Molecular exchange dynamics in partially filled microscale and nanoscale pores of silica glasses studied by field-cycling nuclear magnetic resonance relaxometry

Nuclear magnetic spin-lattice relaxation experiments have been performed in partially filled porous glasses with wetting and nonwetting fluids. The frequency dependence of the spin-lattice relaxation rate in Vycor (4 nm pores) and VitraPOR #5 (1 microm pores) silica glasses was studied as a function of the filling degree with the aid of field-cycling NMR relaxometry. The species of primary interest were water ("polar") and cyclohexane ("nonpolar"). Spin-lattice relaxation was examined in the frequency range from 1 kHz to 400 MHz with the aid of a field-cycling NMR relaxometer and an ordinary 400 MHz NMR spectrometer. Three different mobility states of the fluid molecules are distinguished: The adsorbed state at the pore walls, the bulklike liquid phase, and the vapor phase. The adsorbate spin-lattice relaxation rate is dominated by the "reorientation mediated by translational displacements" (RMTD) mechanism taking place at the adsorbate/matrix interface at frequencies low enough to neglect rotational diffusion of the molecules. The experimental data are analyzed in terms of molecular exchange between the different mobility states. Judged from the dependence of the spin-lattice relaxation rates on the filling degree, limits for slow and fast exchange (relative to the RMTD time scale) can be distinguished and identified. It is concluded that water always shows the features of slow exchange irrespective of the investigated pore sizes and filling degrees. This is in contrast to cyclohexane which is subject to slow exchange in micrometer pores, whereas fast exchange occurs in nanoscopic pores. The latter case implies that the vapor phase contributes to molecular dynamics in this case at low filling degrees while it is negligible otherwise.     

Proton nuclear magnetic resonance lineshapes in lamellar liquid crystals

The nuclear magnetic resonance lineshape of the protons in the amphiphile alkyl chains in lamellar liquid crystals is calculated. It is found that the inclusion of intra molecular dipolar interactions not averaged to zero explains the shape of the experimentally observed spectrum.     

Proton nuclear magnetic resonance characterisation of glycosaminoglycans using chemometric techniques

Various types of glycosaminoglycans (GAGs) including heparins, chondroitin sulfates, dermatan sulfate and hyaluronic acid were studied from their proton nuclear magnetic resonance (1H NMR) spectra using chemometric techniques. Despite the complexity of the 1H NMR signals, data analysis using principal component analysis enabled the different GAG classes to be distinguished and permitted their classification according to their chemical structure. The analysis of the composition of the major disaccharide unit and other relevant chemical structures in the heparin samples was performed using partial least squares regression.     

Brownian dynamics of water confined in AOT reverse micelles: A field-cycling deuteron NMR relaxometry study

Reversed micelles and water in oil micro-emulsions can be used to solubilize biopolymers and genetic materials allowing analyzing their properties in a confined geometry. Nuclear Magnetic Resonance Dispersion (NMRD) provides a powerful and a noninvasive experimental technique to probe the long-term dynamics of these confined systems. However, the first step is to analyze and understand the slow dynamics of water inside these micro-reactors without any guest molecule. This is the aim of this presentation. Experimental results have been obtained for deuteron ²H NMRD of water confined in reverse micelles of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) dispersed in isooctane C₈H₁₈. The water content is expressed as the molar ratio W₀ = [Water]/[AOT]. The radius of the spherical reversed micelles, R_m, increases almost linearly with W₀. In our case, W₀ is chosen in the range 20 ≤ W₀ ≤ 50 (35 ≤ R_m ≤ 80 Å). The frequency dependence for the spin-lattice relaxation rate R₁(ω) exhibits two regimes, for all W₀ values: a plateau at low frequency, proportional to 1/R_m, followed by the beginning of an algebraic decay. These experimental observations are discussed and compared to a numerical simulation of the intermittent Brownian diffusion of a water molecule inside a rotating reverse micelle. The possibility to probe some properties of the confinement, such as the localisation time on the sulfonated palisade and/or the water self-diffusion inside the water pool is emphasised.     

Proton nuclear magnetic resonance and electron paramagnetic resonance investigation of alpha-alpha cross-linked Fe-Co hybrid hemoglobins.

The following asymmetric alpha 1 99 Lys-alpha 2 99 Lys cross-linked Fe(II)-Co(II) hybrid hemoglobins (Hbs) were first prepared from derivatives of hemoglobin C (beta 6 Glu-Lys) and human normal HbA: [alpha(Co)beta(Fe)]A[alpha(Co)beta(Co)]cXL, [alpha(Fe)beta(Co)]A[alpha(Co)beta(Co)]cXL, etc. Their 500 MHz 1H NMR and EPR spectra were measured in order to study the change in their tertiary and quaternary structure under atmosphere of deoxy, oxy and carbon monoxide (with or without IHP). From the change of T and R marks in 1H NMR hydrogen bonding region, it is proved that oxygen molecules are first bonded to alpha(Fe) subunits rather than to beta(Fe). The experimental phenomena provided further evidence that intermediate states of ligation are present in addition to T and R state during process of binding of oxygen to Hb. IHP facilitates transformation of T state to R state. The same conclusion can also be drawn from the results of EPR spectra at 77 K.     

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.     

