Charge distributions on polyatomic ions,and their relationships with cohesive energies of ionic crystals

Some current approaches to the determination of the charge distributions on polyatomic ions are considered. The methods involving cohesive energies of ionic solids on the one hand and those using molecular-orbital calculations on the other hand are compared and discussed. New charge distributions are presented for a series of polyatomic ions.     

Charge distributions in polyatomic ions

The determination of charge distributions in polyatomic ions through both energetic and quantum mechanical (CNDO/2) methods is discussed. Results from both methods are shown to be in good agreement.The author is grateful to Dr. R. Grinter of the University of East Anglia for assistance with the quantum mechanical calculations.     

Mobilities of polyatomic molecular ions in He gas

The mobilities of polyatomic molecular ions in He gas, measured at 83 and 306 K using a low-temperature drift tube, were smaller than the polarization limit K _pol. The mobilities of atomic or small molecular ions tended to show a bump at intermediate reduced field, and then approach the polarization limit at low reduced field; however, the measured mobilities of polyatomic molecular ions were different. We compared the experimental mobilities with values calculated using the MOBCAL program, and found that they agreed well.     

Effect of lithium ions on oxygen reduction in ionic liquid-based electrolytes

The effect of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on the oxygen redox reaction (ORR) in several pyrrolidinium-based ionic liquids (ILs) is investigated by cyclic voltammetry. The results are compared to those found with LiTFSI-tetraglyme and LiTFSI-1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidone solutions. The effect of the addition of tris(pentafluorophenyl) borane (TPFPB) to IL-LiTFSI solutions is also investigated. The results demonstrate that the presence of 0.1 M Li⁺ renders the ORR similar in the investigated ILs, lowers the reduction voltammetric currents and makes ORR electrochemically irreversible. The addition of TPFPB to IL-TFSI solutions favours the solubility of oxygen-based products of Li⁺ and should positively affect the performance of IL-based lithium-air batteries.     

Effect of water on the interfacial structures of room-temperature ionic liquids

The interfacial structures of cyano-based room-temperature ionic liquids play a very important role in reducing friction. However, the presence of water impairs their tribological performance. The interfacial structures and friction forces of 1-ethyl-3-methylimidazolium dicyanamide, [EMIM][DCN], and the effect of water on these structures and forces were investigated using atomic force microscopy. In addition, the interaction of water and [EMIM][DCN] was evaluated using Fourier-transform infrared (FT-IR) spectroscopy. Multiple repulsive layers were observed in the [EMIM][DCN] solution. This solution showed low friction force because these repulsive layers worked as protective layers against friction. On the other hand, the specific repulsive layer characteristics of [EMIM][DCN] could not be observed in a [EMIM][DCN] + 2 wt% H₂O solution. FT-IR results indicated that the layer structure of [EMIM][DCN] was disturbed by the addition of H₂O. Therefore, the solution containing water exhibited a high friction force.     

Dissociation of polyatomic ions in the inductively coupled plasma

A general method for identifying the origin of a particular polyatomic ion is described. Based on a postulated dissociation reaction, measured ion signal ratios (e.g. Ar₂⁺/Ar⁺) are combined with mass bias corrections and estimates of the density of the neutral product (usually Ar, O or H atoms) to determine a gas kinetic temperature T_gas. The temperature can also be measured by the reduction in pressure when the ICP is sampled (compared to room temperature argon), or by other means. Dissociation energies and spectroscopic constants for the ions are necessary. For the particular instrument used, some of the findings of this study are: (a) ArO⁺ and ArN⁺ can be either dissociated (if the plasma potential is high) or created (if the plasma potential is low) by collisions between the sampler and skimmer; (b) the strongly-bound oxide ions O₂⁺ and MO⁺ for the rare earths are observed at levels consistent with T_gas ∼5300 K in a ‘hot’ plasma, but ClO⁺ is formed in excess; and (c) the abundances of most other polyatomic ions like H₂O⁺ and ArH⁺ correspond to higher densities than would be expected in the ICP itself.     

Effect of concomitant elements on the distribution of ions in inductively coupled plasma mass spectrometry — part 2: polyatomic ions

The effect of various concomitant elements in inductively coupled plasma mass spectrometry (ICP-MS) was assessed by measuring the distribution of selected polyatomic singly-charged ions, i.e. oxides of selected analytes (LaO⁺ and CeO⁺) as well as some background ions (Ar₂⁺, ArO⁺ and CO₂⁺), in the plasma. This was accomplished while moving the ICP across and away from the sampling interface with or without a single concomitant element. This study included concomitant elements that were similar in ionization potential, but different in mass (Na, K, Cs and Cl, I), as well as similar in mass but different in ionization potential (K and Cl, Cs and I). Space–charge effects were dominant in many cases. However, the axial profiles of the LaO⁺/(La⁺+LaO⁺) ratio, which should be fairly independent of such effects, were only affected by easily ionized elements (E.I.E.s), i.e. alkalis. A shift of the whole axial profile to lower height A.L.C. was observed in the presence of 0.02 M K or Cs, which is strong evidence for earlier desolvation. However, no shift was observed with 0.02 M Na, where the profile was only suppressed at low heights A.L.C., where the electron density would be most affected. This is consistent with a shift in ionization equilibrium. Further evidence for the latter was observed in the Ar₂⁺ axial profiles, which were shifted to greater height A.L.C. in the presence of E.I.E.s. Finally, CO₂⁺ was found to behave like the analyte oxides and the analytes, in contrast to ArO⁺. This implies that CO₂⁺ originates from dissolved carbon dioxide in the sample.     

