Chemical shift downfield

Although the chemical shifts of most commonly encountered orga-nofluorine compounds are upheld of CFC13 and thus have negative values, there are a number of structural situations for fluorine that lead to positive chemical shifts (downfield from CFC13). These include acyl and sulfonyl fluorides as well as the fluorines of SF5 substituents. 

In the proton NMR, the presence of the electronegative oxygen tends to shift the position of the chemical shift downfield. This can be seen in Table 9-3 and in the proton NMR spectra of propanal (Figure 9-18). 

The O-NMR chemical shift (downfield from water) for dibenzofuran is 158.0 ppm. By comparison, diphenyl ether and furan exhibit the values 101.5 and 228.5 ppm, ° A double bond in the a,)3 position relative to the oxygen atom is observed to produce a large downfield shift compared with saturated analogs probably because of the electronic charge decrease at oxygen produced by resonance. The extent of the shift may be a measure of the participation of the oxygen lone pair in resonance. 

The free energies for forming products 1 and 2 were 99.68 and 99.69 kcal/mol, which are indistinguishable, considering that the uncertainty in these predictions is 4-5 kcal/mol (12). The predicted Al-NMR chemical shifts downfield from A1(N03)3 (aq) were 69.8, 69.7 ppm for product 1 and 62.7, 60.0 ppm for product 2. While the absolute values of the predicted chemical shifts are 30 ppm smaller than the observed chemical shifts, these calculations permit the assignment shown in Table 1. 

The reactants and products show only singlets in their fluorine magnetic resonance spectra with the following chemical shifts (downfield from fluorotrichloromethane Internal standard) 8 TFA, -76.3 TfOH, -77.3 TFAT, -73.3 and -74.8 TFAA, -75.9 triflic anhydride, -72.6 ppm. 

K, force constant 6, chemical shift, downfield from internal TMS in C.H2C12 A<5(mB), complexation shift measured in CHjClj r(B—N), B—N bond distance. 

The chemical shifts (downfield from 85% HoPO ) of sulfonium salts ]X with various Y are very close (a) 5 = +86.1 ppm (temp. 55°C), (b) 6 = +90.1 ppm, (c) 6 = +86.8 ppm, (d) 6 = +86.7 ppm, (e) 6 = +89.2 ppm, (f) 6 = +92.2 ppm. This suggests that all these salts have "true" sulfonium structure with little interaction within the ion pair involved. It is of interest to note that J X (X = Y = Br) reacts with cyclohexene to give 1,2-dibromo-cyclohexane and the starting thiolate. Sulfonium salts J X react readily with external nucleophiles of high P-nucleophi1icity e.g. water and alcohols. The reaction between l (R = Bu1, R = Ph,... 

General Information. The and l c NMR spectra were recorded at 400 MHz and 100 MHz, respectively, with a Varian XL-400 spectrometer on solutions of the compounds in DCCI3 and with TMS as the internal standard. All NMR spectra were obtained with proton decoupling as were the P NMR spectra which were recorded at 161.9 MHz (referenced to external 85% H3PO4). Chemical shifts downfield from the reference are positive and upfield are negative. Some early NMR experiments were done at 300 MHz. All experiments were performed under N2. Off-resonance experiments confirmed the carbon signals. 

Note P=P double bonds have P chemical shifts downfield ofP=C double bonds. 

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