The random contact point model for pair interaction coefficients of unlike solute molecules

The random contact point model described in previous papers is extended to include like-unlike pair interaction coefficients. On the basis of this extension we present the thermodynamics of group interactions involving alkyl, hydroxyl and amide (peptide) groups and water molecules. The values obtained for the Gibbs energy of group interaction (absolute values ranging from 1.5 to 9.8 kJ-mol^–1) indicate that all these groups attract or repel each other in aqueous solutions with comparable strength. Group interaction parameters obtained from aqueous and non-aqueous systems, and based on interaction coefficients and other thermodynamic quantities, agree well. The model also allows for the quantification, though not for the prediction, of the cooperativity of hydrophobic interaction.     

Energy transfer and migration in highly forbidden transitions of lanthanide ion doped crystals

Förster–Dexter theory for resonant energy transfer is extended to higher order and applied to explain the rates of energy transfer and migration processes in highly forbidden transitions for some solid-state lanthanide (Ln) ion systems for which experimental results are available. The second-order two-body energy transfer mechanism involves two inter-ion correlated dipole electrostatic interactions, i.e. dipole dipole–dipole dipole (dd–dd) energy transfer, also termed Axe–Axe energy transfer in view of the similarity of the theoretical formalism with that for two-photon transitions. Each of the dipolar transitions consists of a transition from the 4fⁿ configuration to an opposite-parity configuration, taken to be 4fⁿ⁻¹5d. dd–dd energy transfer is a short-range (R⁻¹²) interaction so that it is most important in systems with short donor Ln–acceptor Ln separations. The energy transfer formalism is extended to include spin-forbidden transitions at one or two sites, the so-called Axe–Judd–Pooler (Axe–JP) and JP–JP energy transfer. In some cases the dd–dd mechanism is the dominant energy transfer process, as exemplified herein for energy migration in the ⁵D₀ state of Sm²⁺ in SrF₂, and also in the ⁵D₀ state of Eu³⁺ in Cs₂NaEuCl₆.     

Theoretical prediction of collision efficiency between adhesion-deficient bacteria and sediment grain surface

Our earlier results concerning bacterial transport of an adhesion-deficient strain Comamonas sp. (DA001) in intact sediment cores from near South Oyster, VA demonstrated that grain size is the principle factor controlling bacterial retention, and that Fe and Al hydroxide mineral coatings are of secondary importance. The experimentally determined collision efficiency (α) was in the range of 0.003–0.026 and did not correlate with the Fe and Al concentration. This study attempts to theoretically predict α, and identifies factors responsible for the observed low α. The modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate the total intersurface potential energy as a function of separation distance between bacterial and sediment surfaces and to provide insights into the relative importance of bacterial and sediment grain surface properties in controlling magnitude of α. Different models for calculating theoretical α were developed and compared. By comparing theoretical α values from different models with previously published experimental α values, it is possible to identify a suitable model for predicting α. When DA001 bacteria interact with quartz surfaces, the theoretical α best predicts experimental α when DA001 cells are reversibly attached to the secondary minimum of the energy interaction curve and α depends on the probability of escape from that energy well. No energy barrier opposes bacterial attachment to clean iron oxide surface of positive charge at sub-neutral pH, thus the model predicts α of unity. When the iron oxide is equilibrated with natural groundwater containing 5–10 ppm of dissolved organic carbon (DOC), its surface charge reverses, and the model predicts α to be on the order of 0.2. The theoretical for DA001 in the natural sediments from South Oyster, VA was estimated by representing the surface potential of the sediment as a patch-wise binary mixture of negatively charged quartz (ζ=−60 mV) and organic carbon coated Fe–Al hydroxides (ζ=−2 mV). Such a binary mixing approach generates α that closely matches the experimental α. This study demonstrates that it is possible to predict α from known bacterial and grain surface properties.     

Theoretical and experimental comparison of the colloid stability of two polystyrene latexes with different sign and value of the surface charge

 The purpose of this paper is to apply the classical DLVO theory to explain the colloid stability of two model colloids with similar size and different sign and value of the surface charge. For this comparison the hydrodynamic interaction and the presence of hydration forces (extended DLVO theory) have been taken into account. The experimental stability factor and the experimental doublet rate constant in diffusion conditions were compared with those evaluated theoretically. The mathematical treatment permits an easy evaluation and interpretation of the different adjustable parameters such as the Hamaker constant, diffuse layer potential and the hydration layer thickness. The theoretical and experimental comparison shows that the “extended DLVO theory” only permits to explain the stability curves Log[W]/Log[KBr] in a semiquantitative way by using, for the evaluation of the total interaction potential V _T, a value of the Hamaker constant (A) similar to the classical theoretical one for polystyrene particles dispersed in water. In the case of the anionic latex, it was necessary to admit the presence of a hydration layer of a thickness similar to the radius of the hydrated/dehydrated counterion. On the other hand, by using the experimental doublet rate constant in diffusion conditions, we obtain a lower value of the Hamaker constant (A), but within the range of the A values usually found in previous studies. Received: 8 September 1997 Accepted: 8 January 1998     

