Isotope effect in gas transport. I. Measurement of the free volume element in four polymers above the glass transition temperature

The experimental observation of an isotope effect in diffusive gas transport across polymeric films is reported. The differences in energies of transport between hydrogen and deuterium are used to estimate the effective dimensions of characteristic minima or “wells” in the potential energy surface of rubbery polymers. The size of a well, determined by assuming it to be a cubical cavity, is interpreted as the effective free volume element v_f, as measured by the hydrogen molecule probe. Estimations are made of the entropy of activation and “jump distance” for the hydrogen diffusion process based on v_f values and experimentally determined pre-exponential factors.     

Quantum effects on adsorption and diffusion of hydrogen and deuterium in microporous materials

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.     

Two-dimensional vibrational analysis of the Lippincott-Schröder potential for OHO,NHO and NHN hydrogen bonds and the deuterium isotope effect

Two-dimensional vibrational analyses [i.e. crude adiabatic approximation, SCF approximation and variational method (crude adiabatic basis function)] are performed on the hydrogen bond systems consisting of the Lippincott-Schröder potentials for the OHO, NHO and NHN bonds. The OHO and NHN systems are supposed to be linear and the bent structure is considered for the NHO system. The frequency shift for the hydrogen bond length variation and its deuterium substitution effects are in good agreement with experiment. The anomalies in the frequency ratio ν_OH/ν_OD at an O—O distance of 2.5 Å, and in the interminimum distance shift on deuteration at 2.5 Å are well explained as the difference of double minimum behavior between the vibrational states of proton and deuterium. It is also shown that the Lippincott-Schröder model for the OHO system supplies the general features for proton tunneling, proton delocalization beyond the barrier and other type processes in hydrogen bonds.     

Segmental mobility model for diffusion of gases in polymers

Diffusion of small molecules in polymers is described quantitatively in terms of segmental mobility processes. The diffusion coefficient depends on a diffusive jump length, which is characteristic of the polymer, and a jump frequency, which is equated to the segmental mobility rate. The presence of a particular solute increases mobility of the surrounding polymer segments by a predictable amount, which is related to the partial molar volume of the solute. The theory is fit to experimental diffusion data, and partial molar volumes are calculated from the fitting parameters. Good agreement with experimental partial molar volumes is obtained.     

The crystal structure of diiodo((o-methylthiophenyl)diphenylphosphine)palladium(II), a complex exhibiting a large structural trans influence

The crystal structure of the title compound has been determined by conventional X-ray diffraction techniques. The red diamond-shaped crystals are monoclinic, space group P2₁/n, a 8.914(4), b 16.090(12), c 14.991(5) Å, β 95.97(2)°, V 2138.44 ų, Z = 4. Refinement by full-matrix least-squares methods employed anisotropic thermal parameters for all non-hydrogen atoms and isotropic temperature factors for hydrogen atoms, and returned final residuals of R = 0.028 and R_w = 0.028.The complex is monomeric, with a square planar coordination geometry comprising the S and P atoms of the bidentate (o-methylthiophenyl)diphenylphosphine ligand and the two iodine atoms. The PdI distance trans to P is 2.658(1) Å whereas that trans to S is 2.602(1) Å. The difference of 56 standard deviations illustrates the greater structural trans influence of phosphine over thioether donors. The PdP and PdS distances of 2.250(2) and 2.288(2) Å, respectively, are normal. Two phenyl hydrogen atoms approach the Pd atom in the axial regions above and below the square plane at distances of 2.98 and 2.99 Å.     

Molecular dynamics simulation study of the mechanisms of water diffusion in a hydrated,amorphous polyamide

Atomistic, molecular dynamics simulations of water diffusion in a hydrated, amorphous polyamide have been carried out to study the effects of polymer dynamics, cross-linking, and hydrogen bonding interactions on the molecular mechanisms of diffusion. This polymer was selected as a model for the discriminating layer of FT-30 reverse osmosis membranes, used commercially for water desalination. Analysis of the configurations generated during the simulations shows that, at the relatively high water content studied, a continuous water phase is formed which permeates the polymer and consists of more than 90% of all waters of hydration. Water diffusion takes place by distinct “jump-like” movements between weakly localized sites in this continuous phase. The jump length is on average ∼3Å, independent of the system studied, and is most likely defined by the local, cooperative rearrangement of water molecules. However, the jump frequency, or equivalently, the rate of water diffusion varies from system to system, and depends on polymer dynamics and cross-linking density, and more significantly, on water–water hydrogen bonding interactions.     

