Vertical Electron Attachment Energies Research Articles

Novel Approach for Predicting Vertical Electron Attachment Energies in Bulk-Solvated Molecules.

When low-energy electrons interact with molecules, they can give rise to transient anion states commonly known as resonances. These states are formed through vertical electron attachment processes and have the potential to induce various forms of DNA lesions, including base damage, single- and double-strand breaks, cross-links, and clustered lesions that are challenging to repair. So far, most experimental and theoretical studies have investigated the formation of resonances of (bio)molecules in the gas phase or in microsolvated environments. Since cellular environments are mainly composed of water molecules, it is crucial to understand how bulk water affects the resonances of (bio)molecules. Given the existing gap in studies on resonances of bulk-solvated molecules, we propose a novel theoretical-computational approach to address this void. Our approach combines the multibasis-set (time-dependent-)density functional theory and self-consistent sequential quantum mechanics/molecular mechanics polarizable electrostatic embedding methods. We apply this combined methodology to predict the vertical electron attachment energies of 1-methyl-5-nitroimidazole (1M5NI), a well-known radiosensitizer model, in bulk water. In addition, we analyze the rapid mutual polarization between the resonances (both shape- and core-excited) of 1M5NI and the surrounding bulk water environment. For comparison, we also studied the isolated and microsolvated 1M5NI. Overall, while the polarization of the environment is clearly sensitive to the solute charge, causing a significant impact on the vertical electron affinity and consequently on the attachment electron energies, it does not have a significant impact on the excitation energies of the anion.

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings.

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B20. In the ground state, the presently focused AB20/EB20 (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A+B20-/E2+B202- frameworks. The excited electronic states of AB20 and EB20+ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB20+ and EB202+. In several excited states, the radical electron is predominantly localized on the B20 frames, which are counterparts of the low-lying states of bare B20-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/A,E d-aug-cc-pVDZ/B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB20 and EB20+ and will serve as a guide for future studies on similar metal-doped boron systems.

Electron binding energies and Dyson orbitals of OnH2n+1 +,0,- clusters: Double Rydberg anions, Rydberg radicals, and micro-solvated hydronium cations.

Ab initio electron propagator methods are employed to predict the vertical electron attachment energies (VEAEs) of OH3 +(H2O)n clusters. The VEAEs decrease with increasing n, and the corresponding Dyson orbitals are diffused over exterior, non-hydrogen bonded protons. Clusters formed from OH3 - double Rydberg anions (DRAs) and stabilized by hydrogen bonding or electrostatic interactions between ions and polar molecules are studied through calculations on OH3 -(H2O)n complexes and are compared with more stable H-(H2O)n+1 isomers. Remarkable changes in the geometry of the anionic hydronium-water clusters with respect to their cationic counterparts occur. Rydberg electrons in the uncharged and anionic clusters are held near the exterior protons of the water network. For all values of n, the anion-water complex H-(H2O)n+1 is always the most stable, with large vertical electron detachment energies (VEDEs). OH3 -(H2O)n DRA isomers have well separated VEDEs and may be visible in anion photoelectron spectra. Corresponding Dyson orbitals occupy regions beyond the peripheral O-H bonds and differ significantly from those obtained for the VEAEs of the cations.

Application of the Stabilized Koopmans’ Theorem to the Temporary Anion States of Chlorosilanes in Long‐Range Corrected Density Functional Theory

AbstractThe stabilized Koopmans’ theorem (SKT) is very successful in predicting relative vertical electron attachment energies in the Hartree‐Fock theory. It is mainly accomplished by systematically scaling the most diffuse functions in the basis set. Recently, the SKT has been extended to the temporary anion states (TASs) of polyatomic molecules in the long‐range corrected density functional theory. In this paper, this method will be further applied to chlorosilanes for their importance in the chemical processes of the semiconductor industry. Their resonance energies and lifetimes are determined by computing the density of resonance states via SKT. The detailed characteristics of resonance orbitals are then analyzed. It turns out that the lowest unfilled orbitals of chlorosilanes are essentially σ*Si‐Cl in character. Moreover, several TASs with strong Si/Cl “d” character have been identified. These results, definitely, will help us in understanding the peculiar bonding and chemical properties of chlorosilanes.

