Hydrogen-bonding contributions to the lattice energy of salts for second harmonic generation

The lattice energies of a series of organic dihydrogenphosphate salts capable of second harmonic generation (SHG) have been calculated. These calculations, coupled with empirical data, indicate that a minimum of 20–25% of the lattice energy arises from hydrogen-bond interactions. Hydrogen bonding is shown to be a strong enough force to have a profound effect on the overall packing and crystal geometry of such ionic materials, and is thus an important factor to consider for crystal engineering.     

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Classical kinetic theories of polymer crystallization were applied to isothermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fitting the isothermal DSC crystallization data of previously self‐nucleated samples. Therefore, we demonstrated that by self‐nucleating the material at the ideal self‐nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crystal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p‐dioxanone), poly(ε‐caprolactone), and a high 1,4 model hydrogenated polybutadiene, we have interpreted the differences in primary nucleation energy barriers as arising from differences in nuclei density. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1478–1487, 2008     

Partial molal compressibilities of salts in aqueous solution and assignment of ionic contributions

Apparent molal adiabatic compressibilities of a number of 1:1, 2:1, 3:1, and 4:1 electrolytes have been accurately determined in the concentration range 0 to 1 m at 25°C. Debye-Hückel limiting-law behavior is observed in the low concentration region, and a scale of individual ionic compressibilities is proposed in terms of semiempirical calculations based on assessment of the contributions of the various types of solute-solvent interaction in the Frank and Wen model for a series of alkali halides. On this scale, K _s ^o (Cl^?, 25°C)=?17×10^?4 ml-mole^?1 bar^?1. Isothermal compressibility values also have been calculated for a number of the salts studied, and specific effects of hydrophobic interaction and hydrogen bonding on compressibility are indicated.     

Volatility and crystal lattice energy of palladium(II) chelates

Temperature dependence of saturated vapor pressure has been measured by gas saturation technique for volatile bis-chelates of palladium(II) with such ligands as acetylacetone, hexafluoroacetylacetone, diethyldithiocarbamate, diisopropyldithiophosphate, and also mixed ligand complex with acetylacetone and cyclooctadiene-2,4. Standard thermodynamic parameters of vaporization ΔH _T ⁰ and ΔS _T ⁰ were calculated. Crystal molecular packings and intermolecular interactions were analyzed basing on structural data. Atomatom potential calculation of van der Waals energy E_cryst in crystal lattice was performed and compared to the experimentally obtained values of sublimation enthalpy for the complexes under study.     

Quadrupole contributions to lattice vibration frequencies of benzene and naphthalene

Calculations have been made of the lattice vibration frequencies of benzene and naphthalene, in which quadrupole interactions have been added to the usual atom—atom terms. A significant improvement between theory and experiment is obtained: in particular the ordering of the highest A_g and B_g frequencies of naphthalene is given correctly for the first time.     

Cationic polyvinylamine binding to anionic microgels yields kinetically controlled structures

Polyvinylamine (PVAm) binding (absorption and adsorption) to carboxylated microgels gave colloidally stable, cationic microgels that can be centrifuged, washed, freeze dried, and redispersed in water with no loss in colloidal stability. Because both PVAm and the carboxylated microgels are pH sensitive, changes in microgel swelling and electrophoretic mobility in response to pH change can be positive or negative depending upon pH and the PVAm content of the microgels. For a given PVAm molecular weight, the steady-state saturated mass fraction of bound PVAm in the microgels varied by a factor of four in our experiments. We proposed that the PVAm content at saturation was controlled by the relative rates of the initial attachment of PVAm chains versus the rate of attached chain spreading on and into the microgel structure. This explanation was further supported by a series of quartz crystal microbalance measurements. Finally, PVAm binding to two types of PNIPAM microgels shows general features recently reported for other polyelectrolyte types. Specifically: (1) for surface localized anionic charges on the microgels, the mass fraction of bound PVAm increased with PVAm molecular weight and vice versa; (2) in virtually all conditions, the quantity of adsorbed cationic ammonium groups was much greater than the carboxylate content of the microgel; and (3) sodium chloride additions lowered the mass fraction of bound PVAm.     

Correlation energy contributions to reaction heats

New, more accurate, Hartree-Fock limit energies (E_HF) for ethane and ethylene are obtained from SCF total molecular energie using Ermler and Kern's procedure. These results, together with E_HF values for other small closed shell molecules, are employed to calculate correlation energy (E_c) contributions to reaction heats. Cancellation to within 98% of the total E_c involved, and often to more than 99%, is found for a wide variety of chemical reactions, which strongly suggests that there are systematic regularities in the contribution to E_c from the different kinds of electron pairs in the valence shell. Assuming trictly localized pairs occupying orbitals having strongly directional character, E_c for the valence shell is evaluated in terms of E_c per lone pair, E_c per X? H bond, and E_c per X/X shared pair for Ne and for molecules containing first row atoms, where X is C, N, O, and F.     

