Structures and positive binding energies of glycine–2Li + in the gas phase: a theoretical study on optimal reaction pathway and proton transfer induced by two lithium cations

Abstract

Six different glycine–2Li + isomers have been obtained at the B3LYP/6-31G* level in the gas phase. Relative energy calculations performed at different levels indicate that the most stable complex holds the structure, in which two lithium cations are bound to the two oxygen sites of the glycine zwitterion. Calculated binding energies show that all of them are positive values, but frequency computations have proven all these complexes to be genuine minima on the potential energy surface (PES). These observations have implied that there should be an energy barrier when a Li + is bound to another active site of the glycine–Li + species. To choose an optimal reaction pathway and avoid the larger barriers as possible as we can in constructions of these complexes, two different combination ways are designed and compared with each other. Only three selected complexes are discussed in detail because the formations of the three complexes represent three different processes. Among them, both the first complex and its glycine–Li + reactant species hold the chain-like geometries, which represent a typical chain-like reaction process. The reactant (glycine–Li +) of the second complex has a structure of five-membered ring and it can be opened when another Li + approaches to the O4 site of the reactants and finally give rise to the product. This process involves a 50.6 kcal/mol energy barrier and a bond cleavage. The last one is the most stable complex. Either of its two generating ways involves an intramolecular proton transfer process induced by approaching of the second Li +. For the two generating ways of the last complex, calculations imply that their largest activation energy barriers of reaction are 43.7 and 45.2 kcal/mol, respectively. While the actual energy barriers of proton transfer at its two corresponding critical points are only within 8.2–11.0 kcal/mol range, similar to that (14.6 kcal/mol) in the solution obtained by experimental method for free glycine. Optimized PES of the proton transfer from O4 site to N3 site of the most stable glycine–2Li + shows that the energy of the complex with proton combined at O4 site is by 22.2 kcal/mol more than that at N3 site, which indicates that the inevitability of proton transfer in the glycine when two lithium cations are bound to its two oxygen ends.