Quantitative analysis by nuclear magnetic resonance with fourier transformation: I—Proton case

For quantitative analysis by NMR spectroscopy with Fourier transform, the usual experimental conditions have to be partly modified. Careful regulation of the delays between the different phases of the process is particularly important. The filters of the spectrometer and the calculations on the free induction decay are important causes of error. Precise results are given.     

Proton magnetic resonance spectra of cinnolines

The ¹H NMR spectra of cinnoline and some 8-substituted derivatives 1 in DMSO-d₆ are reported. A previous assignment of the chemical shifts of the heterocyclic ring protons H-3 and H-4 is confirmed by deuteration studies. The variations in the chemical shifts of H-3 to H-7 are discussed in terms of (a) solvent effects, (b) the perturbation effect of the heterocyclic ring, and (c) the electronic and proximity effects of the 8-substituents, using single-parameter (SCS_o, SCS_m, SCS_p, σ_o, σ_m, σ_p, σ_p⁺, σ_4,1, σ_5,1, σ_6,1Q) and dual-parameter (F, R; σ₁,σ_R) treatments to correlate the data. Marked proximity effects on H-7 are noted for the nitro and acetamido groups, and are correlated with conformational changes due to interaction of the groups with the N-1 lone pair. Variations in the coupling constants are attributed to partial double-bond fixation.     

Proton nuclear magnetic resonance study of phthalide and 2-coumaranone partially oriented in nematic phases

The ¹H NMR spectra of phthalide in Merck Nematic Phase IV and in N-(p-ethoxybenzylidene)-p-n-butylaniline (EBBA) and of 2-coumaranone in Merck Nematic Phase IV were analyzed. Structural parameters were obtained. The planarity of the 5-membered ring and the molecular orientations were discussed.     

Cellulose organic solutions: A nuclear magnetic resonance investigation

Four solvents of cellulose have been studied by using ¹³C-NMR spectroscopy. All these solvents, N-methyl morpholine-N-oxide, methylamine, hydrazine, and paraformaldehyde (PF), contained dimethyl sulfoxide (DMSO) as a cosolvent. Oligomers of cellulose of DP = 10 soluble in hot DMSO have been used as model compounds. ¹³C chemical shifts and line shapes show that three of the mentioned solvents are “true solvents” of cellulose. On the other hand, dissolution of cellulose in DMSO-PF system occurs by the formation of a statistical derivative of cellulose. Enriched ¹³C bacterial cellulose on C-1 and C-6 positions have been used to identify the ¹³C positions mainly in DMSO-N-methyl morpholine-N-oxide system. This solvent has been found to be degradative for the macromolecule when the solution is kept at 100°C over a long period. Viscosity measurements show a reduction of the molecular weight in these conditions. Polarimetry indicates that no glucose is present in solution and hence there is a statistical break of the chain. Enriched cellulose solution in DMSO–N-methyl morpholine-N-oxide has been also used for relaxation time (T₁) determination both of the solvent and of the enriched carbons of the polymer. Nuclear Overhauser enhancement (NOE) was found to be 1.8 for C-1 and 2.1 for C-6 showing that relaxation phenomenon is not purely dipolar. T₁ values of 97 and 65 msec are found for C-1 and C-6 of cellulose, in good agreement with the values known for polysaccharides. Determination of T₁ for the different carbon atoms of the solvent DMSO-N-methyl morpholine-N-oxide with and without cellulose shows a large reduction of T₁ for N-methyl morpholine-N-oxide molecule. This denotes a slower molecular motion of this molecule and a preferential interaction with the cellulose macromolecule.     

Proton magnetic resonance spectra of monosubstituted naphthalenes

The chemical shifts and coupling constants for the ring protons of fourteen monosubstituted naphthalenes in carbon tetrachloride solution have been obtained by analysis and computer simulation of their 100 MHz magnetic resonance spectra. Studies at several concentrations have enabled shifts extrapolated to infinite dilution to be obtained and concentration effects arising from solute–solute interactions to be determined.     

Proton magnetic resonance spectra of the atropisomers of ortho-hexaphenylene

The structures of the atropisomers of o-hexaphenylene are confirmed by their proton magnetic resonance spectra. The centrosymmetrical isomer 1a gives rise to an AA′BB′ spectrum, whereas the helical isomer 1b displays an AA′BB′ spectrum for two of its aromatic rings and an ABCD for the remaining four rings. Long-range coupling between the AA′BB′ and ABCD parts of 1b leads to a tentative assignment of the protons. An analysis of the spectrum of o-tetraphenylene ( 2 ) is also given.     

Proton and carbon nuclear magnetic resonance study on some n- and o-acyl derivatives of monohydroxypyridines

Comparative study on the proton and carbon NMR spectra for a series of $\text{N}$- and $\text{O}$-acyl substituted monohydroxypyridines (C₅H₄NO$\text{R}$: $\text{R}$=-H, -CHO, -COCH₃, -COC(CH₃)₃, -COCF₃, -COC₆H₅, -SO₂CH₃, -SO₂C₆H₄CH₃ is reported. p]Characteristic ¹H, ¹³ NMR and IR spectral features allow simple and unambiguous distinction between the isomeric $\text{N}$- and/or $\text{O}$-acyl-derivatives of 2-, 3- and 4-hydroxypyridines, so that both forms can clearly be identified when tautomeric equilibria occur, since the tautomerism rate is slow on the NMR time scale