Molecular Constant Research Articles

Specific heat of rare earth cobaltates RCoO 3 (R = La, Pr and Nd)

We have reported the temperature dependence (5 K ≤ T ≤ 1000 K) of the lattice contribution to the specific heat of rhombohedral LaCoO 3 and orthocobaltates RCoO 3 (R = Pr and Nd). The strong electron phonon interactions are present in these compounds and lattice distortions can affect them substantially. Thus Rigid Ion Model (RIM) is used for the first time to study the cohesive and thermal properties of the cobaltates RCoO 3 with rare earth cation (R = La, Pr and Nd). The values of specific heat calculated by us have shown remarkably good agreement with corresponding experimental data. In addition, the results on the temperature dependence of cohesive energy ( ϕ), molecular force constant ( f), Reststrahlen frequency ( υ), Debye temperature ( θ D ) and Gruneisen parameter ( γ) are also reported.

Elastic and thermodynamic properties of AVO 3 (A=Sr, Pb) perovskites

We have investigated the elastic and thermodynamic properties for the perovskite type metavanadate SrVO 3 and the multiferroic PbVO 3, probably for the first time by the means of a Modified Rigid Ion Model (MRIM). We present the elastic constants ( C 11, C 12, C 44) and other elastic properties like Bulk modulus ( B), Young′s modulus ( E), shear modulus ( G), Poisson′s ratio ( σ) and wave velocity ( υ l, υ s, υ m). Besides we have reported the thermodynamic properties molecular force constant ( f), Reststrahlen frequency ( ν), cohesive energy ( ϕ), Debye temperature ( θ D) and Gruneisen parameter ( γ). We have also computed the variation of heat capacity ( C P) and there by volume thermal expansion coefficient ( α) in a wide temperature range. We found that the computed properties reproduce well with the available data in literature. To our knowledge some of the properties are reported for the first time.

Transient diffraction grating measurements of molecular diffusion in the undergraduate laboratory

Diffusion is a central process in many biological, chemical, and physical systems. We describe an experiment that employs the interference of laser beams to allow the measurement of molecular diffusion on submillimeter length scales. The interference fringes of two intersecting pump beams within a dye solution create a sinusoidal distribution of long-lived molecular excited states. A third probe beam is incident at a wavelength at which the indices of refraction of the ground and excited states are different, so the probe beam diffracts from the spatially periodic excited-state pattern. After the pump beams are switched off, the excited-state periodicity washes out as the system diffuses back to equilibrium. The molecular diffusion constant is obtained from the rate constant of the exponential decay of the diffracted beam. It is also possible to measure the excited-state lifetime.

Entanglement of polar molecules in pendular states

In proposals for quantum computers using arrays of trapped ultracold polar molecules as qubits, a strong external field with appreciable gradient is imposed in order to prevent quenching of the dipole moments by rotation and to distinguish among the qubit sites. That field induces the molecular dipoles to undergo pendular oscillations, which markedly affect the qubit states and the dipole-dipole interaction. We evaluate entanglement of the pendular qubit states for two linear dipoles, characterized by pairwise concurrence, as a function of the molecular dipole moment and rotational constant, strengths of the external field and the dipole-dipole coupling, and ambient temperature. We also evaluate a key frequency shift, △ω, produced by the dipole-dipole interaction. Under conditions envisioned for the proposed quantum computers, both the concurrence and △ω become very small for the ground eigenstate. In principle, such weak entanglement can be sufficient for operation of logic gates, provided the resolution is high enough to detect the △ω shift unambiguously. In practice, however, for many candidate polar molecules it appears a challenging task to attain adequate resolution. Simple approximate formulas fitted to our numerical results are provided from which the concurrence and △ω shift can be obtained in terms of unitless reduced variables.

Determination of molecular hyperfine-structure constant using the second-order relativistic many-body perturbation theory

The spin-rotational Hamiltonian parameters ${A}_{\ensuremath{\parallel}}$ and ${A}_{\ensuremath{\perp}}$ for the BaF molecule are calculated using four-component relativistic spinors at the second-order many-body perturbation theory (MBPT) level via the Z-vector technique. The second-order MBPT is applied to assess the accuracy of the computed hyperfine-structure constants before studying the problem with the state-of-the-artcoupled cluster with single and double excitations (CCSD) method which is highly accurate but computationally more expensive than MBPT. The hyperfine-structure constants $A$ and ${A}_{d}$ resulted from these calculations agree favorably well with experimental findings and with other correlated calculations. The convergence behavior of $A$ and ${A}_{d}$ with respect to the number of active orbitals used in the perturbative calculations suggests that our estimated $A$ and ${A}_{d}$ values should be accurate.

