Electronic structure analysis and electron detachment energies of polynitrogen pentagonal aromatic anions

Various decouplings of the electron propagator have been employed to provide theoretical comparison to experimental electron detachment energies for the pyrrolide, imidazolide, and pyrazolide anions. Predictions for isoelectronic anions in which CH groups are replaced by N atoms also are reported. The ab initio electron propagator results agree closely with experimental values, and the associated Dyson orbitals provide a detailed catalog of bonding changes as the number and positions of N atoms vary within the set of pentagonal aromatic anions.     

Tautomeric forms of azolide anions: vertical electron detachment energies and Dyson orbitals

Several decouplings of the electron propagator, including the relatively new P3+ approximation for the self-energy, have been used to calculate vertical electron detachment energies of tautomeric forms of closed-shell, pentagonal, aromatic anions in which ring carbons without bonds to hydrogens appear. This study extends previous work in which the most stable forms of anionic, five-member rings with one to five nitrogens were considered. Whereas the lowest electron detachment energies sometimes are assigned by Koopmans's theorem results to pi orbital vacancies, electron propagator calculations always obtain sigma orbital vacancies for the ground states of the doublet radicals. Higher electron detachment energies that correspond to excited doublets with pi vacancies also are presented. The predicted transition energies are in good agreement with low-intensity peaks in recent anion photoelectron spectra that have been assigned to less stable, tautomeric forms of these anions.     

Electronic structures and electron detachment energies of halogen substituted acetate anions, XCH2COO- (X=F,Cl,Br)

The electronic structures and the halogen inductive effects on the acetate anion were investigated in XCH2COO- (X=F,Cl,Br) by photoelectron spectroscopy (PES) and ab initio calculations. The PES spectra indicated that the electron binding energies increased in the order of FCl>Br. These systematic changes of detachment energy and IPs were explained by examining the charge redistributions upon detaching electrons.     

Photoelectron spectrum of valence anions of uracil and first-principles calculations of excess electron binding energies

The photoelectron spectrum (PES) of the uracil anion is reported and discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of the anions of various tautomers of uracil. The PES peak maximum is found at an electron binding energy of 2.4 eV, and the width of the main feature suggests that the parent anions are in a valence rather than a dipole-bound state. The canonical tautomer as well as four tautomers that result from proton transfer from an NH group to a C atom were investigated computationally. At the Hartree-Fock and second-order Moller-Plesset perturbation theory levels, the adiabatic electron affinity (AEA) and the VDE have been converged to the limit of a complete basis set to within +/-1 meV. Post-MP2 electron-correlation effects have been determined at the coupled-cluster level of theory including single, double, and noniterative triple excitations. The quantum chemical calculations suggest that the most stable valence anion of uracil is the anion of a tautomer that results from a proton transfer from N1H to C5. It is characterized by an AEA of 135 meV and a VDE of 1.38 eV. The peak maximum is as much as 1 eV larger, however, and the photoelectron intensity is only very weak at 1.38 eV. The PES does not lend support either to the valence anion of the canonical tautomer, which is the second most stable anion, and whose VDE is computed at about 0.60 eV. Agreement between the peak maximum and the computed VDE is only found for the third most stable tautomer, which shows an AEA of approximately -0.1 eV and a VDE of 2.58 eV. This tautomer results from a proton transfer from N3H to C5. The results illustrate that the characteristics of biomolecular anions are highly dependent on their tautomeric form. If indeed the third most stable anion is observed in the experiment, then it remains an open question why and how this species is formed under the given conditions.     

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (e_aq ⁻) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L⁻¹) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L⁻¹) values. At high pH values, where the HPO₄ ²⁻ ions dominate, the e_aq ⁻ decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·10⁵ L mol⁻¹ s⁻¹). The H₂PO₄ ⁻ ions react with e_aq ⁻ at a rate constant of 2.8·10⁶ L mol⁻¹ s⁻¹ (I = 0), which increases linearly with the parameter in accordance with the Debye-Hückel theory. The rate constant for quenching of e_aq ⁻ by H₃PO₄ at pH ≤ 4 decreases linearly with the parameter due to the secondary salt effect and equals 1.6·10⁹ L mol⁻¹ s⁻¹ at I = 0. The logarithm of the rate constant for quenching of e_aq ⁻ by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1277–1280, July, 2007.     

Differential cross sections for elastic scattering and electron detachment in collisions of halogen anions with noble gas atoms

Electron detachment in collision of halogen anions and noble gas atoms was investigated in the collision energy range of 500-3000 eV. Differential cross sections were determined both for elastic scattering and for the electron detachment process in the angular range of 0-3°. It was found that direct electron detachment, due to the interaction of the discrete ion-atom state with a continuum of states increases with increasing collisional energy. Furthermore, the experimental evidence indicates that direct detachment primarily occurs in a limited range of impact parameters. A model, based on angular coupling, is described which qualitatively explains these two experimentally observed features.     

