Molecular Constant Research Articles

Behavior of the Heterogeneous Electron-Transfer Rate Constants of Arenes and Substituted Anthracenes in Room-Temperature Ionic Liquids

We report the anodic oxidation of several arenes and anthracenes within room-temperature ionic liquids (RTILs). In particular, the heterogeneous electron-transfer rates (k 0 ) for substituted anthracenes and arenes are also investigated in l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2 mim][NTf 2 ]) and found not to obey the outer-sphere Marcus-type behavior of these compounds in contrast to the behavior in traditional organic solvents, in particular the predictions for k 0 with molecular size and solvent static dielectric constant. To obtain the electron-transfer rate for 9-phenylanthracene, the dimerization and heterogeneous electron-transfer kinetics of its electrogenerated radical cations is studied in [C 2 mim][NTf 2 ] and eight other RTILs and are both found to be largely independent of the solution viscosity.

Thermoacoustic parameters of rare earth and reference materials at 298 K

Thermoacoustic properties of various rare earth and reference materials have been evaluated from volume expansivity ( α) data at 298 K. Moelwyn–Hughes parameter ( C 1) has been utilized as a fundamental parameter because of its importance in connection with the lattice Grüneisen parameters ( Γ), the free available volume ( V a/ V), the repulsive exponent of intermolecular potential ( n), the reduced molar volume ( Ṽ), the reduced bulk modulus ( B̃), isochoric acoustical parameter ( Δ), the Anderson–Grüneisen parameter ( δ), and molecular constant. The average value of ( V a/ V) for these materials is about 0.007. It is quite evident from this study that Γ iba > Γ ith for reference and rare earth materials. Sharma's acoustical parameter ( S 0), which is a characteristic of wide variety of substances has been found to be approximately a constant value of 1.104 ± 0.003 in the case of rare earth and reference materials studied.

Heat capacity of EuMnO 3 and Eu 0.7A 0.3MnO 3 (A=Ca, Sr) compounds

We carried out the heat capacity calculation of the magnetoresistance compounds EuMnO 3 and Eu 0.7A 0.3MnO 3 (where A=Ca and Sr) as a function of temperature from 5 to 100 K, using the Rigid Ion Model (RIM). The results on heat capacity for EuMnO 3 and Eu 0.7A 0.3MnO 3 (A=Ca and Sr) obtained by us are in good agreement with the measured values. Although strong electron–phonon interactions are present in these compounds but the lattice part of the specific heat also deserves proper attention. The parent compound EuMnO 3 exhibits two magnetic transitions at 35 and 47 K due to weak ferromagnetic (FM) component and antiferromagnetic (AF) ordering. In addition, we have reported cohesive energy ( φ), molecular force constant ( f), compressibility ( β), Restrahalen frequency ( υ 0), Debye temperature ( θ D) and Gruneisen parameter ( γ) in the temperature range 5 K⩽ T⩽100 K.

Effect of carbon doping on thermodynamic behavior of MgB 2

The thermodynamic behavior of carbon doped MgB 2 has been studied using a rigid ion model (RIM). The model potential consists of the long-range Coulomb, the short-range repulsive and the van der Waals interactions. This model has successfully explained the cohesive and thermodynamic properties of Mg(B 1− x C x ) 2 ( x=0.0, 0.02, 0.05, 0.075, 0.1, 0.2). The properties studied are the cohesive energy, molecular force constant, Restrahlen frequency, compressibility, Debye temperature and Gruneisen parameter. Our results on Restrahlen frequency and Debye temperature are in reasonably good agreement with the available experimental data. In addition, we have computed the specific heat C p for Mg(B 1− x C x ) 2 ( x=0.2) as a function of temperature T in the range 16 K⩽ T⩽1000 K. We have also shown the variation of specific heat C p with doping concentration at room temperature (300 K). The calculated specific heat C p for Mg(B 1− x C x ) 2 ( x=0.2) in the temperature range 16 K⩽ T⩽22 K for which experimental results are available, agrees pretty well with the experimental data.

LOW TEMPERATURE SPECIFIC HEAT OF INTERMETALLICS (Ti/Be)B2

We have applied the Rigid Ion Model (RIM) to study the cohesive and thermal properties of binary intermetallic BeB 2 and TiB 2. The paper reports the calculated results on cohesive energy (ϕ), compressibility (β), molecular force constant (f), Restrahalen frequency (ν0), Debye temperature (Θ D ) and Gruneisen parameter (γ) for the temperature range 50 K ≤ T ≤ 300 K and the effect of van der Waal interaction on these properties are also shown. Our results on Debye temperature are closer to the experimental data. In addition, we have computed the specific heats for BeB 2 and TiB 2 and compared them with the available experimental data.

