Reestimation of the Taft's substituent constant of the pentafluorophenyl group

The Taft's substituent constant of the pentafluorophenyl group (σ*C₆F₅)) was reestimated to be 1.50 by correlation between IR spectral data (v_C=O) and σ* constants for a series of esters (involving the pentafluorobenzyl group) of 3-phenylpropanoic acid and butanoic acid. The possibility of the disturbance of the correlation by the intramolecular π-π interaction between C ₆F₅ and C₆H₅ groups in pentafluorobenzyl 3-phenylpropanoate was excluded by ab initio and DFT calculations of the stable conformations and their carbonyl frequencies. The reestimated σ* (C₆F₅) value was used for calculation of the pK_a value of pentafluorobenzyl alcohol [14.5 (or 14.3)].