Transport of excitation energy at impurities distributed randomly in a three-dimensional crystal lattice

Using the continuous time random walks, the decay probability of an excitation at an impurity site is calculated numerically employing a general fractal distribution function in a three-dimensional disordered lattice. Results agree well with the previously calculated theoretical probabilities and also with experimental results.     

Parametrization of semiempirical models against ab initio crystal data: evaluation of lattice energies of nitrate salts

A method to estimate the lattice energies E(latt) of nitrate salts is put forward. First, E(latt) is approximated by its electrostatic component E(elec). Then, E(elec) is correlated with Mulliken atomic charges calculated on the species that make up the crystal, using a simple equation involving two empirical parameters. The latter are fitted against point charge estimates of E(elec) computed on available X-ray structures of nitrate crystals. The correlation thus obtained yields lattice energies within 0.5 kJ/g from point charge values. A further assessment of the method against experimental data suggests that the main source of error arises from the point charge approximation.     

Cationic polymerization of cyclohexene oxide by pentamethoxytrityl salts

Abstract—Polymerization of cyclohexene oxide (CHO) has been investigated in dichloromethane, using crystalline, soluble, and nonhygroscopic methoxy substituted trityl carbocationic salts having nonnucleophilic anions such as SbF ₆ ^? and PF ₆ ^? without cocatalysts or promoters at 25°C. The effects of salt counterion, photolysis time, and variation in salt concentration on the polymerization of CHO are discussed.     

Calculations of the electrostatic free energy contributions to the binding free energy of sulfonamides to carbonic anhydrase

The interactions between biologically important enzymes and drugs are of great interest. In order to address some aspects of these interactions we have initiated a program to investigate enzymedrug interactions. Specifically, the interactions between one of the isozymes of carbonic anhydrase and a family of drugs known as sulfonamides have been studied using computational methods. In particular the electrostatic free energy of binding of carbonic anhydrase II with acetazolamide, methazolamide,p-chlorobenzenesulfonamide,p-aminobenzenesulfonamide and three new compounds (MK1, MK2, and MK3) has been computed using finite-difference Poisson-Boltzmann (FDPB) [1] method and the semimacroscopic version [2, 3] of the protein dipole Langevin dipole (PDLD) method [4]. Both methods, FDPB and PDLD, give similar results for the electrostatic free energy of binding even though different charges and different treatments were used for the protein. The calculated electrostatic binding free energies are in reasonable agreement with the experimental data. The potential and the limitation of electrostatic models for studies of binding energies are discussed.     

Cationic and anionic states of CF, CCl, SiF and SiCl. Some new information derived using translational energy spectroscopy

Translational-energy spectroscopy was applied to collisional-excitation and charge-inversion reactions of CF⁺, CCl⁺, SiF⁺ and SiCl⁺ in order to gain energetic and bond-length information about the anionic and excited-cationic states of the title molecules. The excitation spectra revealed that the ã³Π state, known in CCl⁺ and SiCl⁺, has a term energy of 4.85 ± 0.15 eV in CF⁺ and 4.70 ± 0.20 eV in SiF⁺, while the 1¹Π state, known in CCl⁺, is not below the dissociation threshold in CF⁺, SiF⁺ and SiCl⁺. These data, and bond-length estimates for the ã³Π states, are consistent with documented ab initio predictions except for r_e of CF⁺(ã³Π) which seems to be larger than 1.21 Å. Charge-inversion spectra indicated that beams of monohalide cations formed from the tetrahalides, contained substantial proportions of ã³Π-state ions, and, in the case of CCl, SiF and SiCl, the broadness of spectral peaks was taken as evidence for the stability of the ã¹Δ-state anion. Adiabatic electron affinities were deduced to be 0.49 ± 0.15 eV, 0.89 ± 0.20 eV, 1.34 ± 0.30 eV and 1.40 ± 0.30 eV for the title molecules, respectively.     

Spectroscopy and crystal structure of anabasine salts

The anabasinium hydrochloride, hydriodide and perchlorate were characterized by IR and NMR spectroscopy as well as by X-ray diffraction. Anabasinium hydrochloride crystallizes with three independent ionic pairs in the asymmetric part of the orthorhombic unit cell, while anabasinium hydriodide and perchlorate crystals, being isostructural, are hexagonal and contain only one symmetry independent ionic pair. Despite these differences in the crystal data, both types of crystals display very similar helical solid-state patterns. The reported results combined with the CSD searches indicate an inherent tendency of anabasinium salts to crystallize with multiple asymmetric units, and to form folded arrangements in crystals. In the solid state the anabasinium cations predominantly adopt either synperiplanar or antiperiplanar conformations with respect to the mutual orientation of C^*–H and pyridine C–C(N) bonds, with deformations towards, respectively, (+) synclinal or (+) anticlinal rotamers.     

