Effect of substituents on the stabilities of multiply-substituted carbon-centered radicals

Abstract

The bond dissociation energies (BDEs) and radical stabilization energies (RSEs) which result from 166 reactions that lead to carbon-centered radicals of the type ∑ CH 2 X, ∑ CHXY and ∑ CXYZ, where X, Y and Z are any of the fourteen substituents H, F, Cl, NH 2 , OH, SH, CH CH 2 , C CH, BH 2 , CHO, COOH, CN, CH 3 , and CF 3 , were calculated using spin-restricted and -unrestricted variants of the double-hybrid B2-PLYP method with the 6-311+G(3df,2p) basis set. The interactions of substituents X, Y, and Z in both the radicals ( ∑ CXYZ) and in the precursor closed-shell molecules (CHXYZ), as well as the extent of additivity of such interactions, were investigated by calculating radical interaction energies (RIEs), molecule interaction energies (MIEs), and deviations from additivity of RSEs (DARSEs) for a set of 152 reactions that lead to di-( ∑ CHXY) and tri-( ∑ CXYZ) substituted carbon-centered radicals. The pairwise quantities describing the effects of pairs of substituents in trisubstituted systems, namely pairwise MIEs (PMIEs), pairwise RIEs (PRIEs) and deviations from pairwise additivity of RSEs (DPARSEs), were also calculated for the set of 61 reactions that lead to trisubstituted radicals ( ∑ CXYZ). Both ROB2-PLYP and UB2-PLYP were found to perform quite well in predicting the quantities related to the stabilities of carbon-centered radicals when compared with available experimental data and with the results obtained from the high-level composite method G3X(MP2)-RAD. Particular selections of substituents or combinations of substituents from the current test set were found to lead to specially stable radicals, increasing the RSEs to a maximum of +68.2 kJ mol -1 for monosubstituted radicals ∑ CH 2 X (X = CH CH 2 ), +131.7 kJ mol -1 for disubstituted radicals ∑ CHXY (X = NH 2 , Y = CHO), and +177.1 kJ mol -1 for trisubstituted radicals ∑ CXYZ (X = NH2, Y = Z = CHO).

NH and NCl homolytic bond dissociation energies and radical stabilization energies: An assessment of theoretical procedures through comparison with benchmark‐quality W2w data

“…The good performance of G3X(MP2)-RAD is consistent with the very good performance shown by this procedure for CAH BDEs [58,61].…”

The spectroscopy and thermochemistry of phenylallyl radical chromophores

“…8 It is convenient in this respect to define the deviation from additivity of the RSEs (DARSE) for a disubstituted radical (cCHXY) by the equation: …”

“…A commonly-used measure of the chemical stability of a hydrocarbon radical of this type (cCH 2 X) is the radical stabilization energy (RSE), [6][7][8] which is often defined as the energy change in the hydrogen-atom-transfer reaction: cCH 2 X + CH 4 / CH 3 X + cCH 3 The RSE measures the effect of the substituent (X) in the radical (cCH 2 X) relative to its effect in the closed-shell molecular precursor (CH 3 X). Defined in this way, a positive RSE corresponds to a relative stabilizing effect in the radical.…”

Photolysis of ortho-nitrobenzylic derivatives: the importance of the leaving group

“…36 Here, one of the radical substituents is the nitrophenyl moiety, and it is necessary to account for the presence of this substituent in the evaluation of the relative stabilization that additional substituents X provide to radicals 9˙.…”