Methyl and silyl mesolytic dissociations in the radical cations and radical anions of but-1-ene,allylsilane, hexa-1,3-diene,and penta-2,4-dienylsilane. CAS-MCSCF and coupled cluster theoretical study

Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.     

The correct McLachlan method in free radicals and radical anions

A recent modification to the McLachlan method is corrected. This modification, when correctly applied to odd alternant hydrocarbons, reproduces McLachlan's results instead of improving their agreement with experiment. The modification is irrelevant and erroneous for radical anions.     

A computational study of solution effects on the disproportionation of electrochemically generated polycyclic aromatic hydrocarbon radical anions. Thermodynamics and structure

Density functional quantum chemical methods are used to predict the thermodynamics of disproportionation of electrochemically generated polycyclic aromatic hydrocarbon radical anions (2) into the corresponding neutral species (1) and dianions (3). The computations reveal the overwhelming influence of solvation effects upon the disproportionation equilibrium. By comparison, the effect of ion pairing between 3 and the cation of the supporting electrolyte (R₄N⁺) is modest but real. The computational results can be combined with a variety of entropy effects to calculate the spacing ΔE between the first and second reduction potentials of 1 within 100-150 mV. The highly asymmetric structures of the ion pairs between 3 and R₄N⁺ show little evidence for steric hindrance to ion pairing, yet the computations do show that the strength of the ion pairs does appear to diminish with increasing size of the R group. The strength of the ion pairs with small cations appears to arise out of the large charge-to-size ratio in such cations.     

13C NMR studies on radical anions. Special solvent effects

The determination of radical ions is crucial in mechanistic studies of reactions in which single electron transfer is suspected. A ¹³C NMR method based on the interaction of radical anions with tetrahydrofuran (THF) molecules is presented; measurements of the broadening of the THF α-carbon signal allow determination of the substrate concentration in the range 0.2–1.2 M . The spectacular effect observed on addition of small amounts of benzene greatly improved the method. In addition, studies of the spin-lattice, T₁, and of the transverse nuclear, T₂, relaxation times and of the effects of added co-solvents allow the proposal of the bonding situation and of the degree of coordination in the radical anion-cation-THF molecules' complex.     

Radical anions of nitrobenzothiazoles: EPR study of conjugative properties of benzothiazolyl systems

The electrochemical reduction in DMSO of the five isomers of nitrobenzothiazole (NBTZ) gave quite persistent radical anions that could be easily characterised by EPR spectroscopy. By contrast, the chemical reduction in alkaline solution, that is by t-BuOK in DMSO or by glucose and MeOK in MeOH, presented some problems with 6- and 4-NBTZ, and in the case of 2-NBTZ did not provide any detectable paramagnetic species. The internal consistency of coupling constants of the nitrobenzothiazole radical anions is in good agreement with the conjugative properties of the various benzothiazolyl systems and allows rectifying a recent EPR characterisation of 6-NBTZ radical anion.     

A theoretical study on the hyperfine coupling constant of the radical cations of aliphatic ethers

Summary The hyperfine coupling constants of the radical cations of dimethylether, oxetane (oxacyclobutane), and tetrahydrofuran (oxacyclopentane) are studied byab-initio molecular orbital theories. The extraordinarily large hyperfine coupling constants of the protons of the ethers that have been found experimentally are analyzed to conclude that an important mechanism of the hole delocalization is the spin polarization in the H-C-O-C-H bond. It is also found that for the ethereal systems conventional molecular orbital calculations give glaringly small spin densities but the SAC-CI calculation gives remarkably improved values.     

