Gas-phase Ionization Energies Research Articles

Gas Phase Ionization Energy of Heptacene.

The ionization energy is a fundamental property that is relevant to charge transport in organic semiconductors. We report adiabatic ionization energies (AIEs) of heptacene at 6.21 and 7.20 eV for the X̃+B2g and Ã+Au states, respectively, as the next larger member of the acene series using mass- and isomer-selective double imaging photoelectron photoion coincidence spectroscopy. The X̃+ state energy decreases monotonically with an increase in size within the homologous series of acenes and approaches an asymptotic limit [AIE(polyacene) = 5.94 ± 0.06 eV] based on a fit with an exponential decay function. As byproducts of heptacene formation from cycloreversion of diheptacenes, 5,18-, 7,16-, and 6,17-dihydroheptacene can be detected, and their AIE is similar to that of their largest acene subunit (anthracene and tetracene, respectively), in very good agreement with computational treatments.

Limiting factors in the accuracy of DFT calculation for redox potentials.

In the present study, we have investigated factors affecting the accuracy of computational chemistry calculation of redox potentials, namely the gas-phase ionization energy (IE) and electron affinity (EA), and the continuum solvation effect. In general, double-hybrid density functional theory methods yield IEs and EAs that are on average within ~0.1 eV of our high-level W3X-L benchmark, with the best performing method being DSD-BLYP/ma-def2-QZVPP. For lower-cost methods, the average errors are ~0.2-0.3 eV, with ωB97X-3c being the most accurate (~0.15 eV). For the solvation component, essentially all methods have an average error of ~0.3 eV, which shows the limitation of the continuum solvation model. Curiously, the directly calculated redox potentials show errors of ~0.3 eV for all methods. These errors are notably smaller than what can be expected from error propagation with the two components (IE and EA, and solvation effect). Such a discrepancy can be attributed to the cancellation of errors, with the lowest-cost GFN2-xTB method benefiting the most, and the most accurate ωB97X-3c method benefiting the least. For organometallic species, the redox potentials show large deviations exceeding ~0.5 eV even for DSD-BLYP. The large errors are attributed to those for the gas-phase IEs and EAs, which represents a major barrier to the accurate calculation of redox potentials for such systems.

Ferrocene/ferrocenium, cobaltocene/cobaltocenium and nickelocene/nickelocenium: from gas phase ionization energy to one-electron reduction potential in solvated medium.

We propose a theoretical procedure for accurate determination of reduction potentials for three metallocene couples, Cp2M+/Cp2M, where M = Fe, Co and Ni. This procedure first computes the gas phase ionization energy (IE) using the explicitly correlated CCSD(T)-F12 method and includes the zero-point energy correction, core-valence electronic correlation, and relativistic and spin-orbit coupling effects. By means of Born-Haber thermochemical cycle, the one-electron reduction potential is obtained as the sum of the gas phase IE and the corresponding Gibbs free energies of solvation (ΔGsolv) for both the neutral and cationic species. Among the three solvent models (PCM, SMD and uESE) investigated here, it turns out that only the SMD model (computed at the DFT level) gives the best estimation of the value for "ΔGsolv(cation) - ΔGsolv(neutral)" and thus, combining with the accurate IE values, the theoretical protocol is capable of yielding reliable values (in V) for , and . These predictions compare favorably with the available experimental data (in V): , , and . We show that our theoretical procedure is reliable for accurate reduction potential predictions of Cp2Fe+/Cp2Fe, Cp2Co+/Cp2Co and Cp2Ni+/Cp2Ni redox couples in aqueous and non-aqueous media; the maximum absolute deviation is as small as ≈120 mV, which outperforms those of the existing theoretical methods.

A Localized-Orbital Energy Evaluation for Auxiliary-Field Quantum Monte Carlo.

Phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) has recently emerged as a promising method for the production of benchmark-level simulations of medium- to large-sized molecules because of its accuracy and favorable polynomial scaling with system size. Unfortunately, the memory footprints of standard energy evaluation algorithms are nontrivial, which can significantly impact timings on graphical processing units (GPUs) where memory is limited. Previous attempts to reduce scaling by taking advantage of the low-rank structure of the Coulombic integrals have been successful but exhibit high prefactors, making their utility limited to very large systems. Here we present a complementary cubic-scaling route to reduce memory and computational scaling based on the low rank of the Coulombic interactions between localized orbitals, focusing on the application to ph-AFQMC. We show that the error due to this approximation, which we term localized-orbital AFQMC (LO-AFQMC), is systematic and controllable via a single variable and that the method is computationally favorable even for small systems. We present results demonstrating robust retention of accuracy versus both experiment and full ph-AFQMC for a variety of test cases chosen for their potential difficulty for localized-orbital-based methods, including the singlet-triplet gaps of the polyacenes benzene through pentacene, the heats of formation for a set of Platonic hydrocarbon cages, and the total energy of ferrocene, Fe(Cp)2. Finally, we reproduce our previous result for the gas-phase ionization energy of Ni(Cp)2, agreeing with full ph-AFQMC to within statistical error while using less than 1/15th of the computer time.

Toward Higher Sensitivity in Quantitative MALDI Imaging Mass Spectrometry of CNS Drugs Using a Nonpolar Matrix.

Tissue-specific ion suppression is an unavoidable matrix effect in MALDI mass spectrometry imaging (MALDI-MSI), the negative impact of which on precision and accuracy in quantitative MALDI-MSI can be reduced to some extent by applying isotope internal standards for normalization and matrix-matched calibration routines. The detection sensitivity still suffers, however, often resulting in significant loss of signal for the investigated analytes. An MSI application considerably affected by this phenomenon is the quantitative spatial analysis of central nervous system (CNS) drugs. Most of these drugs are low molecular weight, lipophilic compounds, which exhibit inefficient desorption and ionization during MALDI using conventional polar acidic matrices (CHCA, DHB). Here, we present the application of the (2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile) matrix for high sensitivity imaging of CNS drugs in mouse brain sections. Since DCTB is usually described as an electron-transfer matrix, we provide a rationale (i.e., computational calculations of gas-phase proton affinity and ionization energy) for an additional proton-transfer ionization mechanism with this matrix. Furthermore, we compare the extent of signal suppression for five different CNS drugs when employing DCTB versus CHCA matrices. The results showed that the signal suppression was not only several times lower with DCTB than with CHCA but also depended on the specific tissue investigated. Finally, we present the application of DCTB and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry to quantitative MALDI imaging of the anesthetic drug xylazine in mouse brain sections based on a linear matrix-matched calibration curve. DCTB afforded up to 100-fold signal intensity improvement over CHCA when comparing representative single MSI pixels and >440-fold improvement for the averaged mass spectrum of the adjacent tissue sections.

How Batteries Store and Release Energy: Explaining Basic Electrochemistry

Batteries are valued as devices that store chemical energy and convert it into electrical energy. Unfortunately, the standard description of electrochemistry does not explain specifically where or how the energy is stored in a battery; explanations just in terms of electron transfer are easily shown to be at odds with experimental observations. Importantly, the Gibbs energy reduction in an electrochemical reaction in a battery also involves atom transfer between different phases. It is shown that, for simple galvanic cells or batteries with reactive metal electrodes, two intuitively meaningful contributions to the electrical energy are relevant: (i) the difference in the lattice cohesive energies of the bulk metals, reflecting metallic and covalent bonding and accounting for the atom transfer, and (ii) the difference in the ionization energies of the metals in water, associated with electron transfer. The ionization energy in water can be calculated as the sum of gas-phase ionization energies and the hydr...

Thermodynamic and Structural Factors That Influence the Redox Potentials of Tungsten–Alkylidyne Complexes

The thermodynamic and structural factors that influence the redox properties of an extensive set of tungsten–alkylidyne complexes (W(CR)L4X) are analyzed by combining synthesis, electrochemistry, and computational modeling based on free energy calculations of oxidation potentials at the density functional theory level. The observed linear correlations among oxidation potentials, HOMO energies, and gas-phase ionization energies are found to be consistent with the approximately constant solvation free energy differences between reduced and oxidized species over the complete set. The W–X bond length, trans to the alkylidyne ligand, is found to be a good descriptor of the positioning of the key frontier orbitals that regulate the redox properties of the complexes.

Theoretical Modeling of the Chirality Discrimination of Enantiomers by Nanotubular Cyclic Peptides using Gas-Phase Photoelectron Spectroscopy: An ONIOM Spectroscopic Calculations.

