Covalent Anion Research Articles

Production of Nitrogen Dioxide, NO2-, Anion from Dissociative Electron Attachment to Nitromethane below 1 eV and Its Temperature Dependence: Direct vs Dipole Bound Mediated Processes.

Reaction induced by slow electrons is implicated in a large field of research and applications. Below 3-4 eV, dissociative electron attachment efficiently fragments molecules via (1) shape resonance or (2) mediated by the formation of a dipole bound anion. While the temperature dependence of process 1 is well-known, that of 2 is not clearly established. Nitromethane is the prototypical molecule for which the electron attachment leads to the formation of both a dipole bound and a covalent anion. We provide here a comprehensive study of the fragmentation of nitromethane by <1 eV electron and the unusual temperature effects attributed principally to process 2.

Substituent Effect on the Circularly Polarized Luminescence of C1‐Symmetric Carbene‐Copper(I) Complexes

AbstractThe substituent effect on the magnitude of the circularly polarized luminescence (CPL) of MentCAAC‐Cu‐X (X=F, Cl, Br, I, BH4, B3H8; CAAC=cyclic (alkyl)(amino)carbenes) complexes is experimentally investigated. This study examines seven pairs of enantiomeric complexes with small anionic substituents (halides, borohydrides, hydride). The complexes are fully characterized, including single crystal X‐ray diffraction studies, and chiroptical measurements show that small covalent anions induce a larger CPL magnitude. These results demonstrate that the magnitude of the CPL can be manipulated without making any modifications to the chiral ligand.

Rich structural topology of the anion formed by the complex of acetonitrile with two water molecules

Ab-initio quantum-mechanical calculations are performed to determine structures and energetic of anions formed by the trimer of an acetonitrile molecule and two water molecules. In total, eight anion configurations corresponding to separate local minima of the anion potential energy surface are identified. All eight anions have positive vertical electron detachment energies. The identified anions belong to the following three categories: dipole-bound anions, anions where the excess electron is suspended between two fragments of the trimer, and solvated covalent anions.

Cation only conduction in new polymer–SiO2 nanohybrids: Na+ electrolytes

Hybrid nanoparticles are functionalized with delocalized covalent anions to form poly-salts for novel electrolytes for use in Li-ion or Na-ion batteries. These redox- and hydrolysis- stable electrolytes can avoid concentration gradients and still maintain the mechanical advantage of nanofillers with conductivities of around 10−5 S cm−1 at 25 °C.

The uracil dimer and trimer covalent anions: An ab initio study

In this work, we present new structures for both the uracil dimer anion ( U 2 - ) and trimer anion ( U 3 - ) calculated at the MP2/6-31++G** level of theory. In these systems, the excess electron is localized at only one uracil unit in a π-state and that molecule shows some off-plane distortion leading to higher anion stabilization. In both structures excess electron attachment leads to the formation of a stable covalent anion, with electron affinity values of 0.25, 0.05 eV for U 2 and U 3, respectively. Experimental implications in relation to the calculated data are also discussed.

Electron localization in negatively charged formamide clusters studied by photodetachment spectroscopy

Size-dependent features of the electron localization in negatively charged formamide clusters (FAn-, n = 5-21) have been studied by photodetachment spectroscopy. In the photoelectron spectra for all the sizes studied, two types of bands due to different isomers of anions were found. The low binding energy band peaking around 1 eV is assigned to the solvated electron state by relative photodetachment cross-section measurements in the near-infrared region. It is suggested that nascent electron trapping is dominated by formation of the solvated electron. The higher energy band originates from the covalent anion state generated after a significant relaxation process, which exhibits a rapid increase of electron binding energy as a function of the cluster size. A unique behavior showing a remarkable band intensity of the higher energy band was found only for n = 9.

Stabilization of the adenine covalent anion by micro-hydration: theoretical study

The Rydberg electron–transfer spectroscopy (RET) experiment [V. Periquet, A. Moreau, S. Carles, J.P. Schermann, C. Desfrançois, J. Electron. Spectrosc. Relat. Phenom., 106, 141 (2000)] showed that only after solvation by two water molecules the adenine molecule can form a stable covalent anion. Anions of adenine–water complexes containing different numbers of water molecules (A·W n , n =1–4) are investigated in this work with the use of quantum mechanical ab initio calculations. The calculations are used to determine the change of stability of the adenine covalent anion as the number of water molecules in the solvation shell increases. The analysis of the anion electron density is used to describe the localization of the excess electron in the A·W n complexes.

