| Abstract
| - Geometry-optimized structures for the most stable conformers ofglycine and protonated glycine were obtainedusing the Hartree−Fock and second-order Møller−Plessetperturbation (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 resultsindicate that the MP2/6-31G*, MP2/6-31+G**, and MP2/6-311+G**levels of theory are more suited forprotonation studies. Considerations were given to potentialapplications of single-point calculations usinghigher 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 203.5kcal/mol at 298.15 K, which was calculated at theMP4/6-31+G(2d,2p) composite level for electronic propertiesand at the MP2/6-31G* level for thermodynamicproperties with corrections of basis-set superposition error andconformational equilibrium effect, is shownto be sufficiently accurate by systematic deductions. Thistheoretical value is in good agreement with thelower 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 andmethylamine reveal that certain protonation properties remain fairlyconstant among molecules undergoingamino N-protonations. Several findings from this study helpformulate practical strategies for calculatingthe GBs of larger molecules, including the use of density functionaltheory.
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