NMR solvent shifts

General procedure for the one-pot Fischer Indole synthesis To allow comparison of the data, a procedure similar to that described in ref. [5] was used. The catalyst (3.00 g) was suspended into a solution of 1 -phenyl-2-butanone (1.48 g, 10.0 mmoles) in p-xylene (30 g). The mixture was then heated to reflux, and after 15 minutes phenylhydrazine (1.08 g, 10 mmoles) was added in one portion. Samples were taken at regular intervals and analyzed by GC using a CP Sil-5 CB capillary column. In several cases, the indole isomer mixture was isolated from the reaction mixture by vacuum distillation after removal of the catalyst and the solvent. NMR shift values and coupling constants for 4 and 5 were identical to those reported in ref. 

In such cases, plots of NMR shifts against infrared band maxima are linear for many pure solvents. A good example is that for EtjPO where the shift is correlated with v(P-0) (Figure 3.14). The only data that fail to fit the linear correlation are those for methanol and ethanol. However, for these solvents two distinct infrared bands are resolved. If the weighted mean of these bands is used, the correlation with the NMR data is good. In order to interpret mixed solvent NMR shifts, we make the assumption that this plot can be used to estimate NMR shifts for specific solvates from v (P-O). It is on this basis that NMR solvent shifts can be reconstructed. The comparison with experi-... 

Instrumentation. H and NMR spectra were recorded on a Bruker AV 400 spectrometer (400.2 MHz for proton and 100.6 MHz for carbon) at 310 K. Chemical shifts (< are expressed in ppm coupling constants (J) in Hz. Deuterated DMSO and/or water were used as solvent chemical shift values are reported relative to residual signals (DMSO 5 = 2.50 for H and 5 = 39.5 for C). ESl-MS data were obtained on a VG Trio-2000 Fisons Instruments Mass Spectrometer with VG MassLynx software. Vers. 2.00 in CH3CN/H2O at 60°C. Isothermal titration calorimetry (ITC) experiments were conducted on a VP isothermal titration calorimeter from Microcal at 30°C. 

Figure 13. Effect of varying concentrations of the NMR shift reagent Eu(fod), on methyl resonances of soyasapogenol B phenyl borate. Euffodjg was dissolved in a minimum amount of acetone-dg and added to a 2 ml solution of soyasapogenol B phenyl borate in the same solvent. Spectra were obtained at 360 MHz.

Fig. 3. 7Li NMR shift measured for 0.05 M LiC104 in acetonitrile— nitromethane solvent mixtures at 25°C (93).

The study of molecular complexation was then extended to other aromatic nitro derivatives125. Although, as was described before, one of the more frequent methods of studying the formation of molecular complexes is by UV-visible spectrophotometry, the author did not observe detectable differences in the UV-visible absorbance spectra between the 2-hydroxypyridine-l-fluoro-2,4-dinitrobenzene (FDNB) mixtures and the sum of their separate components. The author observed that the signals of the 1II NMR spectra of FDNB in apolar solvents were shifted downward by the addition of 2-hydroxypyridine from solutions where [2-hydroxypyridine] [FDNB] he calculated the apparent stability constants, which are shown in Table 13. 

Fig. 11 NMR shifts of 31 (filled square), 32 (filled diamond), and 33 filled circle) showing the presence of aggregation in increasing solvent polarity for 32...

Oxyanions also affect the coordination chemistry of the metal center (84). Molybdate and tungstate are tightly bound noncompetitive inhibitors (Ki s of ca. 4 (iM) (85). These anions bind to the reduced form of the enzyme, changing the rhombic EPR spectrum of the native enzyme to axial (Figure 1) and affecting the NMR shifts observed (84,85). Comparisons of the ENDOR spectra of reduced uterofenin and its molybdate complex show that molybdate binding causes the loss of iH features which are also lost when the reduced enzyme is placed in deuterated solvent (86). These observations suggest that molybdate displaces a bound water upon complexation. 

Generally solvents chosen for NMR spectroscopy do not associate with the solute. However, solvents which are capable of both association and inducing differential chemical shifts in the solute are sometimes deliberately used to remove accidental chemical equivalence. The most useful solvents for the purpose of inducing solvent-shifts are aromatic solvents, in particular hexadeuterobenzene (CgDg), and the effect is called aromatic solvent induced shift (ASIS). The numerieal values of ASIS are usually of the order of 0.1 - 0.5 ppm and they vary with the moleeule studied depending mainly on the geometry of the complexation. 

The chemical shifts of the protons in the H NMR spectra of salicylaldoximes are given in Table 7. The hydroxyl proton varies between 11.61 and 10.82 ppm. The chemical shifts of carbon atoms in the C NMR spectra of the salicylaldoximes are given in Table 8. The signal of C7 shifts downfield when the substiment becomes a stronger electron donor (AS = 149.58 — 144.64 ppm = 4.94 ppm). Comparison of the spectra of 50 and 51 shows that the 2-OH group shifts the signal of C7 upheld. Since the most important interactions between the solvent and the aldoxime probably involve the 2-OH group, the solvent chemical shifts in the spectra of 50 and 51 are not parallel. 

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