Computed Electron Affinities Research Articles

New insights into the reactivity of the triscyclopentadienyl monothiolate uranium(IV) complexes: CS2 and CO2 insertion and redox properties. A DFT theoretical approach

The structural properties of a series of triscyclopentadienyl monothiolate uranium(IV) complexes [U(Cp)3(SR)] (Cp = η5–C5H5; R = Me (1), iPr (2), Ph (3), tBu (4)) as well as their reactions with CO2 or CS2 leading to their insertion into the US bond, have been investigated using relativistic Density Functional Theory (DFT) calculations. The computed activation barriers of these reactions show that insertion of CO2 into the US bond of the thiolate complexes is easier and faster than that of CS2, in agreement with the experimental observation. The study brings to light the electrostatic interactions and steric hindrance effects that play an important role in these processes. The redox behavior of the thiolate [U(Cp)3(SR)] complexes has also been investigated, permitting to find a very nice linear correlation (R2 = 0.99) between the computed electron affinities and the experimental reduction half–wave potentials E1/2. This correlation allowed to estimate the reduction potentials of [U(Cp)3(SMe)] (1) and [U(Cp)3(StBu)] (4) for which the electrochemical measurement failed. Several population analyses were carried out, among them Nalewajski–Mrozek Bond Orders (NMBO) and Hirshfeld charges Analysis (HA) allowing to rationalize the insertion reactions and redox processes and to highlight the driving role of the uranium 5f orbitals.

Metal-Free Molecular Catalysts for the Oxygen Reduction Reaction: Electron Affinity as an Activity Descriptor.

Heteroatom-doped polyaromatic hydrocarbons (or nanographenes) are promising molecular electrocatalysts for the oxygen reduction reaction (ORR). Here, we use density functional theory to investigate the first step of the ORR pathway (chemisorption) for a set of molecules with experimentally determined catalytic activities. Weak chemisorption is found for only negatively charged catalysts, and a strong correlation is observed between the computed electron affinities and experimental catalytic activities for a range of B- and B,N-doped polyaromatic hydrocarbons. The electron affinity is put forward as a simple activity descriptor of charged (activated) catalysts on an electrode.

Inverse Trans Influence in Low-Valence Actinide-Group 10 Metal Complexes of Phosphinoaryl Oxides: A Theoretical Study via Tuning Metals and Donor Ligands.

The recognition and in-depth understanding of inverse trans influence (ITI) have successfully guided the synthesis of novel actinide complexes and enriched actinide chemistry. Those complexes, however, are mainly limited to the involvement of high-valence actinide and/or metal-ligand multiple bonds. Examples containing both low oxidation state actinide and metal-metal single bond remain rare. Herein, more than 20 actinide-transition metal (An-TM) complexes of phosphinoaryl oxide ligands have been designed in accordance with several experimentally known analogs, by changing the metal atoms (An = Th, Pa, U, Np, and Pu; and TM = Ni, Pd, and Pt), actinide oxidation states (IV and III) and metal-metal axial donor ligands (X = Me3SiO, F, Cl, Br, and I). The relativistic density functional theory study of structural (trans-An-X and cis-An-O toward An-TM), bonding (topological electron/energy density), and electronic properties reveals the order of the ITI stabilizing actinide-metal bond. Computed electron affinity (EA) values, related to the electrochemical reduction, linearly correlate with experimentally measured reduction potentials. Although the same ITI order for the ligand donors was shown as in a previous study, the correlation between electrochemical reduction and the ITI was found to be weak when the actinide atoms were changed. For most complexes, the reduction is primarily of an actinide-based mechanism with minor participation of transition metal and phosphinoaryl oxide, whereas that of thorium-nickel complexes is different.

Heptavalent Actinide Tetroxides NpO4- and PuO4-: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO4.

The highest known actinide oxidation states are Np(VII) and Pu(VII), both of which have been identified in solution and solid compounds. Recently a molecular Np(VII) complex, NpO3(NO3)2-, was prepared and characterized in the gas phase. In accord with the lower stability of heptavalent Pu, no Pu(VII) molecular species has been identified. Reported here are the gas-phase syntheses and characterizations of NpO4- and PuO4-. Reactivity studies and density functional theory computations indicate the heptavalent metal oxidation state in both. This is the first instance of Pu(VII) in the absence of stabilizing effects due to condensed phase solvation or crystal fields. The results indicate that addition of an electron to neutral PuO4, which has a computed electron affinity of 2.56 eV, counterintuitively results in oxidation of Pu(V) to Pu(VII), concomitant with superoxide reduction.