Quantum diffusion of hydrogen and muonium atoms in liquid water and hexagonal ice

We have used the ring polymer molecular dynamics method to study the diffusion of muonium, hydrogen, and deuterium atoms in liquid water and hexagonal ice over a wide temperature range (8-361 K). Quantum effects are found to dramatically reduce the diffusion of muonium in water relative to that predicted by classical simulation. This leads to a simple explanation for the lack of any significant isotope effect in the observed diffusion coefficients of these species in the room temperature liquid. Our results indicate that the mechanism of the diffusion in liquid water is similar to the intercavity hopping mechanism observed in ice, supplemented by the diffusion of the cavities in the liquid. Within the same model, we have also been able to simulate the observed crossover in the c-axis diffusion coefficients of hydrogen and deuterium in hexagonal ice. Finally, we have been able to obtain good agreement with experimental data on the diffusion of muonium in hexagonal ice at 8 K, where the process is entirely quantum mechanical.     

Microwave structure determination and quadrupole coupling measurements on acetylene-HCN and ethylene-HCN complexes

Microwave rotational spectra for the complexes HCCH-HCN, HCCH-DCN, DCCD-HCN, H₂CCh₂-HCN and H₂CCH₂-DCN were observed using a pulsed nozzle Fourier transform spectrometer. The distance from the HCN carbon atom the the center of mass is. 3.655(10) Å for acetylene-HCN and 3.709(10) Å for ethylene-HCN. Both complex are T-shaped with the HCN hydrogen atom closest to the CC bond. The HCN axis is perpendicular to the plane of ethylene for ethylene-HCN. Nitrogen and deuterium quadrupole coupling was measured.     

Effect of the Hydrostatic Pressure on the Thermodynamics of Structural Phase Transition in Quasi-One-Dimensional Ferroelectrics Pb(H/D)PO<Subscript>4</Subscript>: Quantum Chemical Analysis

The change in the structural phase transition thermodynamics of lead hydrogen phosphate (LHP) and lead deuterium phosphate (LDP) depending on the external hydrostatic pressure was studied using the Ising pseudo-spin model taking account of tunneling and long-range effects. The same simplified version of the quantum chemical approach that we proposed for studying these systems at atmospheric pressure was used. The critical transition temperatures T_c were estimated for both systems and the considerable decrease in T_c observed experimentally was explained in each case. The calculations gave an estimate for the width of the barrier in LHP (≈0.35 Å); no experimental neutron diffraction value for this barrier is available from the literature.     

Interdiffusion of PMMA and PVF2, studied by solid-state NMR

A previously published new solid-state nuclear magnetic resonance (NMR) method is applied to the interdiffusion of poly(methacrylate) (PMMA) and poly(vinylidene fluoride) (PVF₂) above their T_g. Via a variation of the cross-polarization technique magnetization is transferred from protons to fluorines. When this magnetization is made to disappear at the fluorine sites, only those protons that are distant from fluorines greater than the distance over which cross-polarization functions will retain their magnetization. In this way we detect the fraction of PMMA near (ca. 20 Å) PVF₂. Starting from sheets of PMMA and PVF₂, which are then heated at 190°C for a variable time, and applying the above technique, we can determine the fractions of PMMA and PVF₂ that have diffused within a distance of a few Å of each other. The intrinsic diffusion coefficients of PMMA and PVF₂ determined from such experiments compare well with literature data. Initial attempts to fit the experimental data suggest that the concentration dependence of the diffusion coefficients cannot be neglected. © 1994 John Wiley & Sons, Inc.     

Dynamic Studies on Kinetic H2/D2 Quantum Sieving in a Narrow Pore Metal–Organic Framework Grown on a Sensor Chip

The separation of deuterium from hydrogen still remains a challenging and industrially relevant task. Compared to traditional cryogenic methods for separation, based on different boiling points of H₂ and D₂, the use of ultramicroporous materials offers a more efficient alternative method. Due to their rigid structures, permanently high porosity, tunable pore sizes and adjustable internal surface properties, metal–organic frameworks (MOFs), a class of porous materials built through the coordination between organic linkers and metal ions/clusters, are more suitable for this approach than zeolites or carbon-based materials. Herein, dynamic gas flow studies on H₂/D₂ quantum sieving in MFU-4, a metal-organic framework with ultra-narrow pores of 2.5 Å, are presented. A specially designed sensor with a very fast response based on surface acoustic waves is used. On-chip measurements of diffusion rates in the temperature range 27–207 K reveal a quantum sieving effect, with D₂ diffusing faster than H₂ below 64 K and the opposite selectivity above this temperature. The experimental results obtained are confirmed by molecular dynamic simulation regarding quantum sieving of H₂ and D₂ on MOFs for which a flexible framework approach was used for the first time.     

