Detection of hydrogen bonding in solution: a 2H nuclear magnetic resonance method based on rotational motion of a donor/acceptor complex

The effect of hydrogen bonding on the rotational correlation time of an H-bond acceptor, pyridine N-oxide-d(5), in various solvents was investigated using the (2)H spin-lattice relaxation time (T(1)). The results demonstrate a linear relationship between viscosity and measured rotational correlation times, an example of Stokes-Einstein-Debye behavior. The results also clearly demonstrate reduced rotational rates for the probe in hydrogen bonding solvents in comparison to solvents incapable of forming hydrogen bonds with the probe. The utility of this observation was exploited to estimate the association constant (K(a)) through an NMR titration procedure. These results are presented as a new technique that can be applied to the characterization of hydrogen bonding in similar systems.     

Hydrogen bonding and the cryoprotective properties of glycerol/water mixtures

Molecular dynamics simulations and infrared spectroscopy were used to determine the hydrogen bond patterns of glycerol and its mixtures with water. The ability of glycerol/water mixtures to inhibit ice crystallization is linked to the concentration of glycerol and the hydrogen bonding patterns formed by these solutions. At low glycerol concentrations, sufficient amounts of bulk-like water exist, and at low temperature, these solutions demonstrate crystallization. As the glycerol concentration is increased, the bulk-like water pool is eventually depleted. Water in the first hydration shell becomes concentrated around the polar groups of glycerol, and the alkyl groups of glycerol self-associate. Glycerol-glycerol hydrogen bonds become the dominant interaction in the first hydration shell, and the percolation nature of the water network is disturbed. At glycerol concentrations beyond this point, glycerol/water mixtures remain glassy at low temperatures and the glycerol-water hydrogen bond favors a more linear arrangement. High glycerol concentration mixtures mimic the strong hydrogen bonding pattern seen in ice, yet crystallization does not occur. Hydrogen bond patterns are discussed in terms of hydrogen bond angle distributions and average hydrogen bond number. Shift in infrared frequency of related stretch and bend modes is also reviewed.     

Monohydric alcohol transport in a rubbery poly(urethan)

Steady-state permeation rates and equilibrium sorption measurements were made as a function of temperature for a series of alcohols in a polybutylene adipate polyurethan. The alcohol series included methanol; ethanol; n-propanol; n-, iso-, sec-, and t-butanol; n-pentanol, n-hexanol, and n-heptanol (all at unit activity). Calculated integral diffusivities of the various penetrants correlated well with effective penetrant size. The solubility coefficients for the various alcohols increased as the difference between the solubility parameters of the polymer and penetrant decreased for the normal alcohol series. The solubility coefficients for the branched alcohol isomers were characterized by values lower than those predicted by the correlation for the normal alcohols; possibly steric hindrance limits sorption on specific polymer sites. A detailed analysis of the system ethanol/poly(butylene adipate-polyurethan) revealed a sigmoidal increase of the diffusion coefficient with increasing penetrant concentration, suggesting a competition between penetrant clustering and polymer plasticization. The diffusion process appears to be time dependent as well as concentration dependent. Anomalies in the diffusion behavior that can reasonably be related to polymer segmental relaxation are present to a much larger degree as temperature is increased. Ethanol sorption was further complicated by positive deviation from Henry's law at activities in excess of 0.2.     

A variational biorthogonal valence bond method

The details of a simple and efficient scheme for performing variational biorthogonal valence bond calculations are presented. A variational bound on the energy functional is obtained through the use of a complete configuration expansion in a well-chosen subset of orbitals. The resultant wave functions are clearly dominated by the covalent (spin-coupled) structures, with a negligible contribution from ionic structures. The orbitals obtained compare favorably with overlap enhanced atomic orbitals obtained by other valence bond approaches. The method is illustrated by calculations on water and dioxygen difluoride. © 1994 by John Wiley & Sons, Inc.     

Negatively charged poly(vinylidene fluoride) microfiltration membranes by sulfonation

Negatively charged PVDF microfiltration membranes were prepared using direct sulfonation with chlorosulfonic acid. The effect of sulfonation on the surface chemical properties, morphology, pore size distribution, hydrophilicity, water uptake, pure water flux, fouling and rejection were investigated. As the sulfonation reaction time was furthered, the degree of sulfonation and ion-exchange capacity increased and the membranes became more hydrophilic due to introduction of sulfonyl groups to the membrane surface. Using X-ray photoelectron spectroscopy, the composition of sulfonyl group with respect to sulfur concentration increased with time. From the SEM and porosity measurements, both the untreated and treated membranes did not reveal a substantial change in its morphology. The pure water flux increased significantly having a decreasing intrinsic resistance trend with degree of sulfonation. Both fouling phenomena and rejection were enhanced, with fouling of charged poly(styrene sulfonic acid) molecules on the surface-modified membrane decreased and rejection values increased with increasing degree of sulfonation mainly due to the effective electrostatic repulsion between the negatively charged PSSA and the negatively charged membrane.     