In the present work, the chirality recognition of the enantiomers of a chiral molecule (1-phenyl-1-propanol) interacting with a nanotubular cyclic peptide (E-type cyclic decapeptide) was investigated by their ionization in the gas phase, theoretically. The absolute energy difference between the interaction of the S- and R-enantiomer with the cyclic peptide, calculated at the M06-2X/6-311++G(d, p) level of theory, was 4.70 kcal·mol(-1). Two different schemes of "Our own N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM)" method such as (quantum mechanics (QM):molecular mechanics (MM)) and (QM:QM) were employed to study the effect of the interaction on the gas-phase ionization energies of the enantiomers and cyclic peptide, separately. The symmetry-adapted cluster/configuration interaction (SAC-CI) methodology was used for the calculation of the ionization energies. It was found that the difference between the interactions of R- and S-enantiomer with the cyclic peptide caused different changes in the photoelectron spectrum of each enantiomer so that these changes could be used for the chirality discrimination of the enantiomers in the gas phase. Similarly, the photoelectron spectrum of the cyclic peptide interacting with the R and S-enantiomer were calculated, separately, and it was observed that the difference in the interaction with the R- and S-enantiomer created different changes in the spectrum of cyclic peptide. Finally, it was shown that the difference in the interaction of cyclic peptide with the enantiomers of a chiral molecule in the gas phase can be used for the identification of enantiomers in the gas phase by the direct ionization.

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization.

The efficiencies of charge exchange reaction in dopant-assisted atmospheric pressure chemical ionization (DA-APCI) and dopant-assisted atmospheric pressure photoionization (DA-APPI) mass spectrometry (MS) were compared by flow injection analysis. Fourteen individual compounds and a commercial mixture of 16 polycyclic aromatic hydrocarbons were chosen as model analytes to cover a wide range of polarities, gas-phase ionization energies, and proton affinities. Chlorobenzene was used as the dopant, and methanol/water (80/20) as the solvent. In both techniques, analytes formed the same ions (radical cations, protonated molecules, and/or fragments). However, in DA-APCI, the relative efficiency of charge exchange versus proton transfer was lower than in DA-APPI. This is suggested to be because in DA-APCI both dopant and solvent clusters can be ionized, and the formed reagent ions can react with the analytes via competing charge exchange and proton transfer reactions. In DA-APPI, on the other hand, the main reagents are dopant-derived radical cations, which favor ionization of analytes via charge exchange. The efficiency of charge exchange in both DA-APPI and DA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied (0.05 and 0.1 mL/min). Both DA-APCI and DA-APPI showed the radical cation of chlorobenzene at 0.05-0.1 mL/min flow rate, but at increasing flow rate, the abundance of chlorobenzene M(+.) decreased and reagent ion populations deriving from different gas-phase chemistry were recorded. The formation of these reagent ions explains the decreasing ionization efficiency and the differences in charge exchange between the techniques. Graphical Abstract ᅟ.

Ionization Energies and Aqueous Redox Potentials of Organic Molecules: Comparison of DFT, Correlated ab Initio Theory and Pair Natural Orbital Approaches.

The calculation of redox potentials involves large energetic terms arising from gas phase ionization energies, thermodynamic contributions, and solvation energies of the reduced and oxidized species. In this work we study the performance of a wide range of wave function and density functional theory methods for the prediction of ionization energies and aqueous one-electron oxidation potentials of a set of 19 organic molecules. Emphasis is placed on evaluating methods that employ the computationally efficient local pair natural orbital (LPNO) approach, as well as several implementations of coupled cluster theory and explicitly correlated F12 methods. The electronic energies are combined with implicit solvation models for the solvation energies. With the exception of MP2 and its variants, which suffer from enormous errors arising at least partially from the poor Hartree-Fock reference, ionization energies can be systematically predicted with average errors below 0.1 eV for most of the correlated wave function based methods studies here, provided basis set extrapolation is performed. LPNO methods are the most efficient way to achieve this type of accuracy. DFT methods show in general larger errors and suffer from inconsistent behavior. The only exception is the M06-2X functional which is found to be competitive with the best LPNO-based approaches for ionization energies. Importantly, the limiting factor for the calculation of accurate redox potentials is the solvation energy. The errors in the predicted solvation energies by all continuum solvation models tested in this work dominate the final computed reduction potential, resulting in average errors typically in excess of 0.3 V and hence obscuring the gains that arise from choosing a more accurate electronic structure method.

