Theoretical Electron Affinities Research Articles

DFT calculations of copper complexes mimicking superoxide dismutase and docking studies and molecular dynamics of the transition metal complex binding to serum albumin

Superoxide dismutase (SOD) is a metalloenzyme whose antioxidant activity is mimicked by some transition metal complexes, and such ability can be added in proteins such as the bovine serum albumin (BSA), creating a hybrid protein. In this work, density functional theory (DFT) calculations of three Cu(II)-complexes of general formula [CuL2phen] (phen = phenanthroline; C1, L = mefenamate; C2, L = tolfenamate; C3, L = flufenamate) with SOD-like activity, and docking and molecular dynamics (MD) simulations of these complexes with the BSA were performed. The DFT calculations revealed that the complex reduction involves Cu(II) → Cu(I) reduction, the theoretical electron affinity (EA) correlated with the SOD-like activity (IC50), and the contribution of the phenanthroline ligand and the metal in LUMO it’s related with the complex-protein interaction (KVS). The docking and MD simulations revealed the binding site of the complexes in BSA and the residues involved in the binding. The stability of the Cu(II) and Cu(I) forms of the complexes in the site indicated that the catalysis promoted by these complexes occurs in the same region of the BSA and that their mimetic activity can be incorporated into BSA, creating a hybrid protein (BSA with SOD activity) Communicated by Ramaswamy H. Sarma

ThAu2 -, ThAu2O-, and ThAuOH- anions: Photoelectron spectroscopic and theoretical characterization.

The thorium-gold negative ions ThAu2 -, ThAu2O-, and ThAuOH- have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0K agree with the experimental adiabatic EAs to within 0.02eV for all species. Two separate isomers were located in the calculations for ThAuOH-, and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reported at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th-Au and Th-F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th-F σ bonds are much more ionic compared to Th-Au. This results in an EA for ThF2 that is 10 kcal/mol smaller than that of ThAu2. The EA values for the Th(IV) species, i.e., ThX2O and ThXOH, only differed, however, by 3-4 kcal/mol.

Negative Ion Formation in Low-Energy Electron Collisions with the Actinide Atoms Th, Pa, U, Np and Pu

Here we investigate ground and metastable negative ion formation in low-energy electron collisions with the actinide atoms Th, Pa, U, Np and Pu through the elastic total cross sections (TCSs) calculations. For these atoms, the presence of two or more open d- and f- subshell electrons presents a formidable computational task for conventional theoretical methods, making it difficult to interpret the calculated results. Our robust Regge pole methodology which embeds the crucial electron correlations and the vital core-polarization interaction is used for the calculations. These are the major physical effects mostly responsible for stable negative ion formation in low-energy electron scattering from complex heavy systems. We find that the TCSs are characterized generally by Ramsauer-Townsend minima, shape resonances and dramatically sharp resonances manifesting ground and metastable negative ion formation during the collisions. The extracted from the ground states TCSs anionic binding energies (BEs) are found to be 3.09eV, 2.98eV, 3.03eV, 3.06eV and 3.25eV for Th, Pa, U, Np and Pu, respectively. Interestingly, an additional polarization-induced metastable TCS with anionic BE value of 1.22eV is generated in Pu due to the size effect. We also found that our excited states anionic BEs for several of these atoms compare well with the existing theoretical electron affinities including those calculated using the relativistic configuration-interaction method. We conclude that the existing theoretical calculations tend to identify incorrectly the BEs of the resultant excited anionic states with the electron affinities of the investigated actinide atoms; this demonstrates the great need for experimental verification and unambiguous determination of their electron affinities.