Stability,metastability, and unstability of three-electron-bonded radical anions. A model ab initio theoretical study

The stability of O therefore O, N therefore N, S therefore S, P therefore P, and Si therefore Si three-electron bonds in anionic radicals isoelectronic to dihalogen radical anions is studied by means of ab initio calculations on model systems. The difficulty of generating the dissociation energy profiles of such anions and their rearrangement to neutral species is solved by a practical method which consists of calculating the neutral and anionic energy profiles separately and shifting the curves with respect to each other to match the experimental energy gap between the asymptotes. Here the neutral and anionic reaction profiles are calculated at the CASPT2 and MP2 levels, respectively. The calculations predict that the O therefore O bond is likely to be observed in anions of the type [RO therefore OR](*-), where R is any alkyl substituent or carbon chain. The anion Si(2)H(6)(*-) is found to be a metastable species, with a fair barrier to electron detachment. The barrier is much smaller for N(2)H(4)(*-) and P(2)H(4)(*-), thus precluding experimental observation. However, these species can be stabilized by electron-attractor substituents, the effect of which can be quantitatively estimated by means of the parent anion's diagrams and some fast complementary calculations. An example is given with the [CF(3)HN therefore NHCF(3)](*-) anionic complex.     

Electrochemical study of isopoly and heteropoly anion transfer across the water | nitrobenzene interface. Part II. Vanadium-containing heteropolytungstate anions

The electrochemical transfer behaviour of vanadium-containing heteropolytungstate anions [PW_12−xV_xO₄₀]^(3+x)− (x = 1−4) across the water | nitrobenzene interface has been investigated by cyclic voltammetry and chronopotentiometry with cyclic linear current scanning. The transfer of PW₁₁V₁O⁴⁻₄₀, HPW₁₀V₂O⁴⁻₄₀, H₂PW₁₀V₂O³⁻₄₀, H₃PW₉V₃O³⁻₄₀ and H₄PW₈V₄O³⁻₄₀ across the water | nitrobenzene interface can be observed within the potential window. The effects were observed of pH in the water phase on the transfer behaviour and the formation of vanadium-containing heteropolytungstate anions in solution. Heteropolytungstate anions become more stable due to their involving the vanadium atom. The degree of protonation and the dissociation constant of the trivalent vanadium-containing heteropolytungstate anion of protonation increase with increasing vanadium content. The transfer processes are diffusion-controlled. The standard transfer potential, the standard Gibbs energy and the dissociation constant for vanadium-containing heteropolytungstate anions have been obtained and the transfer mechanisms are discussed.     

Organic anions : IV. A theoretical study of pentadienyllithium

CNDO II calculations suggest that, for all three conformations of the pentadienyl anion and of pentadienyllithium, the central carbon atom should be more reactive than either of the two terminal carbon atoms. They also reveal that, for the sickle- and W-shaped conformations there is more than one potential bonding site for the lithium. Bonding is discussed both in terms of FMO theory and in terms of possible M:obius aromatic character.     

X-ray absorption spectroscopy of ultramarine pigments: A new analytical method for the polysulfide radical anion S3 chromophore

Blue and mauve ultramarine artists' pigments and their heat-treated products have been investigated by sulfur K-edge X-ray absorption. X-ray absorption near-edge structure spectra are dominated by features of reduced sulfur and sulfate species. There is also a pre-peak at about 2468.0 eV which reflects the presence of the unpaired electron on the polysulfide radical anion (S₃⁻). Pre-peak intensity is directly proportional to the depth of blue coloration, and provides a new, independent method for estimating the proportion of ultramarine cage sites occupied by the blue chromophore. The occupancy of the polysulfide radical anion S₃⁻ is estimated to be 33% in an intense ultramarine blue pigment, 22% in a dark blue ultramarine pigment, and 1% in deep royal blue lazurite from Afghanistan. The more efficient development of color in lazurite is attributed to extensive annealing of the mineral structure in the natural environment.     

A DFT study of the interaction between microhydrated anions and naphthalendiimides

The characteristics of the interaction of anions with naphthalendiimides, the basic structural motif of a newly synthesized anion channel based on anion···π interactions, are studied by computational methods. Stable complexes are formed with bromide, chloride, fluoride or hydroxide anions, which exhibit strong anion···π interactions in the gas phase. Following the sequence of the polarizing power of the anions, hydroxide and fluoride complexes are the most strongly interacting. The presence of a small number of water molecules strongly affects the anion···π interactions, especially for hydroxide and fluoride complexes, so the differences in interaction strength among the anions drop significantly. The calculations suggest that a small number of water molecules can be crucial to reducing dehydration cost and contributing to stabilizing interactions with the naphthalendiimide units.     

