A time-dependent wave packet study of the H4 four-center reaction

A quantum model based on the time-dependent initial state selected wave packet approach was developed to study the four-center (4C) reaction, A₂ + B₂ → 2AB, and the competing collision induced dissociation (CID), A₂ + B₂ → A + B₂ + A, as applied to the H₂(v₁) + H₂(v₂) system important in combustion. A reduced three-dimensional model of the reaction with the atoms constrained to an isosceles trapezium and a realistic global potential energy surface of Aguado et al. [J. Chem. Phys. 101 (1994) 2742], following Hernández and Clary [J. Chem. Phys. 104 (1996) 8413], was used. A method to analyse the reaction flux for 4C and CID reaction probabilities is presented. The initial A₂ vibrational excitation is not only more efficient than translational energy in facilitating the 4C and CID processes, it also reduces the threshold energy. Both the 4C and CID processes exhibit similar threshold energy behavior. For low vibrational excitation in the A₂ diatom, the 4C process is dominant; as the A₂ diatom becomes highly excited the CID process becomes more important at low collision energies with B₂, but as the collision energy increases the 4C process is favored again.     

Photophysical properties of 5-hydroxyflavone

To investigate the role of the excited triplet state in the deactivation process of 5-hydroxyflavone (5HF), the photophysical process of 5HF was studied by transient absorption, phosphorescence spectroscopies, and semiempirical calculations. The triplet–triplet absorption (T–T) spectra of 5HF and 5-methoxyflavone (5MF) were observed upon direct and triplet-sensitized excitation. The T–T spectrum of 5HF (λ_max=350 nm, τ_T=2.8 μs) was different from that of 5MF (λ_max=360 nm, τ_T=6.8 μs). Estimations of the triplet energies of 5HF and 5MF by quenching experiments, phosphorescence, and semiempirical (PM3/CI4) calculation revealed that 5HF underwent an intramolecular hydrogen atom transfer and formed the tautomer in the excited triplet state. The triplet energy of the normal form of 5HF was 260 kJ mol⁻¹, while that of the tautomer form (5HF′) was 197 kJ mol⁻¹. The triplet energy of 5MF, the model compound of the normal form of 5HF, was 261 kJ mol⁻¹. The PM3/CI4 calculation supported the experimental observations and suggested that the most stable conformer in the triplet state of 5HF is the tautomer form.     

Heat capacities,third-law entropies and thermodynamic functions of the geometrically frustrated antiferromagnetic spinels GeCo2O4 and GeNi2O4 from T=(0 to 400) K

The molar heat capacities of GeCo₂O₄ and GeNi₂O₄, two geometrically frustrated spinels, have been measured in the temperature range from T=(0.5 to 400) K. Anomalies associated with magnetic ordering occur in the heat capacities of both compounds. The transition in GeCo₂O₄ occurs at T=20.6 K while two peaks are found in the heat capacity of GeNi₂O₄, both within the narrow temperature range between 11.4<(T/K)<12.2. Thermodynamic functions have been generated from smoothed fits of the experimental results. At T=298.15 K the standard molar heat capacities are (143.44 ± 0.14) J · K⁻¹ · mol⁻¹ for GeCo₂O₄ and (130.76 ± 0.13) J · K⁻¹ · mol⁻¹ for GeNi₂O₄. The standard molar entropies at T=298.15 K for GeCo₂O₄ and GeNi₂O₄ are (149.20 ± 0.60) J · K⁻¹ · mol⁻¹ and (131.80 ± 0.53) J · K⁻¹ · mol⁻¹ respectively. Above 100 K, the heat capacity of the cobalt compound is significantly higher than that of the nickel compound. The excess heat capacity can be reasonably modeled by the assumption of a Schottky contribution arising from the thermal excitation of electronic states associated with the CO²⁺ ion in a cubic crystal field. The splittings obtained, 230 cm⁻¹ for the four-fold-degenerate first excited state and 610 cm⁻¹ for the six-fold degenerate second excited state, are significantly lower than those observed in pure CoO.     