Ionization of vitamin C in gas phase: Theoretical study

In this work, the gas phase ionization energies and photoelectron spectra of four important conformers of vitamin C were calculated. Symmetry adapted cluster/configuration interaction methodology employing the single and double excitation operators (SAC-CI SD-R) along with D95++(d,p) basis set were used for the calculations. Thermochemistry calculations were also performed on all possible conformers of vitamin C to find the relative stability of conformers in the gas phase. The calculated ionization bands of each conformer were assigned by calculating the contribution of natural bonding orbital (NBO) in the calculated canonical molecular orbitals involved in the ionization. SAC-CI calculations showed that the first ionization band of vitamin C is related to the π electrons of CC bond of the ring of molecule although, there is the lone electron pairs of oxygen atoms and π electrons of CO bond in the molecule.

What Makes a Strong Organic Electron Donor (or Acceptor)?

Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas-phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor-like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π-system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas-phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas-phase) ionization energy of an electron donor or the (gas-phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.

Dissociation, absorption and ionization of some important sulfur oxoanions (S2On2−n = 2, 3, 4, 6, 7 and 8)

In this work, a systematic theoretical study was performed on the dissociation, absorption and ionization of several important sulfur oxoanions (S2On2− (n=2, 3, 4, 6, 7 and 8)). ΔEelec (thermal corrected energy), ΔH° and ΔG° of the dissociation reactions of the oxoanions to their radical monoanions were calculated using combined computational levels of theories such as Gaussian-3 (G3) and a new version of complete basis set method (CBS-4M) in different environments including gas phase, microhydrated in gas phase and different solvents. Calculations showed S2O72− is the most stable anion against the dissociation to its radical monoanions (SO4−+SO3−). It was also found that S2O42− has more tendency to dissociate to its radical anions (SO2−+SO2−) compared to the other anions. The absorption spectra of the anions were also calculated using the time-dependent density functional theory (TD-DFT) employing M062X functional. The effect of microhydration and electrostatic field of solvent on the different aspects (intensity, energy shift and assignment) of the absorption spectra of these anions were also discussed. It was observed that both hydrogen bonding and electrostatic effect of water increases the intensity of the absorption spectrum compared to the gas phase. Effect of microhydration in shifting the spectra to the shorter wavelength is considerably higher than the effect of electrostatic field of water. Finally, several gas phase ionization energies of the anions were calculated using the symmetry-adapted cluster–configuration interaction methodology (SAC–CI) and found that the first electron detachment energies of S2O22−, S2O32− and S2O42− are negative. Natural bonding orbital (NBO) calculations were also performed to assign the electron detachment bands of the anions.

From gas-phase ionization energies to solution oxidation potentials: Dimolybdenum tetraformamidinate paddlewheel complexes

The gas-phase ionization energies of a series of Mo2(DPhF)4 paddlewheel complexes (DPhF is the N,N′-diphenylformamidinate anion with p-CH3, p-Cl, m-Cl, p-CF3, or m-CF3 phenyl substituents) have been measured by ultraviolet photoelectron spectroscopy (UPS) and compared with the solution oxidation potentials measured by cyclic voltammetry (CV) reported by Ren and coworkers. A linear relationship was found between the gas-phase ionization energies and the solution oxidation potentials. Density functional theory (DFT) computations clarify the individual electronic and thermodynamic factors that contribute to the correlation. The metal–metal delta bond electron energy is the largest factor in determining the solution oxidation potential. The substituents shift the metal–metal orbital energies by changing the through-space field potential at the metals rather than by an inductive change in charge at the metals or orbital overlap effects. The cation solvation energies determine the extent that the potential shifts are attenuated in solution. The results show that substituent field effects and solvation have major roles in determining the dimetal redox chemistry even when the dimetal unit is protected from direct interaction with the substituent and the solvent.

Control of single-molecule junction conductance of porphyrins via a transition-metal center.

Using scanning tunneling microscope break-junction experiments and a new first-principles approach to conductance calculations, we report and explain low-bias charge transport behavior of four types of metal-porphyrin-gold molecular junctions. A nonequilibrium Green's function approach based on self-energy corrected density functional theory and optimally tuned range-separated hybrid functionals is developed and used to understand experimental trends quantitatively. Importantly, due to the localized d states of the porphyrin molecules, hybrid functionals are essential for explaining measurements; standard semilocal functionals yield qualitatively incorrect results. Comparing directly with experiments, we show that the conductance can change by nearly a factor of 2 when different metal cations are used, counter to trends expected from gas-phase ionization energies which are relatively unchanged with the metal center. Our work explains the sensitivity of the porphyrin conductance with the metal center via a detailed and quantitative portrait of the interface electronic structure and provides a new framework for understanding transport quantitatively in complex junctions involving molecules with localized d states of relevance to light harvesting and energy conversion.

