+) clusters (q = 2, 3, and 4; n = 6 and 18) surrounded by continuum solvent. The performance of 10 exchange-correlation functionals, two GGAs, three MGGAs, two HGGAs and three HMGGAs, combined with the MDF10(ECP)/6-31+G(d,p) basis set is assessed for 11 M-O bond distances, 10 vertical ionization energies, 6 adiabatic ionization energies, and the associated reorganization free energies. We find that for divalent cations the HGGA and HMGGA functionals in combination with the 18 water model show the best agreement with experimental vertical ionization energies and geometries; for trivalent ions, the MGGA functionals perform best. The corresponding reorganization free energies (λo) of the oxidized ions are significantly underestimated with all DFT functionals and cluster models. This indicates that the structural reorganization of the solvation shell upon ionization is not adequately accounted for by the simple solvation models used, emphasizing the importance of extended sampling of thermally accessible solvation structures for an accurate computation of this quantity. The photoelectron spectroscopy measurements reported herein provide a comprehensive set of transition-metal redox energetic quantities for future electronic structure benchmarks.

Electronic structure and photoelectron spectra of nickel(II) acetylacetonate

Density functional theory and photoelectron spectroscopy are used to study the electronic structure of nickel acetylacetonate Ni(acac)2. Based on the calculated energies and composition of the molecular orbitals and the pattern of localization of the electron density, the nature of the gas-phase photoelectron spectrum bands of Ni(acac)2 is examined, and a new interpretation of it is proposed. The energies of the Kohn-Sham orbitals are assigned to the experimental vertical ionization energies in the approximation of the extended Koopmans theorem, which enables, with consideration given to the dependence of the Koopmans defect on the nature of the electronic level, to obtain a good agreement between the calculated orbital energies and experimental data on ionization energies.

Sp3 Hybrid orbitals and ionization energies of methane from PNOF5

The Piris Natural Orbital Functional, in its 5th version (PNOF5), has been used to obtain the localized bonding picture of methane that agrees well with the Pauling proposition of hybrid orbitals, and to predict the two-peak structure of its vertical valence ionization energy spectrum. The ionization energies have been calculated using the extended Koopmans’ theorem. The calculated PNOF5 values are in good agreement with the corresponding experimental data. It is demonstrated that it is possible to reconcile sp3 orthogonal hybrid orbitals with the experimental vertical ionization energies observed for the CH4 within the framework of the natural orbital functional theory.

Calculated Vertical Ionization Energies of the Common α-Amino Acids in the Gas Phase and in Solution

The vertical ionization energies of the low-lying conformers of the α-amino acids found in proteins have been calculated. Geometry optimizations were first performed at the B3LYP/6-311G(d,p) level of theory, and then reoptimized at the MP2/6-311G(d,p) level of theory. Vertical ionization energies were then computed by three methods, electron propagator in the partial third-order (P3) approximation, Outer-Valence-Green's Functions, and by evaluating the difference in the total energy between the cation radical and the neutral amino acid in the geometry of the neutral species. When available, the results are compared to the experimental vertical ionization energies. The vertical ionization energies calculated using the MP2/P3 method gave the best overall agreement with the experimental results. Next, the ionization energies in solution are calculated for the zwitterionic forms of the α-amino acids by using IEFPCM methods. To obtain the vertical ionization energy in solution, it is necessary to use the nonequilibrium polarizable continuum model (NEPCM), the results of which are reported here for the α-amino acids.