Electron-induced dissociation of chlorosilanes: Role of aromatic side groups in gas phase and solution chemistry

Dissociative electron attachment (DEA) to chlorotrimethylsilane (Me3SiCl) and chlorodimethylphenylsilane (PhMe2SiCl) was studied to reveal a possible enhancing effect of the aromatic side group on the SiCl bond dissociation. Calculations point to a relevant interaction between the lowest unoccupied π* orbital of the phenyl ring and the σ*(SiCl) orbital and a consequent lowering of the lowest-energy electron attachment channel. It was previously reported that such a situation can lead to a significant enhancement of DEA. However, the lowest energy DEA channel in the investigated chlorosilanes lies below the thermodynamic limit for dissociation of the SiCl bond in the radical anion and thus cannot contribute to the Cl− yield. In consequence, release of chloride ions is in fact the dominant fragmentation channel but the Cl− yield is not noticeably enhanced in PhMe2SiCl. In contrast to the DEA results, comparative experiments on the reductive coupling in THF of Me3SiCl, PhMe2SiCl, and benzyldimethylchlorosilane (BzMe2SiCl) under identical conditions reveal a significant enhancement of the reactivity by aromatic side groups. As this finding does not correlate with the DEA results, an alternative explanation for the different reactivity of Me3SiCl and PhMe2SiCl in solution is suggested based on their vertical electron attachment energies and considering the impact of these quantities on the equilibrium between solvated electrons and neutral reactants on one side and the solvated radical anion on the other.

Gas-phase dissociative electron attachment to flavonoids and possible similarities to their metabolic pathways

The gas-phase empty-level structures and formation of anion states via resonance attachment of low-energy electrons to the flavonoids naringenin (III), quercetin (IV) and myricetin (V) and the smaller reference molecules chromone (I) and flavone (II) are investigated experimentally for the first time. Dissociative electron attachment spectroscopy (DEAS) is used to measure the fragment anion currents generated through dissociative decay channels of the molecular anions of compounds I–V, detected with a mass filter as a function of the incident electron energy in the 0–14 eV energy range. Due to the insufficient volatility of flavonoids III–V, the energies of vertical electron attachment associated with temporary occupation of the lower-lying virtual orbitals are measured with electron transmission spectroscopy (ETS) only in the smaller reference molecules I and II. The experimental findings are interpreted with the support of appropriate density functional theory calculations with the B3LYP functional. The experimental vertical electron attachment energies measured in the ET spectra of I and II are compared with the orbital energies of the neutral molecules scaled using an empirically calibrated linear equation. The vertical and adiabatic electron affinities are evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. The latter theoretical method is also used for evaluation of the most stable conformers of the neutral molecules, O–H bond dissociation energies and thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra. A possible role played by loss of an H(2) molecule from the parent molecular anion in vivo in the mitochondrial respiratory chain is briefly discussed.

Temporary Anion States of Pyrimidine and Halopyrimidines

The empty-level electronic structures of pyrimidine and its 2-chloro, 2-bromo, and 5-bromo derivatives have been studied with electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS) in the 0-5 eV energy range. The spectral features were assigned to the corresponding anion states with the support of theoretical calculations at the ab initio and density functional theory levels. The empty orbital energies obtained by simple Koopmans' theorem calculations, scaled with empirical equations, quantitatively reproduced the energies of vertical electron attachment to π* and σ* empty orbitals measured in the ET spectra and predicted vertical electron affinities close to zero for the three halo derivatives. The total anion currents of the halo derivatives, measured at the walls of the collision chamber as a function of the impact electron energy, presented intense maxima below 0.5 eV. The mass-selected spectra showed that, in this energy, range the total anion current is essentially due to halide fragment anions. The DEA cross sections of the bromo derivatives were found to be about six times larger than that of the chloro derivative. The absolute cross sections at incident electron energies close to zero were evaluated to be 10(-16)-10(-15) cm(2).