A rigidity-enhanced antimicrobial activity: a case for linear cationic α-helical peptide HP(2-20) and its four analogues.

Linear cationic α-helical antimicrobial peptides are referred to as one of the most likely substitutes for common antibiotics, due to their relatively simple structures (≤40 residues) and various antimicrobial activities against a wide range of pathogens. Of those, HP(2–20) was isolated from Helicobacter pylori ribosomal protein. To reveal a mechanical determinant that may mediate the antimicrobial activities, we examined the mechanical properties and structural stabilities of HP(2–20) and its four analogues of same chain length by steered molecular dynamics simulation. The results indicated the following: the resistance of H-bonds to the tensile extension mediated the early extensive stage; with the loss of H-bonds, the tensile force was dispensed to prompt the conformational phase transition; and Young's moduli (N/m2) of the peptides were about 4∼8×109. These mechanical features were sensitive to the variation of the residue compositions. Furthermore, we found that the antimicrobial activity is rigidity-enhanced, that is, a harder peptide has stronger antimicrobial activity. It suggests that the molecular spring constant may be used to seek a new structure-activity relationship for different α-helical peptide groups. This exciting result was reasonably explained by a possible mechanical mechanism that regulates both the membrane pore formation and the peptide insertion.

Electromechanical Properties of Ethylene Propylene Diene Elastomers: Effect of Ethylene Norbornene Content

Ethylene propylene diene elastomers (EPDM) of various side chains and molecular weights were prepared as thin discs and the effects of electric field strength and temperature on the electromechanical properties were investigated. The electrical conductivity, the dielectric constant, the storage and loss moduli (G and G), the storage modulus response ( 1000 Vm m G  ), and the storage modulus sensitivity ( 1000 Vm m G  / 0 G ) of the elastomers of different ethylene norbornene (ENB) contents and molecular weights were measured under electric field strengths varying from 0 V/mm to 1000 V/mm and at temperatures between 300 K and 380 K. The storage modulus response and sensitivity increase with increasing molecular weight and dielectric constant, consistent with the existing theory. However, for the case of EPDMs with different ENB contents, the storage modulus response and sensitivity vary inversely with the dielectric constant. EDPM is potentially a new type of electroactive materials.

Investigations of Thermal Properties of Earth Mantle Silicates: ASiO3 (A = Mg, Ca)

We have investigated the thermal properties and specific heat of earth mantle silicates ASiO3 (A = Mg, Ca) by the means of rigid ion model (RIM) and compared them with the earlier experimental reports. The calculated specific heat of ASiO3 (A = Mg, Ca) obeys T3 law at low temperature (T < θD/50). The temperature dependence of the specific heat of MgSiO3 and CaSiO3 (0 K ≤ T ≤ 2000 K) is presented along with the elastic, cohesive and thermal properties, like Bulk modulus (B), cohesive energy (φ), molecular force constant (f), reststrahlen frequency (ν), Debye temperature (θD) and Gruneisen parameter (γ) at T = 300 K.

Acoustical study of molecular interactions in polymer solutions through various thermodynamical parameters and Flory's theory at 298.15 K

This study intends to portray the nature of polymer solutions by ultrasonic velocity and density data. Acoustical parameters, such as relaxation strength, relative association, molecular constant, fractional free volume, available volume, Beyer's non-linearity parameter, internal pressure, van der Waals’ constants and molecular radius have been computed for the binary mixtures of PEG400 + methanol, PEG400 + ethanol, PPG400 + ethanol and PPG400 + 2-propanol systems at 298.15 K. Flory's theory has been used for computing ultrasonic velocity, surface tension, thermal expansion coefficient, thermal conductivity, excess heat capacity and excess isothermal compressibility. Isothermal compressibilities of mixtures have been theoretically evaluated using different theories. A new relation has been proposed by us for evaluating excess molar volume using ultrasonic velocity of the mixture which gives fairly good result when compared with the experimental values. Excess molar volume has been calculated using Flory's theory and Prigogine–Flory–Patterson theory. The partial molar volume and its excess value at infinite dilution have also been evaluated.

Electronic spectra and molecular constants of zirconium mononitride

The electronic spectra and structure of zirconium mononitride were considered. The electronic vibrational-rotational spectrum of ZrN was obtained by intracavity laser spectroscopy within the 550–750 nm region. The spectrum was analyzed and the molecular constants of the ground X2Σ and excited A2Π electronic states were calculated. On the basis of the obtained experimental data and critical analysis of all the information on ZrN spectrum studies, the most reliable set of molecular constant values was recommended.