Redox Properties of Monocyclooctatetraenyl Uranium(IV) and (V) Complexes: Experimental and Relativistic DFT Studies

The redox properties of a series of mono(cyclooctatetraenyl) uranium(IV) and (V) complexes [(Cot)(Cp)U(NEt2)2] (1) (Cot = η-C8H8, Cp = η-C5H5) [(Cot)U(OiPr)3] (2), [(Cot)U(NMe2)3] (3), [(Cot)U(N{SiMe3}2)2] (4), [(Cot)U(NEt2)3]– (5), and the cyclopentadienyl compound [(C5Me5)U(NEt2)3] (6) have been investigated using cyclic voltammetry and relativistic density functional theory (DFT). Electrochemical measurements of half-wave potentials in tetrahydrofuran were carried out under strictly anaerobic conditions. The calculations of ionization energies (IE) and electron affinities (EA) of these compounds, related to the UIII/UIV, UIV/UV, and UV/UVI redox systems, revealed a very good linear correlation (r2 = 0.99) between calculated ionization energies at the ZORA/BP86/TZP level and the measured E1/2 half-wave oxidation potentials. A similar good linear correlation between the computed electron affinities and the electrochemical reduction potentials (r2 = 0.98) was obtained. It was found to be crucial to take i...

Stability, metastability and spectroscopic properties of some low-lying electronic states of C2H−and N2H−

Accurate ab initio calculations have been carried out and the potential energy surfaces (PESs) and the potential energy curves (PECs) of the lowest states of the neutral C2H and N2H and those of the electronic states of their respective negative ion C2H− and N2H− correlating to the bound asymptotes of these molecular systems determined. These calculations reveal that the ground states of C2H− and N2H− are respectively X1Σ+ and X1A′ and the long range parts of anionic PESs of C2H− and N2H− are stable against the autodetachment processes. In the light of these computations, we also predict some long lived excited states for all these anions. C2H− possesses a linear structure with a ground state located solely and does not interact strongly with other states. N2H− presents a bent structure. The b3A′ electronic state of this anion which equilibrium’s geometry is RNN=2.44 a0, RNH=1.97 a0 and θ=113.5 degree, reveals itself to be the most indicated state in which spectroscopic studies can be done and in which the anion can be described, thanks to its bound nature. The three-dimensional PES of this metastable state is generated using a coupled cluster approach associated with a large basis sets. A set of spectroscopic parameters for N2H− (b3A′) are deduced from a 3D PES. Finally, the 5A′ and 5A″ states that correlate at the linearity form a bent/bent Renner-Teller in the bent structure. The computed electron affinity of C2H is in very good agreement with the available experimental values.

Photophysics, photochemistry and thermal stability of diarylethene-containing benzothiazolium species

Abstract The photophysics, photochemistry and thermal stability of four 1-aryl-2-(N-methyl-2-benzothiazolium) ethene iodides (aryl: phenyl, 1-naphthyl, 9-phenanthryl and 9-anthryl) were studied. Although the absorption spectra are found to be hypsochromic-shifted, fluorescence spectra are bathochromic-shifted. The dipole moment in the relaxed excited state was found to be larger than that in the ground state. To investigate effects of N-methylation and the aryl-ring size, a detailed comparison was made between those in the present work (of charged compounds) and previous studies of their neutral analogues, with computed electron affinity and ionisation potentials serving to rationalise the experimentally observed bathochromic shifts in absorption and emission spectra. The kinetics of thermal isomerisation depend strongly on the nature of the aryl moiety and solvent; the larger the aryl ring, the slower the rate of isomerisation. The fastest isomerisation process was found to take place in MeOH. The anthryl derivative did not isomerize either by light- or heat-exposure, due to high energy barriers of rotation around the ethenic bond. Based on the significant blue-shift of the Z-isomer absorption maximum relative to that of the E-isomer, and the high percentage of Z-isomers in the photostationary state, these compounds may serve as potential promising candidates for optical data-storage applications.