Electron attachment to trans-azobenzene

The temporary anion states of gas-phase trans-azobenzene are characterised by means of electron transmission spectroscopy (ETS) in the 0-6 eV range. The measured energies of vertical electron attachment are compared with the energies of the pi* virtual orbitals of the neutral molecule supplied by HF (at MP2 optimized geometries) and B3LYP calculations. The calculated energies, scaled with empirical equations, reproduce quantitatively the energies of the corresponding spectral features and predict a positive vertical electron affinity of 0.83 eV. The total anion current at the walls of the collision chamber and the mass-selected molecular anion current are also reported as a function of the impact electron energy. In agreement with previous data, long-lived (>1 mus) parent molecular anions are detected at zero eV and near 1 eV. The close similarity of the electron transmission spectrum with the derivatives with respect to energy of the anion currents suggests strongly that shape resonances produced by electron capture into empty pi* orbitals are the initial step in formation of the long lived molecular anions. This appears to rule out mechanisms in which direct formation of core-excited anion states are invoked. However, according to DFT calculations, conversion of the shape resonances around 1 eV to longer-lived sigma-pi* core-excited doublet anion states is possible on energetic grounds.

Electron collisions with furan

The authors report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by C(4)H(4)O (furan) molecules. Their calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and in the static-exchange plus polarization approximations. The authors found two shape resonances located around 2.1 and 4.2 eV that belong to the B(1) and A(2) symmetries of the C(2v) group, respectively. The authors' results are consistent with recent measurements of vertical electron attachment energies.

Temporary Anion States and Dissociative Electron Attachment to Isothiocyanates

The temporary anion states of isothiocyanates CH3CH2=C=S (and CH3CH2N=C=O for comparison), C6H5CH2N=C=S, and C6H5N=C=S are characterized experimentally in the gas phase for the first time by means of electron transmission spectroscopy (ETS). The measured vertical electron attachment energies (VAEs) are compared with the virtual orbital energies of the neutral-state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental VAEs. The first VAE is also closely reproduced as the total energy difference between the anion and neutral states calculated at the B3LYP/6-31+G* level. Due to mixing between the ring and N=C=S pi-systems, C6H5N=C=S possesses the best electron-acceptor properties, and its lowest-lying anion state is largely localized at the benzene ring. The anion states with mainly pi*C=S and pi*N=C character lie at higher energy than the corresponding anion states of noncumulated pi-systems. However, the electron-acceptor properties of isothiocyanates are found to be notably larger than those of the corresponding oxygen analogues (isocyanates). The dissociative electron attachment (DEA) spectra show peaks close to zero energy and at 0.6 eV, essentially due to NCS- negative fragments. In spite of the energy proximity of the first anion state in phenyl isothiocyanate to the DEA peak, the zero-energy anion current in the benzyl derivative is about 1 order of magnitude larger.

Electronic structure of the 6+6 dimer of tropone

AbstractPhotoelectron and electron transmission (ET) spectroscopy were used to determine the ionization potentials and vertical electron attachment energies, respectively, of the 6 + 6 dimer of tropone. These measurements are combined with electronic structure calculations to provide insight into the extent of the coupling between the two cycloheptadienone ring systems. The ground state anion is predicted to be slightly stable as a consequence of the interaction between the π* orbitals of the CO and butadiene groups. Copyright © 2006 John Wiley & Sons, Ltd.

Temporary Anion States and Dissociative Electron Attachment in Diphenyl Disulfide

The temporary anion states of gas-phase diphenyl disulfide are characterized by means of electron transmission (ET) and dissociative electron attachment (DEA) spectroscopies. The measured energies of vertical electron attachment are compared to the virtual orbital energies of the neutral state molecule supplied by MP2 and B3LYP calculations with the 6-31G basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the attachment energies measured in the ET spectrum. The first anion state of diphenyl disulfide is stable, thus escaping detection in ETS. The vertical and adiabatic electron affinities, evaluated with B3LYP/6-31+G calculations as the energy difference between the neutral and anion states, are predicted to be 0.37 and 1.38 eV, respectively. The anion current displayed in the DEA spectrum has a sharp and intense peak at zero energy, essentially due to the C6H5S- negative fragment. In agreement, according to the calculations, the localization properties of the first anion state are strongly S-S antibonding, and the energetic requirement for its dissociation along the S-S bond is fulfilled even at zero energy.

