Predicted Electron Affinities Research Articles

Odd carbon long linear chains HC2n+1H (n = 4-11): properties of the neutrals and radical anions.

The optimized geometries, adiabatic electron affinities, vertical electron affinities, vertical electron detachment energies (for the anions), and IR-active vibrational frequencies have been predicted for the long linear carbon chains HC(2)(n)()(+1)H (n = 4-11). The B3LYP density functional combined with DZP and TZ2P basis sets was used in this theoretical study. These methods have been extensively calibrated versus experiment for the prediction of electron affinities (Chem. Rev. 2002, 102, 231). The computed physical properties are discussed and compared with the even carbon chains HC(2)(n)()H. The predicted electron affinities form a remarkably regular sequence: 2.12 eV (HC(9)H), 2.42 eV (HC(11)H), 2.66 eV (HC(13)H), 2.85 eV (HC(15)H), 3.01 eV (HC(17)H), 3.14 eV (HC(19)H), 3.25 eV (HC(21)H), and 3.35 eV (HC(23)H). These electron affinities are as much as 0.4 eV higher than those for analogous even carbon chains. The predicted structures display an intermediate cumulene-polyacetylene type of bonding, with the inner carbons appearing cumulenic and the outer carbons polyacetylenic.

Homoleptic tetrahydrometalate anions MH(4)(-) (M = Sc, Y, La). Matrix infrared spectra and DFT calculations.

Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH(2) and (H(2))MH(2). The homoleptic tetrahydrometalate anions ScH(4)(-), YH(4)(-), and LaH(4)(-) are formed by electron capture and identified by isotopic substitution (D(2), HD, and H(2) + D(2) mixtures). Doping with CCl(4) to serve as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The observed vibrational frequencies are in agreement with the results of density functional theory calculations, which predict electron affinities in the 2.8-2.4 eV range for the (H(2))ScH(2), (H(2))YH(2), and (H(2))LaH(2) complexes, and indicate high stability for the MH(4)(-) (M = Sc, La, Y) anions and suggest the promise of synthesis on a larger scale for use as reducing agents.

What is the nature of polyacetylene neutral and anionic chains HC(2n)H and HC(2n)H(-) (n = 6-12) that have recently been observed?

The optimized geometries, adiabatic electron affinities, and IR-active vibrational frequencies have been predicted for the long linear carbon chains HC(2n)H. The B3LYP density functional combined with the DZP basis set was used in this theoretical study. The computed physical properties are discussed. The predicted electron affinities form a remarkably regular sequence: 1.78 (HC(12)H), 2.08 (HC(14)H), 2.32 (HC(16)H), 2.53 (HC(18)H), 2.69 (HC(20)H), 2.83 (HC(22)H), and 2.95 eV (HC(24)H). The predicted structures display an alternating triple and very short single bond pattern, with the degree of bond alternation significantly less for the radical anions.

Electron Affinities of Polycyclic Aromatic Hydrocarbons

The electron affinities of benzene and four polycyclic aromatic hydrocarbons (PAHs), naphthalene, anthracene, tetracene, and the perinaphthenyl radical, have been obtained using six common density functional theory (DFT) methods. When compared to experiment, the BHLYP, BLYP, and B3LYP functionals have average absolute errors of 0.17, 0.18, and 0.19 eV, respectively. The success of the BHLYP functional is dubious due to a fortuitous cancelation in error between the tendency for BHLYP to underestimate electron affinities and zero-point vibrational energy (ZPVE) corrections. We recommend the BLYP and B3LYP functionals for future studies of PAH anions. However, the computation of ZPVE corrections may be a limiting factor in the accuracy of any method seeking to predict electron affinities for large PAHs.

