Ultrafast intermolecular zero quantum spectroscopy

Clinical magnetic resonance spectroscopy is typically limited by magnetic inhomogeneities which destroy spectral resolution, but intermolecular zero quantum coherences (iZQCs) are insensitive to such inhomogeneities. iZQC resolution in vivo, however, has been hampered by physiological fluctuations over the time scale of the two-dimensional acquisition. A faster iZQC sequence will allow us to average away these fluctuations, and thus we present a new approach to ultrafast two-dimensional spectroscopy. This communication reports iZQC experiments acquiring up to 31 t1-points per scan, as well as extensions to a broad range of other 2D sequences.     

High-resolution NMR spectroscopy in unstable and inhomogeneous fields via stroboscopic acquisition

In this paper, we demonstrated that despite the insensitivity of intermolecular zero-quantum coherences (iZQCs) to B(0) variations, the influence of unstable fields on the observable single-quantum coherence signals causes strong t(1) noises in the high-resolution iZQC projection spectra. Stroboscopic acquisition was then proposed for noise suppression. The feasibility of the modified sequences with the proposed acquisition scheme was verified by computer simulations and experiments in different unstable fields generated by the Z0 and Z1 coil current oscillations, which mimic the unstable fields of NMR using externally powered magnets and MRS in the presence of physiological motions, respectively.     

Electroosmotically enhanced mass transfer through polyacrylamide gels

We present an internal pumping strategy to enhance solute fluxes in polymer gels. The method is based on electroosmotic flow driven by an electric field applied across a gel that has been doped with charged colloidal inclusions. This work is motivated by the need to enhance the transport in gel-based biosensor devices whose response dynamics are often mass transfer limited. In this case, polyacrylamide gel slabs were doped with immobilized, charged silica colloids, and the flux of a fluorescent tracer was measured as a function of applied field strength, the volume fraction and size of the colloidal silica inclusions, and the bulk electrolyte composition. Significant flux enhancements were achieved with applied electric currents on the order of a few mA. Control experiments indicated that the flux enhancement was not due to any distortion of the gel diffusional properties in response to the presence of the inclusions. At a constant inclusion volume fraction, the electroosmotic solute flux enhancement was strongest for the smallest particle sizes that provide the highest total surface area, consistent with the electroosmotic mechanism whereby fluid flow is generated along the solid/liquid interface.     

High-resolution intermolecular zero-quantum coherence spectroscopy under inhomogeneous fields with effective solvent suppression

Intermolecular zero-quantum coherences (iZQCs) are not susceptible to magnetic field inhomogeneities significantly larger than the dipolar correlation distance and can be used to obtain 1D high-resolution spectra in an inhomogeneous field. However, with the iZQC methods proposed previously, residual conventional single-quantum coherences (SQCs) originating mainly from solvent resonance result in strong t(1) ridge noises. A modified HOMOGENIZED with an intermolecular double-quantum filter (iDQF), named iDQF-HOMOGENIZED, is presented in this work to suppress the residual conventional SQC signals as well as solvent iZQC signals. The solvent-suppression efficiency of the iDQF-HOMOGENIZED is analyzed and a thorough comparison of the new sequence with several relevant pulse sequences is made. Dramatic resolution enhancement and solvent suppression in the measurements of a piece of grape sarcocarp suggest potential applications of the method in in vivo spectroscopy.     

1H NMR study of inclusion compounds of phenylurea derivatives in beta-cyclodextrin

Proton nuclear magnetic resonance spectroscopy ((1)H NMR), which has become an important tool for the study "in situ" of beta-cyclodextrin (beta-CD) complexes, was used to study and structurally characterize the inclusion complexes formed between beta-CD and isoproturon, fenuron, monuron and diuron. The high variation of the chemical shifts from the proton located inside the cavity (H-3, H-5 and H-6) coupled with the non variation of the one located outer sphere of the beta-CD (H-1, H-2 and H-4) provided clear evidence of the inclusion phenomena. Two-dimensional rotating frame Overhauser effect spectroscopy (ROESY) experiments were carried out to further support the proposed inclusion mode.     

Diffusion of spherical micronetworks in polymer diluent systems and melts studied by dynamic light scattering techniques

Spherical polystyrene (PS) micronetworks can be prepared in microemulsion with bulk radii of 5–60 nm and different cross-linking densities. The diffusion of these PS spheres has been studied in polymer diluent systems ranging from dilute solutions to plasticized melts by using forced Rayleigh scattering and photon correlation spectroscopy. On increasing the PS concentration, a colloid glass transition is observed at a volume fraction Φ_C ≈ 0.64 of the swollen spheres. At higher concentration inside the “colloid glass” state the sphere diffusion is slowed down and becomes very complex but can be observed up to the limit of a melt of collapsed spheres.     