Tests of second-generation and third-generation density functionals for electron affinities of the alkylthio radicals

The molecular structures and electron affinities of the R – S / R – S -( R = CH 3, C 2 H 5, n- C 3 H 7, n- C 4 H 9, n- C 5 H 11, i- C 3 H 7, i- C 4 H 9, t- C 4 H 9) species have been studied using 17 pure and hybrid density functionals (five generalized gradient approximation (GGA) methods, six hybrid GGAs, one meta GGA method and five hybrid meta GGAs). The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted by DZP++. The geometries are fully optimized with each DFT method and discussed. Harmonic vibrational frequencies are found to be within 3.5% of available experimental values for most functionals. Three different types of the neutral-anion energy separations have been presented. The theoretical electron affinities of alkylthio radicals are in good agreement with the experiment data. The M06 method is very good for the adiabatic electron affinity calculations, and the average absolute error is 0.0439 eV. The HCTH method performs better for EA prediction. The M06-HF, mPWPW91, VSXC and B98 are also reasonable. The most reliable adiabatic electron affinities are predicted to be 1.864 eV ( CH 3 S ), 1.946 eV ( C 2 H 5 S ), 1.959 eV (n- C 3 H 7 S ), 1.970 eV (n- C 4 H 9 S ), 1.982 eV (n- C 5 H 11 S ), 2.053 eV (i- C 3 H 7 S ), 1.991 eV (i- C 4 H 9 S ) and 2.100 eV (t- C 4 H 9 S ) at the M06/DZP++ level of theory, respectively.

Reassessment of 182Hf AMS measurements at VERA

The radioisotope 182Hf ( t 1/2 = 8.9 Ma) is of great interest for astrophysical applications as a chronometer for the early solar system or as possible live supernova remnant on earth. However, AMS measurements of 182Hf are seriously influenced by the presence of the stable isobar 182W, which cannot be separated at typical AMS energies. Previous studies revealed a possible suppression of 182W against 182Hf by extracting the negatively charged pentafluoride HfF 5 - from the ion source, leading to a detection limit for 182Hf/ 180Hf in the order of 10 −11. However, this suppression behavior is in contrast to theoretical calculations of the electron affinity and recent measurements using SIMS instruments, where the achieved suppression cannot be reproduced. The aim of our study is to determine the effects of ion source background as well as further investigate the suppression of tungsten against hafnium by extracting negatively charged fluoride ions from different sample materials. The previously reported suppression factor of about 6000 could be increased to 36000 by careful tuning of the ion source using HfF 4 as sample material. The trend of the theoretical electron affinities could be reproduced using atomic tungsten and hafnium instead of HfF 4 as sample material. This supports the assumption that the major contribution of the tungsten background is not sputtered from the target matrix but comes from somewhere else in the ion source. Measurements from the second ion source show a higher background of tungsten and a lower suppression factor, i.e. careful design of the ion source is crucial. Moving the sputter beam over the target surface extending over the wheel holding the targets revealed the highest tungsten background was detected outside the sputter target position. Further investigations are necessary to locate the origin of the tungsten background in the ion source. Possible sources are the material used for the ion source construction or contaminations in the cesium used for sputtering.

Studies on structures and properties of protonated pyridoxine, (PN+H) [rad] radicals and (PN+H) − anions with density functional theory

The hydrogenated pyridoxine (PN) species in cationic, neutral, and anionic states have been studied. The structures, energetics, and theoretical electron affinities are predicted by means of the B3LYP/DZP++ method. These radical and anionic species come from consecutive electron attachment to the corresponding protonated (PN+H) + cations in low pH environments. The most reliable proton affinities of PN (protonation at N1) are predicted to be 232.3 kcal/mol. The radicals range within 11.5 kcal/mol in relative energy and radicals r1 (attached H to N1 site of PN) are the lowest energy. Structures r7 and r8 are predicted to extrude water molecule. The theoretical adiabatic electron affinities (AEA), vertical electron affinities (VEA), and vertical detachment energies (VDE) are estimated. The ranges of AEA for six radicals are from 0.44 (or 0.35) to 1.76 eV. Taking into account the solvent effect, protonation of PN in aqueous solution is more stable and neutral hydrogenated PN more readily takes up solvated electrons.