Gas phase ionization energies of some important unsaturated steroids

Gas phase ionization energies of some important biological active unsaturated steroids, known as active molecules in biology, were calculated using an expensive ab initio method for the first time. The calculations were performed using symmetry-adapted cluster-configuration interaction methodology employing a large basis set (6–311++G(2df,pd)), and ten ionization energies were calculated for each steroid. In addition, thermochemistry calculations were performed to determine the Boltzmann population ratios (BPRs) of different conformers of each steroid in the gas phase. The calculated ionization energies, their intensities, and the BPRs of conformers were used to simulate the photoelectron spectrum of each steroid. The calculations showed that for some steroids, more than one conformer contribute to the photoelectron spectrum of steroid. The calculated photoelectron spectrum of each steroid was compared with its corresponding experimental spectrum reported in literature. This comparison showed that the applied theoretical model can predict very well the relative energy of the ionization bands. The spectral ionization bands of each steroid were assigned by natural bonding orbital calculations and the ionization energy of πCC of each steroid was determined.

Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds.

Quantum chemical implicit solvent models are used widely to estimate aqueous redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aqueous single electron oxidation potentials of a diverse test set of neutral organic compounds for which accurate experimental oxidation potential and gas-phase ionization energy data are available. Using a thermodynamic cycle, we decomposed the free energy of oxidation into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aqueous oxidation potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidation potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted experimental data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aqueous oxidation potentials and related properties.

Quantum‐Chemical and Electrochemical Investigation of the Electrochemical Windows of Halogenated Carborate Anions

The range of electrochemical stability of a series of weakly coordinating halogenated (Hal=F, Cl, Br, I) 1-carba-closo-dodecaborate anions, [1-R-CB(11)X(5)Y(6)](-) (R=H, Me; X=H, Hal, Me; Y=Hal), has been established by using quantum chemical calculations and electrochemical methods. The structures of the neutral and dianionic radicals, as well as the anions, have been optimized by using DFT calculations at the PBE0/def2-TZVPP level. The calculated structures are in good agreement with existing experimental data and with previous calculations. Their gas-phase ionization energies and electron affinities were calculated based on their optimized structures and were compared with experimental (cyclic and square-wave) voltammetry data. Electrochemical oxidation was performed in MeCN at room temperature and in liquid sulfur dioxide at lower temperatures. All of the anions show a very high resistance to the onset of oxidation (2.15-2.85 V versus Fc(0/+)), with only a minor dependence of the oxidation potential on the different halogen substituents. In contrast, the reduction potentials in MeCN are strongly substituent dependent (-1.93 to -3.32 V versus Fc(0/+)). The calculated ionization energies and electron affinities correlate well with the experimental redox potentials, which provide important verification of the thermodynamic validity of the mostly irreversible redox processes that are observed for this series. The large electrochemical windows that are afforded by these anions indicate their suitability for electrochemical applications, for example, as supporting electrolytes.

Structure and Charge-State Dependence of the Gas-Phase Ionization Energy of Proteins

Photoionisierung von Protein-Ionen: Die mithilfe von VUV-Synchrotronstrahlung ermittelte Ionisierungsenergie mehrfach protonierter Proteinkationen in der Gasphase scheint mit dem Ladungszustand z des Proteins und seiner Tertiärstruktur zu korrelieren. Ein einfaches elektrostatisches Modell erklärt diese Befunde und ermöglicht Vorhersagen über den mittleren Radius Rm des Protein-Ions aus der Ionisierungsenergie und umgekehrt.

Electronic energy level alignment at metal-molecule interfaces with aGWapproach

Using density functional theory and many-body perturbation theory within a $GW$ approximation, we calculate the electronic structure of a metal-molecule interface consisting of benzene diamine (BDA) adsorbed on Au(111). Through direct comparison with photoemission data, we show that a conventional ${G}_{0}{W}_{0}$ approach can underestimate the energy of the adsorbed molecular resonance relative to the Au Fermi level by up to 0.8 eV. The source of this discrepancy is twofold: a 0.7 eV underestimate of the gas phase ionization energy (IE), and a 0.2 eV overestimate of the Au work function. Refinements to self-energy calculations within the $GW$ framework that account for deviations in both the Au work function and BDA gas-phase IE can result in an interfacial electronic level alignment in quantitative agreement with experiment.