PFG NMR studies of lysine-silica solutions

The formation process of silica nanoparticles in lysine-silica mixtures was studied using dynamic light scattering (DLS) and pulsed-field gradient (PFG) NMR measurements. (1)H NMR shows line broadening of the lysine resonances during TEOS hydrolysis/nanoparticle formation. Analysis of the TEOS hydrolysis kinetics show that TEOS hydrolysis is the rate-limiting step in particle formation, and has an activation energy of 20.5 kJ/mol. Transverse relaxation measurements show a corresponding decrease in T(2) with TEOS hydrolysis, indicating a reduction in the lysine mobility due to lysine-silica interactions. PFG NMR results indicate a systemic decrease in the self-diffusion coefficient of lysine as particle formation proceeds. The results obtained can be described using a two-state model wherein lysine is either free in solution or bound to the nanoparticles. Analysis of the PFG data of samples made at various temperatures show that lysine coverage upon complete hydrolysis is between 2.5 and 2.8 mmol lysine/kg solution, and insensitive to the heating temperature. PFG NMR shows a linear increase in the amount of bound lysine with increasing lysine content, indicating an increase in the surface area present, i.e. more and smaller particles, with increased lysine content. The PFG NMR results presented give quantitative insights that indicate that while pH is likely the primary driver for the rate of particle formation and particle size, lysine is critical for stabilization of the nanoparticles.     

Revealing complex formation in acetone-n-alkane mixtures by MAS PFG NMR diffusion measurement in nanoporous hosts

Magic-angle spinning pulsed field gradient nuclear magnetic resonance (MAS PFG NMR) was applied for selective self-diffusion measurements of acetone-n-alkane (C(6) up to C(9)) mixtures in nanoporous silica gel. Two specimens of silica gel with mean pore sizes of about 4 and 10 nm are considered. In the smaller pores, the n-alkane diffusivities are by about one and the acetone diffusivities by about two orders of magnitude smaller than in the larger pores. In addition, the acetone diffusivities in the narrow-pore specimen exhibit a pronounced oscillation with increasing chain length of the solvent n-alkanes: the diffusivities of acetone dissolved in odd-carbon number n-alkanes exceed those of acetone dissolved in even-carbon number n-alkanes by about 50%! These findings reproduce the tendencies observed in previous macroscopic release studies (Phys. Chem. Chem. Phys. 2003, 5, 2476) and suggest the formation of acetone-n-alkane complex-like assemblages in the narrow-pore silica gel.     

Self-diffusion coefficient measurements at high temperature by PFG NMR

We have developed a new experimental setup for the measurement of self-diffusion coefficients at high temperature (up to 1500 K) in corrosive liquids. It is based on pulsed field gradient NMR using a 10 mm liquid probe with two channels F-H/X modified and coupled with laser heating. The values obtained thanks to this setup are in good agreement with the available data obtained using the reference method, i.e., the capillary method. We present here results on ¹⁹F, ⁷Li, ²³Na, ²⁷Al in molten fluorides. In alkali fluoride mixtures, the self-diffusion coefficients depend weakly on the composition (nature and concentration of the different alkali) but mainly on temperature. In cryolite (Na₃AlF₆), the diffusion evidences AlF_x^3 ? x species.     

Molecular mass estimation of derivatized compounds: a PFG NMR study

Pulsed-field gradient (PFG) 1H and 31P NMR methods were developed to quantitatively estimate the molecular mass of compounds, derivatized with either trichloroacetyl isocyanate (TAI) or 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (chlorophospholane). These agents provide selective analysis with high sensitivity for molecules containing alcohol, amine, carboxylic acid, or thiol functional groups. Tetramethylsilane (TMS) or bisphenol A was used as internal diffusion reference. The empirical relationship between relative diffusivity and molecular mass was established for a set of mono- and difunctional compounds with molecular masses in the range 32-330 g/mol. The utility of the method was demonstrated by analyzing alcohol, phenol, and carboxylic acid components in lubricating oil.     