NMR investigation of non-covalent aggregation of coordination compounds ranging from dimers and ion pairs up to nano-aggregates

This review summarizes the results recently obtained by our research group investigating the non-covalent aggregation of coordination compounds in solution through NMR spectroscopy. First, systems that can undergo only weak non-covalent interactions, such as dispersive and dipole–dipole ones, are considered; successively, coordination compounds that are capable to establish more energetic non-covalent interactions, such as hydrogen bonding and/or extended π–π stacking interactions, are taken into account. The parallelism between the energy of non-covalent interactions and the level of aggregation is highlighted. The results concerning the latter are mainly obtained through diffusion NMR experiments. In some cases, information about the structure of non-covalent aggregation in solution, obtained through intermolecular NOE studies, is discussed and contrasted with that observed in the solid state (by means of X-ray single crystal investigations, mainly carried out by our group) and/or derived from theoretical calculations.     

Reaction potential map analysis of chemical reactivity—III

Reaction potential maps (RPM) have been introduced as a new tool for the study of molecular reactivity. The equipotential energy maps, which are created on given planes around a molecule, define reaction contours towards specific counter-reagent models and are evaluated by perturbation theory. Since the calculated interaction energy involves electrostatic, polarization, exchange, and charge transfer energies, the RPM's can be used to predict site selectivity in a variety of chemical reactions. We found that the calculated RPM's of the SCN^– anion explained well the experimental observations that it reacts at the S atom with soft electrophiles and at the N atom with hard electrophiles. The difference in reactivity between SCN^– and OCN^– was clearly shown by the RPM's of these anions. The ambident nucleophilic nature of the NO ₂ ^– and the CH₂CHO^– anions was also well represented by their RPM's.     

Coadsorption of Halide Anions and 1-Adamantanol Molecules on a Mercury Electrode

Coadsorption of 1-adamantanol (AdOH) and halide anions (F^–, Cl^–, Br^–) at the Hg/H₂O interface is studied systematically. Experimental results are compared with calculations of these systems within a modified Alekseev–Popov–Kolotyrkin model complemented by a set of two Frumkin isotherms. A satisfactory agreement between experimental data and theoretical conclusions concerning dependences of the phase transition potentials and the adsorption-desorption potentials on the concentrations of supporting electrolyte and organic substances is established. The experimental and calculated potential dependences of the differential capacitance in the phase transition region in the case of the AdOH adsorption from electrolytes containing halide anions are compared. It is shown that the shape of experimental capacitance curves corresponds to either a weak attraction of species of different nature during concurrent adsorption or the absence of lateral interaction between them. Accounting for the contribution made by the energy of the diffuse double layer to the overall energy gain during the adsorption removes seeming contradictions in the interpretation of dependences studied.     

Determination of total attenuation cross section of carbon using secondary excitors in the energy range 4.5–60 keV

Experimental total attenuation coefficients of carbon in the photon energy range 4.5–60 keV have been measured. The total attenuation cross section of C rapidly decreased with energy in the range of 4.5–10 keV. The measured values are in agreement with the theoretical values.     

Conformational analysis and vibrational spectroscopic investigation of 3-phenylpropylamine

The possible stable forms of 3-phenylpropylamine (3-PPA) molecule were experimentally and theoretically studied by infrared and Raman spectroscopy. FT-IR and Raman spectra of 3-PPA were recorded in the regions of 4000–400 cm⁻¹ and 3700–60 cm⁻¹, respectively. The potential energy surface corresponding to the internal rotations of the molecule was investigated by semi-empirical quantum mechanical methods, and appropriate conformers defined with B3LYP hybrid density functional theory method along with the basis sets of different size and type. Results from experimental and theoretical data showed the trans–trans–gauche (TTG) to be the most stable form of a 3-PPA molecule.     

Electroreduction of [Fe(CN)6]3– on a Mercury Electrode: Substantiating Activationless Character of the Process at High Overvoltages

Experimental cathodic polarization curves, obtained on a mercury electrode in 0.33 mM K₃[Fe(CN)₆] + 0–1.5 mM KCl solutions, are analyzed quantitatively. On the basis of quantum-chemical calculations of the geometry of species [Fe(CN)₆]^3– and [Fe(CN)₆]^4– and charge distributions in them, it is shown that the species interaction with the EDL field is equivalent to a repulsive interaction between effective point charges localized near anion centers in the diffuse layer. The effective Born radius of ferricyanide anion and the solvent reorganization energy are calculated, and inner-sphere constituents of the energy are estimated. These parameters are used for computing theoretical dependences of the transfer coefficient on the overvoltage via the equation of the quantum-mechanical theory of elementary act. The value of , determined from corrected Tafel plots, is shown to substantially depend on the assumptions adopted when analyzing the system in the framework of the classical slow-discharge theory; it is close to the theoretical value only if the participation of anion–cation associates formed in the bulk solution is taken into account. Such an approach explains the weak temperature dependence of the process rate. The experimental facts do not contradict theoretical prediction that the reaction occurs in the vicinity of the activationless region.     