Energy Levels and Redox Properties of Aqueous Mn2+/3+ from Photoemission Spectroscopy and Density Functional Molecular Dynamics Simulation

Energy-resolved photoemission spectroscopy and density functional molecular dynamics simulations are combined to construct an energy level diagram for the Mn(2+/3+) redox reaction in aqueous solution. Two peaks centered at 8.88 and 10.26 eV electron binding energies can be assigned to the Mn2+ hexa-aquo complex with a peak area ratio of 2:2.83. Using the notation of crystal field theory, the peak at lower energies can be interpreted as arising from ionization from the e(g) levels (highest occupied molecular orbital, HOMO), and the peak at higher energies are from ionization of the t(2g) levels. The difference corresponds to the average crystal field splitting, 1.38 eV. From the position of the HOMO level and the absolute redox potential, an experimental value for the reorganization free energy of the aqueous Mn3+ hexa-aquo complex is estimated to be 2.98 eV. Density functional molecular dynamics simulations can reproduce the experimental vertical ionization energy, redox free energy, and reorganization free energies fairly well, provided that the absolute potential shift in periodic boundary conditions, finite size effects, and inaccuracies of the exchange correlation functional are taken into account. Most strikingly, in the simulations, we observe spontaneous and reversible deprotonation of the aqueous Mn3+ hexa-aquo complex to form MnOH(H2O)5(2+) + H+, in line with the low experimental pKa value of this ion. The interconversion between protonation states leads to interesting redox phenomena for aqueous Mn3+, culminating in a bimodal thermal distribution of the electron affinity.

Study of Pentafluoroethane and its Thermal Decomposition Using UV Photoelectron Spectroscopy and Ab Initio Molecular Orbital Calculations

The UV photoelectron spectrum of CF(3)CHF(2) has been recorded and assigned using EOM-CCSD calculations. For the first band, the adiabatic ionization energy (AIE) and vertical ionization energy (VIE) have been measured as (12.71 +/- 0.05) and (13.76 +/- 0.02) eV, respectively. The measured AIE is higher than the recommended value from state-of-the-art ab initio calculations of (12.26 +/- 0.02) eV because of a large geometry change on ionization, mainly arising from a significant increase in the C-C bond length, which results in poor Franck-Condon factors in the adiabatic region. The experimental VIE also shows poor agreement with the computed value of 14.05 +/- 0.06 eV because, in the higher energy region of the first photoelectron band, dissociation of CF(3)CHF(2)(+) to CHF(2)(+) + CF(3) occurs. This has a calculated thermodynamic onset of (12.89 +/- 0.20) eV. Recommendations are made for the heats of formation, DeltaH(f,298)(slashed circle), of CF(3)CHF(2) and CF(3)CHF(2)(+), based on the results of the ab initio calculations. Pyrolysis of flowing CF(3)CHF(2) diluted in argon shows evidence of production of C(2)F(4) and HF at lower temperatures and CF(2) and CF(3)H at higher temperatures. The relative temperature dependence of the observed photoelectron bands associated with these molecules is interpreted in terms of two decomposition reactions of CF(3)CHF(2) as well as the reaction C(2)F(4) --> 2CF(2).

Theoretical Study of the Structure and Electronic Properties of Si3On(-) and Si6On(-) (n = 1-6) Clusters. Fragmentation and Formation Patterns.

A theoretical study of two series of small clusters, Si3On(-) and Si6On(-) (n = 1-6), has been carried out. The minimum energy structures were produced adding an electron to neutral species followed by relaxation at the B3LYP-6-311G(2d) level. The vertical ionization energies (VIEs) were computed using the electron propagator theory (EPT) in two approximations, Unrestricted Outer Valence Green Functions (UOVGF) and partial third-order approximation (P3). In the series Si3On(-) the theoretical VIEs of the minimum energy structures agree well with experimental data. For the second series there are not experimental VIEs, and the theoretical results are predictions. The performance of EPT methodologies in conjunction with all-electron or pseudopotentials (PP) calculations is analyzed. The conjunction of P3 and PP approximation proves to be the most efficient and economical methodology to calculate the VIEs of small anionic silicon oxide clusters. In the series Si6On(-) different channels of fragmentation have been calculated. The results suggest that the fragments do not have drastic geometric changes and the anionic fragment corresponds to the atoms where the spin density of the initial large cluster is localized. The Fukui function calculated over selected optimized fragments predicts adequately the interaction between them to form large stable clusters.