Measurement of emulsion droplet sizes using PFG NMR and regularization methods

The droplet size distributions of emulsions have been measured using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) for many years. This technique finds particular application with emulsions that are concentrated and/or opaque, since such emulsion systems are difficult to characterize by other methods. Most studies employing PFG techniques assume a lognormal form when extracting the droplet size distribution from the experimental data. It is clearly desirable to retrieve a droplet size distribution from the experimental data without assuming such a functional form. This is achieved for the first time using regularization techniques. Regularization based on the distribution area and on its second derivative are compared and assessed along with the following techniques for selecting the optimal regularization parameter: the L-curve method, generalized cross validation (GCV), and the discrepancy principle. Regularization is applied to both simulated data sets and experimental data. It is found that when the experimental error can be estimated accurately, the discrepancy principle with area regularization is the best approach. When the error is not known the GCV method, with second derivative regularization and allowing only nonnegative values, is most effective.     

NMR investigations of rotenoids

Extensive NMR studies of major rotenoids have (1) verified the cis B/C ring fusion of rotenone; (2) confirmed the structure of the reduction-dehydration product of rotenone; (3) provided considerable evidence regarding the preferred conformations of rotenoids; (4) revealed an array of long-range couplings; and (5) pointed up analytically useful solvent effects. Incidentally, these studies also allowed assignment of NMR signals for essentially all protons of the major rotenoids in deuterochloroform.     

NMR investigations of polymer electrets

1H NMR investigations were performed on thin films of poly(vinylidene fluoride) (PVDF) and polyamide 11 (PA 11). The variation of the angle between the static magnetic B_o-field and preferred directions given by drawing and poling leads to characteristic dependences of T_20ff, which can be discussed in terms of Legendre polynomials of the orientational distribution functions of crystallographic c- and b-axis. A good c-axis alignment can be realized by uniaxial drawing. The tendency for b-axis to orient towards the direction of the applied electric field via the orientation of the electric dipoles is evident. The b-axis alignment can be “switched” periodically.     

NMR spectroscopic investigations with isatin guanylhydrazones

The synthesis of some novel (substituted) guanylhydrazones of isatin, 5-methylisatin and 1-methylisatin is decribed. Moreover, detailed nmr-spectroscopic studies (¹H-nmr, ¹³C-nmr) with these compounds and previously known congeners are presented.     

NMR spectroscopy applied to photochromism investigations

Despite its poor time-resolution and low sensitivity compared with classical spectrophotometric methods, modern NMR is now a highly developed spectroscopy technique, appropriate for photochromism studies owing to its high spectral resolution and two-dimensional (2D) NMR techniques that offer detailed structural and quantitative information. NMR spectroscopy can therefore be applied to answer questions concerning which compounds are produced, how they are formed and how they evolve and behave within the photochromic reaction. In this review, we have chosen among the wide variety of photochromic molecules, some specific examples in the family of spirocompounds, naphthopyrans, dihydropyrenes, diarylethenes and aryldiketones to illustrate the ways in which NMR has proved useful in the kinetic and structural studies of photochromism.     

Indirect Measurement of the Cooperative Hydrogen Bonding of Polymers Using NMR Quadrupole Relaxation and PFG Methods

Summary: A general method of measurement of polymer hydrogen bond (HB) cooperativity using a low-molecular weight model ligand named marker and two independent methods of ²H NMR is presented. As marker, a deutero-compound chemically similar to the functional groups of one of the polymers is used, e.g. pyridine-d₅ in the investigated interaction of poly(4-vinylpyridine) with poly(4-vinylphenol) or acetic acid-d₄ in the interaction of polyacrylic acid with poly(4-vinylphenol) reported here. The method is based on the fact that a substantial fraction of the marker, originally bound to the groups of one of the polymers, is liberated by the cooperative interaction between the two polymers. For the establishment of the fraction of the bound marker before and after mixing the polymers, ²H NMR quadrupolar relaxation or ²H PFG NMR diffusion measurement can be used with comparable precision. In both these methods, the results must be normalized to a standard viscosity using the relaxation or diffusion of an added inert compound such as CDCl₃.     

