Local Effective Charge Research Articles

Magnetoelastic coupling in bulk and nanoscale MnO

Phonons are exquisitely sensitive to finite length scale effects because they are intimately connected to charge, structure, and magnetism, and a quantitative analysis of their behavior can reveal microscopic aspects of spin-lattice interaction. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of nanoscale MnO and compared the results to those of a single crystal. A charge and bonding analysis reveals that Born effective charge, local effective charge, total polarizability, and the force constant are overall lower in the nanoparticles compared to the bulk. Several of these quantities split through the N\'eel transition due to magnetoelastic interactions. We find that the spin-lattice coupling drops from $~$7 cm${}^{\ensuremath{-}1}$ in the single crystal to $l$1 cm${}^{\ensuremath{-}1}$ in the nanoparticles. These results are important for understanding finite length scale effects in simple binary oxides and the more complicated functional oxides that emanate from this parent compound.

Stopped-flow kinetic method for the fluorimetric determination of DNA traces in biological samples based on the interaction long-wavelength fluorophor, surfactant and nucleic acid

A simple and rapid method for the determination of DNA, involving the interaction between a surfactant, a long-wavelength fluorophor (LWF) and the nucleic acid, is presented. Different chemical systems based on the local effective charge of the surfactant/LWF system with DNA were tested, choosing cetyltrimethyl ammonium bromide (CTAB) and indocyanine green (ICG) for the development of the method. The fluorescence of ICG increases in the presence of CTAB, but it rapidly decreases in the presence of deoxyribonucleic acid. The initial reaction-rate ( v 0 ) and signal at a prefixed-time (Δ IF 20) are monitored at 780 and 802 nm as excitation and emission wavelengths, respectively, using stopped-flow mixing technique, which makes the method applicable to automate routine analysis. Each measurement was obtained in about 30 s, being the integration time 0.1 s. The dynamic range of the calibration graph was 10–1500 ng mL −1, with a detection limit of 5 ng mL −1. The precision of the method, expressed as relative standard deviation, ranged between 2.1% and 4.5%. After a sample treatment consisting on a conventional extraction, the method was applied to the determination of DNA in several samples from different biological materials.

Stability constant determination and characterizationn of the complexation of trivalent lanthanides with polyoxometalates

Polyoxometalates (POMs), water-soluble metal oxides, can be considered small fragments of oxide lattices similar to the principle components of clay surfaces. Consequently, POMs may be used to model the thermodynamic aspects of clay–metal interactions to develop an understanding of the importance of clay colloids in the migration of metal cations in an environmental setting. The stability constants for the complexation of lanthanide(III) cations with P 2W 18O 62 6− and the lacunary α 2-P 2W 17O 61 10− polyoxometalates have been measured using laser-induced fluorescence techniques at 25 °C and an ionic strength of 0.10 M (NaClO 4). The values of log β 1 correlate well with the ligand p K a values, reflecting the strong ionic nature of the metal–ligand interaction. The stability constants were used to estimate the localized effective charges sensed by the metal ions. The luminescent lifetimes of the 1:1 and 1:2 complexes were measured to evaluate the residual hydration of the metal ions in these complexes.

Interatomic force constants and localized effective charges in copper halides

The interatomic force constants and effective charges are important lattice dynamical quantities. The magnitude of these quantities is expected to be intimately related with the fast ionic conduction in solids. In order to gain knowledge about the fundamental properties of superionic materials, the pressure dependence of the bond stretching force constant, bond bending force constant and localized effective charge in copper halides is studied theoretically. It is shown that the bond stretching force constant increases with pressure, whereas the bond bending force constant decreases. The localized effective charge in CuI, CuBr and CuCl increases, remains constant and decreases with pressure, respectively. The implication of these results on the superionic properties is discussed and the possibility of ionic conductivity enhancement with pressure in CuI is pointed out.

Interatomic force constants and localized effective charges in sphalerite‐structure compounds

AbstractThe Keating model with one bond‐stretching and one bond‐bending force constant and a localized effective charge to account for long‐range Coulomb forces and dipole‐dipole interactions is used to analyse the lattice vibrational properties of sphalerite‐structure binary compounds. It is shown that the trends in the model parameters can be readily explained in terms of the bond length and the bond ionicity.

The influence of charge on reactions of metalloproteins with small molecules

A procedure for estimating the local effective charge on a protein surface from association constants for surface reactions of metalloproteins with inorganic complexes is outlined; results obtained are consistent with structural information.

Infrared Spectrum and Structure of Hydrogenated Amorphous Silicon

AbstractThe ir absorption spectrum of hydrogenated amorphous silicon is discussed in the context of structural models for this material. It is shown that the integrated strength of the bond stretching bands in hydrogenated amorphous silicon cannot be used to determine the hydrogen concentration because the local effective charge for ir absorption is a function of hydrogen concentration and sample preparation. The effective charge for the wagging—rocking—rolling vibrations at 640 cm−1, however, is independent of hydrogen concentration and sample preparation. Hence the integrated strength of this band can be used to measure the hydrogen concentration provided the proportionality constant is determined empirically. Changes in the ir absorption bands as a function of isochronal annealing temperature show that two different types of bonds contribute to the bond stretching band at 2100 cm−1. The bond bending bands at 890 and 840 cm−1 are associated with one of the bonds producing the 2100 cm−1 stretching band. The relative strengths of the 890 and 840 cm−1 bands to the 2100 cm−1 band depend strongly on the film preparation method: the glow discharge produced films show much stronger bending bands than those produced by rf sputtering. A structural model for hydrogenated amorphous silicon is presented in which a Maxwell‐Garnett‐type local field correction, which is dependent on the dipole location within a microvoid and the shape of the microvoid, is used to explain the data.

Optical Phonon Modes and Localized Effective Charges of Transition-Metal Dichalcogenides

Reflection spectra have been measured in the far infrared region (10 cm -1 –500 cm -1 ) for several kinds of crystals of transition-metal dichalcogenides (TX 2 ). Optical phonon frequencies of infrared active mode ( E ⊥ c ) are determined for the group IV compounds HfSe 2 , ZrSe 2 and TiSe 2 and for the group VI compounds MoS 2 , MoSe 2 , WS 2 and WSe 2 . Using available Raman data the localized effective charges ( e l * ) of these materials are estimated. The nature of the bonding in TX 2 is found to be well described in terms of the estimated e l * rather than by the macroscopic effective charge e T * .