| Abstract
| - Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) have been calculated for a seriesof models that represent a glycine-containing peptide-backbone. High-level methods that have been usedinclude W1, CBS-QB3, U-CBS-QB3, and G3X(MP2)-RAD. Simpler methods used include MP2, B3-LYP,BMK, and MPWB1K in association with the 6-311+G(3df,2p) basis set. We find that the high-level methodsproduce BDEs and RSEs that are in good agreement with one another. Of the simpler methods, RBMK andRMPWB1K achieve good accuracy for BDEs and RSEs for all the species that were examined. Formonosubstituted carbon-centered radicals, we find that the stabilizing effect (as measured by RSEs) of carbonylsubstituents (CXO) ranges from 24.7 to 36.9 kJ mol-1, with the largest stabilization occurring for the CHO group. Amino groups (NHY) also stabilize a monosubstituted α-carbon radical, with the calculated RSEsranging from 44.5 to 49.5 kJ mol-1, the largest stabilization occurring for the NH2 group. In combination,NHY and CXO substituents on a disubstituted carbon-centered radical produce a large stabilizing effectranging from 82.0 to 125.8 kJ mol-1. This translates to a captodative (synergistic) stabilization of 12.8 to39.4 kJ mol-1. For monosubstituted nitrogen-centered radicals, we find that the stabilizing effect of methyland related (CH2Z) substituents ranges from 25.9 to 31.7 kJ mol-1, the largest stabilization occurring for theCH3 group. Carbonyl substituents (CXO) destabilize a nitrogen-centered radical relative to the correspondingclosed-shell molecule, with the calculated RSEs ranging from −30.8 to −22.3 kJ mol-1, the largestdestabilization occurring for the CHO group. In combination, CH2Z and CXO substituents at a nitrogenradical center produce a destabilizing effect ranging from −19.0 to −0.2 kJ mol-1. This translates to anadditional destabilization associated with disubstitution of −18.6 to −7.8 kJ mol-1.
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