Redox properties of biscyclopentadienyl uranium(V) imido-halide complexes: a relativistic DFT study

Calculations of ionization energies (IE) and electron affinities (EA) of a series of biscyclopentadienyl imido-halide uranium(V) complexes Cp*2U(=N-2,6-(i)Pr2-C6H3)(X) with X = F, Cl, Br, and I, related to the U(IV)/U(V) and U(V)/U(VI) redox systems, were carried out, for the first time, using density functional theory (DFT) in the framework of the relativistic zeroth order regular approximation (ZORA) coupled with the conductor-like screening model (COSMO) solvation approach. A very good linear correlation (R(2) = 0.993) was obtained, between calculated ionization energies at the ZORA/BP86/TZP level, and the experimental half-wave oxidation potentials E1/2. A similar linear correlation between the computed electron affinities and the electrochemical reduction U(IV)/U(III) potentials (R(2) = 0.996) is obtained. The importance of solvent effects and of spin-orbit coupling is definitively confirmed. The molecular orbital analysis underlines the crucial role played by the 5f orbitals of the central metal whereas the Nalewajski-Mrozek (N-M) bond indices explain well the bond distances variations following the redox processes. The IE variation of the complexes, i.e., IE(F) < IE(Cl) < IE(Br) < IE(I) is also well rationalized considering the frontier MO diagrams of these species. Finally, this work confirms the relevance of the Hirshfeld charges analysis which bring to light an excellent linear correlation (R(2) = 0.999) between the variations of the uranium charges and E1/2 in the reduction process of the U(V) species.

Accurate Electron Affinities from the Extended Koopmans' Theorem Based on Orbital-Optimized Methods.

The extended Koopmans' theorem (EKT) provides a systematic way to compute electron affinities (EAs) from any level of theory. Although, it is widely applied to ionization potentials, the EKT approach has not been extensively applied to computations of electron affinities. We present the first benchmarking study to investigate the performances of the EKT methods for predictions of EAs. We assess the performances of the EKT approaches based on orbital-optimized methods [Bozkaya, U. J. Chem. Phys. 2013, 139, 154105], such as the orbital-optimized third-order Møller-Plesset perturbation theory and the orbital-optimized coupled-electron pair theory [OCEPA(0)], and their standard counterparts for EAs of the selected atoms, closed- and open-shell molecules. Especially, results of the OCEPA(0) method (with the aug-cc-pVQZ basis set) for EAs of the considered atoms and molecules are very promising, the corresponding mean absolute errors are 0.14 and 0.17 eV, respectively.

Formation and properties of iron-based magnetic superhalogens: A theoretical study

In order to explore new magnetic superhalogens, we have systematically investigated the structures, electrophilic properties, stabilities, magnetic properties, and fragmentation channels of neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters using density functional theory. Our results show that a maximum of six F atoms can be bound atomically to one Fe atom, and the Fe-Fe bonding is not preferred in Fe2F(n)(0/-) clusters. The computed electron affinities (EAs) indicate that FeF(n) with n ≥ 3 are superhalogens, while Fe2F(n) can be classified as superhalogens for n ≥ 5. To further understand their superhalogen characteristic, the natural population analysis charge distribution and the HOMOs of anionic clusters were also analyzed. When the extra negative charge and the content of HOMO are mainly located on F atoms, the clusters could be classified as superhalogens with EAs substantially surpass that of Cl. By calculating the binding energies per atom and the HOMO-LUMO gaps, FeF3, FeF4(-), Fe2F4, Fe2F5(-), and Fe2F7(-) clusters were found to have higher stabilities, corresponding to the Fe atoms that are attained at their favorite +2 and +3 oxidation states. Furthermore, we also predicted the most preferred fragmentation channel and product for all the ground state clusters. Even more striking is the fact that both neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters carry large magnetic moments which mainly come from 3d orbital of iron atom.