Electronic structure of FeF2 and FeF3 studied by x-ray absorption and fluorescence spectroscopy

High resolution investigations of polycrystalline FeF2 and FeF3 were performed by means of x-ray absorption and resonant x-ray fluorescence spectroscopy. The obtained F Kα fluorescence spectra of ionic FeF2 and FeF3 were analysed in comparison with the F Kα spectrum of the covalent anion TiF62- in solid K2TiF6. The FKα spectra of compounds under study consist of the main band accompanied by a low-energy shoulder. The latter can be associated with the valence subband of the Fe 3d – F 2p hybridized electron states. Strong high-energy shake-up satellites were observed in the F Kα spectra of these compounds when the excitation energy exceeds the F 1s ionisation threshold (∼690 eV). Information about local partial densities of states for FeF2 was obtained from a comparative analysis of Fe L and F K x-ray fluorescence and valence-band photoemission spectra.

An excess electron connects uracil to glycine

In recent work Gutowski et al. [Eur. Phys. J. D 20, 431 (2002)] reported photoelectron-spectroscopy and theoretical study of covalent anion of the uracil-glycine complex. In present work we use ab initio calculations to describe an anionic complex of uracil and glycine where the excess electron is localized in a diffuse state between the two monomers. In this system the uracil and glycine molecules are separated by about 4.5 A and the dipoles of the two monomers point at the excess electron located in the middle of the complex. The calculated fragmentation energy of the anion into a dipole-bound uracil anion and a neutral glycine molecule is 1.7 kcal/mol.

Cluster size effects upon stability of adenine–methanolanions. Theoretical study

Rydberg electron transfer spectroscopy (RET) experiments [J. Electr. Spectr. Relat. Phenom. 106 (2000) 141] showed that only after being solvated with three methanol molecules the adenine molecule can form a stable covalent anion. Anions of adenine–methanol complexes (A·M n , n=1–3) are investigated in the present work with the use of quantum mechanical calculations. These complexes are good models to study how solvation affects the stability on the adenine anion. It is shown that among several hydrogen-bonded configurations of the A·M n , n=1−3, complexes there are systems which can form stable dipole-bound (DB) anions. These systems, however, are not the lowest energy structures. The stability of the covalent anion of A·M 3 is also investigated.

Anions of the hydrogen-bonded thymine dimer: ab initio study

Theoretical calculations have been performed to determine the ability of the hydrogen-bonded thymine dimer to form stable anions. The major conclusions of this work are: (i) three of the hydrogen-bonded conformers of the thymine dimer can form stable dipole-bound anions with excess electrons; (ii) thymine dimer can form a covalent anion that has a structure dissimilar from the structures of the neutral dimer; (iii) in the covalent thymine-dimer anion the excess electron is localized in a π-orbital on one of the thymine molecules and this molecule shows an out-of-plane distortion; (iv) the covalent thymine-dimer anion is stable with respect to an adiabatic electron detachment.

Cytosine anions: ab initio study

Theoretical ab initio calculations have been performed to determine the stability of covalent and dipole-bound anions of cytosine. The work is related to the recent anion spectroscopy experiments performed on some nucleic acid bases including cytosine by Schiedt et al. [Chem. Phys. 239 (1998) 511], where two anion states attributed to dipole-bound anions of the amino–hydroxy (A–H) and amino–oxo (A–O) tautomers were detected. The present calculations reveal that (i) the A–O isomer and two rotamers of the A–H isomer have sufficient dipole moments to bind σ excess electrons in stationary dipole-bound states. The calculated adiabatic electron affinities are 58, 22, and 6 meV for the A–O cytosine and the two rotamers of the A–H cytosine, respectively. These values are considerably smaller than the two experimentally determined values of 85±8 and 230±8 meV; (ii) our calculations also describe covalent anions of the A–O and A–H tautomers. In both systems, the six-member ring is noticeably distorted from planarity. According to the calculations only, the A–O covalent anion is vertically stable with respect to the electron attachment, and the corresponding vertical electron detachment energy is 102 meV. However, both A–O and A–H cytosine anions are predicted to be unstable with respect to the adiabatic electron detachment. This finding is consistent with the lack of a covalent anion feature in the experimental spectrum.

