Dynamic NMR spectroscopic

The iH—NMR spectrum of 41 b is temperature-dependent. According to dynamic NMR spectroscopical studies the underlying diastereo-topomerization 67a> involves an Arrhenius energy of 11.1 0.3 kca1/ mol 67 b>. 

Dynamic NMR spectroscopic measurements indicate, that the free enthalpies of activation of 57 and 59 are 10.2 kcal/mol and 11.9 kcal/mol, respectively. Similar conformational behavior is exhibited by the [2.2](2,5)heterophanes 43, 61, and 62. The thiophene-containing hetero-phanes 61 and 62 8b82) are conformationally rigid up to 200 °C and are not subject to ring inversion, whereas the furanophane 43 is conforma-... 

A remarkable effect was observed for salt 28, as no 31P NMR signals could be obtained between —100 and +30 °C. According to theoretical calculations of the 31P NMR shift values and an inversion barrier, this effect is related to molecular dynamics <2003AGE2778>. Dynamic NMR spectroscopic evidence was established for a rapid 1,3-migrational exchange of the phosphorus atoms A and B within the four-membered ring of triphospha bicyclopentene 31 <1998CC1537>. 

Dynamic NMR spectroscopic methods have been used to study more inversions of coordinated sulfur. The rate of S-inversion of compound (2) varies little with the nature of the solvent, in keeping with a process involving no Pt—S bond breaking. Two separate sulfide inversions were detected in compound (3). The... 

Dibenzo[a,e]cyclooctene (19) has a tub-shaped structure in the crystalline state with dihedral angles of 99 between the two benzene rings [21]. The investigation of its flexibility employing dynamic NMR spectroscopic studies showed that the activation energy for the ring inversion in 19 is AG = 12.3 0.2 kcal mol at -5°C [22]. 

The number of dynamic NMR spectroscopic studies devoted to mechanistic aspects of Pd and Pt complexes continues to grow, and a review on Pt NMR spectroscopy has appeared. ... 

Conformational analysis can be performed using a variety of methods.1161 Most important are X-ray analysis and NMR spectroscopic techniques, which allow detailed insights into the topology of the peptides. NMR combined with molecular dynamic calculations provides the spatial structure of the peptide, but also its dynamics in solution.13643491 Additional information can be obtained from FTIR,15,4521 CD,151,53-551 or Raman spectroscopy. 56,571 The results derived from such conformational analysis of cyclic peptides have a considerable impact on the study of the bioactive conformations of peptides and on the design of cyclic peptides as proteinomimetics.124,58,59,6301... 

In this review, we present NMR spectroscopic techniques currently used to study protein dynamics at various time scales. Instead scrutinizing each technique, we put emphasis on their fundamentals. On the other hand, we enumerate a number of NMR-derived parameters and discuss their relation and relevance to macromolecular motions. As a complement, we briefly describe several other techniques capable of capturing protein dynamics, as synthesis of different methods is the most fruitful way to understand biomolecular processes. 

In further studies of ion-pairing, a variety of sec-a-silyl benzylic lithium compounds 39, 40, 41 and 42, were prepared, both externally and internally solvated, the latter by means of a potential ligand attached to the carbanionic moiety. Ion-paired carbanide salts tend to assemble into several arrangements which differ in aggregation, solvation and in the proximity of anion to cation. Many of these species interconvert rapidly relative to the NMR time scale even at quite low temperatures. An internally solvated ion-pair carbanide salt is more likely to assume a single molecular structure, to undergo the latter exchange processes more slowly and thus be more amenable to NMR spectroscopic studies of structure and dynamic behavior. 

To summarize, different conformations of a molecule can exhibit different NMR spectroscopic properties. However, if the conformations interconvert rapidly (as they normally do), they generate a time-averaged spectrum that reflects the most symmetrical conformation of the molecule. In order to observe spectra of the separate conformations, it would be necessary to slow down the rotation of some of the single bonds in the molecule, and this can sometimes be done by cooling the sample to a very low temperature. There will be more about these dynamic processes in Chapter 10. 

Inversion of configuration at the phosphane phosphorus atom of 3a is most likely due to 3lP NMR spectroscopically monitored dynamic phenomena. Keeping 3a at 60°C for 3 hours leads to an irreversible thermodynamically more stable 4aE. The energy of activation is calculated to 80.5 kJ/mol. According to X-ray structure determinations of 3a and 4aE the SS stereoisomeric compound is subject to a Cope rearrangement, resulting in the EE stereoisomeric form (Scheme 9). 

The conformational analysis of oligothiophenes by use of a combined molecular dynamics (MD)/NMR spectroscopic protocol has been carried out. A series of MD simulations were performed for 2-(2-thienyl)-3-hexylthiophene 173, 2,5-bis(3 -hexyl-2 -thienyl)thiophene 176, and 2,5-bis(4 -hexyl-2 -thienyl)-thiophene 177, with a new MM2 torsional parameter set developed earlier for unsubstituted and methyl-substituted 2,2 -bithiophene. 

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