Search results for "MR"
showing 10 items of 3001 documents
Zero-field NMR of urea : spin-topology engineering by chemical exchange
2021
Well-resolved and information-rich J-spectra are the foundation for chemical detection in zero-field NMR. However, even for relatively small molecules, spectra exhibit complexity, hindering the analysis. To address this problem, we investigate an example biomolecule with a complex J-coupling network─urea, a key metabolite in protein catabolism─and demonstrate ways of simplifying its zero-field spectra by modifying spin topology. This goal is achieved by controlling pH-dependent chemical exchange rates of 1H nuclei and varying the composition of the D2O/H2O mixture used as a solvent. Specifically, we demonstrate that by increasing the proton exchange rate in the [13C,15N2]-urea solution, the…
Brownian dynamics of grafted polymer chains: time dependent properties
1995
Results of computer simulations of polymer layers consisting of chains grafted by one end on an unpenetrable plane are presented. Characteristics of translational and rotational motion of different chain segments and correlation functions of chain radii were calculated both for single layers at different grafting densities s and for two interacting layers at different distances D between parallel grafting planes. Two values of grafting density were used in the latter case. The behavior of different correlation times as function of s and D and the interplay between the interpenetration of the brushes and rotational and translational motion are discussed. Both relaxation functions and mean sq…
Atomistic Molecular Dynamics Simulations of the Static and Dynamic Properties of a Polyethylene Melt
1999
I will discuss in this work the atomistic modelling of polymer melts using Molecular Dynamics (MD) simulations. After a general introduction into the problems one faces in the computer simulation of polymers in general and the atomistic simulations in particular, I will present some recent work on the atomistic modelling of an — C 100 H 202 melt,i.e. short chain polyethylene (PE). The aim of this work is to establish a quantitatively validated atomistic model which then can be used in two directions. One direction is the generation of input for the calibration of parameters in coarse-grained models, which in turn can be used to simulate the polymer under investigation over a much broader te…
Studies of network organization and dynamics of e-beam crosslinked PVPs: From macro to nano
2012
Abstract In this work the influence of poly(N-vinyl pyrrolidone) (PVP) concentration in water on the organization and dynamics of the corresponding macro-/nanogel networks has been systematically investigated. Irradiation has been performed at the same irradiation dose (within the sterilization dose range) and dose rate. In the selected irradiation conditions, the transition between macroscopic gelation and micro-/nanogels formation is observed just below the critical overlap concentration (∼1 wt%), whereas the net prevalence of intra-molecular over inter-molecular crosslinking occurs at a lower polymer concentration (below 0.25 wt%). Dynamic–mechanical spectroscopy has been applied as a cl…
Cyclodextrins in Polymer Synthesis: Enantiodiscrimination in Free-Radical Polymerization of Cyclodextrin-Complexed Racemic N -Methacryloyl-D,L -pheny…
2003
The enantiodiscriminating polymerization of racemic cyclodextrin-complexed N-methacryloylphenylalanine methyl ester is investigated 1 H NMR spectra of the complexes with methylated β-cyclodextrin in D 2 O manifest splittings due to chiral recognition. The different stabilities of the diastereomeric complexes influence the kinetics of the homopolymerization, particularly at 0°C. An enrichment of the residual N-methacryloyl-L-phenylalanine methyl ester of 14% was achieved after 21 h of polymerization.
NMR and Quantum-Chemical Study on the Structure of Ester Enolate−Aluminum Alkyl Complexes as Models of the Active Center in the Anionic Polymerizatio…
1999
6Li and 13C NMR on ethyl α-lithioisobutyrate (EiBLi) and quantum-chemical (DFT) calculations on methyl α-lithioisobutyrate (MiBLi) were used to elucidate the structure of the active center in the anionic polymerization of methacrylates in the presence of triethylaluminum (AlEt3) in toluene. This study reveals ester enolate/aluminum alkyl complexes with different degrees of association, (MiBLi·AlEt3)n (n = 1, 2, 4), and different stoichiometries, MiBLi·xAlEt3 (x = 1, 2). In the presence of methyl pivalate (MPiv), which is taken as a model compound for the monomer and polymer, complexes such as (MiBLi·MPiv·AlEt3)n (n = 1, 2) are formed. These complexes can dissociate into MiBLi·2AlEt3 and MPi…
Zum Ligandenaustausch des Ferrocens mit Methyl- und Ethyl-substituierten Naphthalinen
1988
Abstract The replacement of a Cp ligand of ferrocene by each of ten unsymmetrically substituted naphthalenes yields pairs of complex ions [arene-FeCp] + , which have been identified from their 1 H NMR spectra. The influence of the substituent on the ratio of the two isomers, and its temperature dependence implies a complex reaction mechanism involving an η 2 -bound intermediate of the associated naphthalene. The proton resonances of the alkyl groups of the naphthalenes are shifted to lowfield by complexation; this effect decreases with increasing distance from the iron. The resonance shift of the arene protons has a different sign for the complexed (highfield) and the uncomplexed part (lowf…
Interaction of Oxygen Partial Pressure and Energy Metabolism with the Relaxation Rate of Inorganic Phosphate: A 31P NMR Study
1990
It is well known that oxygen molecules present in liquid or solid samples can shorten NMR spin-lattice relaxation times (T1) of the nucleus under investigation. Several studies have shown large decrements of 1H, 13C, and 19F relaxation times mediated by the O2 molecule (Lees and Muller, 1961; Ohuchi et al., 1979; Fishman et al., 1989). The positions of the 3lp atoms in phosphate compounds are stereometrically similar to some 13C atoms in organic compounds, and thus an effect of oxygen on the 31P T1 is expected. Recently, we have shown (Okunieff et al., 1988) that oxygen breathing can significantly reduce the T1 of 31p in the inorganic phosphate molecule (Pi). The degree to which this change…
Zero- to Ultralow-Field NMR Spectroscopy of Small Biomolecules.
2021
Nuclear magnetic resonance (NMR) spectroscopy is a well-established analytical technique used to study chemicals and their transformations. However, high-field NMR spectroscopy necessitates advanced infrastructure, and even cryogen-free benchtop NMR spectrometers cannot be readily assembled from commercially available components. We demonstrate construction of a portable zero-field NMR spectrometer employing a commercially available magnetometer and investigate its applications in analytical chemistry. In particular, J-spectra of small representative biomolecules [13C]-formic acid, [1-13C]-glycine, [2,3-13C]-fumarate, and [1-13C]-d-glucose were acquired, and an approach relying on the prese…
Effect of ramp size and sample spinning speed on CPMAS 13C NMR spectra of soil organic matter
2011
Abstract Cross polarization (CP) magic angle spinning (MAS) 13C NMR spectroscopy is a solid state NMR technique widely applied to study the chemical composition of natural organic matter. In high magnetic fields (>7 T), fast sample spinning is required in order to reduce the influence of spinning sidebands underlying other chemical shift regions. As the spinning speed increases, the Hartmann–Hahn matching profiles break down into a series of narrow matching bands. In order to account for this instability variable amplitude cross polarization techniques (e.g. VACP, ramp-CP) have been developed. In the present study, we experimentally verified the stability of the Hartmann–Hahn condition unde…