Search results for "nuclear magnetic resonance"
showing 10 items of 1743 documents
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…
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…
NMR Matrix Technique for Detecting Self Diffusion in Polymers
1979
A technique is discussed where monodisperse small spheres or cylindrical fibers of a polymer are embedded in a matrix of the corresponding fully deuterated polymer. On annealing the sample at a temperature above the glass transition the protonated macromolecules diffuse into the deuterated matrix and vice versa. Since the NMR spectrum depends upon the intermolecular proton proton dipolar coupling, the decrease of this coupling can be used to monitor the diffusion process. For particle diameters d ∼ 10−4 cm and annealing times t ∼ 105 s, extremely small diffusion constants D < d2/t are attainable. The NMR analysis of prospective diffusion experiments with polystyrene is discussed in some det…
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…
Direct monitoring of spin state in dinuclear iron(II) coordination compounds
2001
So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high-spin (HS) and low-spin (LS) species have been deduced from magnetic susceptibility and zero-field Mossbauer spectroscopy data irrespective of whether they belong to LS–LS, LS–HS and HS–HS pairs. However, the distinction of pairs becomes possible if Mossbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.
13C NMR study of aromatic ring-substituted (E)-3-phenylpropenals and (2E,4E)-5-phenylpenta-2,4-dienals
1990
13C and 1H chemical shifts together with C,H and H,H coupling constants are presented for (E)-3-phenylpropenal, (2E,4E)-5-phenylpenta-2,4-dienal and their o-OCH3- and o-, m- and p-NO2-substituted derivatives. The SCSs calculated for the aldehyde chains show similar effects on the ring carbons, except in the para position where the shorter chain causes a 3.3 ppm deshielding and the longer chain a 1.0 ppm shielding effect. This shift difference is reflected in all the ring-substituted derivatives of the two series of aldehydes, but not in the one-bond C,H coupling constants. The effect of aromatic ring substitution on 2J(C,CHO) seems to be mainly inductive in origin. The importance of the str…
Über tetraaryl-methan-analoga in der gruppe 14
1994
Abstract The title compounds have been synthesized by Grignard reactions or pyrolysis of diplumbanes respectively. The crystal structures of m -Tol 4 Pb and Ph 4 Pb (redetermination) have been determined. All eight compounds (Ph/Tol) 4 (Sn/Pb) are S 4 symmetric and contracted along this unique axis. The 13 C-NMR chemical shifts and the couplings 1 J ( 119 Sn/ 207 Pb 13 C) as well depend additively upon the methyl substituents. The ratios 1 K ( 207 Pb 13 C): 1 K ( 119 Sn 13 C) of the reduced coupling constants are all near to 1.61; a comparison with values from the literature for alkyl, alkenyl and alkinyl substituents is given. The ratio δ( 207 Pb):δ( 119 Sn) results in 2.28. IR (700 dow…
1H,13C,17O and19F NMR spectroscopic study of isomeric ring-substituted monofluoro-(E)-3-phenylpropenals
1993
Isomeric ring substituted monofluoro-(E)-3-phenylpropenals (cinnamaldehydes) were synthesized in order to provide, with the NMR-active 19F nucleus, an insight into the influence of substituents and the transmission of electronic effects between the aromatic ring and the side-chain. Their 1H, 13C and 17O NMR chemical shifts and their nJ(H,H), nJ(H,F), nJ(C,H) and nJ(C,F) values were determined. Generally, the nJ(C,F) coupling constants correlate with the length of the coupling route and thus simplify the assignments of the 13C NMR spectra. Only the side-chain carbons C-2 and C-3 in the ortho-substituted compounds showed an exception to this rule. Many long-range nJ(H,F) (n = 4–7) values were…
Structure of 5,17-dimethyl-11,23-dioctylcalix[4]arene
1991
The title compound C46O4H60 crystallizes in the triclinic space group, P¯1, witha=11.584(2),b=16.261(2),c=11.172(1) A,α=103.15(8),β=95.68(1),γ=96.85(1)°. The structure was solved by direct methods, and refined by weighted full-matrix least squares toR=0.097. This is the second calix[4]arene with two different alkyl substituents atpara positions of the phenolic rings. The macrocycle adopts the cone conformation. Interactions CH3-π are established between two calixarenes related by a center of symmetry. Comparisons are made between the conformation of this molecule and that of symmetrically substituted calix[4]arenes.
Dracoside, a New Steroidal Saponin fromHelleborus purpurascens
1994
Abstract A new steroidal saponin, 1-O-[6-O-acetyl-β-D-galactopyranosyl]-24-O-[β-D-apiofuranosyl-(1→3)-4-O-acetyl-α-L-rhamnopyranosyl-(1→4)-β-D-xylopyranosyl-(1→3)-α-L-arabinopyranosyl]-dracogenin, was isolated from the roots of Helleborus purpurascens and named dracoside. Structure elucidation was performed by chemical transformations and various spectroscopic methods, mainly 2D NMR techniques (COSY, HMQC, HMBC).