Gas-phase Basicity of Glycine

The protonation thermochemistry of gaseous glycine is re-examined. The composite G3, G3MP2, G3B3, CBS-Q and CBS-QB3 methods have been used to estimate the enthalpy component i.e. the proton affinity of glycine, PA(Gly). The so called "protonation entropy", ΔpS(Gly) = S°(GlyH+) − S°(Gly), has been evaluated by calculating contributions to entropy of the internal rotations using the Pitzer hindered rotor model. The resulting theoretical gas-phase basicity, GB(Gly) = PA(Gly) – T[S°(H+) − ΔpS(Gly)] has been then calculated. These computations were done, either by considering only the most stable neutral and protonated conformers of glycine ("most stable conformer" values, denoted "msc") or a population of conformers based on a Boltzmann distribution at 298 K ("molar" values). An isodesmic procedure has been used by anchoring the computed data to the experimental proton affinities of isopropylamine. The results are the following: PAmsciso(Gly) = 889.2 kJ mol−1, PAmolar(Gly) = 890.9 kJ mol−1, ΔpSmsciso(Gly) = 2.3 J mol−1 K−1, ΔpSmolar(Gly) = 1.2 [−6.2] J mol−1 K−1, GBmsciso(Gly) = 857.5 kJ mol−1, GBmolar(Gly) = 858.9 [856.6] kJ mol−1 [values in brackets correspond to data obtained by including the entropy of mixing]. Evaluated values of PA(Gly)= 889 kJ mol−1 and GB(Gly)= 856 kJ mol−1, at 298 K, may be proposed after comparison with the available experimental data