Natural abundance 17O NMR spectra of 4-substituted N-chlorobenzamides were obtained in acetonitrile at 75°C and compared with those for similarly substituted benzamides. An excellent correlation (δ = 5.43σ+ + 350.1, r = 0.998) was obtained when the 17O chemical shifts of N-chlorobenzamides were plotted against σ+ and using σ- for the 4-nitro derivative. Based on the greater electronegativity of the N-chloro group the value of ρ observed for N-chlorobenzamides (5.4) would have been expected to be greater compared to that of benzamides (7.3). It is concluded that the 17O chemical shift in N-chlorobenzamides is very sensitive to torsion angle and that is why a low value of ρ is obtained. Sensitivity (ρ) of the chemical shift to ring substituents cannot be used as an unambiguous measure of electronic effects in systems where the torsion angle is greater than zero.
|Original language||English (US)|
|Number of pages||3|
|Journal||Journal of the Chemical Society. Perkin Transactions 2|
|State||Published - Aug 1997|
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