ELECTRON AFFINITY METHOD FOR DETERMINING THE ACTIVITY OF MONOMERS IN ANIONIC POLYMERIZATION

The electron affinities(EA) of the vinyl monomers with different substituents were used to determine the anionic polymerization activities of the vinyl monomers.To obtain the theoretical EA data of the vinyl monomers,the Gaussian 03 program based on density functional theory was employed.The geometry of the monomers with different substituents was first optimized by a B3LYP method with 6-31G(d) basis sets,then the electron affinities were calculated at the level of(B3LYP/6-311++G(3df,2p).)It is suggested that the method used to calculate the molecular electron affinity is reliable according to the comparison between the predicted and the experimental EA values.The Mulliken charges of(C)(C) double bonds in different monomers is helpful to determine the reactive sites of the anionic polymerization.The monomers with EA values less than-20 kJ·mol~(-1) are mainly conjugated alkenes without polarity,which only react with strong nucleophilic initiators.The EA vaules of the monomers with a relatively strong electron-withdrawing substituent are generally in the ranges of-20～60 kJ·mol~(-1),which can be polymerized with the weak nucleophilic.While the EA values of the monomers with two strong electron-withdrawing substituents are generally more than 60 kJ·mol~(-1),and the corresponding monomers can react with very mild nucleophiles.Therefore,the EA values of the monomers with different substituents could be used to determine the anionic polymerization activities of the monomers.This conclusion was further validated according to the comparison with the Q-e and σ values.

A Gaussian-3 Theoretical Study of Small Silicon−Lithium Clusters: Electronic Structures and Electron Affinities of SinLi− (n = 2−8)

The molecular structures of neutral Si n Li ( n = 2-8) species and their anions have been studied by means of the higher level of the Gaussian-3 (G3) techniques. The lowest energy structures of these clusters have been reported. The ground-state structures of neutral clusters are "attaching structures", in which the Li atom is bound to Si n clusters. The ground-state geometries of anions, however, are "substitutional structures", which is derived from Si n+1 by replacing a Si atom with a Li (-). The electron affinities of Si n Li and Si n have been presented. The theoretical electron affinities of Si n are in good agreement with the experiment data. The reliable electron affinities of Si n Li are predicted to be 1.87 eV for Si 2Li, 2.06 eV for Si 3Li, 2.01 eV for Si 4Li, 2.61 eV for Si 5Li, 2.36 eV for Si 6Li, 2.21 eV for Si 7Li, and 3.18 eV for Si 8Li. The dissociation energies of Li atom from the lowest energy structures of Si n Li and Si atom from Si n clusters have also been estimated respectively to examine relative stabilities.

π and σ-Phenylethynyl Radicals and Their Isomerso-,m-, andp-Ethynylphenyl: Structures, Energetics, and Electron Affinities

Molecular structures, energetics, vibrational frequencies, and electron affinities are predicted for the phenylethynyl radical and its isomers. Electron affinities are computed using density functional theory, -namely, the BHLYP, BLYP, B3LYP, BP86, BPW91, and B3PW91 functionals-, employing the double-zeta plus polarization DZP++ basis set; this level of theory is known to perform well for the computation of electron affinities. Furthermore, ab initio computations employing perturbation theory, coupled cluster with single and double excitations [CCSD], and the inclusion of perturbative triples [CCSD(T)] are performed to determine the relative energies of the isomers. These higher level computations are performed with the correlation consistent family of basis sets cc-pVXZ (X = D, T, Q, 5). Three electronic states are probed for the phenylethynyl radical. In C2v symmetry, the out-of-plane (2B1) radical is predicted to lie about 10 kcal/mol below the in-plane (2B2) radical by DFT methods, which becomes 9.4 kcal/mol with the consideration of the CCSD(T) method. The energy difference between the lowest pi and sigma electronic states of the phenylethynyl radical is also about 10 kcal/mol according to DFT; however, CCSD(T) with the cc-pVQZ basis set shows this energy separation to be just 1.8 kcal/mol. The theoretical electron affinities of the phenylethynyl radical are predicted to be 3.00 eV (B3LYP/DZP++) and 3.03 eV (CCSD(T)/DZP++//MP2/DZP++). The adiabatic electron affinities (EAad) of the three isomers of phenylethynyl, that is, the ortho-, meta-, and para-ethynylphenyl, are predicted to be 1.45, 1.40, and 1.43 eV, respectively. Hence, the phenylethynyl radical binds an electron far more effectively than the three other radicals studied. Thermochemical predictions, such as the bond dissociation energies of the aromatic and ethynyl C-H bonds and the proton affinities of the phenylethynyl and ethynylphenyl anions, are also reported.

