Predicted Electron Affinities Research Articles

The electron affinity of the uranium atom.

The results of a combined experimental and computational study of the uranium atom are presented with the aim of determining its electron affinity. Experimentally, the electron affinity of uranium was measured via negative ion photoelectron spectroscopy of the uranium atomic anion, U-. Computationally, the electron affinities of both thorium and uranium were calculated by conducting relativistic coupled-cluster and multi-reference configuration interaction calculations. The experimentally determined value of the electron affinity of the uranium atom was determined to be 0.309 ± 0.025eV. The computationally predicted electron affinity of uranium based on composite coupled cluster calculations and full four-component spin-orbit coupling was found to be 0.232eV. Predominately due to a better convergence of the coupled cluster sequence for Th and Th-, the final calculated electron affinity of Th, 0.565eV, was in much better agreement with the accurate experimental value of 0.608eV. In both cases, the ground state of the anion corresponds to electron attachment to the 6d orbital.

Structurally Diverse Covalent Triazine-Based Framework Materials for Photocatalytic Hydrogen Evolution from Water.

A structurally diverse family of 39 covalent triazine-based framework materials (CTFs) are synthesized by Suzuki–Miyaura polycondensation and tested as hydrogen evolution photocatalysts using a high-throughput workflow. The two best-performing CTFs are based on benzonitrile and dibenzo[b,d]thiophene sulfone linkers, respectively, with catalytic activities that are among the highest for this material class. The activities of the different CTFs are rationalized in terms of four variables: the predicted electron affinity, the predicted ionization potential, the optical gap, and the dispersibility of the CTFs particles in solution, as measured by optical transmittance. The electron affinity and dispersibility in solution are found to be the best predictors of photocatalytic hydrogen evolution activity.

Accelerated Discovery of Organic Polymer Photocatalysts for Hydrogen Evolution from Water through the Integration of Experiment and Theory.

Conjugated polymers are an emerging class of photocatalysts for hydrogen production where the large breadth of potential synthetic diversity presents both an opportunity and a challenge. Here, we integrate robotic experimentation with high-throughput computation to navigate the available structure–property space. A total of 6354 co-polymers was considered computationally, followed by the synthesis and photocatalytic characterization of a sub-library of more than 170 co-polymers. This led to the discovery of new polymers with sacrificial hydrogen evolution rates (HERs) of more than 6 mmol g–1 h–1. The variation in HER across the library does not correlate strongly with any single physical property, but a machine-learning model involving four separate properties can successfully describe up to 68% of the variation in the HER data between the different polymers. The four variables used in the model were the predicted electron affinity, the predicted ionization potential, the optical gap, and the dispersibility of the polymer particles in solution, as measured by optical transmittance.

IrF8 Molecular Crystal under High Pressure.

An important goal in chemistry is to prepare F-rich transition metal fluorides due to the high oxidation states and potential applications such as oxidating and fluorinating agents. Thus far, the highest F stoichiometry in the neutral transition metal fluorides is 7. Here, we identify a hitherto unknown IrF8 compound through first-principles swarm-intelligence structure search calculations under high pressure. The three identified IrF8 phases exhibit typical molecular crystal characters, showing +8 oxidation state in Ir. The spatial symmetry of the basic building block in the three IrF8 phases gradually increases with pressure (e.g., dodecahedron [Formula: see text] square antiprism [Formula: see text] quasicube). The pressure-induced faster increase of Ir 5d orbital energy level with respect to F 2p provides a strong charge transfer driving force from Ir 5d to F 2p, facilitating the formation of F-rich compounds. More interestingly, the predicted electron affinities of the three predicted IrF8 phases are comparable/larger than that of PtF6, the strongest oxidation agent in the third row transition metal hexafluorides. The built high-pressure phase diagram of Ir-F binary compounds provides useful information for experimental synthesis.