Liposomes from novel photolabile phospholipids: light-induced unloading of small molecules as monitored by PFG NMR

A molecular dithiane-based approach to synthesis of novel photolabile phospholipids is developed. These lipids are used in formulations with egg-POPC and cholesterol to prepare light-sensitive liposomes. Irradiation of such liposomes in PBS buffer (medium pressure mercury lamp, Pyrex filter, lambda > 300 nm) significantly increases the bilayer permeability and accelerates the release of entrapped small organic molecules by an order of magnitude. A simple assay, based on (1)H or (19)F PFG NMR measurements of diffusion coefficients, is developed to monitor light-induced unloading of the probe molecules.     

Total synthesis and NMR investigations of cylindramide

Cylindramide (1) was built up from three components: a hydroxyornithine derivative 7, a tetrazolylsulfone 8, and a substituted pentalene subunit 9. Derivative 7 was prepared in a six-step reaction sequence involving the Wittig reaction and a Sharpless asymmetric dihydroxylation starting from N-Boc-3-aminopropanal (12). Tetrazolylsulfone 8 was accessible in four steps from dioxinone 22. The synthesis of the pentalene fragment 9 started from cycloocta-1,5-diene 26, that was converted into enantiopure bicyclo[3.3.0]octanedione 29. The latter was functionalized to give derivative 9. The total synthesis was accomplished by inducing C-C bond formation by Sonogashira coupling of derivatives 9 and 7 followed by olefination with tetrazolylsulfone 8 under Julia-Kocienski conditions, macrocyclization, and subsequent Lacey-Dieckmann condensation to form the tetramic acid unit. As indicated by extensive 1H and 13C NMR spectroscopic investigations (DQF-COSY, ROESY spectra), the stereochemistry of synthetic cylindramide (1) corresponds with that of the naturally occurring product. ROE data were used for molecular modeling of the lowest-energy structures for cylindramide.     

Calorimetric investigations of hydrogen-bonded liquid crystal binary mixtures

Journal of Thermal Analysis and Calorimetry - Double hydrogen-bonded liquid crystals formed between methyl malonic acid (MM) and p-n-alkyloxy benzoic acids (nBAO) are characterized. Variation in...     

Narcissistic aggregation of steroid compounds in diluted solution elucidated by CSI-MS, PFG NMR and X-ray analysis

Large-scale aggregated chain structures of progesterone, estrone, cortisone, hydrocortisone and cholic acid were observed in diluted solution by means of cold-spray ionization mass spectrometry (CSI-MS) and pulsed field gradient (PFG) NMR. The crystal structures were determined by X-ray crystallography, and the relationship between the crystal and solution structures is discussed. It is suggested that the intermolecular hydrogen bondings observed in the crystal might be partly retained in diluted solution.     

29Si NMR investigations on oligosilane dendrimers

Starting with the high functionalized trisilane SiClMe(SiCl₂Me)₂ and tetrasilane SiMe(SiCl₂Me)₃ several octa- and decasilane dendrimers containing directly neighboured branchings were prepared. In these compounds the ²⁹Si NMR chemical shifts of the different silyl groups are shifted towards lower field compared with those of analogous groups in tetra- or hexasilanes. This observation is a helpful tool for the characterization of further dendritic oligomers by ²⁹Si NMR. Received: 3 June 1996 / Revised: 3 July 1996 / Accepted: 9 July 1996     

Self-diffusion coefficients of organic solvents in linear and branched high density polyethylene particles measured by PFG NMR

