15C-NMR

Co111 complexes with these ligands are rather unstable with respect to Co—N bond rupture and reduction to Co11.861 Nevertheless, (ran.s-CoCl2(3,3,3-tet)+ has been synthesized and characterized862 by 15C NMR and trans- RuC12 (3,3,3-tet)+ has been reported recently.410... 

Recent general spectral studies of such nitriles include the vibration spectra of 2-pyrazinecarbonitrile and a (> 99%)15N-isotopic version 1172 the mass spectra of 2,3-pyrazinedicarbonitrile, its 5,6-diphenyl derivative, and 2,3,5,6-pyrazinetetracarboni-trile for comparison with those of analogous heterocyclic nitriles 1406 and the 15C NMR spectra of 2-pyrazinecarbonitrile and the like for correlation with their reactivities toward acetone enolate anions.251 The structure-activity relationship of pyrazinecarbonitriles as herbicides has been reported.1048... 

The appearance of signals is described by the codes in the table below. i In 15C NMR spectra this means the appearance of the carbon signal S acquired without decoupling and under low resolution so that long- range C-H couplings can be ignored. 

In anhydrous organic solvents, ethene/CO copolymerisation termination occurs exclusively by P-H transfer to give vinyl terminated polyketone and Pd-H (Scheme 7.15c). On the other hand, traces of water are very difficult to eliminate and consequently chain transfer by protonolysis is often observed, together with p-H transfer. Experimental evidence in this sense has been straightforwardly obtained by an in situ NMR study of the chemical stability of the p-chelate [Pd(CH7CH7C(0)-Me)(dppe)]PF5 (7) in wet and anhydrous CD2CI2 [5ej. Figure 7.13 reports a sequence of P H NMR spectra taken after dissolution of the p-chelate in the wet solvent already the first spectrum at room temperature showed the formation of the p-hydroxo binuclear complex [Pd(OH)(dppe)]2(PF )2 (8), that was the only detectable species after 15 h. 

Figure 2. NMR Spectra (60 MHz)15c of 3,3-dimethyl-2-butanol esters of four different nonracemic enantiomeric mixtures of carboxylic acids. Excess of the (R,R)- or (5,5 )-diasLereomers mandelate 22% atrolactate 18% MTPA 7.5% 0-methylmandelate 12.8%.

In 4- and 5-substituted thiazoles the character of the protomerism is not so sharply defined. IR and lH NMR data indicate that in non-polar solvents, as in the solid state, A2-thiazolin-4-ones exist predominantly in the keto form (15b) but that polar solvents such as DMSO shift the keto-enol equilibrium towards the enol form (15a) in acetone equal amounts of (15 R2 = Ph, Rs = H) enol and keto form were found, whereas in DMSO more than 90% of the enol form exists at equilibrium (65ACS1215). The third form (15c) corresponds to the mesoionic 4-hydroxythiazole and its existence has been suggested from reactivity experiments but it could not be established spectroscopically. 

The " Sn NMR spectra confirm the observations made for the highly populated species M. H and Sn spectra reveal identical patterns in C6D6 and CDCI3 and display strong but perfectly reversible temperature dependence (Figures 15a to 15c). 

Elemental analyses were determined by Microanalysis Inc., Wilmington, Delaware oxygen was directly determined by a modified Unterzaucher technique. IR spectra were recorded on a Digilab Model 15C interferometer using KBr pellets. NMR spectra were recorded on a Varian XL-200. 

Prompted by its crystal structure [34a] and the fact that some of these aggregates were formed in water-saturated chloroform solutions, the role of the water molecules in the formation of the hexameric capsules of 15c and 16c was also studied. Diffusion NMR was selected since the chemical shift of water protons in the presence of acidic compounds is very sensitive to the amount of water, the concentration of the acidic material, the exchange rate and solvent pH and is therefore not a predictable parameter. In addition, since water molecules are very small relative to the resorcinarene and pyrogallolarene hexamers, the water diffusion coefficient was anticipated to be a rather sensitive parameter to follow water interaction with these supramolecular structures. Indeed, when the water diffusion coefficient was followed as a function ofthe I5C/H2O ratio, a decrease in the diffusion coefficient of the water peak was observed with the increase of the 15c/H2O ratio, as shown in Fig. 6.12. Fig. 6.12a-d shows the signal decay of the water (Fig. 6.12a, c) and the... 

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