Structural conformation in a poly(ethylene oxide) film determined by X-ray emission spectroscopy

The electronic structure of pol(ethylene oxide) (PEO) in a thin (<1 mu) film sample was experimentally probed by X-ray emission spectroscopy. Both nonresonant and resonant X-ray emission spectra were simulated by using density functional theory (DFT) applied to four different models representing different conformations in the polymer. Calculated spectra were compared with experimental results for the PEO film. It was found that the best fit was obtained with the polymer conformation in PEO electrolytes from which the salt (LiMF6, M = P, As, or Sb) had been removed. This conformation is different from the crystalline bulk polymer and implies that film casting, commonly used to form electrolytes for Li polymer batteries, induces the same conformation in the polymer not depending upon the presence of salt.     

Accurate sizing of nanoparticles using confocal correlation spectroscopy

The ability to accurately size low concentrations of nanoscale particles in small volumes is useful for a broad range of disciplines. Here, we characterize confocal correlation spectroscopy (CCS), which is capable of measuring the sizes of both fluorescent and nonfluorescent particles, such as quantum dots, gold colloids, latex spheres, and fluorescent beads. We accurately measured particles ranging in diameter from 11 to 300 nm, a size range that had been difficult to probe, owing to a phenomenon coined biased diffusion that causes diffusion times, or particle size, to deviate as a function of laser power. At low powers, artifacts mimicking biased diffusion are caused by saturation of the detector, which is especially problematic when probing highly fluorescent or highly scattering nanoparticles. However, at higher powers (>1 mW), autocorrelation curves in both resonant and nonresonant conditions show a structure indicative of an increased contribution from longer correlation times coupled with a decrease in shorter correlation times. We propose that this change in the autocorrelation curve is due to the partial trapping of the particles as they transit the probe volume. Furthermore, we found only a slight difference in the effect of biased diffusion when comparing resonant and nonresonant conditions. Simulations suggest the depth of trapping potential necessary for biased diffusion is > 1 k(B)T. Overcoming artifacts from detector saturation and biased diffusion, CCS is particularly advantageous due to its ability to size particles in the small volumes characteristic of microfluidic channels and aqueous microdroplets. We believe the method will find increasing use in a wide range of applications in measuring nanoparticles and macromolecular systems.     

Hydrogen and deuterium atoms in amorphous ZrNi alloys

The local environment of hydrogen and deuterium atoms dissolved in amorphous alloys with the composition ZrNi and Zr₂Ni was studied using neutron inelastic scattering and neutron diffraction techniques. The frequency distribution functions exhibit a broad peak at about 130 meV. Interatomic DNi and DZr correlations are observed in the radial distribution function curves at correlation lengths of 1.7 Å and 2.1 Å respectively for both alloy compositions. It is concluded that the majority of hydrogen and deuterium atoms in the amorphous alloys are trapped at holes in the tetrahedra which consist of three or four zirconium atoms. The maximum hydrogen content per metal atom can be explained by assuming a statistical configuration of the metal atoms.     

Theoretical and experimental study of lone pair interactions in THF/chloranilic acid system

Crystallization of a multi-component molecular crystal that consists of chloranilic acid with THF as solvate afforded the general formula C₆H₂O₄Cl₂·THF. Its crystal structure is new and reveals new example of cooperative lone pair–π interactions (oxygen of THF to centroid of chloranilic distance of 3.258 Å) beside others (e.g., hydrogen bonding OH···O) with new experimental evidence of receptor/solvent as a lone pair donor. This has been supported by computational methods, mainly, DFT and RIMP2 levels of theory (?48.3 kJ/mol). In addition, several potential curve surfaces are obtained to test the strength and type of every notable interaction in the lattice.     

