NMR spectroscopy variable-temperature

The methodologies for studying both the conformers at equilibrium and their rates of interconversion are mainly based on NMR spectroscopy. Variable temperature measurements are of great help to freeze the conformational equilibria. Also, quantitative determinations of ring interconversion and N-inversion barriers rely exclusively on dynamic NMR methods. In addition to the state of the art, already reviewed previously (vide supra), only a very few methods and applications were added. 

Copper. Variable temperature H NMR spectroscopy and Ti measurements have been used to study the interconversion of conformers of complexes such as [Cu(L-phenylalanine)(phen)(OH2)]. A complex equilibrium involving copper complexes such as [Cu(saccharinate)(PPh3)2] has been studied by NMR spectroscopy. Variable temperature P NMR spectra of [Cu2(p-dppm)3(02CR)2] have revealed at least three dimeric species in equihbrium. ... 

A Relaxation time measurement in the solid (Al) in solution (A2). B Mechanical spectroscopy. C Variable-temperature NMR spectroscopy (coalescence temperature measurement). D Variable-temperature EPR spectroscopy... 

In order to perform the various tasks mentioned in Section 11.1.2, it is necessary to use one or several methods to gather information by NMR spectroscopy. Typically, chemical shift and coupling constant information, 2D-NMR measurements, variable temperature or pressure studies are used. If appropriate, specific examples of the particular topic as applied in homogeneous hydrogenation research are detailed below. 

Triamino-j -triazine derivatives 48-50 have been prepared, and their cationic states have been analyzed electroche-mically by Blackstock and coworkers (Figure 20). At 298 K, 48 has a limited lifetime in CH2CI2 solution however, 49 and 50 are long-lived under such conditions, and quartet states of 49 and 50 are observed by electron spin resonance (ESR) spectroscopy. Variable-temperature ESR analysis and NMR shift susceptibility measurements indicate that 50 is a doublet ground state with a populated quartet state <20000L171>. 

Accounting for this effect, it was possible to apply dynamic NMR spectroscopy to measure energy barriers to the prototropic rearrangements of pyrazoles. Temperature-variable spectra of a series of 4-substituted pyra-zoles 5 and 6 have been studied in methanol-d4 solutions and the free energy barriers of the degenerate type 2a 2b tautomerization reported (93CJC1443). 

Compound 233 was prepared from 144 and 234, and its intramolecular ligand reorganization was investigated by variable temperature NMR spectroscopy (80JA5073). 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

C NMR spectroscopy has been used to establish the structures of intermediates formed during the photooxygenation of 13C-labeled derivatives of 64 related to bioluminescent luciferins <1996T12061>. The mechanism of the formation of 3-oxo derivatives of 64 related to the chemistry of bioluminescence has been studied by low-temperature 13C (together with variable temperature proton) NMR spectroscopy <1997J(P2)1831>. The 13C and 1SN... 

Such chelates have been detected also by variable temperature multinu-clear NMR spectroscopy, which showed that basically the above mechanism is also operating in solution [47,55,58]. 

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