Theoretical investigation of glycine–2Ben+ (n=0,1,2) complexes in gas phase: Origin of negative dissociation energies

Abstract

The negative dissociation energies (or positive binding energies) have first been found in seven different glycine–2Be2+ conformers, one glycine–Be2+Be+ complex, and one glycine–2Be2+ complex. For the seven glycine–2Be2+ conformers, the negative dissociation energies originate from the contributions of electrostatic, dipolar, charge transfer and deformed terms. All of these terms play important roles for the negative dissociation energy during separating one Be2+ from the corresponding complex. Also two dissociation energy barriers have been observed in the course of separating each of the two Be2+ ions from the most stable glycine–2Be2+ conformer. One barrier derives from the deformed effect and another is mainly from electronic effect. For the glycine–Be2+Be+, the positive binding energy (or negative dissociation energy) is also observed when Be+ ion interacts with the oxygen end of zwitterionic glycine. Binding energy contribution analysis (BECA) shows that it mainly stems from the electrostatic effect. For the glycine–2Be+ with two possible multiplicities, only the triplet state tautomer has positive binding energy. BECA indicates that its binding energy results from the contribution of the deformation energy, and from the spin repulsion of two single electrons over three different atoms of the glycine–2Be+ complex. In the course of studying these interesting binding energies, geometries of the seven glycine–2Be2+ conformers, five different glycine–2Ben+ complexes, and seven different glycine–2Ben+ (n=0, 1, or 2) complexes are optimized and characterized at HF(B3LYP)/6-31G* level. The results indicate that the most stable glycine–2Be2+ isomer in the seven glycine–2Be2+ complexes has a Cs symmetry, in which two Be2+ ions are bound to two oxygen ends of the zwitterionic glycine, respectively. For the five glycine–2Ben+ complexes with different valence states, which are yielded according to the coordination mode of the most stable glycine–2Be2+ complex, the geometries of three lower valence-state complexes suffer serious deformation due to the repulsion of lone pairs or parallel electron spin. The distance of two Be ions becomes longer and longer with the increase of their valence states in these complexes. For the separated species of these glycine–2Ben+ complexes, their characteristic geometries are presented and the binding energy of one glycine–Be2+ complex is calculated and compared with other theoretical values.