Modeling of thermal and chemical non-equilibrium in a laser-induced aluminum plasma by means of a Collisional-Radiative model

A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a 4 ns/65 mJ/532 nm Q-switched Nd:YAG laser pulse and an aluminum sample in vacuum. The heavy species considered are Al, Al⁺, Al²⁺ and Al³⁺ on their different excited states and free electrons. The translation temperatures of free electrons and heavy species are assumed different (T_e and T_A respectively). Numerous elementary processes are accounted for as electron impact induced excitation and ionization, elastic collisions, multiphoton ionization and inverse Bremsstrahlung. Atoms passing from the sample to gas phase are described by using classical vaporization theory so that the surface temperature is arbitrarily limited to values less than the critical point one at 6700 K. The laser flux density considered in the study is therefore moderate with a fluence lower than 7 J cm^? 2.This model puts forward the major influence of multiphoton ionization in the plasma formation, whereas inverse Bremsstrahlung turns out to be quasi negligible. The increase of electron temperature is mainly due to multiphoton ionization and T_e does not exceed 10,000 K. The electron induced collisions play an important role during the subsequent phase which corresponds to the relaxation of the excited states toward Boltzmann equilibrium. The electron density reaches its maximum during the laser pulse with a value ≈ 10²², 10²³ m^? 3 depending highly on the sample temperature. The ionization degree is of some percents in our conditions.     

AF molecular rings for quantum computation

Molecular magnets have been recently proposed as possible building blocks for a solid-state quantum computer. In order to substantiate and develop such a proposal, one needs to identify those molecules that are best suited for the qubit encoding and manipulation. Here, we focus on a heterometallic molecular ring, namely Cr₇Ni, where the substitution of one Cr³⁺(S = 3/2) with Ni²⁺(S = 1) provides an extra spin to the otherwise compensated molecule. We show that its ground state consists in an S = 1/2 doublet, energetically well separated (Δ₀/k_B ∼ 13 K at zero magnetic field) from the first excited multiplet. This relatively large value of Δ₀, together with the reduced mixing of the subspaces corresponding to different values of the total spin S, enables a safe encoding of the |0〉 and |1〉 states with the ground-state doublet, and allows to coherently rotate the effective S = 1/2 spin, while keeping the population loss to the excited states negligible. A further, intriguing challenge is represented by the implementation of the conditional dynamics (two-qubit gates). We present here preliminary characterization of molecular “Cr₇Ni-dimers”, i.e., derivatives in which two Cr₇Ni rings are linked with each other by means of delocalized aromatic amines. The resulting intercluster couplings are estimated to be ⩽1 K and are expected to be permanent, i.e., not tuneable during gating, as required by the standard approach to quantum computation. We discuss a computational scheme that allows in principle to overcome this limitation. The most relevant decoherence mechanisms for Cr₇Ni and possible ways to reduce their effects are discussed as well.     

Investigation of the phonon band gap effect on Raman-active optical phonons in SrMoO4 crystal

The phonon dispersions of SrMoO₄ crystal are calculated using the lattice dynamical calculations approach. Spontaneous Raman spectra in the SrMoO₄ were measured in the temperature range from 10 K to 295 K, and the temperature dependence of the linewidth of the B_g (95 cm⁻¹) and A_g (888 cm⁻¹) Raman modes was analyzed using the lattice dynamical perturbative approach. We found that different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the phonon spectrum resulting in different anharmonic interactions. The calculated temperature dependence of the linewidth of A_g (888 cm⁻¹) mode was well accounted for the experimental one by including both down-conversion by the cubic term and the dephasing by quartic term. The dephasing processes are increased only at high temperatures and the effect of dephasing is related to the size of a large phonon band gap.     

