Dagfinn W. Aksnes

Cyanidin 3-[6-(p-coumaroyl)-2-(xylosyl)-glucoside]-5-glucoside and other anthocyanins from fruits of Sambucus canadensis.

From the fruits of Sambucus canadensis four anthocyanin glycosides have been isolated by successive application of an ion-exchange resin, droplet-counter chromatography and gel filtration. The structure of the novel, major (69.8%) pigment, cyanidin 3-O-[6-O-(E-p-coumaroyl-2-O-(beta-D-xylopyranosyl)-beta-D- glucopyranoside]-5-O-beta-D-glucopyranoside, was determined by means of chemical degradation, chromatography and spectroscopy, especially homo- and heteronuclear two-dimensional NMR techniques. The other anthocyanins were identified as cyanidin 3-sambubioside-5-glucoside (22.7%), cyanidin 3-sambubioside (2.3%) and cyanidin 3-glucoside (2.1%).

A Pulsed Field Gradient Spin-Echo Method for Diffusion Measurements in the Presence of Internal Gradients.

Over the past decade several pulsed field gradient stimulated-echo methods have been presented for diffusion measurements in heterogeneous media. These methods have reduced or eliminated the coupling between the applied magnetic field gradient and a constant internal magnetic field gradient caused by susceptibility changes throughout the sample. For many research purposes the z-storage delay between the second and third pi/2 RF pulse has been included in order to increase the decay of the echo attenuation to an appropriate level and to increase the signal-to-noise ratio by avoiding T2 relaxation of the magnetization in parts of the pulse sequence. For these reasons a stimulated-echo method has been applied instead of a spin-echo method. When studying systems where it is necessary to keep the duration of the pulse sequence at a minimum, and one is not dependent on using z-storage time to increase the echo attenuation or to study diffusion as a function of observation time, a spin-echo method should be chosen. Here we propose a bipolar pulsed field gradient spin-echo method which is well suited to this purpose, and preliminary diffusion measurements are presented as illustration. Copyright 1999 Academic Press.

Relaxation and diffusion studies of cyclohexane confined in MCM-41 by NMR

Abstract In this work the rotational and translational dynamics of cyclohexane confined in MCM-41 have been studied as a function of temperature by measuring intra-crystalline diffusivities and T 1 and T 2 relaxation times. The results are compared with values obtained for bulk cyclohexane and cyclohexane confined in 40 A silica. T 1 was measured for three different samples, with three different filling grades of cyclohexane. The T 1 curves are continuous over the studied temperature region, and the slope is practically unchanged. However, a marked increase in T 1 with the degree of pore filling is seen owing to an increasing contribution from the bulk-like molecules at the interior of the pores. Only one surface component, attributed to the molecules at the pore walls, was observed when measuring the T 2 relaxation. The relatively narrow spin-echo signal and the long T 2 indicate that the liquid adsorbed on the pore surface does not freeze at all, even at temperatures far below the plastic-brittle solid transition point of the bulk material. The true intra-crystalline diffusivity was obtained by using the short diffusion time model and extrapolating to zero observation time. A high diffusion rate was observable over a wide temperature region, and the measured diffusivity is about three orders of magnitude larger than in the plastic phase of bulk cyclohexane.

Complete Assignment of the 1H and 13C NMR Spectra of Flavone and its A-Ring Hydroxyl Derivatives

Complete assignment of the 1H and 13C NMR spectra of flavone and three flavone derivatives is reported. Proton–proton and carbon–proton coupling constants of the flavones, including the extreme seven‐bond long‐range coupling between H‐7 and H‐3 in 6‐hydroxyflavone (0.52 Hz) and flavone (0.27 Hz), are presented. Hydrogen bonding between 5‐OH and the keto group (C‐4) in 5,7‐dihydroxyflavone gives rise to a large coupling constant between H‐3 and C‐3 (169.1 Hz), in addition to the expected low‐field shifts of the 5‐OH hydroxyl proton (12.92 ppm) and C‐4 (181.86 ppm). This investigation revises the previously reported carbon assignments of three of the examined flavones and provides new NMR data for 6‐hydroxyflavone.