Structures and Properties of Stable Al4, Al4+, and Al4- Comparatively Studied by ab Initio Theories

Previous high-level theoretical calculations of aluminum clusters mostly relied on density functional theory (DFT) or theories less sophisticated than it. Here, we point out that a second-order Møller-Plesset perturbation (MP2) method is more appropriate and is the minimum level of theory in the predictions of property such as geometries, electronic structures, and IR and Raman spectra of Al4, Al4+, and Al4- clusters. The theoretical electron affinities and ionization potentials predicted with MP2 geometries are closer to experimental ones than those by DFT. The p-electron characters and single valence state of aluminum atoms and IR and Raman spectra of the aluminum clusters were also reliably predicted by MP2 and could be based on for further experimental and theoretical studies.

Theoretical electron affinities of PAHs and electronic absorption spectra of their mono-anions

We present theoretical electron affinities, calculated as total energy differences, for a large sample of polycyclic aromatic hydrocarbons (PAHs), ranging in size from azulene (C10H8) to dicoronylene (C48H20). For 20 out of 22 molecules under study we obtained electron affinity values in the range 0.4-2.0 eV, showing them to be able to accept an additional electron in their LUMO π � orbital. For the mono-anions we computed the absolute photo-absorption cross-sections up to the vacuum ultraviolet (VUV) using an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which has already been proven to yield accurate results for neutral and cationic PAHs. Comparison with available experimental data hints that this is the case for mono-anions as well. We find that PAH anions, like their parent molecules and the corresponding cations, display strong π ∗ ← π electronic transitions in the UV. The present results provide a quantitative foundation to estimate the fraction of specific PAHs which can be singly negatively charged in various interstellar environments, to simulate their photophysics in detail and to evaluate their contribution to the interstellar extinction curve.

Electron affinities of the radicals derived from cytosine

Theoretical studies have been carried out to investigate the electron affinities (EAs) of five radicals derived from cytosine by removing a hydrogen atom (C(-H)*), using carefully calibrated computational methods. Significant structural differences are predicted between cytosine, its five radicals and the five corresponding anions. The largest bond distance differences between the radical and its analogous anion are 0.05 angstroms. The theoretical EAs (predicted using carefully calibrated methods) for the five cytosine radicals range from 2.22 eV to 3.00 eV. Among these the N-centred radical 2 has the largest adiabatic electron affinity (EA(ad) = 3.00 eV), and the carbon-centred radical 8 has the smallest (2.22 eV). These values are much larger than EA(ad) for the neutral closed-shell cytosine molecule. The largest EA(ad) value predicted for the cytosine radical isomers (C(-H)*) is comparable to those for radicals related to other nucleoside bases. The ordering for the four DNA bases is EA(T(-H)*) = 3.46 > EA(A)-(-H)*) = 3.26 > EA(C(-H)*)= 3.00 > EA(G(-H)*) = 2.99 eV.

Competition between linear and cyclic structures in monochromium carbide clusters CrCn- and CrCn (n = 2-8): a photoelectron spectroscopy and density functional study.

Photoelectron spectroscopy (PES) is combined with density functional theory (DFT) to study the monochromium carbide clusters CrCn- and CrCn (n = 2-8). Well-resolved PES spectra were obtained, yielding structural, electronic, and vibrational information about both the anionic and neutral clusters. Experimental evidence was observed for the coexistence of two isomers for CrC2-, CrC3-, CrC4-, and CrC6-. Sharp and well-resolved PES spectra were observed for CrCn- (n = 4,6,8), whereas broad spectra were observed for CrC5- and CrC7-. Extensive DFT calculations using the generalized gradient approximation were carried out for the ground and low-lying excited states of all the CrCn- and CrCn species, as well as coupled-cluster calculations for CrC2- and CrC2. Theoretical electron affinities and vertical detachment energies were calculated and compared with the experimental data to help the assignment of the ground states and obtain structural information. We found that CrC2- and CrC3- each possess a close-lying cyclic and linear structure, which were both populated experimentally. For the larger CrCn- clusters with n = 4, 6, 8, linear structures are the overwhelming favorite, giving rise to the sharp PES spectral features. CrC7- was found to have a cyclic structure. The broad PES spectra of CrC5- suggested a cyclic structure, whereas the DFT results predicted a linear one.

