Glycine Canonical and Zwitterionic Isomers within Zeolites

The glycine zwitterion (Gly(A)) in the gas phase is not a local energy minimum and transforms to the canonical isomer (Gly(B)) 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 Gly(C), the most stable isomer of glycine in the gas phase; Gly(B) 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 Gly(A) < Gly(B) < Gly(C), the same as that in the gas phase. Silicalite-1 stabilizes Gly(A) somewhat, whereas it destabilizes Gly(B) 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.