Structure of carbon fluorochlorides CF n Cl m (n,m ≤ 3) and their singly charged negative ions

The electronic and geometrical structures of carbon fluorochlorides with low coordination numbers (n 3) and their singly charged anions are calculated using the functional density method. The results of the calculations are used to evaluate the electron affinities (EA) of the neutral compounds and the first ionization potentials of the anions as well as the energies of fragmentation through different decay channels of both series. The adiabatic EA of carbon fluorochloride CF _k Cl _3–k is shown to be determined mainly by the presence of a CX₂ unit in these compounds. There are no monotonic changes in stability of either the neutral compounds withn = 3 or the anions withn = 2 or 3 upon successive substitution of one halogen by another.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1044–1049, June, 1993.     

Electron Induced Dissociation of Singly Deprotonated Peptides

Dissociation of singly charged species is more challenging compared with that of multiply charged precursor ions because singly charged ions are generally more stable. In collision activated dissociation (CAD), singly charged ions also gain less kinetic energy in a fixed electric field compared with multiply charged species. Furthermore, ion–electron and ion–ion reactions that frequently provide complementary and more extensive fragmentation compared with CAD typically require multiply charged precursor ions. Here, we investigate electron induced dissociation (EID) of singly deprotonated peptides and compare the EID fragmentation patterns with those observed in negative ion mode CAD. Fragmentation induced upon electron irradiation and collisional activation is not specific and results in the formation of a wide range of product ions, including b-, y-, a-, x-, c-, and z-type ions. Characteristic amino acid side chain losses are detected in both techniques. However, differences are also observed between EID and CAD spectra of the same species, including formation of odd-electron species not seen in CAD, in EID. Furthermore, EID frequently results in more extensive fragmentation compared with CAD. For modified peptides, EID resulted in retention of sulfonation and phosphorylation, allowing localization of the modification site. The observed differences are likely due to both vibrational and electronic excitation in EID, whereas only the former process occurs in CAD.     

Structure and stability of gold-substituted diborane, boranes, and borohydride ions

Pseudopotential ab initio calculations were performed for species of the type BH _n (AuPH₃) _m ^k , where n+m=3 or 4, and the charge k is −2,…,+1. Some derivatives of these and diaurated diboranes were also studied. The structural data agree well with the available experimental evidence. Factors affecting the stability of these systems, including the role of aurophilic attraction, are discussed. The singly charged anions and the diaurated diboranes are predicted to be the most stable members of these series. Received: 22 January 1999 / Accepted: 2 June 1999 / Published online: 4 October 1999     

Validity of <Emphasis Type="Italic">B</Emphasis>-splines as a universal basis set for atomic Hartree–Fock–Roothaan calculations

The validity of B-splines as a universal basis set for atomic Hartree–Fock–Roothaan calculations is studied. In order to accomplish our aim, the ground-state energies of neutral atoms He–Xe, cations Li ⁺–Xe ⁺, and anions H ^-–I ^- with the nuclear charge Z=54 are calculated by the Hartree–Fock–Roothaan method with the B-spline sets. All radial functions of the atoms and singly charged ions are expanded by common B-spline sets regardless of atomic systems and symmetries of atomic orbitals. The energies obtained by the best B-spline set are in excellent agreement with ten-digit numerical Hartree–Fock results.     

Differentiation of Isomeric Hydroxypyridine <Emphasis Type="Italic">N</Emphasis>-Oxides Using Metal Complexation and Electrospray Ionization Mass Spectrometry

Differentiation between two isomers of hydroxypyridine N-oxide according to the metal cation adducts generated by electrospray ionization (ESI) was investigated for different metal cations, namely Mg (II), Al (III), Ca (II), Sc (III), Fe (III), Co (II), Ni (II), Cu (II), Zn (II), Ga (III), besides the diatomic cation VO(IV). Protonated molecules of the isomeric hydroxypyridine N-oxides as well as the singly/doubly charged adducts formed from neutral or deprotonated ligands and a doubly/triply charged cation were produced in the gas phase using ESI, recording mass spectra with different metal ions for each isomer. While complex formation was successful for 2-hydroxypyridine N-oxide with trivalent ions, in the case of 3-hydroxypyridine N-oxide, only peaks related to the protonated molecule were present. On the other hand, divalent cations formed specific species for each isomer, giving characteristic spectra in every case. Hence, differentiation was possible irrespective of the metal cation utilized. In addition, quantum chemical calculations at the B3LYP/6-31 + G(d,p) level of theory were performed in order to gain insight into the different complexation of calcium(II) with the isomers of hydroxypyridine N-oxide. The relative stability in the gas phase of the neutral complexes of calcium made up of two ligands, as well as the singly charged and doubly charged complexes, was investigated. The results of these calculations improved the understanding of the differences observed in the mass spectra obtained for each isomer.     

