Comparative Study on the Modulation of Amino Acid Electronic Properties by Polynuclear and Mononuclear Superhalogens: A High-Level Ab Initio Case Study

The modulation of the electronic properties of amino acid molecules by superhalogens has been investigated through the formation of cysteine–superhalogen composite anions between cysteine (Cys) and both the isolated mononuclear superhalogen ([MgF3]−) and the isolated binuclear superhalogen ([Mg2F5]−), using high-level ab initio methods. Vertical electron detachment energies (VDEs) were computed at the outer valence Green’s function (OVGF), HF and MP2 levels. At the OVGF level, the cysteine–binuclear-superhalogen composite anion ([CysMg2F4]−) yields VDEs exceeding those of the cysteine–mononuclear-superhalogen composite anion ([CysMgF2]−) by 0.45~0.88 eV, implying that polynuclear superhalogens are more efficient in modulating the electronic properties of cysteine. This enhanced modulation effect is primarily governed by the properties of the isolated superhalogens themselves: the OVGF VDEs of the isolated superhalogens are 8.56 eV for [MgF3]− and 9.84 eV for [Mg2F5]−, a difference of 1.28 eV that is of the same sign and comparable in magnitude to the 0.45~0.88 eV increase observed in the cysteine composites. The extra electron in these cysteine–superhalogen composite anions is predominantly localized on the Cys moiety, with partial delocalization into the superhalogen-derived fragment. Comparison between the highest occupied molecular orbital (HOMO) and the extra-electron density distributions shows that the two are qualitatively consistent for the systems studied. The OVGF VDEs are in good agreement with the DLPNO-CCSD(T)/6-311++G**benchmark values for representative structures, with a mean absolute deviation of 0.04 eV and a maximum deviation of 0.09 eV between the two methods. The HF/MP2 average shows trends consistent with the OVGF results, although deviations of up to about 0.38 eV are observed for individual structures. These results suggest that, when the HOMO and the extra-electron density are qualitatively consistent, OVGF may serve as a cost-effective alternative for estimating VDEs, as supported by the DLPNO-CCSD(T) benchmark values for representative structures. In addition, we characterized the structures, relative stabilities, and thermodynamic stabilities of all cysteine–superhalogen composite anions with respect to the two dissociation pathways considered (loss of the neutral superhalogen substituent and release of F−).