Conformers of gaseous protonated glycine

✍ Scribed by Zhang, Kui; Chung-Phillips, Alice

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 385 KB
• Volume: 19
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/(sici)1096-987x(199812)19:16<1862::aid-jcc7>3.0.co;2-j

✦ Synopsis

Ab initio geometry optimizations were performed on gaseous protonated glycine using the second-order Møller᎐Plesset perturbation theory with the 6-31G*, 6-31G**, 6-31qG**, and 6-311qG** basis sets. Eight energy minima and 12 saddle points in the low-energy region of the electronic potential energy surface were characterized. The global minimum was an amino Nq protonated conformer containing an ionic H bond between the NH and 3 O C-groups . The lowest energy O-protonated conformer was stabilized by a conjugative attraction between the nitrogen lone-pair electrons and the q Ž . positively charged planar fragment C OH . Relative electronic energies of 2 the nine N-and 11 O-protonated species fall in the ranges of 0᎐10 and 30᎐40 kcal mol y1 . At room temperature the equilibrium distribution contained the most stable N-protonated conformer almost exclusively. Additional subjects for investigation include the effects of basis set and electron correlation on the predicted structures, nonbonded interactions that influence the relative stability of protonated conformers, conformational interconversions based on intrinsic reaction coordinate calculations, and kinetic pathways for protonation and associated changes in Gibbs free energy. The work provides geometric, energetic, and thermodynamic data pertinent to the study of gas-phase ion chemistry of amino acids and peptides.