Formation energies of molecules and anions of europium bromides

The content of saturated vapors above europium dibromide and Eu-EuBr₂, Eu-Ba-BaBr₂, EuBr₂-LaBr₃ systems is investigated by means of high-temperature mass-spectrometry in the electron ionization and thermoionic emission regimes. On the basis of the measured equilibrium constants for reactions with participation of molecules and negative ions, the enthalpies of formation ??_fH₂₉₈° (kJ/mol) are determined using the method of the third law of thermodynamics: ?59 ± 13 (EuBr), ?349 ± 19 (EuBr₂), and ?861 ± 24 (EuBr ₃ ^? ).     

Vertical ionization energies of halogen anions in solution

Based on the constrained equilibrium state theory, the nonequilibrium solvation energy is derived in the framework of the continuum model. The formula for spectral shift and vertical ionization energy are deduced for a single sphere cavity with the point charge assumption. The new model is adopted to investigate the vertical ionization for halogen atomic and molecular anions X^? (X = Cl, Br, I, Cl₂, Br₂, I₂) in aqueous solution. According to the calculation using the CCSD-t/aug-cc-pVQZ method in vacuum, our final estimated vertical ionization energies in solution are very close to the experimental observations, while the traditional nonequilibrium solvation theory overestimates these vertical ionization energies.     

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.     

Vertical detachment energies of anionic thymidine: Microhydration effects

Density functional theory has been employed to investigate microhydration effects on the vertical detachment energy (VDE) of the thymidine anion by considering the various structures of its monohydrates. Structures were located using a random searching procedure. Among 14 distinct structures of the anionic thymidine monohydrate, the low-energy structures, in general, have the water molecule bound to the thymine base unit. The negative charge developed on the thymine moiety increases the strength of the intermolecular hydrogen bonding between the water and base units. The computed VDE values of the thymidine monohydrate anions are predicted to range from 0.67 to 1.60 eV and the lowest-energy structure has a VDE of 1.32 eV. The VDEs of the monohydrates of the thymidine anion, where the N(1)[Single Bond]H hydrogen of thymine has been replaced by a 2(')-deoxyribose ring, are greater by ～0.30?eV, compared to those of the monohydrates of the thymine anion. The results of the present study are in excellent agreement with the accompanying experimental results of Bowen and co-workers [J. Chem. Phys. 133, 144304 (2010)].     

Collision-induced electron detachment of carbon clusters

The collisional electron detachment process of carbon cluster anions containing from 2 to 72 atoms was investigated on the collision with a MoS₂ surface. The detachment efficiency was classified into several groups and is most likely to reflect the structures or the isomer contents of the incident cluster anions. The cluster anions produced via laser-vaporization of graphite showed quite a larger detachment efficiency than the fullerene C ₆₀ ^- and its fragment anions. The major isomers of these cluster anions were suggested to be chains or rings.     

Comparative dissociation of peptide polyanions by electron impact and photo-induced electron detachment

We compare product-ion mass spectra produced by electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) of multi-deprotonated peptides on a Fourier transform and a linear ion trap mass spectrometer, respectively. Both methods, EDD and EPD, involve the electron emission-induced formation of a radical oxidized species from a multi-deprotonated precursor peptide. Product-ion mass spectra display mainly fragment ions resulting from backbone cleavages of C_α-C bond ruptures yielding a and x ions. Fragment ions originating from N-C_α backbone bond cleavages are also observed, in particular by EPD. Although EDD and EPD methods involve the generation of a charge-reduced radical anion intermediate by electron emission, the product ion abundance distributions are drastically different. Both processes seem to be triggered by the location and the recombination of radicals (both neutral and cation radicals). Therefore, EPD product ions are predominantly formed near tryptophan and histidine residues, whereas in EDD the negative charge solvation sites on the backbone seem to be the most favorable for the nearby bond dissociation.     

Calculation of electron detachment energies for water cluster anions: an appraisal of electronic structure methods, with application to (H2O)20- AND (H2O)24-

We present benchmark calculations of vertical electron detachment energies (VDEs) for various conformers of (H2O)n-, using both wave function and density functional methods, in sequences of increasingly diffuse Gaussian basis sets. For small clusters (n < or = 6), a systematic examination of VDE convergence reveals that it is possible to converge this quantity to within approximately 0.01 eV of the complete-basis limit, using a highly diffuse but otherwise economical Pople-style basis set of double-zeta quality, with 28 atom-centered basis functions per water molecule. Floating-center basis functions can be useful but are not required to obtain accurate VDEs. Second-order M?ller-Plesset perturbation (MP2) theory suffices to obtain VDEs that are within 0.05 eV of the results from both experiment and coupled-cluster theory, and which always err toward underbinding the extra electron. In contrast to these consistent predictions, VDEs calculated using density functional theory (DFT) vary widely, according to the fraction of Hartree-Fock exchange in a given functional. Common functionals such as BLYP and B3LYP overestimate the VDE by 0.2-0.5 eV, whereas a variant of Becke's "half and half" functional is much closer to coupled-cluster predictions. Exploratory calculations for (H2O)20- and (H2O)24- cast considerable doubt on earlier calculations that were used to assign the photoelectron spectra of these species to particular cluster isomers.     