Spin density distributions in radical anions of heterocyclic amine N-oxides

The unrestricted Hartree-Fock (UHF) method of Snyder and Amos is used to calculate, in the -electron approximation, the spin density distributions in radical anions of heterocyclic amine N-oxides. The computed spin densities are observed to be in good agreement with the experimental values. The computed spin density distribution of the radical anion of pyridine N-oxide is consistent with the greater susceptibility of pyridine N-oxide relative to pyridine to electrophilic nitration. Also, the calculations are consistent with the lower basicity of the N-oxides relative to the parent bases.     

A Mixed-Anion System Consisting of a Germyl Anion and Anions Delocalized on Conjugated Carbon Ring Skeletons

A trianion consisting of one germyl anion and two anions delocalized on conjugated carbon ring skeletons was synthesized by trimerization of the germanium analogue of the anthryl anion, which exhibits high germylene character. In addition, the generation of the putative intermediate of this reaction, the radical anion of 9-germaanthracene, was confirmed by the formation of the corresponding dimeric dianion. The isolated trianion and dianion were characterized by X-ray crystallographic analysis together with NMR spectroscopy and theoretical calculations.     

The composition and architecture of hydrated hydroxy anions as observed in the crystalline state – A review

Search of the Cambridge Structural Database reveals that hydrated hydroxide anions can be quite complex in their stereochemistry. As in the case of hydronium cations [I. Bernal. C.R. Chim. 9 (2006) 1454], they can be acyclic as well as cyclic. Moreover, it is documented here that hydrated hydroxide anions exist in the form of geometrical isomers or conformers. A simple example occurs in species of composition H₅O₃⁻ which exist in isomeric forms in which their stereochemistry depends on whether the OH⁻ is the hydrogen donor to waters or vice versa. All illustrations in this survey were generated with DIAMOND.     

The dimerization of radical anions of aromatic carboxylic acids competing with the self-protonation reaction

Quantum-chemical calculations (CNDO/2) of the theoretical relationship between the rate constants for the dimerization and self-protonation of radical anions show that dimer formation in the one-electron electroreduction of aromatic carboxylic acids (benzoic (1), 1-naphthoic (2), and 9-anthroic (3) is most probable for1. It is established that during the constant potential electrolysis (CPE) of1 a mixture of “head-to-tail” dimers is formed in the presence of 0.1M Bu₄NClO₄ (DMF). Their ratio depends on the amount of electricity passed through the solution. The CPE of 2 in the presence of 20 % H₂O affords 1,4-dihydro-1-naphthoic acid in up to 70 % yield. The high yield (∼70 %) of 9,10-dihydro-9-anthroic acid during the CPE of 3 can be accounted for by the decomposition of the dimeric product followed by protonation of the anionic species. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1735–1738, October, 1993.     

Potential of CE for the determination of inorganic and acidic anions in cyanoacrylate adhesives

In this work, a CZE method with indirect UV detection was developed for the simultaneous determination of the inorganic and acidic anions, chloride, sulfate, nitrate, fluoride, formate, phosphate, diethylphosphate, methyl sulfonate, cyanoacetate, and methacrylate present in cyanoacrylate adhesives. Chromate was employed as the probe ion, and the EOF was reversed by incorporating CTAB into BGE. Detection limits of 0.7-4.6 microg/mL were obtained for all the anions studied. The CE method developed is a significant improvement on traditionally used chromatographic methods such as ion chromatography, as it resulted in shorter analysis times with enhanced separation efficiencies. This method was successfully employed for the analysis of inorganic and acidic anions in cyanoacrylate adhesive samples.     

Probing the aromaticity of bis(diazolo)pyrazine radical anions

The aromaticity of a series of heterocyclic radical anions of bis(diazolo)pyrazine type, X(CN)₂N₂(CN)₂Y (X, Y = O, S, Se, Te) was explored by the methods of electron density of delocalized bonds (EDDB) and gauge-included magnetically induced currents (GIMIC). The existence of T-aromaticity that encloses the entire molecule, which was due to delocalization of seven β-electrons, was shown. The degree of aromaticity depends on the nature of the X(Y) heteroatom and varies in the series S > O > Se > Te.     