Geometries and electronic structures of metastable C2N4 and its ions

We carried out the computational studies on the geometric and electronic properties of electronic states of metastable C(2)N(4) (m-C(2)N(4)) and corresponding ions using the CASSCF and DFT(B3LYP)/CCSD(T) techniques. The optimized geometries of electronic states, vibrational frequencies, Mulliken populations, bond orders, and average polarizabilities are computed at the DFT level while the relative energies of the electronic states, ionization energy, electron affinity, binding energy of m-C(2)N(4) are calculated at the CCSD(T) level. The anion photoelectron spectra of m-C(2)N(4)(-) are also predicted. It is interesting to find that the relative energies of the electronic states of m-C(2)N(4) cluster linearly correlate with the amount of charge transfer between N and C atoms and that, however, there is no charge transfer between C and N atoms upon electron ionization or electron attachment.     

Structures of ionic species and electronic interactions in radical ions; ketyls with divalent cations

EPR and optical spectra of magnesium and calcium ketyls were investigated in MTHF and mixtures of DMF and MTHF at various concentrations. It was shown that these ketyls form aggregates in ethereal solvents and dissociate into biradicals in more polar solutions. Models of ion aggregate to explain the observed EPR and optical spectra are suggested.     

Charge exchange ionization in collision cells as a method to detect the presence of long-lived excited electronic states of polyatomic ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C4H6, CS2, CH3Cl) were lower than the recombination energy (approximately 11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH3F,CH4) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.     

Fragment calculation of electronic structures of polyatomic molecules in the ground state. I. A method of intermediate fragment

A method is proposed for fragment calculation of electronic structures of polyatomic molecules in the ground state. The wave function of a molecule in the ground state in single-determinantal representation of a closed shell is employed. The concise formulation allows efficient calculation of the electronic structures of polyatomic molecules taking into account possible charge transfer between interacting molecular fragments. V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 3, pp. 387–394, May–June, 1995. Translated by I. Izvekova     

Reactions of mercury-like ions in potassium halide lattices

Cadmium and zinc enter the alkali halide matrices as M²⁺ ions and exhibit characteristic absorption bands in the uv. Often these divalent species aggregate to form dimers (and polymers). After additive coloration of the doped single crystals, reduction occurs and monovalent monomers or dimers, and neutral atomary centers are formed. The monovalent dimers exhibit a broad ESR signal with a g value of ∼2.2.     

Effect of ionic solvation on the polarographic behaviour of halide ions inN,N-dimethylformaide-water mixtures

The paloragraphic behaviour of chloride, bromide and iodide ions in severalDMF-water mixtures at a temperature of 25°C is reported. The half-wave potentials change linearly to more cathodic potentials with increasingDMF content of the solvent, up toDMF-rich compositions, where deviations are observed for chloride and iodide ions. On the other hand, theD-product varies with solvent composition, showing a sharp maximum at water-rich compositions for the three halide ions and a smooth minimum for chloride and bromide ions at highDMF content of the solvent. These results are interpreted for different solvent mixtures in terms of the solvent structure and the ionic solvation nature of halide ions.

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Effekt der Ionensolvatation auf das polarographische Verhalten von Halogenid-Ionen inN,N-Dimethylformamid-Wasser-Mischungen

Zusammenfassung Es wird über das polarographische Verhalten von Chlorid-, Bromid- und Iodid-Ionen in einigenDMF-Wasser-Mischungen bei 25°C berichtet. Die Halbwellenpotentiale ändern sich mit ansteigendemDMF-Gehalt linear in Richtung zu kathodischen Potentialen, beiDMF-reichen Zusammensetzungen werden für Chlorid und Iodid Abweichungen beobachtet. Andererseits variiert dasD-Produkt mit der Lösungsmittelzusammensetzung: bei wasserreichen Mischungen zeigt es ein scharfes Maximum für die drei Halogenid-Ionen, bei hohemDMF-Gehalt ein glattes Minimum für Chlorid und Bromid. Die Ergebnisse werden in bezug auf Lösungsmittelstruktur und Ionensolvatation der Halogenid-Ionen interpretiert.