Pressure-induced phase transition phenomena in NpSe and NpTe

A three-body interaction potential (TBIP) has been formulated by incorporating the effects of long-range coulomb interaction, three-body interaction and short-range repulsive interactions effective up to second neighbor ions. The three-body interactions arise from the electron-shell deformation when the nearest-neighbor overlaps. This TBIP has been employed for detailed studies of pressure-induced phase transition and high-pressure behavior of NpSe and NpTe. The model has yielded fairly good description of the cohesive energy, compressibility, molecular force constant, reststralhlen frequency and Debye temperature in case of these actinide monochalcogenides.

Elastic and thermal properties of La1−xSrxMnO3 manganites

The elastic and thermal properties of perovskite manganites La1−xSrxMnO3 (x = 0.165, 0.185, 0.20, 0.40, 0.70, 0.80, 0.90, 1.0) have been studied as a function of temperature (1K≤T≤300K) by means of Atoms in Molecules (AIM) theory and a Modified Rigid Ion Model (MRIM) respectively. The specific heat values (Cv) and the Debye temperatures obtained by us from the lattice contributions are in reasonably good agreement with the available experimental data for some concentrations (x) of La1−xSrxMnO3. In addition, the results on the Bulk Modulus (BT), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (vo) and Grüneisen parameter (γ) are also analysed.

The Metal Ions Absorption – and Transportation Properties of L-Aspartic Acid in the Body

The intra molecular interactions in complexes of Aspartic acid with divalent metal ions (Mg, Ca & Zn) M(Asp) 1 were investigated in this work. The acidity constants of Asp and stability constants of M(Asp) are determined and reported as well. The effect of intra molecular interactions can be seen by comparison of these stability constants. Mg and Ca ions form weaker bonds to Asp than Zn ion. Zn(Asp) complex undergoes an intra molecular interaction and builds a chelate in two forms ( an open - isomer M(Asp)op and a closed - isomer M(Asp)cl ). The concentration independent intra molecular interaction constant for two isomers has been calculated and reported. The obtained results demonstrate that for some M(Asp) complexes the stability constant is largely affected by the affinity of metal ions for amine group. This shows a kind of selectivity of metal ions and transfer them via complex - building with the aspartate. Also all three metal ions complexes could be considered as mineral carriers. These complexes can release the minerals in human body in certain conditions ( e,g. pH - range).

Theoretical analysis of static properties of mixed ionic crystal: NH4Cl1−x Br x

In the present paper, we have investigated the static properties of the mixed ionic crystal NH4Cl1ixBrx using three-body potential model (TBPM) by the application of Vegard's law. The results for the mixed crystal counterparts are also in fair agreement with the pseudo-experimental data generated from the application of Vegard's law. The results for the end point members (x = 0 and 1) are in good agreement with the experi- mental data. The results on compressibility, molecular force constant, infrared absorption frequencies and Debye temperature are presented probably for the flrst time for these mixed crystal counterparts.

Bulk modulus and thermodynamic properties of electron-doped calcium manganate—Ca 1−xRE xMnO 3

We have investigated the thermodynamic properties of electron-doped perovskite manganite CaMnO 3 by incorporating the effect of lattice distortions. In this paper the functional relation between the MnO 6 distortions, charge and size mismatch and the thermal properties is determined. In the insulating state, distortions of the Mn–O environment are linear with calcium concentration. In the low-temperature spin-ordered ferromagnetic/anti-ferromagnetic state, at least 50% of the distortion is removed. The lattice contributions to the specific heat at constant volume ( C v (lattice)) of Ca 1− xRE x MnO 3 ( x=0.05, 0.1, 0.15, 0.20) with rare earth cation doping at the A-site has been studied as a function of temperature (10 K≤ T≤500 K) by means of a Modified Rigid Ion Model (MRIM). In addition, the results on the bulk modulus ( B), cohesive energy ( φ), molecular force constant ( f), Reststrahlen frequency ( ν 0) and Gruneisen parameter ( γ) are also presented. Findings indicate an anomalous behavior of some highly Jahn–Teller (JT) distorted Ca 1− xRE x MnO 3.

