Gas-phase basicity of glycine: A comprehensive ab initio study

Geometry-optimized structures for the most stable conformers of glycine and protonated glycine were obtained using the Hartree-Fock and second-order Moller-Plesset perturbation (MP2) methods with the 3-21G*, 6-31G*, 6-31G**, 6-31+G**, 6-311G**, 6-311+G**, and 6-311++G** basis sets. Analyses of results indicate that the MP2/6-31G*, MP2/6-31+G**, and MP2/6-311+G** levels of theory are more suited for protonation studies. Considerations were given to potential applications of single-point calculations using higher correlation methods such as MP4, QCISD(T), and CCSD(T) and larger basis sets including 6-311+G(3df,2p) and aug-cc-pVTZ. An ideal gas basicity of 203.5 kcal/mol at 298.15 K, which was calculated at the MP4/6-31+G(2d,2p) composite level for electronic properties and at the MP2/6-31G* level for thermodynamic properties with corrections of basis-set superposition error and conformational equilibrium effect, is shown to be sufficiently accurate by systematic deductions. This theoretical value is in good agreement with the lower of the two mass spectrometric values, 202.5 and 207.0 kcal/mol, assigned as the gas-phase basicity (GB) of glycine based on two different basicity scales. Comparisons with GB calculations on ammonia and methylamine reveal that certain protonation properties remain fairly constant among molecules undergoing amino N-protonations. Several findings from this study help formulate practical strategies for calculating the GBs of larger molecules, including the use of density functional theory.