Thermodynamic properties of 4-amino-3-furazanecarboxamidoxime

4-Amino-3-furazanecarboxamidoxime (AAOF) is an important precursor for synthesizing new furazano (furoxano) energetic compounds. Its thermal behaviour was studied under a non-isothermal condition by DSC methods. The results of this study show that there are one melting process and two exothermic decomposition processes. Its kinetic parameters of the intense exothermic decomposition process are obtained from analysis of the DSC curves. The apparent activation energy (E_a), pre-exponential factor (A) and the mechanism function (f(α)) were (146 ± 18) kJ · mol⁻¹, (10^10.9±1.8) s⁻¹ and (1 − α)², respectively. The specific molar heat capacity (C_p,m) of AAOF was determined by a continuous C_p mode of micro-calorimeter. The self-accelerating decomposition temperature (T_SADT), thermal ignition temperature (T_TIT) and critical temperatures of thermal explosion (T_b) were obtained to evaluate its thermal safety.     

Thermodynamic properties of soddyite from solubility and calorimetry measurements

The release of uranium from geologic nuclear waste repositories under oxidizing conditions can only be modeled if the thermodynamic properties of the secondary uranyl minerals that form in the repository setting are known. Toward this end, we synthesized soddyite ((UO₂)₂(SiO₄)(H₂O)₂), and performed solubility measurements from both undersaturation and supersaturation. The solubility measurements rigorously constrain the value of the solubility product of synthetic soddyite, and consequently its standard-state Gibbs free energy of formation. The log solubility product (lg K_sp) with its error (1σ) is (6.43 + 0.20/−0.37), and the standard-state Gibbs free energy of formation is (−3652.2 ± 4.2 (2σ)) kJ mol⁻¹. High-temperature drop solution calorimetry was conducted, yielding a calculated standard-state enthalpy of formation of soddyite of (−4045.4 ± 4.9 (2σ)) kJ · mol⁻¹. The standard-state Gibbs free energy and enthalpy of formation yield a calculated standard-state entropy of formation of soddyite of (−1318.7 ± 21.7 (2σ)) J · mol⁻¹ · K⁻¹. The measurements and associated thermodynamic calculations not only describe the T = 298 K stability and solubility of soddyite, but they also can be used in predictions of repository performance through extrapolation of these properties to repository temperatures.     

Photophysical and photochemical studies of thifensulfuron-methyl herbicide in aqueous solution

The photophysical and photochemical studies of a sulfonylurea herbicide, thifensulfuron-methyl (THM), have been investigated in a buffered aqueous solution. In the first part, the influence of pH on the spectroscopic properties was studied. This allowed the determination of the ground and excited state acidity constants, pK_a = 4 and 4.4, respectively, thus exhibiting the potential existence of a photoinduced protonation in the singlet state. In the second part, the photolysis kinetics was studied at different pH and varying oxygen concentrations, using an HPK 125 W lamp and followed up by the identification of photoproducts formed under continuous photo-irradiation. The kinetics results suggest that the photolysis process is faster in acidic (k = 3 × 10^?4 s^?1) than in basic medium (k = 9.8 × 10^?5 s^?1). The photolysis products were identified by high performance liquid chromatography HPLC-DAD, HPLC–MS and HPLC–MS–MS. In order to obtain a better understanding of the photodegradation mechanism, a laser flash photolysis study was performed. By comparing the quenching rate constant (k_q = 9.64 × 10⁸ mol^?1 l s^?1) obtained from triplet state quenching by molecular oxygen and from the Stern–Volmer relation (k_q = 0.41 × 10⁸ mol^?1 l s^?1), the role of the singlet state in the photodegradation process was demonstrated. The photoproducts originating from both singlet and triplet excited states have been identified and hypothetical photodegradation pathways of the thifensulfuron-methyl in aqueous solution are proposed.     

Direct ab initio molecular dynamics study of the two photodissociation channels of formic acid

A total of ∼1200 trajectories have been integrated for the two photodissociation channels of formic acid, HCOOH → H₂O + CO (1) and HCOOH → CO₂ + H₂ (2), which occur with 248 and 193 nm photons, using the direct ab initio molecular dynamics method at the RMP2(full)/cc-pVDZ level of theory. It was found that the percentage of the energy distributed to a relative translational mode in reaction (2) is much larger than that in reaction (1). This is mainly due to the difference in the geometry of transition state (TS); the H₂O geometry in the TS of reaction (1) was predicted to significantly deviate from the equilibrium one, whereas the CO₂ and H₂ geometries in the TS of reaction (2) were found to be more similar to their equilibrium ones. It was also found that the product diatomic molecules, CO and H₂, are both vibrationally and rotationally excited. The calculated relative population of the vibrationally excited CO for the 248 nm photodissociation was consistent with experiment.     