Bromine Halides: The Neutral Molecules BrClFn (n = 1− 5) and Their Anions Structures, Energetics, and Electron Affinities

Density functional methods were used for predictions of the structures and energetics of molecules and anions in the series BrClFn (n = 1−5). Very little is known from experiment about these potentially important interhalogen molecules. Energy minima were found for all neutral molecules with the exception of BrClF5. However, the dissociation limits for ClF elimination from the BrClF3 (BHLYP and B3LYP only) and BrClF4 (all methods) molecules lie below the energy of the respective ground states. All the BrClFn anions are bound with respect to the corresponding neutrals; energy minima were found for anions up to BrClF5-. Structures of isomers with the chlorine atom in the central position were also optimized. However, these local minima lie substantially above the bromine-centered structures, and the energy gap between isomers grows with n. For the neutral molecules two paths of dissociation were found to possess the lowest energies, i.e., chlorine atom and ClF elimination. The latter channel becomes predominant at n > 1. For the anions the lowest energy dissociation pathways are Cl- or Cl elimination, but for n = 3, 5, ClF elimination becomes predominant. The adiabatic electron affinities, the vertical electron affinities of the BrClFn molecules, and the vertical detachment energies of the BrClFn- anions were predicted in this work. The adiabatic electron affinities of the closed-shell BrClFn molecules significantly exceed those of the corresponding BrFn species, while for molecules with an odd number of electrons they are similar or slightly lower than electron affinities in the BrFn series. Many new experiments are suggested by this researcher, in light of the large predicted electron affinities and the fact that none of these has been measured in the laboratory.

Medium ring compounds and their anions: a systematic density functional theory study

The optimized geometries, harmonic vibrational frequencies, and adiabatic electron affinities of eight ring compounds: o-benzyne, cyclohexanone enolate, cyclopentadienyl radical, cyclopentanone enolate, maleic anhydride, phenoxyl radical, phenyl radical, and pyrrolyl radical have been determined with six different density functional or hybrid Hartree–Fock density functional methods using a double-zeta basis set with polarization and diffuse functions (DZP+ +). The optimized geometric structures are discussed. Harmonic vibrational frequencies were generally found to be within 4% of available experimental fundamentals for most functionals. Of the six functionals, BP86 predicted electron affinities remarkably close to the experimental values for six of the eight molecules studied, with an overall average absolute error with respect to experiment of just 0.12eV. The B3LYP and BLYP functionals also gave excellent predictions, with absolute average errors of 0.18 and 0.20eV, respectively.

A QUANTUM CHEMICAL INVESTIGATION OF STRUCTURES, VIBRATIONAL SPECTRA AND ELECTRON AFFINITIES OF THE RADICALS OF QUINONE MODEL COMPOUNDS

In the present study, the first quantum chemical calculations of structures and vibrational spectra of radicals of 1,4-naphthoquinone and 2-methoxy-1,4-benzoquinone that account for electron correlation are presented. In the case of 1,4-naphthoquinone a good agreement between calculated vibrational frequencies and 18O-shifts of the 1,4-naphthoquinone radical (protonated radical anion) with experimental data of a species detected after irradiation of vitamin K1 in solution is found. Our calculations, thus, support the previous assignment. In the case of 2-methoxy-1,4-benzoquinone we have localized the stable conformations with respect to the orientation of the methoxy group and we have determine the harmonic force fields for these structures. Our calculation suggest that, while the frequencies of the two conformers are similar, the 18O-shift of the most intensive absorptions in the spectral region between 1400 and 1700 cm−1 of the two conformers differ significantly and might serve as a tool to distinguish between the two conformers. The applied DFT method is shown to predict electron affinities which are systematically underestimated by 10%.

Coupled cluster spectroscopic properties and isomerization pathway for the cyanate/fulminate isomer pair, NCO−/CNO−

