Temperature effects nuclear magnetic resonance

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

Morrow et al. also examined the effects of pressure and temperature on DPPC and DMPC choline headgroup conformation using deuterium nuclear magnetic resonance. The electric dipole resulting from charge separation in... 

Changes In nuclear magnetic resonance measurements of an extensive suite of Australian coals on heating and exposure to pyridine are used to elucidate the molecular conformation of coal macerals Two types of fusible material are Identified In these coals One Is associated with llptlnltes of all ranks and Is typified by fusion commencing at temperatures below 475 K. The other Is associated with vltrlnltes and some Inertlnltes of bituminous coals only and Is characterized by a sharp onset of fusion at temperatures above 625 K. The temperature of onset of fusion Increases with rank for both types The effect of pyridine on the molecular stability of bituminous coals at ambient conditions Is strongly dependent on maceral composition at 86% C and on rank at higher carbon contents ... 

Below that temperature this thermally activated motion is frozen out. There is a characteristic change in the solid-state lineshape that clearly demonstrates the effect. Nuclear magnetic resonance can in such cases provide important information about molecular structure and dynamics in solids. 

In this experiment, nuclear magnetic resonance (NMR) techniques will be used to determine the specific rate constants and for the forward and reverse reactions as well as the value for the equilibrium constant K. Like the hydrolysis of many other organic compounds, this reaction can be acid catalyzed and the effect of hydrogen-ion concentration on the kinetics can be studied. Furthermore, the dependence of y, , and K on temperature will be measured and used to evaluate activation energies. 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

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