Electron affinities of At and its homologous elements Cl, Br, and I

The multiconfiguration Dirac-Hartree-Fock method is applied to study the electron affinities of At and its homologous elements Cl, Br, and I. Our method of calculation is validated through the comparison with the available experimental electron affinities of Cl, Br, I, and other theoretical values. The agreement between our predicted electron affinities and the available experimental values for Cl, Br, and I is within 0.2%, which is an improvement of more than a factor of 10 over previous theoretical studies. Applying the same method to At, the electron affinity of At is predicted to be 2.3729(46) eV.

A high-resolution photoelectron imaging and theoretical study of CP- and C2P.

The discovery of interstellar anions has been a milestone in astrochemistry. In the search for new interstellar anions, CP- and C2P- are viable candidates since their corresponding neutrals have already been detected astronomically. However, scarce data exist for these negatively charged species. Here we report the electron affinities of CP and C2P along with the vibrational frequencies of their anions using high-resolution photoelectron imaging. These results along with previous spectroscopic data of the neutral species are used further to benchmark very accurate quartic force field quantum chemical methods that are applied to CP, CP-, C2P, and two electronic states of C2P-. The predicted electron affinities, vibrational frequencies, and rotational constants are in excellent agreement with the experimental data. The electron affinities of CP (2.8508 ± 0.0007 eV) and C2P (2.6328 ± 0.0006 eV) are measured accurately and found to be quite high, suggesting that the CP- and C2P- anions are thermodynamically stable and possibly observable. The current study suggests that the combination of high-resolution photoelectron imaging and quantum chemistry can be used to determine accurate molecular constants for exotic radical species of astronomical interest.

Synthesis and electrochemical evaluation of 2‐substituted imidazolium salts

AbstractHerein, we report the synthesis, electrochemical, and computational evaluation of six 2‐substituted imidazolium bromides and six 2‐substituted imidazolium triflates. All final compounds were obtained in 2 or fewer synthetic steps from inexpensive starting materials and display a single, irreversible electrochemical reduction. The reduction potentials span a range greater than 1 V depending on the electron withdrawing power of the 2‐substituent. Imidazolium bromides such as Bn2(H)ImBr reduce with E1/2 = −2.70 V vs Fc/Fc+, whereas the electron‐withdrawing Br‐containing analog Bn2(Br)ImBr reduces at only −1.58 V vs Fc/Fc+. The reduction potential of imidazolium bromides obeys a linear free energy relationship to σm Hammett constants, whereas imidazolium triflates correlate better with the σp Hammett constants. These results indicate that the stabilizing effect of the 2‐substituent is anion‐sensitive, changing from induction to resonance upon exchanging bromide for triflate. Predicted electron affinities from density functional theory–optimized structures of imidazolium cations and reduced species more closely match experimental data for the triflates, suggesting that a triflate anion does not electronically perturb the imidazolium core as much as a bromide. Taken together, these data highlight the dual modularity of imidazolium salts by changing both 2‐substituent and anion.

Evaluating the GW Approximation with CCSD(T) for Charged Excitations Across the Oligoacenes.

Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technological applications, are widely studied and therefore provide an excellent system for benchmarking theoretical methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approximation relative to new high-quality CCSD(T) reference data for charged excitations of the acenes. We compare GW calculations with a number of hybrid density functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with molecule length increasing from benzene to hexacene. We find that GW calculations with starting points based on an optimally tuned range-separated hybrid (OTRSH) density functional and eigenvalue self-consistency can yield quantitative ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from reference values. Our work paves the way for predictive and cost-effective GW calculations of charged excitations of molecules and identifies certain limitations of current GW methods used in practice for larger molecules.