Isotope shift in the electron affinity of beryllium

The study of the isotope shift in the electron affinity is interesting for probing correlation effects. Experiments that allow this property to be measured are rare, being difficult to realize, while accurate calculations remain a challenge for atomic theory. The present work focuses on the theoretical estimation of the isotope shift in the electron affinity of Be (2s2p 3Po), using correlated electronic wavefunctions obtained from multiconfiguration Hartree–Fock and configuration interaction variational calculations. The reliability of the correlation models is assessed from a comparison between the observed and theoretical electron affinities, and between theoretical isotope shift values for the 2s2p 3Po 2s2 1S transition of neutral beryllium. The sign and the magnitude of the difference between the mass polarization term expectation values obtained for the neutral atom and the negative ion are such that the resulting isotope shift in the electron affinity is ‘anomalous’, corresponding to a smaller electron affinity for the heavier isotope.

Structural and electronic properties of iron monoxide clusters FenO and FenO− (n=2–6): A combined photoelectron spectroscopy and density functional theory study

We report a combined anion photoelectron spectroscopy and density functional theory (DFT) study on a series of iron monoxide clusters, FenO (n=2–6). Well-resolved photoelectron spectra were obtained for FenO− at variable detachment energies, allowing the ground state and numerous low-lying excited states of FenO to be observed. Sharp threshold photoelectron features were obtained for each species, which suggest rather small geometry changes between the anion and neutral ground states for the monoxide clusters and allows the electron affinities of the neutral clusters to be measured accurately. Extensive DFT calculations using the generalized gradient approximation were carried out for both FenO and FenO−. Optimized geometries of the ground and lowest excited states of both the anion and neutral species are reported along with the ground-state vibrational frequencies and fragmentation energies. Theoretical electron affinities were compared with the experimental measurements to verify the ground states of the iron monoxide clusters obtained from the DFT calculations.

Cyclopentadiene annulated polycyclic aromatic hydrocarbons: investigations of electron affinities.

The adiabatic electron affinities of cyclopentadiene and 10 associated benzannelated derivatives have been predicted with both density functional and Hartree-Fock theory. These systems can also be regarded as benzenoid polycyclic aromatic hydrocarbons (PAHs) augmented with five-membered rings. Like the PAHs, the electron affinities of the present systems generally increase with the number of rings. To unequivocally bind an electron, cyclopentadiene must have at least two conventionally fused benzene rings. 1H-Benz[f]indene, a naphthalene-annulated cyclopentadiene, is predicted to have a zero-point energy corrected adiabatic electron affinity of 0.13 eV. Since the experimental E(A) of naphthalene is negative (-0.19 eV), the five-membered ring appendage contributes to the stability of the naphthalene-derived 1H-benz[f]indene radical anion significantly. The key to binding the electron is a contiguous sequence of fused benzenes, since fluorene, the isomer of 1H-benz[f]indene, with separated six-membered rings, has an electron affinity of -0.07 eV. Each additional benzene ring in the sequence fused to cyclopentadiene increases the electron affinity by 0.15-0.65 eV: the most reliable predictions are cyclopentadiene (-0.63 eV), indene (-0.49 eV), fluorene (-0.07 eV), 1H-benz[f]indene (0.13 eV), 1,2-benzofluorene (0.25 eV), 2,3-benzofluorene (0.26 eV), 12H-dibenzo[b,h]fluorene (0.65 eV), 13H-indeno[1,2-b]anthracene (0.82 eV), and 1H-cyclopenta[b]naphthacene (1.10 eV). In contrast, if the six-membered ring-fusion is across the C(2)-C(3) cyclopentadiene single bond, only a single benzene is needed to bind an electron. The theoretical electron affinity of the resulting molecule, isoindene, is 0.49 eV, and this increases to 1.22 eV for 2H-benz[f]indene. The degree of aromaticity is responsible for this behavior. While the radical anions are stabilized by conjugation, which increases with the size of the system, the regular indenes, like PAHs in general, suffer from the loss of aromatic stabilization in forming their radical anions. While indene is 21 kcal mol(-1) more stable than isoindene, the corresponding radical anion isomers have almost the same energy. Nucleus-independent chemical shift calculations show that the highly aromatic molecules lose almost all aromaticity when an extra electron is present. The radical anions of cyclopentadiene and all of its annulated derivatives have remarkably low C-H bond dissociation energies (only 18-34 kcal mol(-1) for the mono-, bi-, and tricyclics considered). Hydrogen atom loss leads to the restoration of aromaticity in the highly stabilized cyclopentadienyl anion congeners.