Interaction of the uracil dipole-bound electron with closed-shell systems (Ar and N2)

A very diffuse, but spatially confined, electron trapped in a dipole–bound state of a polar molecule provides an excellent target for testing the interaction of a localized electron positioned outside the molecular frame of its host molecule with other atomic or molecular systems. In this work we use ab initio calculations to investigate systems where a dipole–bound electron attached to a uracil molecule is interacting with an N₂ molecule and an Ar atom. Neither of the two systems forms a stable anion and in the aducts they form with the dipole–bound electron the electron becomes suspended between the uracil molecule and Ar or N₂. Calculations are performed to determine the vertical electron detachment energies of these anions and to determine the molecular rearrangements occurring when the excess electron is removed from them.Contribution to the Jacopo Tomasi Honorary Issue     

Lead‐enhanced gas‐phase stability of multiply charged EDTA anions: a combined experimental and theoretical study

Besides their fundamental importance, multiply charged anions (MCAs) are considered as promising molecular capacitors for which their intrinsic stabilities are of great significance. Herein, the gas‐phase stabilities of ethylenediaminetetraacetic acid (EDTA) anions (i.e. [EDTA‐nH]^n?, n = 1–4) and their Pb(II) complexes (i.e. [EDTA + Pb‐nH]^(2‐n)?, n = 3, 4) have been investigated using an approach that combines extractive electrospray ionization mass spectrometry (EESI‐MS) measurements, Car–Parrinello molecular dynamics simulations and density functional theory/Tao–Perdew–Staroverov–Scuseria calculations. The EESI‐MS data showed that the doubly charged EDTA anions in the form of [EDTA‐2H]^2? and [EDTA + Pb‐4H]^2? were much more abundantly observed than the singly charged species such as [EDTA‐H]^? and [EDTA + Pb‐3H]^?, respectively. The calculation results indicated that [EDTA‐2H]^2? and [EDTA + Pb‐4H]^2? anions were thermodynamically more stable than the [EDTA‐H]^? and [EDTA + Pb‐3H]^? species in the gas phase, respectively. The [EDTA + Pb‐3H]^? anions preferred five‐coordinated structure, whereas [EDTA + Pb‐4H]^2? anions formed either five‐coordinated or six‐coordinated structures. The calculations further revealed that significant electron clouds drifting from the ligand EDTA to the metal Pb(II) ions and the large distances between the carboxylic groups reduced the Coulomb repulsion among the excess electrons of these MCAs. Our data demonstrated that EESI‐MS combined with theoretic calculations were able to provide a deep insight into the fundamental behavior of stability of MCAs in the gas phase and, thus, might be useful tools for studying MCAs for potential molecular capacitors. Copyright © 2012 John Wiley & Sons, Ltd.     

ESR Spectra of radicals produced by reduction of pyridine and pyrazine

Radicals produced by reaction of e^?_aq with pyridine, pyrazine, and pyrazinedicarboxylic acid have been studied by electron spin resonance using the in situ radiolysis steady-state ESR technique. The radical anions initially produced have been found to undergo rapid protonation on nitrogen to form pyridinyl and pyrazinyl radicals. The NH proton of pyridinyl radical does not dissociate even at pH 13.7. The radical from pyrazine has been observed only in the doubly protonated positively charged form in acid and neutral solutions, but no spectrum was observed in alkaline media. With 2,3-pyrazinedicarboxylic acid the doubly protonated radical has been observed at pH 4–8 and the singly protonated one at pH 11–12. The pK for this dissociation is 9.2. The hyperfine constants of the pyridinyl radical are compared with those obtained from INDO molecular orbital calculations.     

Expansion coefficients and moments of electron momentum densities for singly charged ions

Numerical Hartree–Fock calculations of the first three coefficients of the MacLaurin expansion and the leading coefficient of the large-p asymptotic expansion of the electron momentum densities Π(p) are reported for 54 singly charged atomic cations from He⁺ (atomic number Z = 2) to Cs⁺ (Z = 55) and 43 anions from H⁻ (Z = 1) to I⁻ (Z = 53) in their experimental ground states. We also report all the finite moments <p ^k > (−2≤k≤+4) of the momentum densities Π(p) for the above-mentioned 97 ionic species. The results are compared with the previous ones for neutral atoms [Koga and Thakkar (1996) J Phys B 29: 2973], and the dependence of the expansion coefficients and moments on nuclear charge is discussed among isoelectronic species. Received: 20 November 1998 / Accepted: 15 January 1999 / Published online: 7 June 1999     

Nitrosonium complexes of triptycene

According to the ¹H and ¹³C NMR data, triptycene reacts with nitrosonium tetrachloroaluminate to form positively charged π-complexes with one and two nitrosonium ions, which are rapidly (on the NMR time scale) converted into each other. DFT quantum-chemical calculations with Λ02 basis set indicated higher stability of singly charged complexes relative to doubly charged. Addition of three nitrosonium ions to triptycene molecule is unfavorable from the energy viewpoint.     