Feshbach resonances of the H − 3 and D − 3 anions

Based on a series of ab initio calculations two Feshbach resonances of the H − 3 negative ion having λ(1 sa′1)2(2sa′1)2−μ(1sa′1)2(2pa′′2)2: 1 A′1 and (1sa′1)2(2sa′1)(2pa′′2): 3 A′′2 electronic structure and estimated lifetimes of more than 10−14 s are predicted. The potential energy surfaces of the Feshbach states are roughly parallel to those of the Rydberg states of neutral H 3 and show minima located between the dissociative 2pE′ ground state surfaces and the lowest Rydberg states 2sA′1 and 2pA′′2 of H 3. We have determined a number of vibrational levels of the non-rotating molecules H − 3 and D − 3 and we predict electron affinities for the mentioned two Rydberg states.

A Recipe for Designing Molecules with Ever-Increasing Electron Affinities

Halogens possess among the highest electron affinities of elements in the periodic table. Superhalogen molecules with electron affinities higher than those of halogen atoms have been known to form when a metal atom is surrounded with halogen atoms. Recently, it was discovered that a new class of molecules called hyperhalogens with electron affinities higher than those of superhalogens can form when the latter serve as the building block. By use of density functional theory and B3LYP hybrid exchange-correlation functional we show that molecules with electron affinities even higher can be formed by using hyperhalogens as building blocks. We demonstrate this by using Na and Li as metal atoms and F, BF(4), and Na(BF(4))(2) as halogen, superhalogen, and hyperhalogen building blocks. The predicted electron affinities of Na[Na(BF(4))(2)](2) and Li[Li(BF(4))(2)](2) are 9.18 and 9.01 eV, which are, respectively, 0.85 and 0.5 eV higher than those of their hyperhalogen [Na(BF(4))(2) and Li(BF(4))(2)] counterparts.

Stability of the valence anion of cytosine is governed by nucleobases sequence in the double stranded DNA π-stack: A computational study

The stabilities of the valence anion of cytosine (C(-)) in model trimers of complementary base pairs that possess the B-DNA geometry but differ in base sequence are reported. In order to estimate the energetics of electron attachment to the middle cytosine incorporated in the trimer, a thermodynamic cycle employing all possible two-body interaction energies in the neutral and anionic duplex as well as the adiabatic electron affinity of isolated cytosine were developed. All calculations were carried out at the MP2 level of theory with the aug-cc-pVDZ basis set. We have demonstrated that contrary to the literature reports, concerning single stranded DNA, the sequence of nucleic bases has a profound effect on the stability of the cytosine valence anion. The anionic 3(')-CCC-5(') complex is the most stable configuration (EA=0.399 eV) and the 3(')-GCG-5(') trimer anion is the most unstable species (EA=-0.193 eV). Moreover, with the energetic correction for the presence of sugar-phosphate backbone all possible double stranded DNA sequences lead to the stable C(-). The predicted electron affinities of the cytosine anion have been compared to the results of analogous studies on the thymine anion published recently [M. Kobyłecka et al., J. Am. Chem. Soc. 130, 15683 (2008)]. The consequences of low-energy barrier proton transfer in the GC anion have been discussed in the context of induced by electrons DNA single strand breaks. The DNA sequences that should dramatically differ in their vulnerability to be damaged by low energy electrons have been proposed.

Ladder-Type Polyenazine Based on Intramolecular S···N Interactions: A Theoretical Study of a Small-Bandgap Polymer

Ladder-type polyenazine (LPEA, 2), a nitrogen-containing polymer analogous to polyacetylene (PA), is proposed, based on molecule 1 synthesized and characterized earlier.8 By using density functional theory calculations, we show that the ladder-type structure of the proposed polymer arises from the intramolecular S···N interactions, which reduce the bond length alternation of the polymer and hence lead to a significantly narrower bandgap compared to the parent PA. A σ-conjugated pathway is found along the ···S−S···N−N···S--S··· linkage due to the noncovalent S···N interactions and NN through-space interactions. The theoretically predicted electron affinity of the proposed LPEA lies in the range for n-type semiconductors. We examine the energy barrier from the equilibrium geometry to the geometry of uniform bond length pattern and find that the estimated energy barrier is very similar to that of the parent PA.