Comparative quantum chemical studies in the framework of Gaussian approximations of the acidic characteristics of the simplest zeolite clusters and of the sulfuric acid molecule

Different variants of the Gaussian approximation, giving the energetic characteristics of molecules with chemical accuracy (±2 kcal mol^–1), are applied to calculations of the deprotonation energy of the sulfuric acid molecule in the gas phase as well as to the simplest clusters modeling the bridging hydroxyl groups in zeolites, The conclusion is made that the bridging hydroxyls am more acidic than the sulfuric acid molecule. The estimated range of deprotonation energy in zeolites (275±15 kcal mol^–1), is in good agreement with experimental data and with results ofab initio calculations for extended models including several tens of atoms. The effects of the quality of the basis set and electron correlation on deprotonation energy are also discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1641–1647, July, 1996.     

Stark mixing by ion-Rydberg atom (molecule) collisions

Classical molecular dynamics simulations are performed to cover Stark mixing transitions (nℓ→nℓ^′) in Rydberg atoms by collision with slow ions. Accuracy is tested by comparison with the exact analytical classical probabilities previously obtained when the ion–atom potential is taken as the long-range ion–dipole interaction. The results are provided not only for the ion–dipole interaction but also for the full electrostatic interaction. It is shown, by comparison, that the ion–dipole potential alone provides reliable probabilities. The method is highly accurate and is very amenable to ready inclusion of other processes competing with Stark mixing.     

Dissolution of ZnO nanoparticles at circumneutral pH: a study of size effects in the presence and absence of citric acid

Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.     

Total (elastic + inelastic) cross sections for electron scattering with bound and free germanium and lead atoms

Spherical complex optical potential (SCOP) approach has been used to compute the differential, total (elastic + inelastic) and momentum transfer cross sections for electrons scattering from the bound and free germanium and lead atoms in the energy range from 100–5000 eV. We find that the present calculated differential scattering cross sections (DCS) exhibit all important features (such as forward peaking, dip at middle angles and enhanced backward scattering) observed in other theoretical calculations and experimental measurements. The effect of absorption potential is generally to reduce the elastic cross section.     

Particle-size analysis of latexes by classical and small-angle spectrodissymmetry

The spectral dependence of classical and small-angle dissymmetryZ in polydispersions containing spherical particles with lognormal distribution was theoretically studied. It has been found, for instance, for polystyrene latex that small-angle dissymmetry may be used for determining the distribution parameters in systems with particle medial radius 60–700 nm on the basis of measurements ofZ (10°/5°( at two wavelengths of radiation. For particles outside this interval and by use of working relationships of classical dissymmetry only the application of a wide range of wavelengths comes into consideration by distribution analysis.     

A structurally driven spectral transform Lanczos method for mixed quantum—classical canonical simulations (MQCC): the thermodynamics of Kr10–H and the energies of Krn–H, (n:1→9)

We develop and test an approximate approach for canonical simulations of weakly bound atomic or molecular systems for which some degrees of freedom can be treated separately by quantum mechanics. The system chosen for testing is Kr₁₀–H, for which the adiabatic approximation applied to separate the hydrogen degrees of freedom works reasonably well. The hydrogen atom is bound to the Kr clusters at cold temperatures and we calculate several bound states for clusters in the n=1–9 range, in the global minimum configuration. The structural character of the mixed quantum classical simulation is substantially different than the classical simulation for Kr₁₀–H as a result of zero point energy effects. When quantum effects are included, the low temperature dynamics of Kr₁₀–H are dominated by a significant well to well hopping about an incomplete icosahedral krypton core.     

MRCI potential energy curves and spectroscopic constants of the ground and low-lying excited states of HgCd

The multireference configuration interaction (MRCI) electronic energy calculations with different basis sets have been performed on the ground state (X¹Σ) and three low-lying excited states (³Σ, ¹Π and ³Π) of HgCd dimer. The obtained potential energy curves (PECs) are fit to analytical potential energy functions (APEFs) using the Murrell–Sorbie potential function. Spectroscopic constants are calculated using the APEFs. Based on the PECs, the vibrational levels of each state are predicted. Our equilibrium positions of the X¹Σ state and ³Π state are in excellent agreement with the experimental reports.     

Ab initio and density functional theory based studies on collagen triplets

An understanding of the amino acid sequence dependent stability of polypeptides is of renowned interest to biophysicists and biochemists, in order to identify the nature of forces that stabilize the three-dimensional structure of proteins. In this study, the role of various collagen triplets influencing the stability of collagen has been addressed. It is found from this study that proline can stabilize the collagen triplet only when other residues are also in the polyproline II conformation. Solvation studies of various triplets indicate that the presence of polar residues increases the free energy of solvation. Especially the triplets containing arginine residues displays a higher solvation free energy. The chemical hardness of all the triplets in collagen-like conformation has been found to be higher than that in the extended conformation. Studies on Gly–X–Y, Gly–X–Hyp, and Gly–Pro–Y triplets confirm that there will be local variations in the stability of collagen along the entire sequence.