Molecular dynamics simulation of pressure dependence of cluster growth in inert gas condensation

The growth speed of nanoclusters during inert gas condensation has been studied for copper, silver, aluminum, and platinum by using molecular dynamics simulations. We determine the condensation time for vapor atoms in a particular volume to be inversely proportional to the initial partial vapor pressure. We further find that the condensation time depends on the molecular mass and lattice constant, but not on other material properties. An analytical model for the condensation time is derived from kinetic gas theory by using the basic approximations of classical nucleation theory for a homogeneous vapor.

Dynamics of deuterium implanted in boron coating film for wall conditioning

Retention and desorption behavior of deuterium implanted into pure boron films has been studied by means of the secondary ion mass spectroscopy. It was found that the process controlling deuterium desorption depended on the temperature. At stage 1, below 573 K, the desorption of deuterium from B–D–B bonds dominated and its diffusion was the rate-determining process. Above 573 K, deuterium was mainly desorbed from B–D bonds, and recombination was the rate-determining process. The effective molecular recombination rate constant of deuterium bound to boron associated with B–D bond was determined by an isothermal annealing experiment.

Spectrophotometric, conductometric and thermal studies of Co(II), Ni(II) and Cu(II) complexes with 2-(2-hydroxynaphthylazo)-4-hydroxy-6-methyl-1,3-pyrimidine

The electronic absorption spectra of 2-(2-hydroxynaphthylazo)-4-hydroxy-6-methyl-1,3-pyrimidine in pure organic solvents of different polarities and in buffer solutions of varying pH are studied. The important bands in the IR and the main signals in the 1H NMR spectra are assigned. The observed UV–vis absorption bands are assigned to the corresponding electronic transitions. The molecular stoichiometry, stability constant, absorption maximum, molar absorptivity and Sandell's sensitivity of the complexes are calculated. Obeyence to Beer's law and Ringbom optimum concentration ranges are also determined. The ability of using the titled azodye as metalochromic indicator in complexometric titrations was also studied. The effect of Co(II), Ni(II) and Cu(II) ions on the fluorescence of the azodye is also considered. The solid Cu(II) complexes of the titled azodye have been prepared and characterized by elemental, IR, UV–vis spectra as well as by conductometric and magnetic measurements. The data suggest square planar geometry for 1:1 and 1:2 (M:L) complexes. The thermal behaviour of the complexes has been studied. The kinetic parameters ( n, E, A, Δ H, Δ S and Δ G) of the thermal decomposition steps are computed using Coats–Redfern equations.

Enhancing molecular formation in a Rb-MOT

In this paper, we report observation of an enhancement of the ground state molecule Rb2 formation in a Rb magneto-optical trap due to a photoassociation laser. Such an enhancement effect is due to flux enhancement of atomic pairs at short internuclear distances. Our experimental observations consist in the measurement of the molecular formation rate constant due to a probe laser as a function of the trap laser intensity. The results are compared with a simple semi-classical model, showing good agreement. We conclude that the production of cold Rb2 may be enhanced using appropriate laser parameters; this may be useful for future experiments involving production and trapping of ultracold ground state molecules.

Effect of doping on cohesive and thermophysical properties of MgB 2

The effects of doping Al and Mn on the cohesive and thermophysical properties of MgB 2 have been investigated using a Rigid Ion Model (RIM). The interatomic potential of this model includes contributions from the long-range Coulomb attraction and the short-range overlap repulsion and the van der Waals attraction. This model has been applied to describe the temperature dependence of the specific heat of MgB 2, Mg 1− x Al x B 2 ( x = 0.1–0.9) and Mg 1− x Mn x B 2 ( x = 0.01–0.04) in the temperature range 5 K ⩽ T ⩽ 1000 K. The calculated results on cohesive energy ( ϕ), Bulk modulus ( B T), molecular force constant ( f), Restrahalen frequency ( ν 0), Debye temperature ( Θ D) and Gruneisen parameter ( γ) are also reported for these materials. Our results on Bulk modulus, Restrahalen frequency and Debye temperature are closer to the available experimental data. The comparison between our calculated and available experimental results on the specific heat at constant volume for MgB 2 and Mg 1− x Al x B 2 ( x = 0.1–0.4), particularly, at lower temperatures has shown almost an excellent agreement. The trend of variation of the specific heat with temperature is more or less similar in pure and doped MgB 2.