     

Effect of meta electron-donating groups on the electronic structure of substituted phenyl nitrenium ions

Density functional theory (UB3LYP/6-31G(d,p)) was used to determine substituent effects on the singlet-triplet-state energy gap for 21 meta-substituted phenylnitrenium ions. It was found that strongly electron-donating substituents stabilize the triplet state relative to the singlet state. With sufficiently strong meta electron donors (e.g., m,m'-diaminophenylnitrenium ion) the triplet is predicted to be the ground state. Analysis of equilibrium geometries, Kohn-Sham orbital distributions, and Mulliken spin densities for the triplet states of this series of nitrenium ions leads to the conclusion that there are two spatially distinct types of low-energy triplet states. Simple arylnitrenium ions such as phenylnitrenium ions as well as those having electron-withdrawing or weakly donating meta substituents have lowest-energy triplet states that are n,pi in nature. That is, one singly occupied molecular orbital is orthogonal to the plane of the phenyl ring and one is coplanar. These n,pi triplets are generally characterized by large ArNH bond angles (ca. 130-132 degrees ) and an NH bond that is perpendicular to the plane of the phenyl ring. In contrast, meta donor arylnitrenium ions have a lowest-energy triplet state best described as pi,pi. That is, both singly occupied molecular orbitals are orthogonal to the aromatic ring. Such pi,pi states are characterized by NH bonds that are coplanar with the phenyl ring and have ArNH bond angles that are more acute (ca. 110-111 degrees ). These triplet nitrenium ions have electronic structures analogous to those of meta-benzoquinodimethane derivatives.     

Model calculation on the pump-probe measurement of ultrafast electronic population decay in polyatomic molecules

We consider the common situation of strong vibronic coupling of an optically bright (in absorption from the ground state) excited electronic state to a lower-lying dark electronic state in a polyatomic molecule. It is shown that for sufficiently short pump and probe laser pulses a time-resolved experiment measures the total time-dependent population probability P(t) of the bright state. For a realistic model problem (representing the three lowest electronic states of the benzene cation) a conical intersection of the potential energy surfaces of the bright and the dark state causes an ultrafast initial decay of P(t) on a femtosecond time scale, followed by quasiperiodic recurrences. These recurrences show up as femtosecond quantum beats in the time-resolved pump-probe signal. The beating frequency is related to the vibrational frequency of the dominant accepting mode of the system.     

Fragment calculation of electronic structures of polyatomic molecules in the ground state. II. A method of delocalized states of fragments

A fragment method is proposed to calculate the electronic structures of polyatomic molecules in the ground state. Localization and delocalization of the electronic states of molecular fragments are calculated simultaneously. The compact formulation of the method allows algorithmically efficient calculations of the electronic structures of interacting molecular fragments as well as of the whole molecules. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 3, pp. 395–400, May–June, 1995. Translated by I. Izvekova     

Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers

This study addresses the effect of ionic strength and type of ions on the structure and water content of polyelectrolyte multilayers. Polyelectrolyte multilayers of poly(sodium-4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared at different NaF, NaCl and NaBr concentrations have been investigated by neutron reflectometry against vacuum, H(2)O and D(2)O. Both thickness and water content of the multilayers increase with increasing ionic strength and increasing ion size. Two types of water were identified, "void water" which fills the voids of the multilayers and does not contribute to swelling but to a change in scattering length density and "swelling water" which directly contributes to swelling of the multilayers. The amount of void water decreases with increasing salt concentration and anion radius while the amount of swelling water increases with salt concentration and anion radius. This is interpreted as a denser structure in the dry state and larger ability to swell in water (sponge) for multilayers prepared from high ionic strengths and/or salt solution of large anions. No exchange of hydration water or replacement of H by D was detected even after eight hours incubation time in water of opposing isotopic composition.     

Substituent effect on electronic structures of halonitrobenzenes

The electronic structures and substituent effects of o-, m-, and p-chloronitrobenzene and bromonitrobenzene have been studied by ultraviolet photoelectron spectroscopy (UPS). It was found that the o-isomer possesses particular electronic properties. This characteristic depends on the conjugation between the benzene ring pi orbital and the nitro group pi orbital and the interaction of the halogen and nitro groups in the adjacent position.     

Lattice modes of alkali halides activated by polyatomic impurity ions of various symmetries

Raman and lattice side band spectra of the external modes of ReO⁻₄, NO⁻₃ and NCO⁻ isolated wuthin KI have been recorded at 300K and 77K. Similarities and differences shown by these spectra are interpreted in terms of (i) highly specific properties of the impurity-occupied lattice site (eg. torsional and translational oscillations of the impurity ion), (ii) a very general but weak activation of host lattice modes due to the disorder imposed by the randomly distributed impurity ions and (iii) selective enhancement of specific host lattice modes which, as a result of having the same symmetry properties, can resonate with (and draw intensity from) the modes described in (i) above. It is in order to investigate this third point, (which has been totally ignored in previous work on the external mode spectra of polyatomic impurity ions) that three ions with different symmetries have been used.