Variable-temperature NMR

Some of the trends indicated by the 13C-NMR study97 are summarized in Table XX. The conformers were readily recognized by means of y-substituent effects (Section II,B,5). For the simple 3-alkyltetrahydro-l,3-oxazines, the slowing of ring-inversion processes are not detectable by the 13C-NMR variable-temperature technique, and entries 1 and 5 show the axial preference for the 2-methyl and 2-ethyl derivatives with an increased... 

H-NMR variable-temperature studies of 2,5-dimethyltetrahydro-l,2,5-oxadiazine351-354 established a high energy barrier process (AG 14.4 + 0.1 kcal mol-1 and AS —1.2 + 0.4 kcal mol-1) comparable to that in N-methyltetrahydro-l,2-oxazine. From studies of a deuterated tetrahydro-1,2-oxazine, it was concluded that this should be attributed to slowing of 2-AT-Me inversion rather than ring reversal.353... 

The substituted triazine, a dye, shown in Figure 8, undergoes internal rotation of the substituents about the triazine ring. This is reflected in the H NMR variable-temperature spectra of the H(3 )H(5 )H(6 ) system, an analysis of which has been performed by Birkett et al for the H3 proton only yielding the relevant four- and five-bond proton-proton couplings. The... 

In [Ag(TpCF3)(PiBu3)], the silver atom is in a distorted tetrahedral coordination geometry. This compound has been investigated by 31 Pf1 H NMR variable temperature NMR studies and ESI MS spectroscopy.3... 

Lyerla J R, Yannoni C S and Fyfe C A 1982 Chemioal applioations of variable-temperature CPMAS NMR speotrosoopy in solids Accounts Chem. Res. 15 208-16... 

I still did not have suitable low-temperature instrumentation of my own to carry out the low-temperature NMR studies, but Martin Saunders at Yale did. Thus our samples now traveled the Massachusetts Turnpike from Boston to New Haven, where with Marty we were able to study solutions of the norbornyl cation at increasingly lower temperatures using his home-built variable-temperature NMR instrumentation housed in the basement of the old Yale chemistry building. We... 

Three different conformations are possible for monomeric chalcogen diimides (Eig. 10.1). Variable-temperature NMR spectra indicate that the cis,trans isomer of S(NR)2 is most stable in solution for small organic groups (R = Me, Bu). With bulkier organic substituents, small amounts of the trans,trans isomer exist in equilibrium with the cis,cis isomer. " The cis,cis isomer is observed in solutions of certain sulfur diimides with... 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

Simple examples of diazoalkylideneamine-l,2,3-triazole equilibria have been demonstrated for a series of l,2,3-triazolo[l,5-a]pyrimidines by variable-temperature NMR [74JCS(CC)671]. Tautomers A, B, and C interconvert rapidly at elevated temperatures the energy barrier for these ring-opening-ring closure processes was found to be AG = 76 kJ mol (for = H, Me R = CONH2) (Scheme 111). 

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

The tautomeric behavior of six-membered azaphospha heterocycles 136 has been studied in solution by variable-temperature NMR [82JCS(D)1549]. 

The structural environments of cations absorbed onto clays Cs variable-temperature MAS NMR spectroscopic... 

The selectivity for (/ ,/ )( ,S)-10 has been rationalized by invoking a synperiplanar enolate species whose conformation is enforced by a donor(enolate oxygen)- acceptor) peril uo-rophenyl) interaction depicted in structure N47. Infrared and variable temperature NMR spectroscopic studies of the neutral precursor complex 8 support the existence of such a donor-acceptor interaction. 

Variable temperature NMR studies of [IrCl2(PMe2Ph)4]+ (XXV L = PMe2Ph) show a broad line in the 31P spectrum at room temperature but on cooling to 80°C a mass of sharp lines are observed, owing to three to four rotational isomers (Figure 2.89) the predominant ABCD pattern is... 

Variable-temperature H and 13C NMR spectroscopic study of this seemingly crowded molecule 38 revealed that the ring rotation about the iron ring axis occurs freely down to — 95 °C, although at lower temperatures the rotation of the... 

A variable-temperature NMR spectroscopic study of the titanium(IV) complex 43 also indicated free rotation of the five-membered rings, but, as in the ferrocene derivative 38 allowed the determination of the activation barrier for the phenyl ring rotation (AG (-90 °C) = 9.8 0.5 kcal mol1). 

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... 

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

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