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Reestimation of the Taft's Substituent Constant of the Pentafluorophenyl Group

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journal contribution
posted on 15.10.2004, 00:00 by Toshinobu Korenaga, Koichi Kadowaki, Tadashi Ema, Takashi Sakai
The Taft's substituent constant of the pentafluorophenyl group (σ*(C6F5)) was reestimated to be 1.50 by correlation between IR spectral data (vCO) 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 C6F5 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 pKa value of pentafluorobenzyl alcohol [14.5 (or 14.3)].