Photoelectron spectroscopic study of the anionic transition metalorganic complexes [Fe1,2(COT)]− and [Co(COT)

The gas-phase, iron and cobalt cyclooctatetraene cluster anions, [Fe(1,2)(COT)](-) and [Co(COT)](-), were generated using a laser vaporization source and studied using mass spectrometry and anion photoelectron spectroscopy. Density functional theory was employed to compute the structures and spin multiplicities of these cluster anions as well as those of their corresponding neutrals. Both experimental and theoretically predicted electron affinities and photodetachment transition energies are in good agreement, authenticating the structures and spin multiplicities predicted by theory. The implied spin magnetic moments of these systems suggest that [Fe(COT)], [Fe(2)(COT)], and [Co(COT)] retain the magnetic moments of the Fe atom, the Fe(2) dimer, and the Co atom, respectively. Thus, the interaction of these transition metal, atomic and dimeric moieties with a COT molecule does not quench their magnetic moments, leading to the possibility that these combinations may be useful in forming novel magnetic materials.

Thermal electrons and Watson Crick AT(−)

The adiabatic electron affinity of adenine/thymine, AT, is >1.1 eV from the onset in the anion photoelectron spectrum. We demonstrate that the Watson Crick hydrogen bonded base pair is consistent with this onset using the experimental adiabatic electron affinities of adenine, 1.08(5) eV, and thymine, 0.93(5) eV, and the quantum mechanically predicted electron affinity of AT, 1.4 eV. This differs from the literature assignment to a non-biologically significant proton transferred form. The anion spectra of thymine and hydrated thymine are compared with that of AT. Herschbach Ionic Morse Person Empirical Curves are calculated for thymine anions.

Electron affinities of polynuclear aromatic hydrocarbons and negative-ion chemical-ionization sensitivities

Negative-ion chemical-ionization mass spectrometry (NICI MS) has the potential to be a very useful technique in identifying various polycyclic aromatic hydrocarbons (PAHs) in soil and sediment samples. Some PAHs give much stronger signals under NICI MS conditions than others. On the other hand, positive-ion signals are largely comparable under the same source conditions. An extensive set of newly re-evaluated experimental electron affinities (EAs), or free energies of electron attachment, are now available, as well as reliable predicted electron affinities from quantum theoretical calculations or from solution reduction potentials and theoretically predicted solvation energies. In order to show a high negative-ion sensitivity, a PAH must have an EA that exceeds a threshold of approximately of 0.5 eV. Comparisons between the negative-ion to positive-ion sensitivities ( N/ P ratios) and these new electron affinities show a rough correlation between the two, but naphthacene and perylene are exceptions to this relationship with much lower sensitivities than expected from their high EA values. By calculating the EA for a PAH, one can predict whether a sensitivity enhancement under NICI MS conditions is to be expected. Since aliphatic hydrocarbons and many other substances have negative or very low EAs, NICI MS is expected to be a good technique for detecting PAHs in samples contaminated with other hydrocarbons or compounds with low EAs.

Effects of Fluorine on the Structures and Energetics of the Propynyl and Propargyl Radicals and Their Anions

[reaction: see text] The adiabatic electron affinity (EA(ad)) of the CH(3)-C[triple bond]C(*) radical [experiment = 2.718 +/- 0.008 eV] and the gas-phase basicity of the CH(3)-C[triple bond]C:(-) anion [experiment = 373.4 +/- 2 kcal/mol] have been compared with those of their fluorine derivatives. The latter are studied using theoretical methods. It is found that there are large effects on the electron affinities and gas-phase basicities as the H atoms of the alpha-CH(3) group in the propynyl system are substituted by F atoms. The predicted electron affinities are 3.31 eV (FCH(2)-C[triple bond]C(*)), 3.86 eV (F(2)CH-C[triple bond]C(*)), and 4.24 eV (F(3)C-C[triple bond]C(*)), and the predicted gas-phase basicities of the fluorocarbanion derivatives are 366.4 kcal/mol (FCH(2)-C[triple bond]C:(-)), 356.6 kcal/mol (F(2)CH-C[triple bond]C:(-)), and 349.8 kcal/mol (F(3)C-C[triple bond]C:(-)). It is concluded that the electron affinities of fluoropropynyl radicals increase and the gas-phase basicities decrease as F atoms sequentially replace H atoms of the alpha-CH(3) in the propynyl system. The propargyl radicals, lower in energy than the isomeric propynyl radicals, are also examined and their electron affinities are predicted to be 0.98 eV ((*)CH(2)-C[triple bond]CH), 1.18 eV ((*)CFH-C[triple bond]CH), 1.32 eV ((*)CF(2)-C[triple bond] CH), 1.71 eV ((*)CH(2)-C[triple bond]CF), 2.05 eV ((*)CFH-C[triple bond]CF), and 2.23 eV ((*)CF(2)-C[triple bond]CF).