A new sulfate-selective electrode and its use in analysis

The improvement of the steric accessibility of exchange centers of higher quaternary ammonium salts (QAS) by replacing several long-chain alkyl substituents at the nitrogen atom with methyl groups enhanced the selectivity of ion-selective electrodes (ISEs) based on the specified ion exchangers for the SO ₄ ^2? ion up to six orders of magnitude in the presence of singly charged anions. This can be qualitatively explained by the specific features of ion pair formation between quaternary ammonium cations and singly and doubly charged anions. The effect of the steric accessibility of the QAS exchange center on the selectivity of ISEs is partially retained in the presence of a neutral anion carrier, hexyl p-trifluoroacetylbenzoate (HTFAB), which is used for enhancing the selectivity for the sulfate ion in the presence of singly charged anions. A sulfate-selective electrode with a reasonable selectivity for practical purposes was proposed. It is based on the HTFAB-higher QAS ion pair bearing three methyl substituents at the nitrogen atom. The ISE was used in the analysis of natural water.     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

Geometries and electronic structures of negatively charged carbon clusters

Self-consistent field molecular orbital calculations have been performed on neutral and negatively charged clusters of carbon atoms using an extended basis set designed to obtain correct electron affinity. Correlation effects have been included perturbatively up to second order. The optimized geometries of theC _n ^? (n ≤ 7) anions are all linear chains as observed in experiments. The calculated electron affinities are comparable with experimental data. Studies of the stabilities of doubly charged anions show that clusters uptoC ₇ ^?? are unstable.     

Density functional theory study of the structure and energetics of negatively charged oligopyrroles

First‐principles calculations are used to investigate the electronic properties of negatively charged n‐pyrrole oligomers with n = 2–18. Chains of neutral oligomers are bent, whereas the negatively charged oligomers become almost planar due to accumulation of negative charge at the end monomers. Isomers of short oligomers (n < 6) display negative electron affinity although the corresponding anions are energetically stable. For longer oligomers with n ≥ 6, the electron affinity is small and positive, slowly increasing with oligopyrrole length. Doping of 12‐pyrrole with lithium atoms shows that negative oxidation states are possible due to electron transfer from dopant to oligomer at locations close to dopant. These 12‐pyrrole regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid structure in the C? C backbone conjugation. Comparison between neutral and doped polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the band gap energy and the appearance of dopant‐based bands in the gap for a 50% per monomer doping level. It is predicted that Li‐doped PPy is not metallic. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The structural significance of doubly charged ion spectra. Phenylenediamine derivatives

The distribution of doubly charged ions found in the mass spectra of various N, N′-alkyl substituted phenylenediamines is compared to that of their singly charged counterparts. Structural and electronic requirements for the resonance stabilization of doubly charged organic ions are suggested and it is shown that doubly charged ions could be used to differentiate between certain structural isomers which give practically identical singly charged ions. The doubly charged ions were distinguished from singly charged ions of the same integral m/e value by high resolution techniques.     

Electron capture and target excitation in slow ion-alkali atom collisions

Experimental cross sections for single electron capture and target atom resonance line emission in impact of singly charged ions (He⁺, Ne⁺, Ar⁺, C⁺, N⁺, O⁺) on respectively Li(2s) and Na(3s) are presented and compared with semiempirical calculations of the Demkov-Olson type. It is shown that such calculations can still be useful despite involvement of metastable primary ion admixtures and several final states. In addition, observed undulations in the impact energy dependence of electron capture — and target excitation cross sections are discussed.     

Analytical Hartree-Fock electron densities for singly charged cations and anions

The Hartree-Fock (HF) electron density has an important property that it is identical to the unknown exact density to the first order in the perturbation theory. We generate the spherically averaged HF electron density ρ(r) by using the numerical HF method for the singly charged 53 cations from Li⁺ to Cs⁺ and 43 anions from H⁻ to I⁻ in their ground state. The resultant density is then accurately fitted into an analytical function F(r), which is expressed by a linear combination of basis functions r ⁿⁱ exp(−ζ _i r). The present analytical approximation F(r) has the following properties: (1) F(r) is nonnegative, (2) F(r) is normalized, (3) F(r) reproduces the HF moments <r ^k > (k=−2 to +6), (4) F(0) is equal to ρ(0), (5) F ^′(0) satisfies the cusp condition and (6) F(r) has the correct exponential decay in the long-range asymptotic region. The present results together with our previous ones for neutral atoms provide a compilation of accurate analytical approximations of the HF electron densities for all the neutral and singly charged atoms with the number of electrons N≤54. Received: 11 July 1997 / Accepted: 27 August 1997     