Pulsed field gradient nuclear magnetic resonance （PFG NMR） has been performed to study the diffusion of organic solvents into semicrystalline polyethylene particles. Self-diffusion coefficients in different domains of the sample can be extracted through a bi- exponential fit to the echo intensity attenuation, which allows the precise determination of the tortuosity of the polyethylene particles. Further exploration comes from the measurements with branched polyethylene particles and it was found that the diffusion in polymer phase contributed significantly to the slow component of the exponential decay curve. 2007 Jing Dai Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

29Si NMR investigations on oligosilane dendrimers

Starting with the high functionalized trisilane SiClMe(SiCl₂Me)₂ and tetrasilane SiMe(SiCl₂Me)₃ several octa- and decasilane dendrimers containing directly neighboured branchings were prepared. In these compounds the ²⁹Si NMR chemical shifts of the different silyl groups are shifted towards lower field compared with those of analogous groups in tetra- or hexasilanes. This observation is a helpful tool for the characterization of further dendritic oligomers by ²⁹Si NMR. Received: 3 June 1996 / Revised: 3 July 1996 / Accepted: 9 July 1996     

Solvent Effects in the Homogeneous Catalytic Reduction of Propionaldehyde with Aluminium Isopropoxide Catalyst: New Insights from PFG NMR and NMR Relaxation Studies

Solvent effects in homogeneous catalysis are known to affect catalytic activity. Whilst these effects are often described using qualitative features, such as Kamlet-Taft parameters, experimental tools able to quantify and reveal in more depth such effects have remained unexplored. In this work, PFG NMR diffusion and T₁ relaxation measurements have been carried out to probe solvent effects in the homogeneous catalytic reduction of propionaldehyde to 1-propanol in the presence of aluminium isopropoxide catalyst. Using data on diffusion coefficients it was possible to estimate trends in aggregation of different solvents. The results show that solvents with a high hydrogen-bond accepting ability, such as ethers, tend to form larger aggregates, which slow down the molecular dynamics of aldehyde molecules, as also suggested by T₁ measurements, and preventing their access to the catalytic sites, which results in the observed decrease of catalytic activity. Conversely, weakly interacting solvents, such as alkanes, do not lead to the formation of such aggregates, hence allowing easy access of the aldehyde molecules to the catalytic sites, resulting in higher catalytic activity. The work reported here is a clear example on how combining traditional catalyst screening in homogeneous catalysis with NMR diffusion and relaxation time measurements can lead to new physico-chemical insights into such systems by providing data able to quantify aggregation phenomena and molecular dynamics.     

Thermodynamic investigations of methyl tert-butyl ether and water mixtures

Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning calorimetry, isothermal titration calorimetry, densitometry, IR-spectroscopy, and gas chromatography. The solubilization of MTBE in water at 25 °C at infinite dilution has ΔH° = -17.0 ± 0.6 kJ mol(-1); ΔS° = -80 ± 2 J mol(-1) K(-1); ΔC(p) = +332 ± 15 J mol(-1) K(-1); ΔV° = -18 ± 2 cm(3) mol(-1). The signs of these thermodynamic functions are consistent with hydrophobic interactions. The occurrence of hydrophobic interaction is further substantiated as IR absorption spectra of MTBE-water mixtures show that MTBE strengthens the hydrogen bond network of water. Solubilization of MTBE in water is exothermic whereas solubilization of water in MTBE is endothermic with ΔH° = +5.3 ± 0.6 kJ mol(-1). The negative mixing volume is explained by a large negative contribution due to size differences between water and MTBE and by a positive contribution due to changes in the water structure around MTBE. Henry's law constants, K(H), were determined from vapor pressure measurements of mixtures equilibrated at different temperatures. A van't Hoff analysis of K(H) gave ΔH(H)° = 50 ± 1 kJ mol(-1) and ΔS(H)° = 166 ± 5 J mol(-1) K(-1) for the solution to gas transfer. MTBE is excluded from the ice phase water upon freezing MTBE-water mixtures.