Water-assisted interconversions and dissociations of the acetaldehyde ion and its isomers. An experimental and theoretical study

The gas-phase reactions of the ion [CH(3)CHO/H(2)O](+*) have been investigated by mass spectrometry. The metastable ion (MI) mass spectrum reveals that this ion-molecule complex decomposes spontaneously by the losses of H(2)O, CO, and (*)CH(3). The structures of stable complexes and transition states involved in the potential energy surface (PES) have been studied by the G3//B3-LYP/6-31+G(d) computational method. Hydrogen-bridged water complexes have been found to be the major products of the losses of CO and (*)CH(3). The CO loss produces the [(*)CH(3)...H(3)O(+)] ion and involves a "backside displacement" mechanism. The products corresponding to (*)CH(3) loss have been assigned by theory to be [OC...H(3)O(+)] and [CO...H(3)O(+)], and their 298 K enthalpy values, calculated at the G3 level of theory, are Delta(f)H[OC...H(3)O(+)] = 420 kJ/mol and Delta(f)H[CO...H(3)O(+)] = 448 kJ/mol. The PES describing the interconversions among water-solvated CH(3)CHO(+*), CH(3)COH(+*), and CH(2)CHOH(+*) have been shown to involve proton-transport catalysis (PTC), catalyzed 1,2 H-transfer, and an uncatalyzed H-atom transfer mechanism, respectively.     

EPR study of ceria–silica and ceria–alumina catalysts: Localization of superoxide radical anions

Oxygen adsorption experiments were performed on evacuated and prereduced CeO₂/SiO₂ and CeO₂/Al₂O₃ catalysts with and without platinum. Considerable amounts of the superoxide radical ions were stabilized on all the samples. Signal parameters suggest Ce⁴⁺–O₂⁻ positioning for all detectable superoxide species. Physisorbed oxygen broadens O₂⁻ signal beyond detection for all the alumina-based samples, while the same procedure for all the silica-based samples did not change signal shape of O₂⁻ species. Detectable O₂⁻ species are localized in the bulk of ceria and the nature of support (silica or alumina) determines the number of oxygen vacancies and the rate of electron transfer. XRD data suggest that for alumina-based samples small and/or thin islands of ceria dominate, while comparatively large ceria particles are stabilized on the surface of silica-based samples with the same ceria content. Average size of ceria crystallites is still not determining factor and cannot account for the observed differences. Higher concentrations of paramagnetic species may be stabilized on alumina-based samples and thus, sensor-like behavior towards gaseous oxygen at room temperature was detected for them—oxygen admission reversibly changes superoxide lineshape. For silica samples, only minor changes of O₂⁻ lineshapes were typical upon the change of the partial pressure of oxygen at ambient and low temperatures. Addition of platinum has little effect on parameters of the O₂⁻ signal, except an enhancement of the superoxide decay in the reducing media. Possible site for O₂⁻ stabilization inside the lattice of CeO₂ was proposed and relevance of the observed effects to the redox catalysis discussed.     

A theoretical study of structures and electron affinities of radical anions of guanine-cytosine, adenine-thymine, and hypoxanthine-cytosine base pairs

Adiabatic electron affinities (AEA) and structural perturbations due to addition of an excess electron to each of the neutral guanine-cytosine (G-C), adenine-thymine (A-T), and hypoxanthine-cytosine (HX-C) base pairs were studied using the self-consistent charge, density functional tight-binding (SCC-DFTB-D) method, augmented by the empirical London dispersion energy term. Performance of the SCC-DFTB-D method was examined by comparing the calculated results using it with those obtained from experiment as well as ab initio and other different density functional theoretical studies. An excellent agreement between the SCC-DFTB-D results and those obtained by the other calculations regarding the structural modifications, hydrogen bonding, and dissociation energies of the neutral and radical anion base pairs was found. It is shown that adiabatic electron affinity can be better predicted by considering reaction enthalpies of formation of the respective neutral and anionic base pairs from their respective molecular components instead of taking the difference between their total energies. The calculated AEAs of the base pairs were compared with those obtained by the bracketing method from Schaefer and coworkers, where a satisfactory agreement was found. It shows applicability of the SCC-DFTB-D method to study charged DNA models at a highly economical computational cost.