Calculation of electrostatic and polarization energies from electron densities

We investigate procedures for calculating the electrostatic and polarization energies, Ees and Epol, associated with noncovalent interactions. The starting points are the electron densities of the isolated components and the complex; these could be obtained either computationally or experimentally. A slightly modified version of a scheme proposed by Gavezzotti is used to carry out numerical integrations over these electron densities. Our approach to estimating Epol is based upon partitioning the charge distributions of the components into overlapping and nonoverlapping regions. The effects of varying the integration parameters, computational techniques and basis sets are examined in detail for several noncovalently bound molecular dimers. Our results are in good agreement with the values of Ees and Epol produced by other methods, which require analytical integrations over interaction Hamiltonian matrix elements.     

A theoretical study of the photodetachment and intramolecular hydrogen-bonding energies of hydrogen maleate anions

Three low-lying conformers of the hydrogen maleate anions (HMAs) regarding cis-HMA(HB) having the O-...HO intramolecular hydrogen bond (HB), cis-HMA(nHB) without the HB, and trans-HMA are studied by density functional theory (B3LYP) combined with natural bond orbital (NBO) and atoms-in-molecules (AIM) analyses. The photoelectron spectra of cis- and trans-HMA conformers recorded by Woo et al. (J. Phys. Chem. A 2005, 109, 10633) are reassigned on the basis of the present electron propagator theory calculations, indicating the significant energy differences between the Dyson orbitals and canonical molecular orbitals due to the electron-correlation and orbital relaxation effects considered in the electron propagator theory. The NBO associated with the natural resonance theory analyses and AIM electron topological study show that the strong O-...HO in cis-HMA(HB) has the remarkable characteristics of three-center four-electron hyperbond, and the bonding strength of ca. 30 kcal/mol is recommended with the reference calculations of the HO-...HOH complex. The further calculations for the microhydrated cis-HMA(HB) clusters indicate that the O-...HO bonding strength decreases in water solution.     

Density functional calculations of structures and ionization energies for heavy group V cluster anions

The structure and electronic structure of heavy-group V cluster anions (Sb, Bi) are calculated with density functional methods within the local spin density approximation (LSDA ). The influence of gradient corrections of the exchange and correlation energy is investigated. The calculated vertical and adiabatic ionization energies are in very good agreement with data from photoelectron spectroscopy (PES ) for Sb, whereas the relatively large deviations for Bi can be reduced by the consideration of relativistic effects in a scalar-relativistic manner. Concerning the structures, a strong similarity to the corresponding P clusters was found. In particular, the negatively charged pentamers are planar rings (with similarities to the aromatic [C₅H₅]^? anion) with especially high ionization energies. © 1995 John Wiley & Sons, Inc.     

Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.     

NMR enantiodifferentiation of triphenylphosphonium salts by chiral hexacoordinated phosphate anions

BINPHAT anion—rather than TRISPHAT—is an efficient NMR chiral shift reagent for triphenylphosphonium salts containing stereogenic centers on the aliphatic side-chain.     

Interactions between cyclic phosphate anions and various cations in aqueous solution

Interactions of tetrameta-, hexameta- and octameta-phosphate ions with proton, alkali metal ions and hexamminecobalt(III) ion were studied by the conductivity and potentiometric method. It was shown that the logarithms of thermodynamic stability constants of 1 : 1 proton associated molecules with these cyclic phosphates increase linearly with the anion charges contrary to those with the linear phosphates. The anomalous order of the stability constants obtained for alkali metal ion-pairs with a cyclic phosphate suggests the possibility of partial dehydration of the alkali metal ions with smaller atomic weights in the ion-pair formation. By the use of ion-selective electrodes, it was shown that in a solution containing an excess of sodium, potassium or rubidium ions compared with cyclic phosphate ions, more than one cation is associated with a cyclic phosphate ion. The apparent association constants of these highly associated species were determined. Interactions of cyclic phosphate ions with hexamminecobalt(III) ion, which was selected as an example of tervalent cations, are discussed in comparison with those for uni- and bi-valent cation complexes.     

On the relation between core electron binding energies of ions in solution and their solvation energies

Experimental and computational results from the study of positive and negative ions in solution are presented. The importance of short-range interactions between ion and solvent is studied with regard to core ionization of the ion. Exchange repulsion is found to be a significant factor in the interpretation of data for both cations and anions. Experimental results are presented for the core ionization of the OH^? ion in solution. The data show a strong similarity with corresponding data for the F^? ion, resulting in a large negative solvation energy for the final core hole state. The Be²⁺ ion shows large solvation energies for both ground- and core-ionized states, which is interpreted as due to charge transfer effects between solvent and ion.