Comment on “Ionization potentials and electron affinities from the extended Koopmans’ theorem applied to energy derivative density matrices: The EKTMPn and EKTQCISD methods” [J. Chem. Phys. 107, 6804 (1997)

In the subject article, it is stated that “the EKTMP2 formalism is certain to replace the Koopmans’ theorem as the ‘black box’ estimator of ionization potentials” and by inference, for electron affinities. It is shown that semiempirical MINDO/3 multiconfiguration interaction ionization potentials for the five aromatic hydrocarbons considered in the article are as accurate as the EKT values. No theoretical EKT electron affinities are presented in the subject article. Experimental and theoretical electron affinities for a number of aromatic hydrocarbons calculated using MINDO/3 with MCCI are presented to be matched by any other estimator.

Study of low-lying electronic states of ozone by multireference Mo/ller–Plesset perturbation method

The geometry and relative energy of the seven low-lying electronic states of ozone and the ground state of ozonide anion have been determined in C2v symmetry by the complete active space self-consistent field (CASSCF) and the multireference Mo/ller–Plesset perturbation (MRMP) methods. The results are compared with the photodetachment spectra of O−3 observed recently by Arnold et al. The theoretical electron affinity of ozone is 1.965 eV, which is 0.14 eV below the experimental result of 2.103 eV. The calculated adiabatic excitation energies (assignment of Arnold et al. in parentheses) of ozone are 3A2 0.90 eV (1.18 eV), 3B2, 1.19 eV (1.30 eV), 3B1, 1.18 eV (1.45 eV), 1A2, 1.15 eV (∼1.6 eV), 1B1, 1.65 eV (2.05 eV), and 1B2, 3.77 eV (3.41 eV), respectively. Overall the present theory supports the assignment of Arnold et al. However, the simple considerations of geometry and energy are insufficient to determine a specific assignment of the 3B2 and 3B1 states. The dissociation energy of the ground state of ozone is computed to be 0.834 eV at the present level of theory. The present theory also predicts that none of the excited states lies below the ground state dissociation limit of O3.

Theoretical study of simple electron transfer reactions involving oxo radicals and anions

The heat of electron transfer reactions involving oxo radicals and anions is calculated using the first-order correlation orbitals and the coupled cluster method. Despite systematic errors which remain in the theoretical electron affinities versus the experimental affinities, the agreement in the electron transfer energy is much more plausible.

Theoretical study of the structures and electron affinities of the dimers and trimers of the group IB metals (Cu, Ag, and Au)

The molecular structure of both the neutral and negatively charged diatomic and triatomic systems containing the Cu, Ag, and Au metals are determined from ab initio calculations. For the neutral triatomic systems, the lowest energy structure is found to be triangular. The relative stability of the 2A1 and 2B2 structures can be predicted simply by knowing the constituent diatomic bond distances and atomic electron affinities (EAs). The lowest energy structure is linear for all of the negative ions. For anionic clusters containing Au, the Au atom(s) preferentially occupy the terminal position(s). The EAs of the heteronuclear systems can be predicted relatively accurately from a weighted average of the corresponding homonuclear systems. Although the theoretical EAs are systematically too small, accurate predictions for the EAs of the triatomics are obtained by uniformly scaling the ab initio results using the accurate experimental EA values available for the atoms and homonuclear diatomics.