Isomerism of the covalent anion of the dimer of uracil and 1-methyl-cytosine: ab initio study

Theoretical ab initio calculations have been performed to determine the distribution of an excess electron in the covalent anion of the dimer of uracil and 1-methyl-cytosine (U–MC). Due to the multitude of possible isomers, this system may present an interesting model for future experimental investigations of the electron attachment and transfer in dimers of nucleic acid bases. The major conclusions of this work are (i) three isomeric structures of the covalent U–MC anion have been found in the calculations. The anion where the excess electron is located at the MC molecule was found to be the most stable; (ii) a noticeable ring distortion was found in the molecule where the excess electron is localized; (iii) the covalent U–MC anions are non-planar, hydrogen-bonded systems; and (iv) the out-of-plane deformed site of the base molecule where the excess electron localizes in the dimer anion is positioned opposite to the site involved in the hydrogen bond.

Covalent Anion of the Canonical Adenine−Thymine Base Pair. Ab Initio Study

Theoretical ab initio calculations have been performed to determine the stability of the covalent anions of adenine and the adenine−thymine (AT) Watson−Crick base pair. The conclusions of this work are as follows: (i) the covalent anion of adenine is predicted to be a stable system with respect to a vertical electron detachment, but unstable with respect to adiabatic detachment; (ii) the covalent anion of the adenine−thymine dimer (A−T-) has similar properties, although in this system the presence of the second base provides an additional stabilization to the excess electron; (iii) in A−T- the excess electron is localized at the thymine molecule, and this molecule's ring is puckered; (iv) no valence A−T anion was found in the calculations with the excess electron located at the adenine molecule; and (v) in view of the above results, we predict that the AT base pair is not an effective trap of excess electrons.

Anions of the Hydrogen-Bonded Uracil Dimer. Ab Initio Theoretical Study

Theoretical ab initio calculations have been performed to determine the ability of the hydrogen-bonded uracil dimer to form stable anions. The major conclusions of this work are (i) three of the hydrogen-bonded conformers of the uracil dimer can form stable dipole-bound anions with excess electrons; (ii) uracil dimer can form a covalent anion that has a structure dissimilar from the structures of the neutral dimer; (iii) in the covalent uracil dimer anion the excess electron is localized on one of the uracil molecules and this molecule shows an out-of-plane distortion; (iv) the covalent uracil dimer anion is stable with respect to a vertical electron detachment, but at the level of theory (MP2) used in the calculations the anion is marginally unstable with respect to an adiabatic electron detachment.

Isomerism of the Covalent Anion of the Uracil−Thymine Dimer. Ab Initio Study

Theoretical ab initio calculations have been performed to determine the ability of the uracil−thymine (U·T) dimer to form stable covalent anions. The are two major conclusions of this work (i) Two isomeric structures of the covalent U·T dimer have been found in the calculations. The anion, where the excess electron is located at the uracil molecule, is 1.4 kcal/mol more stable than the form where the excess electron is located at the thymine molecule. (ii) The ring of the molecule where the excess electron is localized in noticeably nonplanar.

Computer Modelling of Phosphate Biominerals: Parameterisation for Perfect Lattice Calculations

Abstract We are extending the use of the method of static simulation of ionic lattices to modelling phosphates and condensed phosphates, the mineral components of many biominerals whose structure, crystallinity and reactivity can be influenced by substitutions and other defects. Parameters for the interatomic potentials of some phosphates and pyrophosphates have been determined and we present our results for the modelling of α-magnesium pyrophosphate, a representative compound of moderate complexity. The approach has been to treat the compound as an ionic solid with Mg2+ as the cation and P2O7 4 as a predominately covalent anion which is, however, ionically bound to the metal. Good agreement was found between the calculated and experimental structures for this ionic model using formal or residual charges, a Born-Mayer term for non-bonded repulsive interactions, whose parameters were calculated using the electron gas technique, a van der Waals dispersion term and three body terms to account for the directionality of bonding in the phosphate groups in the tetrahedra.