Three-dimensional near-equilibrium potential energy surfaces and dipole moment functions have been calculated for the ground states of NCO− and CNO−, using the coupled cluster method with single and double substitutions augmented by a perturbative estimate of triple excitations [CCSD(T)] with a quadruple zeta basis set consisting of 150 contracted Gaussian type orbitals. The corresponding equilibrium bond distances at their linear geometries are re(NC)=1.1934 Å and re(CO)=1.2306 Å for NCO−, and re(CN)=1.1866 Å and re(NO)=1.2741 Å for CNO−. Full three-dimensional variational calculations have also been carried out using the CCSD(T) potential energy and dipole moment functions to determine the rotation–vibrational energy levels and dipole moment matrix elements for both NCO− and CNO−. The predicted band origin of the ν3 band in the NCO− isomer (2114.4 cm−1) agrees well with the gas phase diode laser infrared result (2124.4 cm−1). The variational analysis suggests possible revisions in the assignment of the two experimentally observed hot bands which are affected by Fermi resonance. The calculated dipole moments of NCO− and CNO− in their ground vibrational states are 1.504 and 1.482 D, respectively. The CCSD(T) method with a triple zeta basis set was employed to more broadly explore the isomerization path between the two isomers. In agreement with previous lower level calculations a broad and shallow minimum corresponding to a cyclic oxazirine form was found. The immediate vicinity of this local minimum in the potential energy surface and those of the two saddle points separating it from the linear isomers were further refined using the same quadruple zeta basis used for the two near-equilibrium regions. The equilibrium structures and energies of the two neutral isomers, NCO, and CNO, were also calculated at the same level of theory. For NCO, whose photoelectron spectrum has recently been studied in detail, the predicted electron affinity and neutral-ion bond distance changes agree well with the photoelectron results. The reported spectroscopic structure of NCO, however, is not supported by the present CCSD(T) calculations.

The electron affinities of the silicon fluorides SiFn (n=1–5)

Several independent density functional methods have been employed to determine the molecular structures and total energies of SiFn and SiF−n (n=1–5). Three significant measures of neutral–anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first Si–F ligand dissociation energies D(Fn−1Si–F), D(Fn−1Si−–F), and D(Fn−1Si–F−) as well as the harmonic vibrational frequencies of SiFn and SiF−n are also reported. Trends in the predictions of the different DFT methods are discussed. Self-consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double-ζ plus polarization basis set augmented with diffuse s-type and p-type functions. The method (BHLYP) based upon the Becke half-and-half exchange functional and the Lee–Yang–Parr correlation functional predicts molecular geometries in best agreement with experiment, while the other methods tend to produce bond lengths that are slightly longer. The BHLYP vibrational frequencies are also superior to those obtained via the other three DFT methods utilized. In previous studies of AFn molecules, the density functional methods have predicted electron affinities a few tenths of an electron volt above experiment with the BHLYP value being the best. Although experimental information concerning the electron affinities of the SiFn series is scarce, the BHLYP method does continue to predict electron affinities that are lower than the other methods. The BHLYP adiabatic electron affinities are 0.83 eV (SiF), 0.42 eV (SiF2), 2.50 eV (SiF3), and −0.22 eV (SiF4). The (unknown) Si–F bond distances for the anions are 0.076–0.088 Å longer than their respective neutral counterparts. The SiF−3 anion is more strongly pyramidal than SiF3. The SiF−4 species is predicted to lie energetically above SiF4, despite its experimental identification. No significantly bound minimum was found for the SiF5 molecule, although SiF−5 is predicted to exist and M+SiF−5 has been experimentally observed. The predicted vertical detachment energy of SiF−5 is 8.54 eV with the BHLYP method.

Electronic structure of small copper oxide clusters: From Cu2O to Cu2O4.

We study the electronic structure of copper oxide clusters, ${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ (x=1\char21{}4), using anion photoelectron spectroscopy and density-functional calculations. The experiment is used to successfully guide a computational search for the cluster geometries. The predicted electron affinities at the obtained cluster structures reproduce exactly the trend observed experimentally. The definitive determination of the cluster structures enables a detailed analysis of the chemical bonding and electronic structure involving Cu atoms in different oxidation states exhibited by these clusters. \textcopyright{} 1996 The American Physical Society.