Double-exchange contributions to the first-order interaction energy between closed-shell molecules

Explicit formulas are presented for the double-exchange first-order interaction energy of two closed-shell molecules. Calculations on He₂ show that the double-exchange terms contribute significantly for R < 3.5 au.     

Cationic polymer adsorption on bilayer membrane containing anionic and cationic lipids

Microelectrophoresis, dynamic light scattering, fluorescence, and microcalorimetry are used to study the adsorption of a synthetic polycation, poly-N-ethyl-4-vinylpyridinium bromide, on the surface of three-component liposomes formed from electrically neutral phosphatidylcholine, anionic diphosphatidylglycerol (cardiolipin), and cationic dicetyldimethylammonium bromide, with the two latter being taken in equal amounts. The adsorption of the polycation on the liposomal membrane results in the generation of a positive charge, which provides the polycation-liposome complex with aggregation stability. Increasing salt concentration in the suspension causes the complex to dissociate into its components. According to the microcalorimetry data, the membranes of the initial three-component liposomes consist of two microphases, with one of them being enriched with the neutral lipid and another one, with the ionic components. The polycation adsorption does not lead to noticeable structural rearrangements in the liposomal membranes.     

Cationic polymerization of vinyl compounds in the presence of tetra-n-butylammonium salts

The effects of salts were examined in cationic polymerization of vinyl compounds. Cationic polymerization of styrene was carried out at 0°C, with acetyl perchlorate, stannic chloride, stannic chloride–trichloroacetic acid and boron trifluoride etherate as catalysts. Tetra-n-butylammonium perchlorate, fluoroborate and iodide were used as salts. The presence of small amounts of the salts changed both the polymerization rate and the molecular weight of polymer considerably. The consideration of various effects led to the conclusion that the results are explicable principally on the basis of counterion exchange. To confirm this, the copolymerization of 2-chloroethyl vinyl ether with γ-methylstyrene was investigated at ?78°C. The copolymer composition curve when stannic chloride was used as catalyst was changed and coincided with that of polymer obtained with acetyl perchlorate catalysis when the perchlorate salt was added. This supports the concept of counterion exchange.     

Approaches to supramolecular structures with various topologies in the crystal lattice

Simple molecular building blocks for creating supramolecular architectures in the crystal lattice are described. Combination of hydrogen bonding and alkyl chain crystallization is used to explore self-assembly¹ of molecular building blocks 1, 2, 3 and 4 in the crystal lattice. The crystal lattice of pure acids and its equimolar mixtures with bifunctional hydrogen bond acceptors are characterized by singe crystal analysis.     

Cationic photopolymerization with the aid of pyridinium‐type salts

Pyridinium‐type salts containing an N‐ethoxy group belong to the family of onium salts and are photoinitiators appropriate for the polymerization of monomers such as oxiranes and vinyl ethers which are not polymerizable by a free radical mechanism. The initiation is accomplished by direct or indirect (sensitized) photolysis of the onium ion, with the former being restricted to the wavelength range of self absorption, the latter being applicable at wavelengths of visible light. An additionally useful tool, namely free radical‐mediated generation of initiating species enlarges the versatility of pyridinium salts as photoinitiators. In this connection, the oxidation of free radicals by pyridinium‐type ions and the free radical‐induced fragmentation of alkoxy pyridinium ions are addressed in this article. Moreover, an interesting application is noted concerning the synthesis of novel block copolymers with the aid of the onium salt‐based photopolymerization technique.     

Sodium polyacrylate adsorption onto anionic and cationic silica in the presence of salts

Sodium polyacrylate is well known for its application as a scale inhibitor in common household products, and the effects of both monovalent and divalent metal cations on its structure have been covered by a range of previous publications. In the present article, we extend this work by using solvent relaxation NMR to look at the adsorption of the polyelectrolyte onto both positively and negatively charged silica and how this is altered by calcium chloride. In the anionic case, we found that polyacrylate adsorption was predictably very weak, and interestingly, perhaps counterintuitively, it was further reduced by calcium ions. This is probably linked to NaPA-Ca2+ binding, which changes the conformation and charge of the polyelectrolyte. In contrast, NaPA adsorbs very strongly on cationic silica, to the point that precipitation often occurs, particularly on addition of salt.