Charge Transfer ΔQ Research Articles

Predicted rebound effects of insulator-metal transition temperature in tungsten doped VO2 via first-principles method

A first-principles study of insulator-metal transitions (IMTs) in the W-doped monoclinic phase and the rutile phase of VO2 has been carried out. The doping concentration was achieved by substituting atoms of vanadium by tungsten in various supercells of monoclinic and rutile phased VO2. We found a fascinating rebound behavior of transition temperature, which exhibited an unobstructed relation to crystal and electronic structures. The results showed that by increasing the doping concentration of W into VO2, the IMT tuned to lower temperature at the beginning and then anomalously shifted to a higher temperature. This rebound effect was confirmed by the calculated V-atomic shift dV, V-atomic charge transfer Δq, and density of states. We attributed this effect to the consequences of “off-chain” and “in-chain” substitutive doping of W atoms. This alteration affects the capability of the VO2 crystal to tolerate the perturbations caused by dopants, and, as an upshot, the IMT properties of VO2 depressed initially and recovered later. This unique behavior might have great applications for smart windows, sensors, and other switching devices.

The effect of anion-π interactions on the properties of pyrazinamide and some related compounds

The effects of anion-π interaction on the protonation tendency and amide functional group properties of pyrazinamide (PZA) and related compounds have been investigated by the quantum mechanical methods. The pyrazine ring were replaced by pyrimidine and triazine rings in order to investigate the effect of second and third nitrogen heteroatoms and their positions on the amide functional group. The increase in the number of nitrogen heteroatoms of the ring rises the anion-π interaction energy. Thermodynamic properties calculated in the water solvent by the SCRF continuum solvation method using the IEF-PCM model, confirm the decrease in the proton affinity and basicity with solvation, while the anion-π interactions increase those in both gas phase and solvation media. The effect of anion type, via the anion-π interaction, on the basicity is approximately negligible. Well-defined relations are established between the anion-π interaction energies ΔE and the valence natural orbital energies ∑NAO of protonated site, charge transfer between two units and the minimum value of electrostatic potential Vmin around the oxygen atom. In addition, linear relationships are observed between the ΔE value and the distance between anion and the center of heterocyclic ring (ranion···π), the topological properties of electron charge density ρ and the 1H NMR absolute shielding constant (δ1 H) of proton. In agreement with the ΔE changes, the effects of second and third heteroatoms are additive on the charge transfer Δq, |∑NAO| and proton affinity.

Ab initioand DFT studies on the structures, binding energies and nature of bonds in X2Y3metal clusters (X+=Li+,Na+andK+; ${\rm Y}_{3}^{2-} = {\rm Zn}_{3}^{2-}, {\rm Cd}_{3}^{2-}$ and ${\rm Hg}_{3}^{2-}$)

A comprehensive theoretical study on the structure and stability of linear and triangular isomers of anionic clusters of zinc, cadmium and mercury [Formula: see text] and their binding with one and two alkali metal cations ( X+= Li+, Na+, K+) has been investigated at the density functional (BP86 and B3LYP) and ab initio (MP2, MP4and coupled cluster single and double (CCSD)) methods. The results showed that in all cases, the triangular geometry with D3hsymmetry is more stable than linear one. The calculated values of interaction energies (IE) between [Formula: see text] anions and two X+cations, Wiberg bond indices (WBI) and the electron densities at bond critical points (BCP), ρ(BCP), for Y–X bonds show that among all complexes investigated here at all levels of theory Zn3Li2and Hg3K2have the largest and the smallest values of IE, WBI and ρ(BCP), respectively. The natural charges of the atoms and WBI involved in the bonding as well as the global value of the charge transfer ΔQ from [Formula: see text] to X+cation in X2Y3clusters, evaluated through natural population analysis, confirmed that covalent contribution in Y–X bond formation increases from K+to Li+. Also the energy decomposition analyses (EDA) were used to detect the nature of interaction in the complexes. The results confirmed that the contribution of electrostatic interactions in present complexes is almost more than 70%.

