Glycine Canonical and Zwitterionic Isomers within Zeolites

Abstract

The glycine zwitterion (GlyA) in the gas phase is not a local energy minimum and transforms to the canonical isomer (GlyB) via a barrierless process. Within ZSM-5 zeolite, it is rendered geometrically stable and even has a lower energy of 7.57 kcal mol(-1) than GlyC, the most stable isomer of glycine in the gas phase; GlyB represents the lowest energy minimum, which is facile to transform into the zwitterion with low-energy barrier (4.46 kcal mol(-1)). In addition, the zwitterion can be efficiently obtained by adsorption of glycine in the deprotonated form at the acidic site of HZSM-5 zeolite. The relative stability of glycine isomers in silicalite-1 increases in the order GlyA<GlyB<GlyC, the same as that in the gas phase. Silicalite-1 stabilizes GlyA somewhat, whereas it destabilizes GlyB greatly. The negative charges of ZSM-5 zeolite created by Al doping are indispensable to the stabilizations of the zwitterion; however, the lattices also play an important role and approximate 74.1% of the contributions of the negative charges.