Activity coefficients of CaCl2 in (serine or proline + water) mixtures at T = 298.15 K

Activity coefficients for the (CaCl₂ + amino acid + water) system were determined at a temperature of 298.15 K using ion-selective electrodes. The range of molalities of CaCl₂ is (0.01 to 0.20) mol · kg^?1, and that of amino acids is (0.10 to 0.40) mol · kg^?1. The activity coefficients obtained from the Debye–Hückel extended equation and the Pitzer equation are in good agreement with each other. Results show that the interactions between CaCl₂ and amino acid are controlled mainly by the electrostatic interactions (attraction). Gibbs free energy interaction parameters (g_EA) and salting constants (k_S) are positive, indicating that these amino acids are salted out by CaCl₂. These results are discussed based on group additivity model.     

Inhibitors for magnesium corrosion: Metal organic frameworks

Electrochemical measurements demonstrate that magnesium surfaces can be protected by alkyl carboxylate. In a nearly neutral pH solution of sodium decanoate, the reduced corrosion rate and a passivation behaviour are attributed to the formation of Mg(C₁₀H₁₉O₂)₂(H₂O)₃ (Mg(C10)₂) at the magnesium surface whereas heptanoate Mg(C₇H₁₃O₂)₂(H₂O)₃ (Mg(C7)₂) is not efficient in such media. The crystal structures of the two metal carboxylates Mg(C7)₂ and Mg(C10)₂ are determined by X-ray diffraction. Single crystal data: Mg(C7)₂, P2₁/a, a = 9.130(5) Å, b = 8.152(5) Å, c = 24.195(5) Å, β = 91.476(5)°, V = 1800.3(15) Å³, D_x = 1.242 g cm⁻³, Z = 4. Synchrotron powder data: Mg(C10)₂, P2₁/a, a = 9.070(3) Å, b = 8.165(1) Å, c = 32.124(1) Å, β = 98.39(1)°, V = 2353.85(8) Å³, D_x = 1.188 g cm⁻³, Z = 4. Their layered structures are quite similar and differ mainly by the length of the hydrophobic chains. They consist of two planes of O-octahedra centred by Mg atoms, parallel to (001). The distorted octahedra are constituted by three oxygen atoms from carboxylate groups and by three oxygen atoms coming from water molecules. The layers are connected by hydrogen bonds. The carboxylate chains are located perpendicularly and on both sides of these planes. One carboxylate chain is bridging the Mg atom along [010] while the other is monodendate. The presence of structural water is confirmed by thermal analyses.     

Theoretical study of the exchange coupling interactions in a polyoxometalate Fe9W12 complex

Theoretical methods based on density functional theory have been employed to analyze the exchange interactions in a Fe₉W₁₂ polyoxometalate complex. The calculated values of the seven exchange coupling constants required by the molecular structure agree well with those reported previously for other Fe^III polynuclear complexes and give an S = 15/2 single determinant ground state, with a first excited state that has S = 5/2.     

