Gas-Phase Lithium Cation Affinity of Glycine

International audienceThe gas-phase lithium cation binding thermochemistry of glycine has been determined theoretically by quantum chemical calculations at the G4 level and experimentally by the extended kinetic method using electrospray ionization quadrupole time-of-flight tandem mass spectrometry. The lithium cation affinity of glycine, ∆ Li H° 298 (GLY), i.e. the ∆ Li H° 298 of the reaction GlyLi + ® Gly + Li + , given by the G4 method is equal to 241.4 kJ mol-1 if only the most stable conformer of glycine is considered or to 242.3 kJ mol-1 if the 298 K equilibrium mixture of neutral conformers is included in the calculation. The ∆ Li H° 298 (GLY) deduced from the extended kinetic method is obviously dependent on the choice of the Li + affinity scale; thus, ∆ Li H° 298 (GLY) is equal to 228.7 ± 0.9(2.0) kJ mol-1 if anchored to the recently re-evaluated lithium cation affinity scale, but shifted to 235.4 ± 1.0 kJ mol-1 if G4 computed lithium cation affinities of the reference molecules are used. This difference of 6.3 kJ mol-1 may originate from a compression of the experimental lithium affinity scale in the high ∆ Li H° 298 region. The entropy change associated with the reaction GlyLi + ® Gly + Li + reveals a gain of approximately 15 J mol-1 K-1 with respect to monodentate Li + acceptors. The origin of this excess entropy is attributed to the bidentate interaction between the Li + cation and both the carbonyl oxygen and the nitrogen atoms of glycine. The computed G4 Gibbs free energy, ∆ Li G° 298 (GLY), is equal to 205.3 kJ mol-1 ; a similar result, 201.0 ± 3.4 kJ mol-1 , is obtained from the experiment if the ∆ Li G° 298 of the reference molecules is anchored on the G4 results