Empty Level Structure and Dissociative Electron Attachment Cross Section in (Bromoalkyl)benzenes

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for the series of (bromoalkyl)benzenes C6H5(CH2)nBr (n = 0-3), where the bromine atom is directly bonded to a benzene ring or separated from it by 1-3 CH2 groups, and the dihalo derivative 1-Br-4-Cl-benzene. The relative DEA cross sections (essentially due to the Br- fragment) are reported, and the absolute cross sections are also evaluated. HF/6-31G and B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral state molecules. The pi* VOEs, scaled with empirical equations, satisfactorily reproduce the corresponding experimental vertical electron attachment energies (VAEs). According to the calculated localization properties, the LUMO (as well as the singly occupied MO of the lowest lying anion state) of C6H5(CH2)3Br is largely localized on both the benzene ring and the C-Br bond, despite only a small pi*/sigma*C-Br interaction and in contrast to the chlorine analogue where the LUMO is predicted to possess essentially ring pi character. This would imply a less important role of intramolecular electron transfer in the bromo derivative for production of the halogen negative fragment through dissociation of the first resonant state. The VAEs calculated as the anion/neutral energy difference with the 6-31+G* basis set which includes diffuse functions are relatively close to the experimental values but do not parallel their sequence. In addition the SOMO of some compounds is not described as a valence MO with large pi* character but as a diffuse sigma* MO.

Donor–acceptor properties of isonitriles studied by photoelectron spectroscopy and electron transmission spectroscopy

Some alkyl and aryl isonitriles, considered as CO analogue sigma-donor and pi-acceptor ligands in transition metal chemistry, were studied by means of HeI photoelectron spectroscopy and electron transmission spectroscopy, in order to evaluate their donor-acceptor properties from the measured ionization energies (IE) and vertical electron attachment energies (VAE). The investigated molecules were 2-propyl, 1-butyl, tert-butyl, 1-pentyl, cyclohexyl, 2,6-dimethylphenyl, 4-methoxyphenyl and 4-chorophenyl isonitrile. By interpreting the spectra on the basis of literature data and quantum chemical calculations, the spectral features associated with the molecular orbitals mainly involved in coordination and back-donation were identified. The results show that the IE (10.62-10.95 eV) of the sigma electron pair (n(c)) responsible for the sigma-donor capability is substantially lower than that of CO. The VAEs of the empty pi* orbitals involved in the d/pi* back-donation indicate that aryl isonitriles are better acceptors (VAE <0.3 eV) than their aliphatic counterparts (VAE >2.7 eV). In the case of aryl derivatives, the pi-donor ability could also play some role in metal-ligand bonding (IE 8.74-9.34 eV). Isonitrile coordination characteristics are also compared with those of CO, N(2) and CH(3)CN.

Anionic States of Six-Membered Aromatic Phosphorus Heterocycles As Studied by Electron Transmission Spectroscopy and ab Initio Methods

The electron transmission spectra of 1,3,5-tri-tert-butylbenzene, 2,4,6-tri-tert-butylpyridine, 2,4,6-tri-tert-butylphosphabenzene, and 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene have been investigated and interpreted by means of quantum chemical calculations. Scaled virtual orbital energies obtained from calculations without employing diffuse functions provide good numerical values for the vertical electron attachment energies (VAEs). B3LYP/6-311+G* VAEs, calculated as the energy difference between the anion and the neutral molecule, were in good agreement with experiment, with the molecules investigated here having VAEs of less than about 1 eV. The first anion state of phosphabenzene is predicted to lie at the edge of stability. The gradual replacement of CH units by phosphorus in π systems results in a significant stabilization of the anionic states, which contrasts with the relative invariance of the electron donor properties. This behavior can be explained by considering that for the PC π system bo...

Electron Attachment to the Aza-Derivatives of Furan, Pyrrole, and Thiophene

The temporary anion states of gas-phase furan, isoxazole, oxazole, pyrrole, pyrazole, imidazole, thiophene, isothiazole, and thiazole are characterized by means of electron transmission spectroscopy. The measured energies of vertical electron attachment are compared with the virtual orbital energies of the neutral state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental attachment energies. Replacement of a ring CH group with a nitrogen atom increases the electron-acceptor properties, although the stabilization of the π* anion states is not as large as that of the π cation states, in line with the bond length variations caused by aza-substitution. In the spectra of thiophene and isothiazole the first π* resonances display sharp vibrational structure with energy spacing of about 80 meV. The spectrum of isothiazole presents clear evidence for a low-energy (1.61 eV) resonance ascribed ...