Paramagnetic relaxation reagents as a probe for structure of alkyl and alkylbenzene hydrocarbons

Abstract Use of the paramagnetic relaxation reagents, tris-(acetylacetonato)chromium(III) [Cr(acac) 3 ], in 13 C spin-lattice relaxation time measurements proves capable of differentiating the alkyl groups of short-chain alkylbenzenes and paraffinic hydrocarbons. A specific hydrogen-bond interaction between Cr(acac) 3 and a very weak hydrogen-bond donor, such as an α-benzylic group, is the most important factor controlling the observed electron-nuclear spin-lattice relaxation times ( T 1 e s) of carbon- 13 nuclei.

Analysis of pore size distribution by 2H NMR

The pore size distributions of four controlled pore glasses with mean diameters ranging from ca. 7.9 to 23.9 nm were analysed by measuring the (2)H NMR signals from the liquid fraction of confined benzene-d(6) and cyclohexane-d(12) as a function of temperature, in steps of ca. 0.1-1 K. The liquid and solid components of the adsorbates were distinguished, on the basis of the spin-spin relaxation time T(2), by employing a spin-echo sequence. The experimental intensity curves of the non-frozen liquids are well represented by a sum of two error functions. The observed melting point depressions are well represented by the simplified Gibbs-Thompson equation DeltaT = k(p)/R where R is the pore radius and k(p) is a characteristic property of the adsorbate. The k(p) value mainly affects the position of the pore size distribution curve, i.e., the mean pore radius, while the slope of the intensity curve determines the width of the distribution curve. In practice, the NMR method can only be used to determine pore sizes with reasonable accuracy in the mesoporous range unless liquids undergoing larger melting point depressions than the ones investigated so far can be found.

Temperature dependence of 13C1H one-bond coupling constants of methyl groups in plastic crystals

Abstract The temperature dependence of the one-bond 13 C 1 H coupling constant of the methyl groups in pivalic acid, tert -butyl chloride and hexamethylethane has been studied in the liquid and plastic crystalline phases. A steady decrease in the coupling constant with falling temperature in the plastic crystalline phase has been observed for these organic solids. A maximum change in the 13 C 1 H coupling constant of 25 Hz has been found after deduction of the effect of overlap of the broadened lines in the methyl quartet. The CNDO/2 calculations indicate that the temperature dependence of the coupling constant is not caused by intramolecular transitions. The significant reduction of the 13 C 1 H coupling constant is largely attributed to intramolecular dipole-dipole interactions due to a slight anisotropic tumbling of the molecules in the plastic phase.

High-field magnetic resonance studies of the molecular motion of tert-butyl compounds in liquid and solid phases: the binary system tert-butyl iodide–carbon tetrachloride

The binary system tert-butyl iodide–carbon tetrachloride was studied in the liquid and solid phases using 1H and 13C nuclear magnetic resonance spectrometry. An analysis of the methyl 1H and 13C linewidths and spin–lattice relaxation times is reported. The 1H linewidths in the disordered phase (solid I) are governed by 1H–1H dipolar interactions even in the most dilute sample (6.7 mol-%), whereas the 13C linewidths appear to be dominated by 13C–35,37Cl interactions at all concentrations below 70 mol-%. The line narrowing of the 1H resonances is largely caused by translational diffusion of the tert-butyl iodide (TBI) molecules, whereas translational diffusion of the CCl4 molecules dominates the 13C line-narrowing mechanism in most instances. The T1(13C) minimum of solid II moves to lower temperature with increasing dilution, indicating that the reorientational freedom of the tert-butyl group is enhanced by the addition of CCl4. There is a discontinuity in T1 at the melting-point and an abrupt change of T1 at the transition point of the neat and dilute samples of TBI. The measured activation energies and correlation times in solid I suggest that the over-all tumbling motion of the TBI molecules in the inhomogeneous region (solute concentrations higher than 50 mol-%) is markedly retarded by the addition of CCl4.