The alkylethynyl radicals, [rad]C [tbnd]C [sbnd]C nH 2 n + 1 ( n = 1−4), and their anions

Alkyl derivatives of the ethynyl radical ( C 2H) have been studied using density functional theory (DFT) with DZP++ basis sets. Adiabatic electron affinities (EA ad) and ZPVE-corrected electron affinities for the alkylethynyl series C C C n H 2 n + 1 ( n = 1–4) have been computed using six different DFT functionals, i.e., BHLYP, BLYP, B3LYP, BP86, BPW91 and B3PW91. These methods have been carefully calibrated for the prediction of electron affinities [J.C. Rienstra-Kiracofe, G.S. Tschumper, H.F. Schaefer, S. Nandi, G.B. Ellison, Chem. Rev. 102 (2002) 231]. The electron affinity of C 2H (2.969 ± 0.006 eV) has been compared with that of the alkylethynyl series with an attempt to determine the effect of the alkyl chain length on the electron affinities of the acetylenic species. The predicted electron affinities are 2.70 eV ( C 2CH 3, experiment = 2.718 ± 0.008 eV), 2.74 eV ( C 2C 2H 5), 2.75 eV ( C 2 n-C 3H 7), and 2.75 eV ( C 2 n-C 4H 9).

Cyclic perfluorocarbon radicals and anions having high global warming potentials (GWPs): structures, electron affinities, and vibrational frequencies.

Adiabatic electron affinities, optimized molecular geometries, and IR-active vibrational frequencies have been predicted for small cyclic hydrocarbon radicals C(n)H(2)(n)(-)(1) (n = 3-6) and their perfluoro counterparts C(n)F(2)(n)(-)(1) (n = 3-6). Total energies and optimized geometries of the radicals and corresponding anions have been obtained using carefully calibrated (Chem. Rev. 2002, 102, 231) density functional methods, namely, the B3LYP, BLYP, and BP86 functionals in conjunction with the DZP++ basis set. The predicted electron affinities show that only the cyclopropyl radical tends to bind electrons among the hydrocarbon radicals studied. The trend for the perfluorocarbon (PFC) radicals is quite different. The electron affinities increase with expanding ring size until n = 5 and then slightly decrease at n = 6. Predicted electron affinities of the hydrocarbon radicals using the B3LYP hybrid functional are 0.24 eV (C(3)H(5)/C(3)H(5)(-)), -0.19 eV (C(4)H(7)/C(4)H(7)(-)), -0.15 eV (C(5)H(9)/C(5)H(9)(-)), and -0.11 eV (C(6)H(11)/C(6)H(11)(-)). Analogous electron affinities of the perflurocarbon radicals are 2.81 eV (C(3)F(5)/C(3)F(5)(-)), 3.18 eV (C(4)F(7)/C(4)F(7)(-)), 3.34 eV (C(5)F(9)/C(5)F(9)(-)), and 3.21 eV (C(6)F(11)/C(6)F(11)(-)).