Negative Ion Thermochemistry: The Sulfur Fluorides SFn/SFn- (n = 1−7)

The molecular structures and total energies of SFn and SFn- (n = 1−7) have been predicted using density functional methods. Three significant measures of electron affinity are reported: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy of the anion (VDE). The first S−F ligand dissociation energies D(Fn-1S−F), D(Fn-1S-−F), and D(Fn-1S−F-) are also reported. Self-consistent Kohn−Sham orbitals were obtained using various functional forms and a double-ζ plus polarization (DZP) basis set. The addition of diffuse s- and p-type functions to the basis set lowered the energies of the anions relative to their neutral species and significantly improved the values of the electron affinities. The method (BHLYP) based upon Becke's half-and-half exchange functional and the Lee−Yang−Parr correlation functional predicted molecular geometries and electron affinities in best agreement with experiment, while the other methods tended to produce bond lengths that were slightly longer and electron affinities which were larger. The BHLYP electron affinities are typically a few tenths of an electronvolt above experiment. Neutral SF7 was found to have no structures that were significantly bound with respect to dissociation. SF7- structures with C4v-symmetry and C3v-symmetry were found to lie very close in energy. The adiabatic electron affinities calculated here are in good agreement with experimental results for SF and SF3, but the predicted electron affinities for SF6 vary widely between functionals and are much larger than the accepted experimental value. The density functional results favor a value EAad(SF4) near 1.8 eV. This is a slightly larger value than that predicted by recent experiments which concluded that EAad(SF4) = 1.5 ± 0.2 eV. The neutral bond dissociation energies D(Fn-1S−F) tend to confirm experiment except for SF5 where the DFT methods predict D(F4S−F) = 1.09−1.72 eV, which is less than an experimental value of 2.30 ± 0.26 eV. The bond dissociation values of the anions D(Fn-1S-−F) and D(Fn-1S−F-) are in agreement with experimentally determined values with the exception of D(F5S−F-) which is predicted to be at least 1 eV larger than an experimental value of 1.10 eV. All density functional schemes predict a vertical detachment energy for SF6- that is at least 0.5 eV more than the recently reported experimental value of 3.16 eV. It is concluded that the density functional methods, while very useful in establishing trends, are not yet quantitatively reliable. However, additional (SFn−SFn-) experiments are required to precisely establish the reliability of the different density functional methods.

Dipole-Bound Excess-Electron States of Adenine Tautomers. A Theoretical ab Initio Study

Ab initio calculations performed in this work render a near-zero but positive electron affinity of the major N(9)H tautomer of adenine. However, the electron affinity of the N(7)H tautomer, which has not been detected in the gas phase but coexists with the N(9)H tautomer in the polar solution, is predicted to be equal to 0.12 eV. ln both cases the excess electron is bonded to the dipole field of the molecule. No conventional stable `valence` anionic states have been found. Due to a significant difference between the predicted electron affinities of the N(7)H and N(9)H tautomers, one should expect a noticeable shift in the tautomeric equilibrium when electrons attach to adenine molecules in the gas phase. 23 refs., 2 figs., 4 tabs.

Contributions to the electron affinity of calcium and scandium

Theoretical predictions of the electron affinity of Ca vary from 0 to 130 meV. Not all calculations have included the same effects. In this paper, the different approaches are reviewed, the effect of assumptions estimated whenever possible, and some new ab initio results reported that estimate the effect of core polarization on electron affinity for both Ca and Sc. For the latter our predicted electron affinity is in good agreement with the experimental value for the lowest 4s23d4p1D state and underestimates the electron affinity for 4s23d4p3D, where the calculation of outer correlation is more demanding and the core-polarization effect is small.

Variational predictions of transition energies and electron affinities: He and Li ground states and Li, Be, and Mg core-excited states.

Variational procedures for predicting energy differences of many-electron systems are investigated. Several different calculations for few-electron systems are considered that illustrate the problems encountered when a many-electron system is modeled as a core plus outer electrons. It is shown that sequences of increasingly more accurate calculations for outer correlation may converge yielding wrong transition energies. At the same time, accurate core-polarization calculations overestimate the binding energy, requiring a core-valence correction. For the high-spin, core-excited states of Li, it was found that outer correlation only predicted electron affinities as accurately as full-correlation studies. This observation suggested a prediction of the core-excited $^{4}$P${\mathrm{\ensuremath{-}}}^{4}$S transition in ${\mathrm{Be}}^{\mathrm{\ensuremath{-}}}$, based on observed $^{3}$${\mathit{P}}^{0}$${\mathrm{\ensuremath{-}}}^{3}$P transition energies of the neutral species, predicted electron affinities including only outer correlation, and a core-valence correction, that is shown to be in good agreement with experiment. A similar calculation for ${\mathrm{Mg}}^{\mathrm{\ensuremath{-}}}$ predicts a wavelength of 2895.1 \AA{} for this transition.

