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(BQ) Part 2 book "Organic structure determination using 2-D NMR spectroscopy" has contents: Through-Space efects: The nuclear overhauser efect, molecular dynamics, strategies for assigning resonance to atoms within a molecule, strategies for elucidating unknown molecular structures, simple assignment problems,.and other contents. | Chapter 8 Molecular Dynamics Molecular dynamics covering a wide range of time scales produce an array of effects in NMR spectroscopy. In large molecules, motion of different segments of a molecule may yield measurably distinct relaxation times, thus allowing us to differentiate between signals from different parts of a molecule. Conformational rearrangements can change the chemical shifts of NMR-active nuclei and the J-couplings observed between various spins. Rapid molecular motions average shifts and/or J-couplings, whereas slower motions may make discovering the underlying mechanistic motions difficult. In many cases, molecular motion and chemical exchange may give broad NMR lines devoid of coupling information. Fortunately, most modern NMR spectrometers include variable temperature (V T ) controlling equipment that allows the sample temperature to cover a wide range. Varying the sample temperature may allow us to observe signals that would be poorly suited to supplying desired information at ambient temperature. Probes containing pulsed field gradient (PFG) coils, however, can often only tolerate a more limited range of temperatures compared to their PFG-coil-lacking counterparts; this reduced operating temperature range is attributable to the limitations associated with the materials used to construct these technologically sophisticated probes and the need to minimize thermal stress. Typical temperature ranges for a normal liquids NMR probe are from about Ϫ100°C to ϩ120°C, and PFG probes may only tolerate temperatures in the range of Ϫ20°C to ϩ80°C. Individual vendors list the temperature range recommended for each of their probes. For the purposes of structural elucidation and resonance assignment, a cursory understanding of molecular dynamics and relaxation is 151 152 CHAPTER 8 Molecular Dynamics useful, but often not essential. Recognizing when a particular resonance is broadened as a result of exchange and knowing what step or steps we might take to .
