On the structure and stability of Ar n and Ar n + clusters at finite temperature

For Ar_2–29 and Ar _2–29 ⁺ clusters at 20 K in the polarization model presented here the electrodynamical dipole-dipole many-body problem is solved selfconsistently with the Monte-Carlo method (MC) at 20 K, i.e. the instantaneous dipole-dipole interaction is solved to infinite perturbation order and in cluster expansion to the order of the cluster size. The long range many-body dipole-dipole interaction is coupled to exchange interaction by a modified effective dipole polarizability. This model will be compared to the dimer model and classical MC simulation of Ar _n . The resulting different magic numbers in the binding energies are discussed in this connection with different experimental techniques of cluster ionization. By the mean square cluster diameter a shape parameter is introduced and it is found that with this parameter structural form transition in cluster growth can be resolved, and surprisingly do not correlate with the magic numbers.     

Dynamics and isotherms of water vapor sorption on mesoporous silica gels modified by different salts

The mesoporous silica gels impregnated with different metal salts were prepared and studied. The pore structure and specific surface area of adsorbents were evaluated using nitrogen adsorption. Then, the sorption isotherms and dynamics of water vapor were carried out at 303 K and different relative humidity (RH). The temperature programmed desorption experiments were conducted to estimate the activation energy (E _d) of water desorption on the silica gels. The results showed that the sorption capacity for water decreased with the increase of the ionic radius (except the calcium ion) and that CaCl₂ and LiCl were particularly suitable for use in modification of the mesoporous silica gel to improve their sorption rates and capacities for water vapor at the lower and medium RH (RH < 80%). The larger the average pore diameter and pore volume of the initial silica gels, higher the accrual rates of the water vapor sorption rate and capacity were after modification with hygroscopic salts. The activation energy of the water desorption on the mesoporous silica gel modified by CaCl₂ were much higher than that on the silica gel modified by LiCl, because the polarizability of the Ca²⁺ was higher than that of Li⁺.     

The preparation and characterization of the solid solution series CuFexGe1−xS2 (0.5 < x < 1.0)

Single crystals of the solid solution series CuFe_xGe_1?xS₂ (0.5 < x < 1.0) have been prepared by the chemical vapor transport technique. X-ray diffraction analysis and density measurements have indicated that all members of this system crystallize with the chalcopyrite structure. M?ssbauer spectra show that these crystals contain both iron(II) and iron(III) on tetrahedral sites and that the iron concentration agrees with that determined by chemical analysis. Magnetic susceptibilities for x = 0.53 display antiferromagnetic behavior. The Néel temperature of 12°K and an effective moment of 5.0 BM is observed, which approaches the calculated spin-only moment of 4.92 BM. As x increases, deviations from spin-only behavior occur, indicating complex magnetic interactions.     

Inverse grand-canonical Monte Carlo investigation of the activity coefficient of polar fluids

The inverse grand-canonical Monte Carlo (IGCMC) simulation is applied to calculate the activity coefficient of polar fluid. Molecules of the fluid are represented by hard spheres with a point electric dipole moment immersed in the centre. The electron polarizability of the fluid is described by the permittivity ?_∞. Good agreement between the IGCMC technique and Widom method data is obtained. Comparison with the results of the mean spherical approximation shows that this theory underestimate the Coulombic interactions while the agreement with PT is good. The dependence of the activity coefficient on the number density, electric dipole moment, molecular diameter, and temperature is presented. The Coulombic contribution to the activity coefficient is separated.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

Dielectric properties of 1,1,1-trifluoroethane (HFC-143a) in the liquid phase

The relative permittivity (?_r) data of 1,1,1-trifluoroethane (HFC-143a), (CAS N# 420-46-2), a hydrofluorocarbon (HFC) developed as a refrigerant that has zero ozone depletion potential, is reported. The relative permittivity of HFC-143a in the liquid phase was measured using a direct capacitance method at temperatures from T = 218 to 294 K and at pressures up to P = 15 MPa, for a frequency of 10 kHz. The uncertainty of the ?_r measurements is estimated to be better than ±1.2 × 10⁻². A complete set of tables of experimental data as a function of temperature, pressure and density, is presented that covers the dielectric property needs for most engineering applications. To study the dependence of ?_r on density and temperature on a molecular basis, the theory developed by Vedam et al. and adapted by Diguet was applied to analyse the data. The Kirkwood modification of the Onsager equation was used to obtain the value of its dipole moment in the liquid phase (μ*). The apparent dipole moment obtained was μ* = 3.293 D. The effective dipole in the liquid state predicted by the Kirkwood–Frölich theory is 2.530 D. The measured values are compared with density functional and density functional self-consistent calculations (SCIPCM) of the electronic distribution and of the dipole moment of HFC-143a. Finally, the values of the isobaric thermal expansion and isothermal compressibility were estimated from the reported measurements.     