Bonding trends in pyrites and a reinvestigation of the structures of PdAs2, PdSb2, PtSb2 and PtBi2

PdAs₂, PdSb₂, PtSb₂ and PtBi₂ have been formed from reactions of 1-gram samples of the elements in quartz tubes at 923 K, and their crystal structures have been reinvestigated. These materials crystallize with the pyrite structure. The lattice constant, a, and the positional parameter, x, of the nonmetal atoms are the following: PdAs₂ 5.9790(3) Å, 0.38317(3); PdSb₂ 6.464(1) Å, 0.3733(1); PtSb₂ 6.4423(9) Å, 0.3753(2); PtBi₂ 6.7014(8) Å, 0.3710(1). The metal-nonmetal and the closest nonmetal-nonmetal bond lengths are the following: PdAs₂, 2.4949(2), 2.4198(3) Å; PdSb₂, 2.6765(9), 2.838(2) Å; PtSb₂, 2.671(1), 2.782(2) Å; PtBi₂, 2.7706(8), 2.995(2) Å. The platinum-group dipnictides show a strengthening of the nonmetal-nonmetal bond with heavier nonmetal atoms, as evidenced by a bond-valence analysis. These dumbbell bonds are compared to other observed dumbbells in crystals. Comparisons are made to the transition-metal pyrite disulfides, where electronic effects are correlated with the nonmetal-nonmetal bond strength. The valence-matching principle is shown to correlate well with the observed structures of the alkali metal superoxides and alkaline earth metal peroxides. Bond valence analysis for a hypothetical ?cubic”? MnO₂ is given along a fluorite to pyrite pathway.     

Synthesis and X-ray crystal structure determination of the first Copper(II) complexes of tetraazamacrocycle-glyoxal condensates

Novel Cu(II) complexes CuLCl(2) (L = 1-4) have been synthesized containing the metal bound to a well-known type of tetracyclic bisaminal formed from the condensation of glyoxal and tetraazamacrocycles (1 = cyclam-glyoxal condensate, 2 = [13]aneN4-glyoxal condensate, 3 = cyclen-glyoxal condensate, 4 = isocyclam-glyoxal condensate). The four-coordinate complexes were characterized by X-ray crystallography, electronic spectroscopy, solid-state magnetic moments, and electron spin resonance spectroscopy. The tetracyclic bisaminals, although having four potential donor atoms, are bound in a cis-bidentate fashion to Cu(II) with two additional cis-chloride donors. The ligands take up folded conformations, and with the exception of ligand 4, only nonadjacent nitrogen atoms coordinate. As expected, ligand 2 in Cu(2)Cl(2) has a folded structure similar to those of the previously characterized 1 and 3. The conformation of 4 in the complex Cu(4)Cl(2) differs from 1-3 in that three nitrogens direct their lone pairs to one side of the folded tetracycle, with adjacent nitrogen atoms coordinated to Cu(II). This difference is probably caused by the presence of the more flexible seven-membered ring rather than the five- to six-membered rings in 1-3. Air oxidation of Cu(I) in the presence of 1 or 3 results in bis(mu-hydroxo) dimers as characterized by X-ray crystal structures, suggesting dioxygen binding, followed by O-O bond splitting to give the Cu(2)O(2) diamond core.     

Fluoride‐terminated hyperbranched poly(arylene ether phosphine oxide)s via nucleophilic aromatic substitution

A series of AB_x‐type triarylphosphine oxide monomers, bis‐(4‐fluorophenyl)‐(4‐hydroxyphenyl)phosphine oxide ( 4a ), bis‐(3,4‐difluorophenyl)‐(4‐hydroxyphenyl)phosphine oxide ( 4b ), and 4‐hydroxyphenyl‐bis‐(3,4,5‐trifluorophenyl)phosphine oxide ( 4c ) were prepared, characterized, and polymerized under nucleophilic aromatic substitution conditions [N‐methylpyrrolidone (NMP), K₂CO₃] to provide the corresponding hyperbranched poly(arylene ether phosphine oxide)s with number‐average molecular weights ranging from 9200 to 14,600 Da. NMR spectroscopic analysis indicated the presence of highly branched products with an approximate degree of branching of 0.57. The polymers were soluble in a variety of typical organic solvents and displayed excellent thermal stability. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1456–1467, 2002