The purpose of presenting the electrophilicity (ω) and the bonding dissociation energies (Table 2) is to assist in the prediction of the antioxidant behavior, via free radical scavenging chemical activity, especially for by hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms. Figure 4 presents the eH-DAMA (e lectron and h ydrogen d onating a bilitym ap for a ntioxidants) for the rasagiline derivatives. This graphical tool has been recently designed to simultaneously account for the likeliness of molecules as H donors (formal HAT reactions) and electron donors (SET reactions).82-84 It is constructed taking into account both the bond dissociation energies (related to HAT feasibility), and electrophilicity (related to SET viability). The pole strength (PS) values, arising from the EPT calculations (used to obtain electrophilicities) are all in the recommended range (Table S7). The complete set of BDE values for each species is provided as Supporting Information (Table S8). eH-DAMA employs electrophilicity (ω) over IE, since molecules with very low IE, are actually not expected to be very effective as free radical scavengers acting as electron donors in SET reactions since they are located in the inverted region of the Marcus parabola. In those cases, electrophilicity (ω) should be used instead of IE (Figure S3 and S4).140-142
As Figure 4 shows, the most promising acid-base species for deactivating free radicals via SET are the anions. They have the lowest ω values, followed by the neutral and the protonated species, respectively. In Figure 4, the parent molecule and Trolox are also included for comparison purposes, and the H2O2/O2•−pair as the potential oxidant target. The latter has been chosen because it is usually harder to scavenge peroxyl radicals than other ROS. In the eH-DAMA, the species with low BDE are likely to be especially competent for scavenging free radicals acting as H donors via formal HAT, while species with low ω are expected to be efficient for scavenging free radicals acting as electron donors via SET. Hence, the species located at the bottom and left side of the eH-DAMA are assumed to act both ways. Thus, they can be considered a priori as good antioxidants. dR-113 is likely the most promising antioxidant from rasagiline derivatives followed by the neutral species of RII-22 and RI-49 which can be efficient as H donors but not perform particularly well for electron transfer reactions.
Table 2 . First ionization energy (IE, eV) and electron affinities (EA, eV), electrophilicity (ω and bond dissociation energies (BDE, kcal/mol) for rasagiline and the selected subset of derivatives.