Zexpansions of the total electronic energy for isoelectronic atoms: A method for determiningV¯enandV¯eefrom experimental total electronic energies

The $Z$ dependence of the total electronic energy for an atomic isoelectronic series, was investigated in several different ways, first by the consideration of a Taylor-series expansion of ${\overline{V}}_{\mathrm{en}}$, the average electron-nuclear attractive potential energy, used in conjunction with the virial and Hellmann-Feynman theorems and, secondly, a Taylor-series expansion of ${\overline{V}}_{\mathrm{ee}}$, the average electron-electron repulsive potential energy, used in conjunction with the same two theorems. A direct Taylor-series expansion of the total energy $E$ was also studied. Convergence of these expansions was inferred from the rate of convergence of least-squares fits of tabulated values of the total electronic energies as a function of the increasing number of terms in the expansion. Theoretical nonrelativistic Hartree-Fock energies, accurate theoretical nonrelativistic energies, and experimental total energies, the latter approximately corrected for relativistic and quantum electrodynamic effects, were used in the fits. These expansions were compared with $\frac{1}{Z}$ expansion results. All Taylor-series variants studied showed comparable or superior convergence properties to the $\frac{1}{Z}$ expansion. The choice of a ${\overline{V}}_{\mathrm{en}}$ or a ${\overline{V}}_{\mathrm{ee}}$ expansion forces certain behavior on the total electronic energy $E$ as a function of $Z$. From a least-squares rate-of-convergence criterion and other considerations it appears that the Taylor-series expansion of ${\overline{V}}_{\mathrm{en}}$ offers slightly better results. It is pointed out that by the use of the virial and Hellmann-Feynman theorems it is possible to obtain accurate estimates of ${\overline{V}}_{\mathrm{ee}}$ and ${\overline{V}}_{\mathrm{en}}$ from a knowledge of total electronic energies of isoelectronic series alone. The technique is applied to accurate values for the He isoelectronic series and experimental data corrected for relativistic and radiative contributions for the He through Ne isoelectronic series in order to determine the effect of electron correlation on the quantities ${\overline{V}}_{\mathrm{en}}$ and ${\overline{V}}_{\mathrm{ee}}$. Comparisons are made with available experimental and theoretical results in the case of Ne. The $Z$ expansions for the energy were also used to fit individual ionization potentials across an isoelectronic series. These fits were used to predict electron affinities and improved values for certain ionization potentials.

Semi-empirical calculations of ?-electron affinities for some conjugated organic molecules

Energy levels have been calculated for some conjugated systems containing C, N, and O atoms using a semi-empiricalmethod based upon a variableβ-γ modification of the Pariser-Parr-Pople approximation to the Hartree-Pock equation. Koopmans' theorem is used to relate the calculated energy of the lowest vacant molecular orbital, ɛLVMO, to the adiabatic electron affinity of a molecule. The approach is identical to that used previously by Kunii and Kuroda [13]. An excellent correlation is found between electron affinities deduced from recent beam experiments and ɛLVMO. This relationship is used to predict electron affinities for over 100 other organic molecules. In addition, excited state energies for negative ions are calculated, and good agreement is found with the available experimental data. Bound excited states are also predicted for some organics which contain the =C(CN)2 substructure. The additive contribution of group substitutions to the electron affinity is discussed for the case of CN substitutions to ethylene, benzene, and naphthalene.

Semi-empirical prediction of the electron affinities of gaseous radicals

The valence-state promotion energies calculated by Hinze and Jaffe for both atoms and negative ions may be combined to predict electron affinities for atoms in specified valence states. These predicted electron affinities agree closely with the experimental electron affinities for radicals derived from the same valence states and substitution in the radical appears to have little effect. The electron affinities of a number of radicals not as yet studied are predicted.