Unsaturated monocyclic compounds. Part XLVIII. A study by nuclear magnetic resonance of the energy barrier for conformational interconversion in annulenes and dehydro-annulenes

1967 ◽  
pp. 660 ◽  
Author(s):  
I. C. Calder ◽  
P. J. Garratt
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Energy Barrier ◽  
Conformational Interconversion ◽  
Nuclear Magnetic

A 13C nuclear magnetic resonance study of the reversible substituent migration in several 2-acyl and 2-silyl derivatives of tropolone. Troponoid-IV

1982 ◽  
Vol 60 (3) ◽  
pp. 231-238 ◽  
Author(s):  
D. Ménard ◽  
M. St-Jacques ◽  
J. F. Bagli
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Energy Barrier ◽  
Bromine Atom ◽  
Nuclear Magnetic Resonance Study ◽  
Magnetic Resonance Study ◽  
13C Nuclear Magnetic Resonance ◽  
Silyl Derivatives ◽  
Derivatives Of ◽  
Nuclear Magnetic

The energy barrier for the reversible substituent migration was determined for a number of 2-acyl and 2-silyl derivatives of tropolone. The energy barrier is found to be dependent on the nature of the migrating group. Asymmetric monosubstitution on the ring shifts the equilibrium in favor of one dynamic isomer. In the two cases studied (3-bromotropolone and 3-bromotropolone acetate) it is found that the equilibrium is shifted towards the isomer bearing the bromine atom at the 7-position.

Nuclear Magnetic Resonance Evidence for High Barrier of Conformational Inversion in Hexahydro-3,3,7,7-tetramethyl-1,2-oxazepine-5-one

1975 ◽  
Vol 53 (3) ◽  
pp. 414-418 ◽  
Author(s):  
Roderick E. Wasylishen ◽  
Kenner C. Rice ◽  
Ulrich Weiss
Keyword(s):  
Activation Energy ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Nitrogen Atom ◽  
Variable Temperature ◽  
Membered Ring ◽  
Rate Process ◽  
Conformational Interconversion ◽  
Nuclear Magnetic

Variable temperature proton n.m.r. spectra indicate an activation energy of 19.0 ± 0.5 kcal/mol for conformational equilibration in hexahydro-3,3,7,7-tetramethyl-1,2-oxazepine-5-one. It is suggested that the rate process involves slow conformational interconversion of the seven-membered ring and not slow inversion at the nitrogen atom. The synthesis of several related 1,2-oxazepines is described.

An emerging technology: Nuclear magnetic resonance microscopy

1988 ◽  
Vol 46 ◽  
pp. 1000-1001
Author(s):  
M.J. Hennessy ◽  
E. Kwok
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Relaxation Times ◽  
Basic Research ◽  
Local Environment ◽  
Magnetic Resonance Images ◽  
Nmr Microscopy ◽  
Mri Scans ◽  
Temperature Relaxation ◽  
Nuclear Magnetic

Much progress in nuclear magnetic resonance microscope has been made in the last few years as a result of improved instrumentation and techniques being made available through basic research in magnetic resonance imaging (MRI) technologies for medicine. Nuclear magnetic resonance (NMR) was first observed in the hydrogen nucleus in water by Bloch, Purcell and Pound over 40 years ago. Today, in medicine, virtually all commercial MRI scans are made of water bound in tissue. This is also true for NMR microscopy, which has focussed mainly on biological applications. The reason water is the favored molecule for NMR is because water is,the most abundant molecule in biology. It is also the most NMR sensitive having the largest nuclear magnetic moment and having reasonable room temperature relaxation times (from 10 ms to 3 sec). The contrast seen in magnetic resonance images is due mostly to distribution of water relaxation times in sample which are extremely sensitive to the local environment.

Nuclear magnetic resonance microscopy

1989 ◽  
Vol 47 ◽  
pp. 828-829
Author(s):  
Paul C. Lauterbur
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Magnetic Fields ◽  
Signal To Noise ◽  
Field Gradients ◽  
Useful Signal ◽  
Magnetic Field Gradients ◽  
Observation Times ◽  
Nuclear Magnetic

Nuclear magnetic resonance imaging can reach microscopic resolution, as was noted many years ago, but the first serious attempt to explore the limits of the possibilities was made by Hedges. Resolution is ultimately limited under most circumstances by the signal-to-noise ratio, which is greater for small radio receiver coils, high magnetic fields and long observation times. The strongest signals in biological applications are obtained from water protons; for the usual magnetic fields used in NMR experiments (2-14 tesla), receiver coils of one to several millimeters in diameter, and observation times of a number of minutes, the volume resolution will be limited to a few hundred or thousand cubic micrometers. The proportions of voxels may be freely chosen within wide limits by varying the details of the imaging procedure. For isotropic resolution, therefore, objects of the order of (10μm) may be distinguished.Because the spatial coordinates are encoded by magnetic field gradients, the NMR resonance frequency differences, which determine the potential spatial resolution, may be made very large. As noted above, however, the corresponding volumes may become too small to give useful signal-to-noise ratios. In the presence of magnetic field gradients there will also be a loss of signal strength and resolution because molecular diffusion causes the coherence of the NMR signal to decay more rapidly than it otherwise would. This phenomenon is especially important in microscopic imaging.

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