Superhalogens as building blocks of complex hydrides for hydrogen storage

Superhalogens are species whose electron affinity (EA) or vertical detachment energy (VDE) exceeds those of halogens. These species typically consist of a central electropositive atom with electronegative ligands. The EA or VDE of species can be further increased by using superhalogens as ligands, which are termed as hyperhalogens. Having established BH₄⁻ as a superhalogen, we have studied BH_4 − x(BH₄)_x⁻ (x = 1–4) hyperhalogen anions and their Li-complexes LiBH_4 − x(BH₄)_x using density functional theory. The VDE of these anions is larger than that of BH₄⁻, which increases with the increase in number of peripheral BH₄ moieties (x). The hydrogen storage capacity of LiBH_4 − x(BH₄)_x complexes is higher but binding energy is smaller than that of LiBH₄, a typical complex hydride. The linear correlation between the dehydrogenation energy of LiBH_4 − x(BH₄)_x complexes and the VDE of BH_4 − x(BH₄)_x⁻ anions is established. These complexes are found to be thermodynamically stable against dissociation into LiBH₄ and borane. This study demonstrates the role of superhalogens in designing new materials for hydrogen storage and should also motivate experimentalists to synthesize LiBH_4 − x(BH₄)_x (x = 1–4) complexes.     

Fluorescence studies on the interaction of hydrophobic ligands with Momordica charantia (bitter gourd) seed lectin

The interaction of Momordica charantia (bitter gourd) seed lectin (MCL) with several nucleic acid bases has been investigated by monitoring changes induced in the protein fluorescence by ligand binding. Values of the binding constant, K_a were obtained as 1.1 × 10⁴, 1.56 × 10⁴ and 2.2 × 10³ M^?1 for adenine, cytosine and uracil, respectively. In addition, binding of 8-anilinonaphthalene 1-sulfonate (ANS) with MCL was investigated by fluorescence spectroscopy. Interaction with MCL at low pH results in a large enhancement of the fluorescence intensity of ANS with a concomitant blue shift in the emission λ_max, whereas at neutral and basic pH changes in both fluorescence intensity and emission maximum were very small, clearly suggesting that the MCL–ANS interaction is stronger at lower pH values. When excited at 295 nm in the presence of ANS, the protein fluorescence decreased with a concomitant increase in the emission intensity of ANS, suggesting resonance energy transfer from the tryptophan residues of MCL to ANS. Gel filtration profiles of MCL at pH values 2.0 and 7.4 are similar indicating that the tetrameric nature of MCL is retained even at low pH. Addition of lactose or adenine to MCL–ANS mixture did not alter the change in ANS fluorescence suggesting that lactose, adenine and ANS bind to MCL at independent and non-interacting sites. These results are relevant to understanding the functional role of MCL in the parent tissue.     

Up-scaling the production of modified a-C:H coatings in the framework of plasma polymerization processes

Hydrogenated amorphous carbon (a-C:H) films with silicon and oxygen additions, which exhibit mechanical, tribological and wetting properties adequate for protective coating performance, have been synthesized at room temperature in a small- (0.1 m³) and a large-scale (1 m³) coaters by low-pressure Plasma-Activated Chemical Vapour Deposition (PACVD). Hence, a-C:H:Si and a-C:H:Si:O coatings were produced in atmospheres of tetramethylsilane (TMS) and hexamethyldisiloxane (HMDSO), respectively, excited either by radiofrequency (RF – small scale) or by pulsed-DC power (large scale). Argon was employed as a carrier gas to stabilize the glow discharge. Several series of 2–5 μm thick coatings have been prepared at different mass deposition rates, R_m, by varying total gas flow, F, and input power, W. Arrhenius-type plots of R_m/F vs. (W/F)^?1 show linear behaviours for both plasma reactors, as expected for plasma polymerization processes at moderated energies. The calculation of apparent activation energy, E_a, in each series permitted us to define the regimes of energy-deficient and monomer-deficient PACVD processes as a function of the key parameter W/F. Moreover, surface properties of the modified a-C:H coatings, such as contact angle, abrasive wear rate and hardness, appear also correlated to this parameter. This work shows an efficient methodology to scale up PACVD processes from small, lab-scale plasma machines to industrial plants by the unique evaluation of macroscopic parameters of deposition.     