1,2,4,5-Tetra([5]trovacenyl)benzene: an organometallic tetraradical displaying pronounced electro- and magnetocommunication

The organometallic tetraradical 1,2,4,5-[(eta)7-C7H7)V(all eta]5-C5H4)]4C6H2 has been prepared and structurally characterized. The isotropic EPR spectrum displays 29 a(51V) hyperfine lines, the intensity distribution slightly deviating from binomial. Exchange coupling therefore approaches the strong exchange limit, J(ortho) not equal to J(meta) not equal to J(para) greater or approximately 50 a(51V) with a(51V)= 0.0067 cm(-1). According to magnetic susceptometry, the interaction is antiferromagnetic. While redox splittings deltaE(1/2) are resolved for the four reduction steps this is not the case for oxidation.     

Purity testing of organometallic catalysts by packed capillary supercritical fluid chromatography

A packed capillary column supercritical fluid chromatography system with flame ionization detection has been used for purity testing of candidates for homogeneous catalysis such as methyl tricarbonyl pentamethylcyclopentadienyl tungsten [Cp*W(CO)3Me], methyl tricarbonyl cyclopentadienyl tungsten [CpW(CO)3Me], tetramethyl pentamethylcyclopentadienyl iridium (Cp*IrMe4), trimethyl (1,4,7-trimethyl-1,4,7-triazocyclononane) rhodium (CnRhMe3), trimethylphosphine hydride dicarbonyl cyclopentadienyl molybdenum [eta5-CpMoH(CO)2PMe3] and triphenylphosphine hydride dicarbonyl cyclopentadienyl molybdenum [eta5-CpMoH(CO)2PPh3]. A mass limit of detection of 240 pg was found for eta5-CpMoH(CO)2PMe3 when using a 60-nl injection volume and pure CO2 as mobile phase on a 5 microm Kromasil C18 column. The stability of the catalysts in solution has been examined. After 24 h more than 70% of eta5-CpMoH(CO)2PMe3 and 50% of eta5-CpMoH(CO)2PPh3 had decomposed. Due to the instability of the compounds the purity testing had to take place rapidly after sample dissolution.     

Diffusion of nanoparticles in monolayers is modulated by domain size

Langmuir monolayers are often used as simple models for biological membranes. The possibility to change their composition and phase state in a very controlled manner as well as access to a large observation area makes them a versatile tool for the investigation of membrane-related interactions. Inspired by experiments in our group, we investigate the interaction of single, partially charged nanoparticles with lipid microdomains by Monte Carlo simulations. Condensed domains in inhomogeneous Langmuir monolayers exhibit an electric dipole field interacting attractively with the nanoparticle's dipole moment. With increasing domain size, the resulting electric field changes from single dipole to semi-infinite domain characteristics, significantly influencing the motion of the particle. Small immobile domains (R = 1 microm) confine the movement of the tracer to the boundary of the domain whereas for large domains (R > or = 10 microm) its motion is only temporarily hindered. This suggests a powerful mechanism for controlling diffusive transport in lipid membranes.     

Effects of solute structure on local solvation and solvent interactions: results from UV/vis spectroscopy and molecular dynamics simulations

Solvation of heterocyclic amines in CO(2)-expanded methanol (MeOH) has been explored with UV/vis spectroscopy and molecular dynamics (MD) simulations. A synergistic study of experiments and simulations allows exploration of solute and solvent effects on solvation and the molecular interactions that affect absorption. MeOH-nitrogen hydrogen bonds hinder the n-pi* transition; however, CO(2) addition causes a blue shift relative to MeOH because of Lewis acid/base interactions with nitrogen. Effects of solute structure are considered, and very different absorption spectra are obtained as nitrogen positions change. MD simulations provide detailed solvent clustering behavior around the solute molecules and show that the local solvent environment and ultimately the spectra are sensitive to the solute structure. This work demonstrates the importance of atomic-level information in determining the structure-property relationships between solute structure, local salvation, and solvatochromism.     

Equilibrium equation of state of a hard sphere binary mixture at very large densities using replica exchange Monte Carlo simulations

We use replica exchange Monte Carlo simulations to measure the equilibrium equation of state of the disordered fluid state for a binary hard sphere mixture up to very large densities where standard Monte Carlo simulations do not easily reach thermal equilibrium. For the moderate system sizes we use (up to N = 100), we find no sign of a pressure discontinuity near the location of dynamic glass singularities extrapolated using either algebraic or simple exponential divergences, suggesting they do not correspond to genuine thermodynamic glass transitions. Several scenarios are proposed for the fate of the fluid state in the thermodynamic limit.     