The germanium clusters Ge n (n = 1–6) and their anions: structures, thermochemistry and electron affinities

Six different hybrid and pure density functional theory (DFT) methods have been employed to study the structures, electron affinities, and dissociation energies of the (n = 1–6) species. The basis set used is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Located were 20 structures of the neutral Ge n clusters and 12 structures of the anionic clusters. The ground states of Ge n and clusters in this work are in good agreement with available experiments. Three types of electron affinities reported are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Ge–Ge dissociation energies D e(Ge n −1–Ge) for Ge n and both D e(–Ge) and D e(Ge n −1–Ge−) for the species have also been reported. Previously observed trends in the prediction of bond lengths by DFT methods are also demonstrated for the germanium clusters and their anions. Generally, the Hartree–Fock/DFT hybrid methods predict shorter and more reliable bond lengths than the pure DFT methods. The most reliable adiabatic electron affinities, obtained at the DZP++ BP86 level of theory, are 1.55 (Ge), 1.97 (Ge2), 2.24 (Ge3), 2.08 (Ge4), 2.29 (Ge5) and 2.07 eV (Ge6), among which those for Ge2, Ge3 and Ge6 are in good agreement with experiment. However, for Ge5 the predicted electron affinity is somewhat smaller than the available experimental values. Average electron affinity differences with the best experiments are 0.10 eV for the B3LYP method and 0.12 eV for BP86. The first dissociation energies for the neutral germanium clusters predicted by the B3LYP method are 2.87 (Ge2), 3.22 (Ge3), 3.80 (Ge4), 3.12 (Ge5) and 3.37 eV (Ge6). Compared to the available experimental dissociation energies for Ge2, the theoretical predictions are very reasonable. For the vibrational frequencies of the Ge n series, the DZP++ B3LYP method produces reliable predictions with a relative error of ∼2% with respect to available experimental values. In addition to the earlier finding that the BHLYP method is the most reliable for geometries, we conclude that the BP86 or B3LYP methods do the best for electron affinities, with B3LYP preferable for dissociation energies and vibrational frequencies among the six DFT methods.

III: PROPERTIES OF COMPLEX SYSTEMS

The molecular structures, vibrational frequencies, dissociation energies and electron affinities of the Br2/Br− 2, Br2O/Br2O−, Br2O2/Br2O− 2, Br2O3/Br2O− 3, and Br2O4/Br2O− 4 systems have been investigated using density functional theory (DFT) and hybrid Hartree-Fock/density functional theory. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities, (Rienstra-Kiracofe, J. C., Tschumper, G. S., Shaefer, H. F., Nandi, S. and Ellison, G. B., 2002, Chem. Rev., 102, 231). Four different types of neutral/anion energetic difference are reported in this work: the adiabatic electron affinity (EAad), the zero-point corrected EAad (EAzero), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The basis set used in this work is of double-zeta plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP++. The BHLYP method does well in reproducing the very limited experimental energetics, while the B3LYP method does well for the few known vibrational frequencies. The final predicted electron affinities with the BHLYP method are 3.41 eV (Br, experiment 3.36 eV), 3.02 eV (Br2, experiment 2.6 ± 0.2 eV), 3.27 eV (Br2O), 2.93 eV (Br2O2), 3.07 eV (Br2O3), and 2.54 eV (Br2O4). The global minimum structures for several of the larger dibromine oxides and their anions are unusual. For neutral Br2O2 the peroxide structure (BrOOBr) lies lowest, but for the anion a loosely bound Cs symmetry BrO-BrO− structure lies lowest for the hybrid functionals, while a C2 symmetry peroxide BrOOBr− structure lies lowest for the pure functionals. Furthermore, the C2 structures are found to exhibit an inverse symmetry breaking problem, and should be interpreted with caution. For neutral Br2O3, a chain structure Br-O3-Br lies lowest, while the complex Br···O3···Br− lies lowest for the negative ion. For neutral Br2O4, a chain structure Br-O4-Br lies lowest, while the complex BrO−···BrO3 lies lowest for the negative ion.