Calculation of molecular electric polarizabilities and dipole moments. III. BH molecule

We use a previously proposed variation-perturbation method to calculate the electric polarizabilities and the electric dipole moment at equilibrium nuclear distance of the BH molecule. We obtain 3.56 × 10^?24 cm³ for the perpendicular polarizability α_xx and 3.22 × 10^?24 cm³ for the parallel polarizability α_zz. Our result for the electric dipole moment μ₀ is 1.734 debye units; there is no reliable experimental result to compare it with.     

Electric properties of hydrogen iodide: Reexamination of correlation and relativistic effects

The electric dipole moment and the static dipole polarizability of the hydrogen iodide molecule were studied using sophisticated correlated and relativistic methods. Both scalar and spin–orbit relativistic effects were carefully accounted for. We conclude that the large differences between the theoretical and experimental dipole moment, the dipole moment derivative and the polarizability cannot be reconciled by improved account of electron correlation and relativistic effects. The most striking difference between theory and experiment is observed for the polarizability anisotropy. We believe that experimental data, namely the experimental dipole moment (the most recent value is 0.176 au as compared to our best theoretical estimate, 0.154±0.003 au), the parallel polarizability (44.4 and 38.47±0.05 au) and the anisotropy (11.4 and 2.33±0.05 au) must be inaccurate. Experimental and theoretical perpendicular polarizability components (33.0 and 36.14±0.05 au,) and the mean polarizability (36.8 and 36.92±0.05 au) agree better. Our vibrationally corrected relativistic CCSD(T) results represent the most sophisticated predictions of electric properties of HI obtained so far.Contribution to the Björn Roos Honorary Issue     

Étude lcao-mo-cndo/2 des liaisons po et no dans des composés X3PO ET X3NO

The authors have studied the electronic structure of X₃PO and X₃NO compounds (with X = F, Cl, CH₃), using the semi-empirical CNDO/2 method. All the calculations have been made with and without 3d functions on the phosphorus atom. The comparison between the calculated and experimental values, especially in the case of bond length, dipole moment, and orbital level order, shows the influence of the 3d orbitals in the PO bond, which contains a sigma donation P → O and a pπ(O)-dπ(P) back bonding. The NO bond has sigma character in trimethylamine oxide, but is partially a double bond in trifluoramine oxide.     

Evaluation of the 1-octanol/water partition coefficient of nucleoside analogs via free energy estimated in quantum chemical calculations

The partition coefficients (logP) of nucleoside analogs determined by the difference in the free energies of hydration and solvation in water-saturated octanol using the thermodynamic integration method are reported. The logP values calculated in this approach are closer to the experimental values compared to other ab initio methods. Solvation free energy in water and octanol, free energy of cavity formation in water and Henry’s constants, and some other parameters are estimated at the density functional theory (DFT) and Hartree-Fock level with 6–31G*, 6–31G, and 6–31+G basis sets. Surface area, mass, refractivity, volume, polarizability, and dipole moment are calculated for some drugs with HF and DFT methods. The results show that log P decreases with the decrease in polarizability and the increase in dipole moment.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

The monomer−dimer equilibria of liquid aliminum alkyls : III. Trimethylaluminum: the monomer−dimer equilibria of liquid and gaseous trimethylaluminum and triethylaluminum

There has been a dichotomy of opinion concerning the values for the heats of dissociation of trimethylaluminum (TMA) and triethylaluminum (TEA) in the liquid phase. It is believd that the result reported herein will enable these differences to be resolved. In the present study, the heat of dissociation (ΔH^o_d of liquid TMA is evaluated as 19.4₀±0.3₀-kcal · (mole of dimer)⁻¹ from the previously determined ΔH⁰_d of liquid TEA and new heat of mixing data for TMA−TEA. Essentially the same result is derived from the established ΔH⁰_d of gaseous TMA by application of a thermodynamic relationship between heats of dissociation and heats of vaporization (the “Dissociation−Vaporization Rule”). It is concluded that the experimental ΔH⁰_d values for liquid TEA (16.9₃ kcal · (mole of dimer)⁻¹) and gaseous TMA (20.4₀ are thermodynamically consistent. The ΔS⁰_dof liquid TMA is evaluated as 29.3±0.3 cal · K⁻¹ · (mole of dimer)⁻¹. The ΔH⁰_d valuesfor liquid adn gaseous TMA are shown to be thermodynamically consistent with vapor pressure data. Boiling points derived for pure TMA monomer and pure dimer are 8.1±2.1 and 131.95±0.02°C, respectively. Degrees of dissociation of TMA in the pure liquid state and at various mole fractions in aliphatic hydrocarbon solution are tabulated over a wide temperature range. The results indicate that the extent of dissociation of liquid TMA is 0.0047% at 20, 0.053% at 70, and 0.32% at 120°C.     