Equation of state and structural properties of the Weeks-Chandler-Andersen fluid

Molecular dynamics simulations have been carried out for the equation of state and percolation properties of the Weeks-Chandler-Andersen (WCA) system in its fluid phase as functions of density and temperature. The compressibility factor Z collapses well for the various isotherms, using an effective particle diameter for the WCA particle which is (in the usual WCA reduced units) sigma(e)=2(16)(1+T)(16), where T is the temperature. A corresponding "effective" packing fraction is zeta(e)=pisigma(e) (3)N6V, for N particles in volume V, which therefore scales out the effects of temperature. Using zeta(e) the simulation derived Z can be fitted to a simple analytic form which is similar to the Carnahan-Starling hard sphere equation of state and which is valid at all temperatures and densities where the WCA fluid is thermodynamically stable. The data, however, are not scalable onto the hard sphere equation of state for the complete packing fraction range. We explored the continuum percolation behavior of the WCA fluids. The percolation distance sigma(p) for the various states collapses well onto a single curve when plotted as sigma(p)sigma(e) against zeta(e). The ratio sigma(p)sigma(e) exhibits a monotonic decrease with increasing zeta(e) between the percolation line for permeable spheres and the glass transition limit, where sigma(p)sigma(e) approximately 1. The percolation packing fraction was calculated as a function of effective packing fraction and fitted to an empirical expression. The local coordination number at the percolation threshold showed a transition between the soft core and hard core limits from ca. 2:74 to 1:5, as previously demonstrated in the literature for true hard spheres. A number of simple analytic expressions that represent quite well the percolation characteristics of the WCA system are proposed.     

How hydrophobic buckminsterfullerene affects surrounding water structure

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C60) is a mesoscale sphere; C60 also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C60 in a periodic box of water, and compare this to methane. A C60 molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C60 solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C60 and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C60 first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C60 surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 A from the solute surface. We conclude that, with a diameter of approximately 1 nm, C60 behaves as a large hydrophobic solute.     

Whispering-gallery mode resonators: Surface enhanced Raman scattering without plasmons

Calculations based on the Mie theory are performed to determine the locally enhanced electric fields due to whispering-gallery mode resonances for dielectric microspheres, with emphasis on electromagnetic "hot spots" that are located along the wavevector direction on the surface of the sphere. The local electric field enhancement associated with these hot spots is used to determine the surface enhanced Raman scattering enhancement factors for a molecule, here treated as a classical dipole, located near the surface of the sphere. Both incident and Raman emission enhancements are calculated accurately using an extension of the Mie theory that includes interaction of the Raman dipole field with the sphere. The enhancement factors are calculated for dielectric spheres in vacuum with a refractive index of 1.9 and radii of 5, 10, and 20 microm and for wavelengths that span the visible spectrum. Maximum Raman scattering enhancement factors on the order of 10(3)-10(4) are found at locations slightly off the propagation axis when the incident excitation but not the Stokes-shifted radiation is coincident with a whispering-gallery mode resonance. The enhancement factors are found to vary inversely with the resonance width, and this determines the influence of the mode number and order on the results. Additional calculations are performed for the case where the Stokes-shifted radiation is also on-resonance and Raman enhancement factors as large as 10(8) are found. These enhancement factors are typically a factor of 10(2) smaller than would be obtained from /E/4 enhancement estimates, as enhancement of the Raman dipole emission is significantly reduced compared to the local field enhancement for micron size particles or larger. Conditions under which single-molecule or few-molecule measurements are feasible are identified.     

Viscosity of hard sphere suspensions

A simple functional representation of the concentration dependence of the low-shear viscosity eta of hard sphere suspensions is proposed. The representation, which agrees with published literature at all volume fractions phi, has a hitherto-unremarked transition in its functional form at phi approximately 0.42 identical with phi(t). phi(t) is definitely less than the volume fraction 0.49 of the hard sphere melting transition.     

Investigations on the kinetics of electron transfer reactions in magnetic fields

There is a controversial debate if a magnetic field can influence the rate of electron transfer (ET) reactions. In this paper, we report kinetic measurements of the ET rate constants for the redox couples [IrCl6]2-/[IrCl6]3-, [Fe(CN)6]3-/[Fe(CN)6]4-, and [Fe(H2O)6]3+/[Fe(H2O)6]2+ in magnetic fields up to 1 T. To reduce effects arising from magnetically induced mass transport (magnetohydrodynamic effect), disk microelectrodes with a diameter of 50 microm were used in potentiodynamic (cyclic and linear sweep voltammetry) and in electrochemical impedance spectroscopy experiments. None of the investigated redox couples showed a magnetic field effect on the ET rate constant.     