Calculated ionization energies for a series of sesquiterpenes: comparisons with experimental vertical ionization energies and comments on related structure–activity relationships (SARs)

The energies of the highest-occupied molecular orbitals (HOMOs) are known to be excellent predictors of the reactivities of biogenic hydrocarbons, such as terpenes, with reactive atmospheric oxidants including O3, OH, and NO3. Structure–Activity Relationships (SARs) have also been effectively employed in such studies and related to HOMO energies and lowest ionization energies (ionization potentials). This study employs density function theory (DFT), at the B3LYP/6-31G** level, to predict vertical ionization energies (IPv) for a structurally diverse group of sesquiterpenes, each of which has been reported in air samples collected in the lower troposphere. The availability of published UV photoelectron spectra for nine sesquiterpenes permits comparison of experimental and theoretical vertical ionization energy data. The experimental and theoretical data show a good correlation (average discrepancy ± 0.07 eV). This enables predictions of reactivities for sesquiterpenes whose tropospheric lifetimes may last only a few hours before their transformations into secondary organic aerosols (SOA) close to their emission sources.

Assignment of the First Photoelectron Band of CH3CHBr(X2A) Using Ab-initio and Density Functional Theory (DFT) Computational Calculations

HeI photoelectron spectra have been recorded for the reaction of atomic fluorine with ethyl bromide at different reaction times. A structured band associated with a short-lived primary reaction product has been recorded at a mixing distance of 12 mm above the photon beam. The adiabatic and vertical ionization energies of this band was measured as 7.78 ± 0.01 and 8.05 ± 0.01 eV, respectively . The average vibrational separation of 700 ± 30 cm−1 was observed in this band. Vertical ionization energies were computed in this work for CH3CHBr(X2A) and CH2CH2Br(X2A) via ΔSCF, ΔMP2 (full) and Δ(B3LYP) levels of theory using different basis sets. Mulliken population analysis and force constant calculations have also been carried out for CH3CHBr(X2A) and CH2CH2Br(X2A) and their singlet cationic states. Comparison between the experimental vertical ionization energies and the corresponding values computed for CH3CHBr (X2A) and CH2CH2Br(X2A) at different levels of theory led to the assignment of the observed first photoelectron band to the ionization of CH3CHBr(X2A). The observed vibrational structure was assigned to the excitation of C–Br stretching mode in CH3CHBr+ (X1A).

Theoretical interpretation of photoelectron spectra for transition metal compounds. The He(I) and He(II) spectra of MF 2 molecules (M = Co, Ni and Cu)

Calculated vertical ionization energies (VIEs) and relative intensities (RIs) have been used in interpreting He(I) and He(II) photoelectron spectra of MF 2 molecules, M being Co, Ni and Cu. VIEs have been obtained by the Green's function technique applying the semiempirical CNDO-UHF method. Within the framework of the Gelius-Siegbahn model, but utilizing the theoretical atomic cross-sections, the molecular photoionization cross-sections (and the corresponding relative intensities of spectral bands) have been calculated for He(I) and He(II) spectra of MF 2 molecules. A comparison of the theoretical and experimental VIEs and RIs shows that the procedure is useful in ascribing photoemission peaks at particular binding energies of He(I) and He(II) spectra to the set of molecular orbitals.

Uv photoelectron spectrum of cyclobutadiene. free cyclobutadiene stable up to high temperatures

The Hel photoelectron spectrum of cyclobutadiene (CB) has been obtained under conditions which demonstrate that free CB is stable up to temperatures of several hundred °C. A new experimental argument for the rectangular geometry of CB is presented. Shake-up structures are unimportant for the interpretation of the PE spectrum of CB. LNDO/S PERTCI, MNDO PERTCI and previous experimental vertical ionization energy estimates accord with the experimental data.