Collisions between linear polar molecules trapped in a microwave field

The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. We demonstrate that the collisional dynamics is mostly controlled by two ratios ν/B and x=μ0E0/ħ B (ν is the microwave frequency, B is the molecular rotational constant, μ0 is the dipole moment, and E0 is the electric field amplitude). We discuss the dependence of collision cross sections on these ratios in order to find an advantageous condition for evaporative cooling.

Crystallization kinetics of poly(glycolic acid- alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process. The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman–Weeks plot. One crystallization regime is detected by using the Lauritzen–Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10 5 K 2, respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).

Molecular-optic modulator

An ultrafast light-intensity modulator, based on stimulated Raman scattering, is described. The intensity of a continuous wave laser is fully modulated at 17THz using hydrogen in a high-finesse cavity. The modulation frequency is determined by the molecular constant of the Raman medium, i.e., the Raman shift frequency. The modulation frequency can be changed in the tetrahertz range by replacing the Raman medium. Due to the accurate modulation frequency and the high beam coherence, this device is amenable to a variety of applications such as in basic science and also in advanced industrial technology.

Molecular Shielding of Electric Field Complex Dissociation

We have previously demonstrated the ability of electric fields to dissociate ascorbate and catecholamines and shown that the electric field generated by cell membranes is sufficient to produce dissociation of these complexes up to 8 nm from the cell membrane. We show here that this process is applicable to a wide range of biological complexes including small molecules (norepinephrine-morphine sulfate), protein-protein complexes (insulin-glucagon), and small molecule-protein complexes (epinephrine-bovine serum albumin). The extrapolation of the slope of the electric field dependence to zero electric field can be used to estimate the log of the dissociation constant ( K D) of a complex and, by multiplying the log( K D) by −2.303RT, the association energy ( E) of the complex. The slope of the electric field dependence is inversely related to the molecular radii, with the best fit of the slope related to E*(1/ r 1 + 1/ r 2), where r is the estimated radius of each molecule in the complementary pair. This indicates that the binding site of the pair is shielded by the remaining parts of the molecules, and the larger the molecule the greater the shielding. When the slope of the electric field dependence goes to 0 as r goes to infinity and 1/ r goes to 0, the molecular shielding constant is 7.04 × 10 −8 cm 2/V. Very large complexes will be minimally affected by the electric field due to molecular shielding and reduced electric field as their radius restricts approach to the membrane. Large protein receptors will deflect the membrane electric field and allow agonist binding.

On the behaviour of thermo-acoustic parameters in different liquids

The thermo-acoustical parameters of liquids are evaluated in terms of the coefficient of thermal expansion on the assumption that the Moelwyn-Hughes parameter is the dominant factor. The Moelwyn-Hughes parameter has been utilized to establish some simple relations between the lattice Gruneisen parameter, Beyer's nonlinearity parameter, Rao's acoustical parameter, Sharma's thermo-acoustical parameter, the fractional available volume, the repulsive exponent of intermolecular potential, the Anderson–Gruneisen parameter and the molecular constant. A relationship between the isothermal, isochoric and isobaric Gruneisen parameters has also been studied. The parameter Y (compressibility) is found to be close to 0.09 from which one may infer that the normal modes of vibrations contributing to heat capacities are very small. Sharma's parameter has been found to be approximately constant in the case of liquids studied.

Interlayer jump diffusion in the smectic-CA*phase by angular-dependentH2NMR study

A detailed 2H nuclear magnetic resonance (NMR) study at 61.4 MHz of two chiral smectic C phases of a smectogen 10B1M7, one ferroelectric (SmC*) and the other antiferroelectric (SmC), is reported. Simulation of angular-dependent spectral patterns and/or signal intensities of the methyl deuterons are shown to provide information on the synclinic or anticlinic ordering, as well as the jump diffusion rates at different temperatures in the SmC phase. The molecular self-diffusion constant is determined to be in the range of 0.2-1.2 x 10(-14) m2/s over the temperature range of the SmC phase. Molecular self-diffusion in the SmC* phase should be faster, but remains undetectable by the present approach. Furthermore, the phase biaxiality in these phases is found to be vanishingly small. The critical magnetic field for unwinding the helical structure of 10B1M7 is found to be relatively high.