Empty Level Structure and Dissociative Electron Attachment Cross Sections in Saturated and Unsaturated Bromohydrocarbons

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for nine (normal, secondary, and tertiary) bromoalkanes and ten bromoalkenes where the bromine atom is directly bonded to an ethene carbon atom or separated from the double bond by 1−4 CH2 groups. The relative DEA cross sections (essentially due to the Br- fragment) are reported and compared with those of the corresponding chlorides. B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral states of the bromohydrocarbons. The calculated VOEs satisfactorily reproduce the trends of the vertical electron attachment energies (VAEs) measured in the ET spectra. Electron attachment to the σ*C-Br MO of the saturated bromides occurs at about 1.2 eV, the energy of the resonance being slightly stabilized with increasing branching. The corresponding peak in the DEA cross section occurs at about 0.6 eV in the normal bromoalkanes and 0.9 eV in the secondary and tertiary bromoalkanes. In vinyl bromide, the lowest resonance is associated with the ethene π* LUMO, whereas in the CH2CH(CH2)nBr alkenes with n > 2, the LUMO is the σ*C-Br MO. Consistently, in the latter compounds the energy at the peak of the DEA cross section and the magnitude of the latter are comparable to those of the normal bromoalkanes.

Electron attachment to the mixed dimers (CH3)3M–M′(CH3)3, with M,M′=Si, Ge, Sn, and correlation with the calculated virtual orbital energies

The energies of vertical electron attachment to the permethylated group 14 dimers (CH3)3M–M′(CH3)3, with M,M′=Si, Ge, Sn, were measured by means of electron transmission spectroscopy. The tin derivatives show the best electron-acceptor properties. The use of Koopmans’ theorem calculations for prediction of the energies of σ∗ anion states was investigated. The LUMO energies from HF and B3LYP calculations were found to correlate linearly with the experimental energies of the first resonance observed in the present mixed dimers, the homonuclear analogues and the tetramethyl monomers.

Temporary Anions and Empty Level Structure in Cyclobutanediones: Through-Space and Through-Bond Interactions

The energies of vertical electron attachment to cyclobutanone, its 2- and 3-oxa derivatives, 1,2-cyclobutanedione, and 2,2‘,4,4‘-tetramethyl-1,3-cyclobutanedione have been measured in the 0−6 eV energy range by means of electron transmission spectroscopy. The energies of temporary anion formation are nicely reproduced by shifting and scaling the virtual π* orbital energies calculated for the MP2/6-31G*-optimized geometries of the neutral molecules. The linear correlation between experimental and calculated energy values reported in the literature for the empty π*CC orbitals fits the present data on the π*CO orbitals with the same accuracy. The two π* anion states observed in the ET spectrum of the 1,3-dione (split by 1.25 eV) are assigned to the in-phase and out-of-phase combinations in order of increasing energy, i.e., the sequence dictated by through-space interaction. The calculations indicate a large through-space splitting (>2 eV) of the empty carbonyl π* orbitals, reduced by through-bond destabiliza...

Is 9-acridinamine anion a dispersion-bound anion?

The possibility of electron binding to 9-acridinamine (9-AA) was studied at the second order Møller–Plesset perturbation theory level with the aug-cc-pVDZ basis set augmented with a diffuse 6s6p4d set that has proven appropriate in earlier studies of weakly bound anions. It was found that both the amino and imino tautomers of 9-AA bind an excess electron to form stable anions. The vertical electron attachment energies corresponding to the amino and imino form were calculated to be 20 and 41 cm−1, respectively. It was found that while the imino 9-AA tautomer forms a typical dipole-bound anion, the electron binding energy for the amino tautomer calculated at the electrostatic Koopmans’ theorem level appears to be cancelled when the correlation correction to the dipole moment of the neutral is taken into account at the MP2 level. Therefore, the stability of the latter anion may be caused only by additional electron correlation effects, which are dominated by dispersion interactions. For this reason, we suggest that this anion may be termed a dispersion-bound anion.