Substituent effects in donor-acceptor complexes with D→A coordination bonds (D = N, O, S; A = B, Al, Ga, Sn, Sb) and related systems

The effect of substituent on the enthalpy ΔH0 and free energy ΔG0 of complexation, on the dipole moments of complexes μC and coordination bonds μDA, and on the degree of charge transfer Δq was analyzed for 20 series of complexes with D→A coordination bonds (D = N, O, S; A = B, Al, Ga, Sn, Sb), hydrogen bonds, and charge transfer. It was found that ΔH0, ΔG0, μC, μDA, and Δq depend not only on the inductive and resonance effects, but also on the polarization effect of substituents; its contribution varies in a wide range and can exceed 50%.

Hydrogen Chemisorption on Supported Semiconductor Films

AbstractHydrogen chemisorption on metal supported semiconductor ZnO films is studied by using the Green function method and complex‐energy‐plane integration approach. The Anderson‐Newns model is used to calculate the chemisorption energy ΔE and the adatom charge transfer Δq. The variations of the adatom state level Ead and the corresponding localized adatom density of states (LDOS) with increasing ZnO film thickness are investigated. It is found that the hydrogen chemisorption energy and the adatom charge transfer depend on both the thickness of the ZnO film and the species of the metal support. The calculations show that ten layers of ZnO film can mirror the chemisorption properties of bulk ZnO. The adatom state level Ead which corresponds to a peak in Ldos, is found shifting upwards to a constant value as the ZnO layer number N increases from 1 to 10.

Mechanistic aspects of chain initiation in radical addition reactions induced by metal ions

Bersuker's concept of redox charge transfer in catalysis has been adopted to study initiating activity of metal ions in radical addition reactions of model organic addends with alkenes. It was found that the most important factor determining the activity of the metal ions was the magnitude of the redox charge transfer δq. Additionally, the spin-allowedness or spin-forbiddenness of the redox step was essential. Two-valent metal ions with configuration d5-d9 do not exhibit, regardless of the spin-allowedness of the redox step, a significant oxidation activity owing to low values of δq. Three-valent metal ions with configurations d1-d3 are also inactive because of relatively lowest values of δq and spin-forbiddenness of the redox step. A transient position is occupied by the Mn3+ (d4) ion for which the redox step is still spin-forbidden, but the value of δq is high enough to evoke satisfactory oxidation activity. Three-valent ions Fe3+, Ni3+ with configurations d5-d7 as well as four-valent ions Pb4+ and Mn4+ show, according to the magnitude of δq, the highest oxidation activity in chain initiation.

Donor–acceptor interactions in biomolecules. II. Spectroscopic and molecular orbital studies of amino acid – nicotinamide adenine dinucleotide complexes

The nature of complex formation between aromatic amino acids and some of their components with NAD+ (nicotinamide adenine dinucleotide) have been studied by spectrophotometric methods. A comparison of the stability of these complexes with that of the complexes of the same donors with a model compound of NAD+i.e. N-methyl-3-carboxamide pyridinium chloride (MCP) has been carried out by examining the type of complexes formed and their geometry. Simple molecular orbital calculations have been done to determine the quantity of charge transfer δQ and stabilization energy δE and to show the significance of mutual orientation of donor and acceptor in determining the stability. The enhanced stability of tryptophan–NAD+ complex in comparison to indole–NAD+ complex has also been explained.

Loss of charge and mass from raindrops falling in intense electric fields

Studies have been made of the relationship between the charge transfer ΔQ and the mass loss ΔM that accompany the disintegration of water drops of radius R, mass M, and charge Q falling through an intense electric field. Calculations show that, if the resultant drop, of radius r, is on the verge of disintegration, the ratio of the charge transfer to the fractional mass loss will be given by the equation MΔQ/ΔM = [4.6(R3T)1/2(γ2 − γ3/2) + (1 − γ2)Q]/(1 − γ3), where γ equals r/R and T is the surface tension. Studies in which ΔQ and ΔM were measured directly showed that the experimental values of MΔQ/ΔM were only slightly greater than the predicted minimum values. This finding suggests that raindrops falling through the strongly electrified lower regions of thunderclouds may undergo a series of disintegrations during which appreciable loss of mass and charge may occur. This conclusion is supported by photographic evidence.