Variable temperature 13C NMR

The ring-inversion process in Af,N-dimethylhexahyd ropyrimidine has been estimated by H-NMR spectral studies as AG. 3r 11.6kcal mol-1 297 and AGi29 4" 11-3 kcal mol"1 298 Variable-temperature 13C-NMR work287 permits the estimation of AG values (Table XXVII) for the N-inversion process in the /V,/V-dimethyl- and JV,JV-diethylhexahydropyrimidines. 

Variable-temperature 13C-NMR studies of 2,4,6-trimethyltetrahydro-1,2,4-oxadiazine also show two coalescences. At low temperature, AG° =0.6 kcal mol-1 and AG x-,ts = 6.8 kcal mol-1 for inversion at the 4-N-Me (eee conformer preferred), Study of the 2-isopropyl-4-methyl derivative shows that it is easier to place the 2-isopropyl group axial (AG° 1.2 kcal mol-1) than the 2-methyl group in the analog 451 (AG° 1.6 kcal mol-1).350... 

Variable temperature 13C NMR spectroscopy has shown that the 1,2,3,4,5,6-hexamethyl-hexahydro-l,2,4,5-tetrazine (57) is the trans isomer with the two C-methyl groups equatorial and the N-methyl groups symmetrically diaxial and diequatorial. The observed dynamic 13C NMR effects are consistent with (57a) and (57b) as the major conformation and probably (57c) as the minor one. The first coalescence represents the freezing out of (57c) while the (57a) (57b) interconversion remains fast. The nitrogen inversion barrier was found to be 32.2 kJ moF1 <79JCS(P2)98l). The He(I) photoelectron spectrum of (57) confirmed the NMR data (80JCS(P2)9l). 

To date only one such cluster has been reported. The tetranuclear cluster 33 is unstable in solution and over a period of several days eliminates the elements of Pt(COD)2, giving the Pt bridged cluster Pt[Ru3(/i-H)(ji4- 72-C=CtBu)(CO)9]2 (75) in reasonable yields (40-50%) (152). The C2 metal core (Fig. 15) is chiral, and a variable-temperature 13C NMR study showed that the cluster undergoes enantiomerization, with AG 66 = 57 kJ mol-1. The enantiomerization presumably proceeds via an intermediate or transition state with a planar coordination at the Pt atom. 

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

Variable-temperature 13C NMR spin-lattice relaxation time measurements have been used to probe the motional behavior of 2,3 5,6-di-O-isopropylidene-a-D-mannofuranose (22) in dimethyl sulfoxide solution.67 This carbohydrate deriva-... 

Fallis and Heuft have used a metal-templated synthesis to form the helical 1,10-phenanthroline-capped metal-complexed SPMs 87 and 88 (Scheme 6.20) and their demetalated analogs (not shown). The addition of [Cu(OAc)2] (0.5 equiv) to a solution of 89 in pyridine and diethyl ether templated the formation of intermediate 90. Following the addition of excess [Cu(OAc)2] (5.5 equiv), the Cu(I)-complexed SPM 87 was isolated in an excellent yield of 84%. Treatment of 87 with aqueous KCN then provided the Cu-free SPM in 70% yield (not shown). The N,N-dibutylamine substituted analog 88 was formed in an analogous manner and taken on directly to the Cu-free SPM (39%) in a one-pot procedure. It was shown by variable temperature 13C NMR spectroscopy that the helical Cu(I) complex 88 had a barrier to racemiza-tion of 4 kcal mol 1 higher than that of the Cu-free analog [83]. 

The conformational equilibria of 2-substituted 1,3-dithianes 63 were determined by application of variable temperature 13C NMR spectroscopy (cf. Scheme 20) (99T359). The thermodynamic data, given in Table IX, provide strong evidence that the predominance of the axial conformer in 63 is enthalpic in origin more polar solvents stabilize the more polar equatorial conformation. Both ab initio and DFT calculations reproduce the experimental results (99T359). 

Variable temperature 13C NMR data of the Y derivative were consistent with a monomer/dimer equilibrium for compounds 85 in solution as depicted in Scheme... 

A number of variable-temperature NMR studies were reported for [Cp (DPVP)2Ru=C=CH2]+ (DPVP = Ph2PCH=CH2), showing that the barrier to vinylidene rotation was very low (<6.7 kcal moN1).916 The barrier to rotation about the P-C(aryl) bond in Hif/o-Ru3(CO)9[(i-P-C6I l2-2,4,6-(CF3)3]2 (AG1) was shown by variable-temperature 19F NMR to be 82.3 kJ mol-1 at 141 °C.917 Variable-temperature 13C NMR examined the fluxional characteristics of (r 1, r 5-CI l2C5Me4)Ru2(CO)6. Five carbonyl ligands interchange in a... 

Fig. 3. Variable-temperature 13C NMR chemical shifts at C-l and C-2 of acenaphthylene dianion (52 )83al...

Shifts in the resonances on going from the monomer to the polymer have been used to elucidate information on the nature of structural effects. Data on the dihedral angle have been obtained from the three-bond —P coupling constants. The preferred molecular conformations of aqueous poly U (MW 130 000) were found to be the 4 -trans rotamer for the 5r-phosphate and the 2 -trans rotamer for the 3 -phosphate (Mantsch and Smith, 1972 Smith et al., 1973a). Spectral changes in a variable-temperature 13C nmr study of poly U in 0 5 M CsCl have been interpreted in terms of a helix-coil transition (Govil and Smith, 1973). 

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