Chemical shift values medium effects

The measurement of pA bh+ and m values is sometimes complicated by the fact that the spectroscopic methods used (generally H and 13C NMR spectroscopy, and UV-vis) are subject to medium effects. Water and (say) 80 wt% H2SO4 are very different media, and it is not very surprising that spectral peaks for the same species in the two media can occur at different wavelengths or different chemical shifts. Several methods have been devised for handling this problem 24,99,100 the excess acidity method lends itself to dealing with medium effects quite well.101... 

Calculations on 125Te shielding have been very successful in reproducing the 4000 ppm range of isotropic chemical shifts in a wide variety of compounds in solutions or neat liquids (25).although no corrections of medium effects had been made in comparing with experimental values. 

It should be noted that the nitrogen chemical shifts of azines are sensitive to medium effects, and that usually the values for aqueous solutions depart from those in organic solvents by 10-20 ppm to lower frequencies (Table XXI). 

The 3-values of some commonly employed standard compounds are collected in Table 1 and can be used to convert reported Se chemical shifts relative to Me2Se in CDCI3 (60% v/v 3 = 0). However, this procedure implies inaccuracies of several ppm since re-referencing of literature data often has to ignore medium effects on the Se chemical shift. 

From INDOR spectra of Pb(CH3)4 measured In various solvents, the Influence of the solvents on the chemical shifts 5 ° Pb, determined as the total effect of the solvent on the nucleus, Is analyzed and compared with effects of the medium on 5 SI and values... 

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