Structures and reactivity of gaseous glycine and its derivatives

B3LYP/6-31++G** has been used to model the conformers of glycine, protonate d glycine, the unimolecular fragmentation, the proton transfer and the bimo lecular proton exchange reactions with NH3. B3LYP/6-31++G** has located all the conformers-except one-located previously with electron correlation met hods. The results show that the performance of B3LYP/6-31++G** is significa ntly 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 r eactions of glycine with NH3. The proton has been found to be mobile over a ll basic sites when a threshold of similar to 33 kcal/mol is reached, A bar rier of similar to 50 kcal/mol exists for the fragmentation reaction, H2O a nd CO being sequentially eliminated. The amino protons are exchanged with a n onium mechanism and the carboxylic proton is exchanged via a salt bridge complex. (C) 2000 Elsevier Science B.V.