Predictive control and verification of conversion kinetics and polymer molecular weight in semi-batch free radical homopolymer reactions

Polymer molar mass distributions critically affect macroscopic characteristics and performance of polymeric materials. While multi-detector methods coupled to size exclusion chromatography (SEC) are widely used to measure endproduct mass distributions, less progress has been made in simultaneously controlling and verifying the evolution of these distributions during synthesis. This work focuses on quantitative predictions and online verification of conversion kinetics and of molecular weight during free radical homopolymerization of acrylamide, where reagents were fed into the reactor during the reaction. The central task is to establish and experimentally test a formalism combining free radical polymerization kinetics with time dependent processes related to flows of material into and out of the reactor. Monomer feed experiments were performed that alternately hold molecular weight constant and ramp the weight up, in contrast to batch reactions, where molecular weight decreases. Three types of initiator feed ‘tapers’ were also used to produce predictable conversion kinetics and mass distributions: (i) constant initiator feed, (ii) linearly stepped feed to produce Gaussian conversion kinetics, and (iii) booster shots to produce multi-modal masses. Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) was used to follow the conversion and evolution of the average mass distribution, and multi-detector SEC was used to cross-check results and measure full distributions of endproducts. In general, there was good agreement between the predictions and results. In future work this approach can be used as an Ansatz for reaction trajectory prediction, and the online monitoring signals exploited to make feedback controlled corrections to the reagent flows and other reaction conditions.

Effect of heavy cation doping on thermal properties of LaMnO3

Effect of heavy cation doping (Ca2+ at the A-site) on the thermal properties of perovskite LaMnO3 has been investigated using the Rigid Ion Model (RIM)). As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. The specific heat of magnetoresistance compound La0.25Ca0.75MnO3 as a function of temperature (10 K ≤ T ≤ 300 K) is reported. Our results on specific heat are in good agreement with the measured values of specific heat at lower temperatures. In addition, the results on the cohesive energy (ϕ), molecular force constant (f), Restrahalen frequency (ν 0), Debye temperature (Θ D) and Gruneisen parameter (γ) are also discussed.

Calculation of the regions of solid phase precipitations in the metal ion-water-complexing agent systems

The calculation the regions of solid phase precipitations with the different compositions by use of solubility product and molecular solubility expressions has been suggested. Advantage of use of molecular solubility expression has been showed, consisting in insignificant difference of values of the molecular solubility constant for metal ions, having same oxidation level and same counter ion. A value determination task of the system parameters, which composition of the solid phases is changed on, has been decided.

Kinetic Study of Humic Acid Ozonation in Aqueous Media

AbstractA pseudo first‐order kinetic model that describes the ozonation of two sources of humic acid (HA) in aqueous media is developed and investigated in a semi‐batch system. Further investigation shows that the degradation rates of the two HAs are highly dependent on the initial pH, temperature of the ozonation, the source of HA, and the overall kinetic rate constant, which is mainly composed of two parts, i. e., direct oxidation by molecular ozone and indirect oxidation by hydroxyl radicals (•OH). The contributions of the two pathways to HA ozonation can be traced using tert‐butanol as the radical scavenger at different pH values (3, 7, and 11), temperatures (283–303 K), and the results show that •OH oxidation is more sensitive to the operating variables than molecular ozone oxidation and exerts a contribution of ca. 50 to 60% on the overall reaction at the different operating parameters. Furthermore, the direct molecular ozone oxidation rate constant is correlated with temperature by a modified Arrhenius equation with an activation energy, Ea, of 33.57 kJ mol–1.

Molecular Mobility and Physical Stability of Amorphous Irbesartan

Amorphous systems have attracted considerable attention due to their favorable properties; however, their stability issues still pose a major challenge. The purpose of the present work was to investigate the role of molecular mobility and moisture in the physical stability of a selected pharmaceutical amorphous system. Irbesartan (IBS), a relatively stable glass, was chosen as the model drug, as it exhibits a good physical stability (resistance to crystallization) at temperatures below the glass transition (Tg-50 K). The amorphous system was annealed at temperatures 298 K (25 °C) and 313 K (40 °C) at 0 and 75 % RH to study the effect of temperature and moisture on its relaxation behavior. Differential scanning calorimetry (DSC) was used to characterize both the crystalline and the freshly prepared glass, and to monitor the extent of relaxation at temperatures below glass transition (Tg) as well as heat capacity changes as a function of temperature. Molecular relaxation time constant (τ) decreased drastically from 302 years to 68 hours with the increase in annealing temperature as determined by Kohlrausch-William-Watts (KWW) equation. IBS was found to be ‘relatively’ stable in the amorphous state and presented a challenge for temporal measurements. Hence, at low annealing temperatures, (Tg-50 K or below) initial relaxation time (τ0) was estimated using the calorimetric based approach. Amorphous IBS was non-hygroscopic and retained its glassy nature under the accelerated stability conditions. The extent of relaxation in the amorphous drug in the presence of moisture was also estimated.