Effect of intraligand π-delocalization on the photophysical properties of two new Ru(II) complexes

Two new Ru(II) complexes, [Ru(bpy)₂(1-COO-iqu)]⁺ (2; bpy = 2,2′-bipyridine, 1-COO-iqu^? = isoquinoline-1-carboxylate) and [Ru(bpy)₂(3-COO-iqu)]⁺ (3; 3-COO-iqu^? = isoquinoline-3-carboxylate), were prepared and their crystal structures solved. The ground and excited state properties of 2 and 3 were characterized and compared to those of [Ru(bpy)₃]²⁺ (1). The presence of the oxygen atom in the Ru(II) coordination sphere makes 2 and 3 easier to oxidize than 1. The Ru → bpy MLCT absorption and emission of 2 and 3 are red-shifted relative to that of 1 in CH₂Cl₂, and the E₀₀ energies were estimated to be 1.89 eV and 1.95 eV from the low temperature emission of 2 and 3, resulting in excited state oxidation potentials of ?1.03 V and ?1.10 V vs SCE, respectively. In addition to the short-lived emissive ³MLCT state, a long-lived species is observed in the transient absorption of 3 in DMSO (τ = 49 μs) and pyridine (τ = 44 μs), assigned to a solvent-coordinated complex. This intermediate is not observed for 3 in non-polar solvents or for 2. The absence of the solvent coordinated intermediate in 2 is explained by the stronger Ru–O bond afforded by the lower conjugation in that extends onto the carboxylic acid in the 1-COO-iquo^?ligand, compared to that in the 3-COO-iqu^?ligand in 3. Transient absorption experiments also show that the ³MLCT excited state of 3 is able to reduce methyl viologen.     

A calorimetric and thermodynamic investigation of A2[(UO2)2(MoO4)O2] compounds with A = K and Rb and calculated phase relations in the system (K2MoO4 + UO3 + H2O)

A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A₂[(UO₂)₂(MoO₄)O₂], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K₂[(UO₂)₂(MoO₄)O₂] was determined using HF-solution calorimetry giving Δ_fH° (T = 298 K, K₂[(UO₂)₂(MoO₄)O₂], cr) = −(4018 ± 8) kJ · mol⁻¹. The low-temperature heat capacity, С_р°, was measured using adiabatic calorimetry from T = (7 to 335) K for K₂[(UO₂)₂(MoO₄)O₂] and from T = (7 to 326) K for Rb₂[(UO₂)₂(MoO₄)O₂]. Using these С_р° values, the third law entropy at T = 298.15 K, S°, is calculated as (374 ± 1) J · K⁻¹ · mol⁻¹ for K₂[(UO₂)₂(MoO₄)O₂] and (390 ± 1) J · K⁻¹ · mol⁻¹ for Rb₂[(UO₂)₂(MoO₄)O₂]. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Δ_fG°, for both phases giving: Δ_fG° (T = 298 K, K₂[(UO₂)₂(MoO₄)O₂], cr) = (−3747 ± 8) kJ · mol⁻¹ and Δ_fG° (T = 298 K, Rb₂[(UO₂)₂(MoO₄)], cr) = −3736 ± 5 kJ · mol⁻¹. Smoothed С_р°(Т) values between 0 K and 320 K are presented, along with values for S° and the functions [H°(T) − H°(0)] and [G°(T) − H°(0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K₂MoO₄ + UO₃ + H₂O) system with and without CO₂ at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach.     

Thermodynamic features associated with intercalation of some n-alkylmonoamines into barium phosphate

Elemental analysis for the synthesized crystalline lamellar compound conforms to the formula Ba(H₂PO₄)₂ and the X-ray diffraction patterns is in agreement with the lamellar structure for this compound. The precursor host was intercalated with a series of n-alkylmonoamines of the general formula H₃C(CH₂)_n-NH₂ (n = 1 to 4) in aqueous solution. The lamellar host was calorimetrically titrated with an aqueous amine solution at T = (298.15 ± 0.02) K and the enthalpy, Gibbs free energy and entropy were calculated. The enthalpic values increased, although not uniformly, with the number of carbon atoms is the amine chain, to give (−13.96 ± 0.12, −14.00 ± 0.48, −15.75 ± 0.23, −16.05 ± 0.11) kJ · mol⁻¹, from n = 1 to 4. The exothermic enthalpy, the negative Gibbs free energy and positive entropic values are in agreement with the favourable energetic process of intercalation for this system.