Classical and quantum mechanical/molecular mechanical molecular dynamics simulations of alanine dipeptide in water: comparisons with IR and vibrational circular dichroism spectra

We have implemented the combined quantum mechanical (QM)/molecular mechanical (MM) molecular dynamics (MD) simulations of alanine dipeptide in water along with the polarizable and nonpolarizable classical MD simulations with different models of water. For the QM/MM MD simulation, the alanine dipeptide is treated with the AM1 or PM3 approximations and the fluctuating solute dipole moment is calculated by the Mulliken population analysis. For the classical MD simulations, the solute is treated with the polarizable or nonpolarizable AMBER and polarizable CHARMM force fields and water is treated with the TIP3P, TIP4P, or TIP5P model. It is found that the relative populations of right-handed alpha-helix and extended beta and P(II) conformations in the simulation trajectory strongly depend on the simulation method. For the QM/MM MD simulations, the PM3/MM shows that the P(II) conformation is dominant, whereas the AM1/MM predicts that the dominant conformation is alpha(R). Polarizable CHARMM force field gives almost exclusively P(II) conformation and other force fields predict that both alpha-helical and extended (beta and P(II)) conformations are populated with varying extents. Solvation environment around the dipeptide is investigated by examining the radial distribution functions and numbers and lifetimes of hydrogen bonds. Comparing the simulated IR and vibrational circular dichroism spectra with experimental results, we concluded that the dipeptide adopts the P(II) conformation and PM3/MM, AMBER03 with TIP4P water, and AMBER polarizable force fields are acceptable for structure determination of the dipeptide considered in this paper.     

Detection of HCl and HF by TTFMS and WMS

In this work we discuss on a compact spectrometer based on DFB diode lasers for detection of chloridric and fluoridric acids. HCl and HF concentrations are determined through optical absorption of the P(4) line (lambda=1.7 microm) and the R(3) line (lambda=1.3 microm), respectively. Both lines belong to first overtone vibrational bands and their line strengths are 7.8 x 10(-21)cm/molecule for HCl and 2.8 x 10(-20)cm/molecule for HF. We chose these lines for their relative high intensities and because they are quite far from water vapour lines which represent the main interfering gas for trace-gases analysis. To detect these species we used two different high frequency modulation techniques: two-tone frequency modulation spectroscopy (f(1)=800 MHz and f(2)=804 MHz) was used for HCl while for HF we followed a simpler approach based on wavelength modulation spectroscopy (f=600 kHz). We demonstrate that the two techniques provide comparable detection limit of about 80 ppbV at atmospheric pressure. Positive testing of our spectrometer makes it suitable for in situ measurements of exhaust gases coming from waste incinerators.     

Adsorption behavior of oxyethylated surfactants at the air/water interface

The low-shear viscosity eta(0) of colloidal suspensions of acrylic latex or silica in aqueous gelatin has been measured at a temperature above the sol-gel transition. Measurements were made on dilution of a concentrated suspension with water or a gelatin solution. Thus, either the gelatin : colloid ratio was maintained or it was varied at constant aqueous gelatin concentration. Systems were studied with four lime-processed gelatins of different molecular weights at two concentrations of added salt. In addition, the latex particle size and the thickness of the adsorbed gelatin layer were measured by photon correlation spectroscopy (PCS) under dilute conditions. The dependence of the low-shear viscosity eta(0) on particle concentration was exponential and did not follow the well-established Krieger-Dougherty model for simple hard-sphere suspensions over the concentration range studied. A simple phenomenological model, eta(0)=eta(o)10(phi(e)/phi(s)), was found to predict the behavior well. Here, eta(o) is the viscosity of a gelatin solution of the corresponding solution concentration, phi(e) is proportional to the volume fraction of the particles, and phi(s) is a scaling factor, which was determined to have a value of 0.85. With this value of phi(s), the dimensions determined from PCS could be used to predict the viscosity values.     

The influence of potential softness on the transport coefficients of simple fluids

This study explores the effects of interaction softness on the transport properties of simple fluids. The transport coefficients of soft-sphere fluids in which the particles interact via the potential, phi(r)=epsilon(rsigma)(-n), with n in the range from 6 to 1152, have been calculated by molecular-dynamics computer simulation. The self-diffusion coefficient D shear viscosity eta(s), bulk viscosity eta(b), and thermal conductivity lambda were computed over a wide packing fraction range. It was found that the Batschinski-Hildebrand expressions, in which D, eta(s) (-1), eta(b) (-1), and lambda(-1) are assumed to have a linear dependence on the molar volume, represent the data quite well for all n, although least well for the thermal conductivity. The density for which, on extrapolation, each of these quantities is zero, increases with the softness of the interaction (or approximately n(-1)), suggesting that the effective hard-sphere diameter decreases with increasing softness in the small n limit. This treatment leads to simple empirical formulas for the effect of density and n on the effective hard-sphere diameter and packing fraction (in an intermediate range) and the four transport coefficients of these fluids.