Structures and reactivity of gaseous glycine and its derivatives

B3LYP/6-31++G** has been used to model the conformers of glycine, protonated glycine, the unimolecular fragmentation, the proton transfer and the bimolecular proton exchange reactions with NH₃. B3LYP/6-31++G** has located all the conformers - except one - located previously with electron correlation methods. The results show that the performance of B3LYP/6-31++G** is significantly better than that of the HF method and in most cases as good as the ab initio theories such as MP2, CCSD, and CISD. We have thus used B3LYP/6-31++G** in order to understand the unimolecular fragmentation and bimolecular reactions of glycine with NH₃. The proton has been found to be mobile over all basic sites when a threshold of ~33 kcal/mol is reached. A barrier of ~50 kcal/mol exists for the fragmentation reaction, H₂O and CO being sequentially eliminated. The amino protons are exchanged with an onium mechanism and the carboxylic proton is exchanged via a salt bridge complex. (C) 2000 Elsevier Science B.V.