Radiative lifetimes of spin forbidden a1Δ → X 3Σ− and spin allowed A3Π → X 3Σ− transitions and complete basis set extrapolated ab initio potential energy curves for the ground and excited states of CH−

The spin forbidden transition a(1)Δ → X(3)Σ(-) in CH(-) has been studied using the Breit-Pauli Hamiltonian for a large number of geometries. This transition acquires intensity through spin-orbit coupling with singlet and triplet Π states. The transition moment matrix including more than one singlet and triplet Π states was calculated at the multi-reference configuration interaction/aug-cc-pV6Z level of theory. The computed radiative lifetime of 5.63 s is in good agreement with the experimental (5.9 s) and other theoretical (6.14 s) results. Transition moment values of the spin allowed A(3)Π → X(3)Σ(-) transition have also been calculated at the same level of theory. Calculations show that the corresponding radiative lifetime is considerably low, 2.4 × 10(-7) s. Complete basis set extrapolated potential energy curves for the ground state of CH and the ground state and six low lying excited states (a(1)Δ, b(1)Σ(+), two (3)Π, and two (1)Π) of CH(-) are reported. These curves are then used to calculate the vibrational bound states for CH and CH(-). The computed electron affinity of CH supports the electron affinity bounds reported by Okumura et al. [J. Chem. Phys. 85, 1971 (1986)].

Density Functional Theory Investigation of the Redox Properties of Tricyclopentadienyl- and Phospholyluranium(IV) Chloride Complexes

The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L(3)UCl with L = C(5)H(5) (Cp), C(5)H(4)Me (MeCp), C(5)H(4)SiMe(3) (TMSCp), C(5)H(4)(t)Bu ((t)BuCp), C(5)Me(5) (Cp*), and C(4)Me(4)P (tmp), has been investigated using relativistic density functional theory calculations, with the solvent being taken into account using the conductor-like screening model. A very good linear correlation (r(2) = 0.99) has been obtained between the computed electron affinities of the L(3)UCl complexes and the experimental half-wave reduction potentials E(1/2) related to the U(IV)/U(III) redox systems. From a computational point of view, our study confirms the crucial importance of spin-orbit coupling and solvent corrections and the use of an extended basis set in order to achieve the best experiment-theory agreement. Considering oxidation of the uranium(IV) complexes, the instability of the uranium(V) derivatives [L(3)UCl](+) is revealed, in agreement with experimental electrochemical findings. The driving roles of both the electron-donating ability of the L ligand and the U 5f orbitals on the redox properties of the complexes are brought to light. Interestingly, we found and explained the excellent correlation between variations of the uranium Hirschfeld charges following U(IV)/U(III) electron capture and E(1/2). In addition, this work allowed one to estimate theoretically the half-wave reduction potential of [Cp*(3)UCl].

Theoretical Study of Negative Molecular Ions

Although this review provides references to tabulations of molecular electron affinities, primarily it focuses on explaining why theory plays an important role in understanding the behavior of anions, explaining the challenges that anions pose to theory, making connections between the theories used to compute electron affinities and the potentials (e.g., charge-dipole, charge-quadrupole, valence attraction and exchange repulsion, dispersion, and polarization) that govern the electron-molecule interaction, and discussing how species with negative electron affinities may possess metastable anion states and how such states should be treated. In addition to references to published literature, many links are given to websites of practicing theoretical chemists who study molecular anions; these links (which appear in boldface) offer the reader a broad avenue to access much more information about molecular anions than can be covered in a review or even through conventional literature sources.

Substitutent Effects in the Periphery of 2,9-Bisaryl-tetraazaperopyrene Dyes

A series of 2,9-bisaryl-1,3,8,10-tetraazaperopyrene (TAPP) derivatives has been synthesized by reacting 4,9-diamino-3,10-perylenequinone diimine with a large excess of the corresponding benzoyl chloride in refluxing nitrobenzene. Among all derivatives only ortho-substituted phenyl congeners were sufficiently soluble for studying solutions of defined concentration in organic solvents. The molecular structures of the crystallized compounds, determined by X-ray diffraction of four derivatives, are determined by the planar tetraazaperopyrene core and the interplanar angle of the phenyl rings, which depends on the size of the ortho substituent (40-70°). The intermolecular packing pattern of all compounds is characterized by parallel stacks of molecules with the substituted phenyl rings rotated out of the peropyrene plane to reduce the steric repulsion. Crystals of a TAPP derivative suitable for X-ray diffraction were grown from trifluoroacetic acid (TFA) for the first time, establishing a 2-fold protonated species. The ground-state geometries of the TAPP derivatives were calculated by DFT [B3PW91/6-31g(d,p)] and the lowest unoccupied molecular orbital (LUMO) energies of derivatives possessing electron-withdrawing groups were decreased, as were the computed electron affinities. The results of the modeling study were confirmed experimentally by cyclic voltammetry to evaluate the substituent effects on the highest occupied molecular orbital (HOMO) and the LUMO of the peropyrene core. The UV-vis absorption spectra of all compounds recorded in trifluoroacetic acid are almost superimposable and display a characteristic visible absorption band between 460 and 490 nm (log ε = 4.64-5.01) with a strong vibrational progression of 1173-1475 cm(-1). Their fluorescence spectra are characterized by bands between 490 and 530 nm that are the mirror images of the absorption spectra (Stokes shifts of 10-50 nm). The luminescence quantum yields range from <0.01 to 0.30, thereby indicating a quenching effect for some substitution patterns.