Controlled incorporation of deuterium into bacterial cellulose

Isotopic enrichment has been widely used for investigating the structural and dynamic properties of biomacromolecules to provide information that cannot be carried out with molecules composed of natural abundance isotopes. A media formulation for controlled incorporation of deuterium in bacterial cellulose synthesized by Gluconacetobacter xylinus subsp. sucrofermentans is reported. The purified cellulose was characterized using Fourier Transform Infra-Red spectrophotometry and mass spectrometry which revealed that the level of deuterium incorporation in the perdeuterated cellulose was greater than 90 %. Small-angle neutron scattering analysis demonstrated that the overall structure of the cellulose was unaffected by the substitution of deuterium for hydrogen. In addition, by varying the amount of D-glycerol in the media it was possible to vary the scattering length density of the deuterated cellulose. A large disk model was used to fit the curves of bacterial cellulose grown using 0 and 100 % D-Glycerol yielding a lower bound to the disk radii, R _min = 1,132 ± 6 and 1,154 ± 3 Å and disk thickness, T = 128 ± 1 and 83 ± 1 Å for the protiated and deuterated forms of the bacterial cellulose, respectively. This agrees well with the scanning electron microscopy analysis which revealed stacked sheets in the cellulose pellicles. Controlled incorporation of deuterium into cellulose will enable new types of experiments using techniques such as neutron scattering to reveal information about the structure and dynamics of cellulose and its interactions with proteins and other (bio) polymers.     

Enumeration of non-labile oxygen atoms in dissolved organic matter by use of 16O/18O exchange and Fourier transform ion-cyclotron resonance mass spectrometry

We report a simple approach for enumeration of non-labile oxygen atoms in individual molecules of dissolved organic matter (DOM), using acid-catalyzed ¹⁶O/¹⁸O exchange and ultrahigh-resolution Fourier-transform ion-cyclotron-resonance mass spectrometry (FTICR-MS). We found that by dissolving DOM in H₂ ¹⁸O at 95 °C for 20 days it is possible to replace all oxygen atoms of DOM molecules (excluding oxygen from ether groups) with ¹⁸O. The number of exchanges in each molecule can be determined using high-resolution FTICR. Using the proposed method we identified the number of non-labile oxygen atoms in 231 molecules composing DOM. Also, using a previously developed hydrogen–deuterium (H/D)-exchange approach we identified the number of labile hydrogen atoms in 450 individual molecular formulas. In addition, we observed that several backbone hydrogen atoms can be exchanged for deuterium under acidic conditions. The method can be used for structural and chemical characterization of individual DOM molecules, comparing different DOM samples, and investigation of biological pathways of DOM in the environment.     

Crystal structure of β-CaNi5D0.77 by rietveld analysis of neutron powder-diffraction data

β-CaNi₅D_0.77 is orthorhombic (a = 8.6033(13) b = 5.0810(9) c = 7.8557(15)Å, Pmcm, Z = 4, D_x = 6.48 g cm^?3, V = 343.4ų, FW = 1339.8 amu. Rietveld analysis of a neutron powder-diffraction pattern shows that D atoms occupy the centers of [Ni₄Ca₂] square dipyramids (“octahedra”) linked through the Ca atoms at opposite apices and forming chains. About 80% of the deuterium atoms occupy fairly regular sites (DNi = 1.53, 1.80, 1.86, 1.86 Å; DCa = 2.42, 2.45 Å). The remaining 20% occupy very distorted sites (DNi = 1.21, 1.38, 2.07, 2.07 Å; DCa = 2.54, 2.60 Å). The average DD distance in a chain is 4.3 Å.     

Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise‐corrected second‐order Møller‐Plesset (MP2) perturbation theory. Single‐point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6‐311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc‐pVXZ and aug‐cc‐pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon–carbon separation was sampled in a step 0.1 Å for a range of 3–9 Å, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well‐established analytical extrapolation schemes. A 4‐site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen–hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom‐wise radial distribution functions and the self‐diffusion coefficients over a wide range of experimental conditions. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

Increasing Electron‐Transfer Rates with Increasing Donor–Acceptor Distance

Electron transfer can readily occur over long (≥15 Å) distances. Usually reaction rates decrease with increasing distance between donors and acceptors, but theory predicts a regime in which electron‐transfer rates increase with increasing donor–acceptor separation. This counter‐intuitive behavior can result from the interplay of reorganization energy and electronic coupling, but until now experimental studies have failed to provide unambiguous evidence for this effect. We report here on a homologous series of rigid rodlike donor‐bridge‐acceptor compounds in which the electron‐transfer rate increases by a factor of 8 when the donor–acceptor distance is extended from 22.0 to 30.6 Å, and then it decreases by a factor of 188 when the distance is increased further to 39.2 Å. This effect has important implications for solar energy conversion.