Low-temperature photoelectron spectroscopy of aliphatic dicarboxylate monoanions, HO2C(CH2(nCO2- (n = 1-10): hydrogen bond induced cyclization and strain energies

Photoelectron spectra of singly charged dicarboxylate anions HO(2)C(CH(2))(n)CO(2)(-) (n = 1-10) are obtained at two different temperatures (300 and 70 K) at 193 nm. The electron binding energies of these species are observed to be much higher than the singly charged monocarboxylate anions, suggesting that the singly charged dicarboxylate anions are cyclic due to strong intramolecular hydrogen bonding between the terminal -CO(2)H and -CO(2)(-) groups. The measured electron binding energies are observed to depend on the chain length, reflecting the different -CO(2)H...(-)O(2)C- hydrogen bonding strength as a result of strain in the cyclic conformation. A minimum binding energy is found at n = 5, indicating that its intramolecular hydrogen bond is the weakest. At 70 K, all spectra are blue shifted relative to the room-temperature spectra with the maximum binding energy shift occurring at n = 5. These observations suggest that the cyclic conformation of HO(2)C(CH(2))(5)CO(2)(-) (a ten-membered ring) is the most strained among the 10 anions. The present study shows that the -CO(2)H...(-)O(2)C- hydrogen bonding strength is different among the 10 anions and it is very sensitive to the strain in the cyclic conformations.     

BaxC60 fullerides: π Electronic peculiarities of the C60 molecule and their consequences for the solid state

The electronic structure of Ba_xC₆₀ fullerides was studied theoretically under special consideration of π electronic effects in the C₆₀ molecule. Band structure data were derived by an intermediate neglect of differential overlap (INDO) crystal orbital (CO) approach. Different electronic configuration were evaluated in the Ba-doped C₆₀ fullerides. Ba_xC₆₀ solids with x=0, 3, 4, 6 are insulators. For a Ba₅C₆₀ model extrapolated from the crystal structure of Ba₆C₆₀, a finite band gap is also predicted. For a Ca₅C₆₀-like structure of Ba₅C₆₀, a quasi-degeneracy between a metallic configuration and an insulating Mott-like state was found. With an increasing Ba-to-C₆₀ charge transfer (CT), sizable changes in the π system of C₆₀ occur. In the neural molecule and for not too high an electron count, the π electrons form more or less electronically isolated hexagon–hexagon (6–6) “double” bonds with only minor hexagon–pentagon (6–5) “double-bond” admixtures. In the vicinity of C₆₀¹²⁻, the 6–6 bonds have lost most of their double-bond character while it is enhanced for the 6–5 bonds. In highly charged anions, the π electron system of the soccer ball approaches a configuration with 12 decoupled 6π electron pentagons. For electron numbers between C₆₀ and C₆₀¹²⁻, the net π bonding is not weakened. The INDO CO results of the Ba_xC₆₀ solids are supplemented by INDO MO and ab initio (3-21 G* split-valence basis) calculations of molecular C₆₀ and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C₆₀ with short 6–6 and long 6–5 bonds is inverted in C¹²⁻₆₀. The high acceptor capability of C₆₀ is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C₆₀ and C₆₀¹²⁻, the influence of the Pauli antisymmetry principle (PAP) is minimized. The quantum statistics of (π) electron ensembles with a deactivated PAP is of the so-called hard-core bosonic (hcb) type. In these ensembles, the on-site interaction is fermionic while the intersite interaction is bosonic. Energetic consequences of the quantum statistical peculiarities of π systems are explained with the aid of simple model systems; we selected annulenes and polyenes. Computational tools in this step are Green's function quantum Monte Carlo (GF QMC) and full configuration interaction (CI) calculations for the π electrons of the model systems. These many-body techniques were combined with a Pariser–Parr–Pople (PPP) Hamiltonian. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 333–373, 1997     

Average inner product sums of electron linear momenta for atoms and ions

From the second moments of the electron-pair densities in momentum space, accurate Hartree–Fock values of the average inner product sum 〈∑ _i<j p _i ·p _j 〉 of electron linear momenta are evaluated for the 102 neutral atoms from He to Lr, the 53 singly charged cations from Li⁺ to Cs⁺, and the 43 stable anions from H⁻ to I⁻ in their experimental ground states. The present results are new for 38 species and improve the literature values for 68 species. Received: 18 July 2002 / Accepted: 4 September 2002 / Published online: 8 November 2002 Acknowledgement. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. Correspondence to: H. Matsuyama e-mail: hisashi@mmm.muroran-it.ac.jp