One-Electron Electron−Molecule Potentials Consistent with ab Initio Møller−Plesset Theory†

Ab initio electronic structure methods such as Møller-Plesset (MP) theory can be used to compute electron affinities (EAs) of molecules or clusters of molecules to reasonable accuracy (+/-a few tenths of an electron volt). For systems in which an electron is bound to a closed-shell neutral in a manner that primarily localizes the excess electron exterior to most of the electron density of the neutral, we investigate how a given level of ab initio description can be accurately described by a one-electron potential governing the excess electron's interaction with the neutral. We show what ingredients such a potential must possess not only to reproduce the ab initio EA but also to have long-range electrostatic, polarization, and other contributions identical to the ab initio potential. In particular, we show that using Hartree-Fock level electrostatic moments and polarizability can produce a one-electron potential consistent with MP2 theory. To be consistent with MP3 theory, MP2-level electrostatics and polarizabilities must be used. The long-range components of the ab initio potential are shown to embody both orbital relaxation induced by the excess electron and the dispersion interactions between the excess electron and the other electrons of the neutral. Even though these individual contributions do not necessarily scale as r(-4), they are shown to combine into a total potential that can be represented in the familiar polarization form -(1)/(2)alphar(-4). These findings suggest that electrostatic potentials combined with polarization potentials scaling as r(-4) can indeed describe the relaxation (induction) and dispersion energies of an excess electron. Finally, how these observations might assist in constructing new electron-molecule potentials is also discussed.

Electrochemical Properties of a Disilene, a Tetrasila‐1,3‐butadiene, and Their Germanium Analogues

The redox properties of aryl-substituted disilene (1), tetrasila-1,3-butadiene (2), digermene (3), and tetragerma-1,3-butadiene (4) (in which the aryl group is 2,4,6-triisopropylphenyl) have been studied by cyclic voltammetry (CV) in two different solvents. In o-dichlorobenzene, disilene 1 exhibited reversible redox couples in both oxidation and reduction processes, whereas tetrasilabutadiene 2 showed a reversible behavior only in its oxidation and an irreversible wave in its reduction. Tetragermabutadiene 4 afforded a reversible couple in its reduction and an irreversible wave in its oxidation, whereas digermene 3 showed only an irreversible oxidation wave but no reduction one. In comparison, all observed oxidation and reduction waves in THF were irreversible. In both solvents, the same trend of decreasing ease of oxidation has been observed: 2>4>3>1. The calculated vertical ionization energies (E(i)) for the 2,6-dimethylphenyl-substituted model compounds 9-12 afforded a trend of ease of oxidation that decreases in the series tetrasilabutadiene 10>tetragermabutadiene 12>disilene 9>digermene 11. This predicted trend parallels the measured oxidation potentials in the sense that the heavy butadienes are easier to oxidize than the heavy alkenes. The trend in the reduction direction has been found to be different in the two solvents studied. The computed electron affinities (E(ea)) for compounds 9-12 indicated that the ease of reduction decreased in the sequence tetragermabutadiene 12>tetrasilabutadiene 10>digermene 11>disilene 9. However, no straightforward relation between the computed electron affinities and the experimentally measured reduction potentials has been found.

