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-CH(3), R-Cl and R-R BDEs (R = CH(2)X, CH(CH(3))X, C(CH(3))(2)X and X = H, BH(2), CH(3), NH(2), OH, F, SiH(3), PH(2), SH, Cl, Br, N(CH(3))(2), NHCH(3), NHCHO, NHCOCH(3), NO(2), OCF(3), OCH(2)CH(3), OCH(3), OCHO, OCOCH(3), Si(CH(3))(3), P(CH(3))(2), SC(CH(3))(2)CN, SCH(2)COOCH(3), SCH(2)COOCH(3), SCH(2)Ph, SCH(3), SO(2)CH(3), S(O)CH(3), Ph, C(6)H(4)-pCN, C(6)H(4)-pNO(2), C(6)H(4)-pOCH(3), C(6)H(4)-pOH, CF(2)CF(3), CF(2)H, CF(3), CCl(2)H, CCl(3), CH(2)Cl, CH(2)F, CH(2)OH, CH(2)Ph, cyclo-CH(CH(2))(2), CH(2)CH[double bond, length as m-dash]CH(2), CH(2)CH(3), CH(CH(3))(2), C(CH(3))(3), C[triple bond, length as m-dash]CH, CH[double bond, length as m-dash]CH(2), CH[double bond, length as m-dash]CHCH(3), CHO, CN, COCH(3), CON(CH(2)CH(3))(2), CONH(2), CONHCH(3), COOC(CH(3))(3), COOCH(2)CH(3), COOCH(3), 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.