A comparison of methods for measuring relative radical stabilities of carbon-centred radicals

Abstract

This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on corrected carbon–carbon bond energies, and more direct measures of the extent of radical delocalisation. As part of this work, a large set of R–H, R–CH₃, R–Cl and R–R BDEs (R˙ = ˙CH₂X, ˙CH(CH₃)X, ˙C(CH₃)₂X and X = H, BH₂, CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl, Br, N(CH₃)₂, NHCH₃, NHCHO, NHCOCH₃, NO₂, OCF₃, OCH₂CH₃, OCH₃, OCHO, OCOCH₃, Si(CH₃)₃, P(CH₃)₂, SC(CH₃)₂CN, SCH₂COOCH₃, SCH₂COOCH₃, SCH₂Ph, SCH₃, SO₂CH₃, S(O)CH₃, Ph, C₆H₄–pCN, C₆H₄–pNO₂, C₆H₄–pOCH₃, C₆H₄–pOH, CF₂CF₃, CF₂H, CF₃, CCl₂H, CCl₃, CH₂Cl, CH₂F, CH₂OH, CH₂Ph, cyclo-CH(CH₂)₂, CH₂CHCH₂, CH₂CH₃, CH(CH₃)₂, C(CH₃)₃, CCH, CHCH₂, CHCHCH₃, CHO, CN, COCH₃, CON(CH₂CH₃)₂, CONH₂, CONHCH₃, COOC(CH₃)₃, COOCH₂CH₃, COOCH₃, COOH, COPh), and associated radical stability values are calculated using the high-level ab initio molecular orbital theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure–reactivity trends.