Multiple-Bond Kinetics from Single-Molecule Pulling Experiments: Evidence for Multiple NCAM Bonds

The kinetic parameters of single bonds between neural cell adhesion molecules were determined from atomic force microscope measurements of the forced dissociation of the homophilic protein-protein bonds. The analytical approach described provides a systematic procedure for obtaining rupture kinetics for single protein bonds from bond breakage frequency distributions obtained from single-molecule pulling experiments. For these studies, we used the neural cell adhesion molecule (NCAM), which was recently shown to form two independent protein bonds. The analysis of the bond rupture data at different loading rates, using the single-bond full microscopic model, indicates that the breakage frequency distribution is most sensitive to the distance to the transition state and least sensitive to the molecular spring constant. The analysis of bond failure data, however, motivates the use of a double-bond microscopic model that requires an additional kinetic parameter. This double-bond microscopic model assumes two independent NCAM-NCAM bonds, and more accurately describes the breakage frequency distribution, particularly at high loading rates. This finding agrees with recent surface-force measurements, which showed that NCAM forms two spatially distinct bonds between opposed proteins.

Temperature Softening of a Protein in Single-molecule Experiments

Mechanical flexibility is crucial for the function of proteins. However, such material properties are not easily accessible experimentally. We used single-molecule force spectroscopy to study the stiffness of a single domain of Dictyostelium discoideum filamin (ddFLN4) in a temperature range from 5 degrees C to 37 degrees C. Analyzing the distributions of unfolding forces allowed us to extract transition barrier heights and positions of the underlying energy landscape. We found a marked narrowing of unfolding force distributions with increasing temperature. This narrowing reflects an increase in transition state position from 2.7 A to 7.8 A and thus a reduction of the molecular spring constant of the protein by a factor of 7. We suggest this temperature softening reflects a shift in the nature of the interactions responsible for mechanical stability from hydrogen bonds to hydrophobic interactions. This result has important consequences for all interpretations of protein mechanical studies if experimental results obtained at room temperature are to be transferred to physiological temperatures.

Effects of Organelle Shape on Fluorescence Recovery after Photobleaching

The determination of diffusion coefficients from fluorescence recovery data is often complicated by geometric constraints imposed by the complex shapes of intracellular compartments. To address this issue, diffusion of proteins in the lumen of the endoplasmic reticulum (ER) is studied using cell biological and computational methods. Fluorescence recovery after photobleaching (FRAP) experiments are performed in tissue culture cells expressing GFP–KDEL, a soluble, fluorescent protein, in the ER lumen. The three-dimensional (3D) shape of the ER is determined by confocal microscopy and computationally reconstructed. Within these ER geometries diffusion of solutes is simulated using the method of particle strength exchange. The simulations are compared to experimental FRAP curves of GFP–KDEL in the same ER region. Comparisons of simulations in the 3D ER shapes to simulations in open 3D space show that the constraints imposed by the spatial confinement result in two- to fourfold underestimation of the molecular diffusion constant in the ER if the geometry is not taken into account. Using the same molecular diffusion constant in different simulations, the observed speed of fluorescence recovery varies by a factor of 2.5, depending on the particular ER geometry and the location of the bleached area. Organelle shape considerably influences diffusive transport and must be taken into account when relating experimental photobleaching data to molecular diffusion coefficients. This novel methodology combines experimental FRAP curves with high accuracy computer simulations of diffusion in the same ER geometry to determine the molecular diffusion constant of the solute in the particular ER lumen.

Determination of the molecular complexation constant between alprostadil and alpha-cyclodextrin by conductometry: Implications for a freeze-dried formulation

The binding constant between alprostadil (PGE 1) and α-cyclodextrin (α-CD) was determined at four temperatures using conductance measurements. Alpha-cyclodextrin is an excipient material in Caverject dual chamber syringe (DCS) that was added to enhance stability. The binding constant was used to calculate the amount of PGE 1 free upon reconstitution and injection, since only the free drug is clinically active. The conductivity measurement is based on a decrease in specific conductance as alprostadil is titrated with α-CD. The change in conductivity was plotted versus free ligand concentration (α-CD) to generate a binding curve. As the value of the binding constant proved to be dependent on substrate concentration, it is really a pseudo binding constant. A value of 742 ± 60 M −1 was obtained for a 0.5 mM solution of alprostadil at 27 °C and a value of 550 ± 52 M −1 at 37 °C. These results compare favorably to values previously obtained by NMR and capillary electrophoresis. Calculation of the fraction PGE 1 free upon reconstitution and injection show it to approach the desired outcome of one. Hence, the amount of drug delivered by Caverject DCS is nominally equivalent to that delivered by Caverject S. Po., a predecessor product that contains no alpha-cyclodextrin.