Marked Increments of Stability and Proton Affinity of the Protonated, Zwitterionic Glycine Induced by the Attachment of Two Excess Electrons

Abstract

The most stable protonated glycine (1GlyH1) attached by different charges (0, ±1, ±2, −3, and −4) in the gas phase has been comprehensively studied by using density functional theory (DFT/B3LYP) and the CCSD method. Results show that, on the basis of protonation, the zwitterionic glycine (GlyZW) can be further stabilized with a 90.4 kcal/mol energy drop by attaching an excess electron in the dipole-bound mode (2GlyH0). The corresponding vertical electron affinity is −86.9 kcal/mol. Interestingly, two-electron-attached 1GlyH1 [1GlyH(−1)] is more stable by 5.4 kcal/mol than the one-electron-attached one (2GlyH0). The analyses for the probable dissociation modes of the series mGlyHn (n, charge; m, spin multiplicity) species also confirm this conclusion. The additional stability mainly stems from the contribution of the deformation energy induced by the two excess electrons. Results show that the deformation contribution induced by attaching one, two, or three electrons can favor the stability of each corresponding system, while either attaching four electrons or ionizing one electron would make the corresponding system unstable. On the other hand, the greater the number of attached electrons is, the more the GlyZW species combines a proton readily, i.e., with larger proton affinity (PA). For example, the CCSD(T)-calibrated PA of 2GlyZW(−1) is 316.9 kcal/mol, larger by 104.3 kcal/mol than that of 1Gly0. The PA of 1Gly0 calibrated at the same level in the present paper is 212.6 kcal/mol, in excellent agreement with the theoretical (211.1 kcal/mol) and experimental (211.8 kcal/mol) results.