Electron propagator studies of molecular anions

Electron propagator (EP) or Green's function (GF) methods have been successfully employed to compute electron affinities for a large number of molecules having either a closed-shell or single-valence-hole dominant electronic configuration. The accuracy of such calculations, if carried out through third order, has often been comparable to that of reasonable configuration interaction (CI) calculations. However, as a computational tool, EP methods have not yet been adequately developed in relation to general open-shell molecules and atoms, and only recently have they begun to be generalized to describe states having two or more dominant configurations. Thus although GFs look promising as methods for ab initio calculation, much formal and programming work remains to be done before this approach can be said to compete with CI methods. On the other hand, GFs provide us with a mechanism for focusing on the one-electron (EP) or two-electron [polarization propagator (PP)] features of any problem. It is this fact that makes GFs an attractive route for developing chemical models which may or may not make use of experimental data. Although such a viewpoint has been widely used in solid-state physics, not enough work has been done on making models based upon the EP or PP. In this article, both the computational and formal history of GFs, as they apply particularly to molecular anion studies, are overviewed. The author's opinions concerning the current status and future development of the area constitute a large part of the presentation.

Structural evolution and properties of subnanometer Tcn (n = 2–15) clusters

All-electron scalar relativistic calculations of the geometric and electronic structures of neutral subnanometer Tc(n) (n = 2-15) clusters have been performed using spin-polarized density functional theory. It is shown that the structural evolution of Tc(n) clusters from n = 3 to n = 8 is similar to Mn clusters, which also belong to the group of VIIB transition metals. The calculated binding energy per atom varies gradually from 0.65 eV for Tc(2) to 2.35 eV for Tc(15). The computed electron affinity and ionization potential follow the classical electrostatic metallic droplet model for clusters larger than Tc(6) and converge asymptotically to the experimental value of the bulk Tc work function. A study of the kinetic stability of Tc clusters in terms of the second difference of binding energies and the energy gap between the frontier orbital states predicts Tc(3)(D(3h)), Tc(4)(T(d)), Tc(6)(O(h)), Tc(12)(I(h)), and Tc(13)(I(h)) to be the most stable clusters. Vibrational spectra of Tc clusters are also provided for further experimental identification.

Theoretical estimation of the electron affinity of enolate radicals

AbstractThe electron affinity of some enolate radicals coming from methyl substituted five‐ and six‐member carbon rings is estimated by the use of the density functional theory model B3LYP/6‐311++g(3df,3pd). The computed electron affinities show an excellent agreement with recent gas phase experimental results (Walthall et al., J Phys Chem A, 2005, 109, 8785). The comparison shows differences around 1 kcal/mol for most of the compounds, except for four substituted compounds whose methyl group is neighbor to the double bond, in this case the variation increases up to 3 kcal/mol. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Structure and relative stability of Si n , Si n − , and doped Si n M clusters ( M = Sc − , Ti , V + ) in the range n = 14 – 18

We report on first-principles quantum mechanical optimizations of the minimum energy equilibrium structure of neutral, Si$_{n}$, and anionic, Si$_{n}^{-}$, pure silicon clusters, as well as the isoelectronic Si$_{n}$M doped clusters (M =Sc$^{-}$, Ti, V$^{+}$) for n=14-18. We have published previously some of these results, but additional analysis is contributed here for the first time, particularly for the pure anionic silicon clusters and doped Si$_{n}$Ti compounds. The lowest energy isomer of the anionic Si$_{n}^{-}$ cluster shows different geometry than the neutral cluster, except for n=15, 17. The geometries of a few low-lying energy isomers of doped Si$_{n}$M does not relate to those of pure silicon clusters in the range of sizes considered in this work. For both pure and doped Si clusters, we analyze the trend of several electronic properties with the cluster size, like the binding energy, the addition energy of the impurity M to pure Si clusters, the second difference of total energy, the Homo-Lumo gap, the average Si-Si and Si-M distance, and the electron affinity. For Si$_{16}$M doped clusters we found the largest binding energy, the highest second difference of energy, and the highest Homo-Lumo gap. These facts are manifestations of the special stability of Si$_{16}$M clusters found in recent experimental mass spectra, which was rationalized in previous works as a combination of geometrical (near spherical cage-like structure) and electronic effects (l-selection rule of the spherical potential model). Here we present additional arguments, by comparing the partial orbital density of states of the near-spherical Frank-Kasper isomer of Si$_{16}$Ti, with that of a non-spherical isomer of Si$_{16}$Sc$^{-}$ anion. We have also computed the adiabatic electron affinity of pure and doped Si clusters. For doped clusters, the computed electron affinities are in very good agreement with available estimations from experimental photoelectrons spectra, but for pure neutral clusters the calculations underestimate by more than 18% the experimental values.