Polarizability effect in silatranes and related compounds

The dipole moments (μ) of the molecules, the dipole moments (μ_DA) and the length (d_DA) of the Si ← N bonds, and the electrochemical oxidation potentials (E_p) of Si-substituted silatranes, the dipole moments (μ^hs) of the molecules of Si-substituted 3-homosilatranes as well as the enthalpies of formation (ΔH⁰) of the intermolecular complexes of SiF₄ with aniline derivatives depend not only on the inductive and resonance effects, but also on the polarizability of substituents which can be characterized by the σ_α constants.     

Li2 Trapped inside Tubiform [n] Boron Nitride Clusters (n=4–8): Structures and First Hyperpolarizability

The geometries and electronic properties of tubiform [n] boron nitride clusters entrapping Li₂ (Li₂@BN‐cluster(n,0); n=4–8), obtained by doping BN‐cluster(n,0) with Li₂ molecules, are investigated by means of DFT. The effects of tube diameter n on the dipole moment μ₀, static polarizability α₀, and first hyperpolarizability β₀ are elucidated. Both the dipole moment and polarizability increase with increasing tube diameter, whereas variation of the static first hyperpolarizability with tube diameter is not monotonic; β₀ follows the order 1612 (n=4)<3112 (n=5)<5534 (n=7)<8244 (n=6)<12 282 a.u. (n=8). In addition, the natural bond orbital (NBO) charges show that charge transfer takes place from the Li₂ molecule to the BN cluster, except for BN‐cluster(8,0) with larger tube diameter. Since the large‐diameter tubular BN‐cluster(8,0) can trap the excess electrons of the Li₂ molecule, Li₂@BN‐cluster(8,0) can be considered to be a novel electride compound.     

Preface

The anisotropic rototranslational scattering spectra of nitrogen gas at high frequency up to 700 cm⁻¹ for several temperatures and from low densities are analyzed in terms of new site–site (M3SV) intermolecular potential and interaction-induced pair polarizability models, using quantum spectral shapes computations. Our theoretical calculations take into account multipole contributions from the mean value and anisotropy of the dipole–dipole polarizability tensor α, two independent components of the dipole–octopole polarizability tensor E and dipole–dipole–quadrupole hyperpolarizability tensor B. The high-frequency wings are discussed in terms of the collision-induced rotational Rayleigh effect and estimates for the dipole–octopole polarizability |E₄| are obtained and checked with recent ab initio theoretical value. Good comparison is found in the frequency range 0–400 cm⁻¹ between the theoretical and experimental spectra. When an exponential contribution [exp(−ν/ν₀)] with ν₀ = 425 cm⁻¹ is considered to model very short-range light scattering mechanisms at room temperature, good agreement is found over the whole frequency range.     

Molecular structure,vibrational spectra and nonlinear optical properties of orthoarsenic acid–tris-(hydroxymethyl)-aminomethane DFT study

In this work, we report a theoretical study on molecular structure, vibrational spectra and nonlinear optical properties of orthoarsenic acid–tris-(hydroxymethyl)-aminomethane (OATA). The theoretical geometrical parameters in the ground state have been investigated by density functional method (B3LYP and BLYP) with 6-311G(d,p) basis set. The influence of intermolecular interactions effects on molecular properties has been considered by calculation performed on (OATA) dimer. The optimized geometric bond lengths and bond angles are in well agreement with the experimental data. As compared to theoretical frequencies of the monomer, the calculated values obtained for (OATA) dimer are in much better agreement with the experiment. All experimental vibrational bands have been discussed and assigned to normal modes on the basis of our theoretical calculations. B3LYP method has shown better fit to experimental ones than BLYP in calculation vibrational frequencies. To investigate nonlinear optical behaviour, the electric dipole moment μ, the polarizability α and the hyperpolarizability β were computed using DFT//B3LYP/6-311G(d,p) method.     

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association,Solvent and Kinetic H/D Isotope Effects

Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d₈ and in CD₂Cl₂. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d₈ and CD₂Cl₂, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD₂Cl₂. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model.     

Calculation of molecular electric polarizabilities and dipole moments. II. The LiH molecule

We use a variation–perturbation method to calculate the electric polarizabilities and the electric dipole moment of the LiH molecule. We obtain 4.455 for the perpendicular polarizability and 4.001 (×10^?24 cm³) for the parallel polarizability. Our result for the electric dipole moment at equilibrium nuclear distance is 5.866, which is in excellent agreement with the experimental value 5.828 debye units.