Study of 13C Chemical Shifts in Substituted Benzenes

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Abstract
Carbon‐13 chemical shifts of several monosubstituted benzenes and para‐substituted fluorobenzenes have been determined. The shifts of the para carbons in monosubstituted benzenes are found to correlate with Taft's resonance parameters, especially when the shifts are corrected by the amount of the meta shifts. A much less precise correlation is found between the 19F chemical shifts in the para‐substituted fluorobenzenes and the 13C–F chemical shifts in the same set of compounds than between these 19F shifts and the para 13C shifts in monosubstituted benzenes. The relationships between the 13C and 19F shifts are considered in terms of the available theories of Karplus and Pople, Karplus and Das, and Prosser and Goodman, and are discussed in terms of π‐electron densities. It is concluded that both 13C shifts in monosubstituted benzenes and 19F shifts in para‐substituted fluorobenzenes will be useful indices of the π‐electron densities in para positions.