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. Author manuscript; available in PMC: 2022 Jul 16.
Published in final edited form as: ACS Catal. 2021 Jun 28;11(14):8456–8466. doi: 10.1021/acscatal.1c01860

Table 5.

Activation free energies for the radical dissociation of pyridyl radical 2 to pyridine 4 (Figures1 and2)

graphic file with name nihms-1720405-t0024.jpg
calc ΔG‡rdb Expt ΔG‡rdc
(kcal/mol)
Entry Ar σ a R1 = i-Pr R1 = Cy R1 = Cy
1 4-MeOC6H4, 2a –0.27 15.9 17.7 18.0
2 4-MeC6H4, 2b –0.17 16.5 18.2 18.4
3 Ph, 2c 0.00 17.3 18.5 18.8
4 4-FC6H4, 2d 0.06 16.9 18.3 18.5
5 4-CF3C6H4, 2e 0.54 17.5 19.0 19.0
6 3,5-F2C6H3, 2f 0.68d 17.4 19.1 nde
7 3,5-Me2C6H3, 12g – nd nd 18.7
a

Hammett σp parameters for substituents.

b

Gibbs free energies of activation computed using UM06/6–311+G(d,p), SMD: DMF // UB3LYP/6–31G(d).

c

Experimental ΔG‡rd obtained from rate constants using the Eyring equation.

d

For 3,5-difluoro substitution, the σ parameter was estimated additively as a sum of two fluorine σm values of 0.34.

e

Insolubility of this pyridinium salt prevented accurate measurement.