Abstract
Discrimination between the enantiomers of 1-phenylethylamine (PhEt) and α(1-naphthyl)ethylamine (NapEt) by the chiral ligand protonated dimethyldiketopyridino-18-crown-6 was studied using Fourier transform ion cyclotron resonance mass spectrometry to perform variable-temperature equilibrium (van't Hoff) experiments in the gas phase. The heterochiral complexes [(S,S)-ligand with (R)-amine, for example] have more-favorable enthalpy in both studied cases than the homochiral complexes; the differences are −6.7 ± 0.7 kJ mol-1 for the PhEt enantiomers and −10.0 ± 1.2 kJ mol-1 for the NapEt enantiomers. Entropy disfavors the heterochiral complexes by −14.8 ± 2.2 J mol-1 K-1 for PhEt and by −20.0 ± 3.9 J mol-1 K-1 for NapEt; entropy−enthalpy compensation is evident. These results suggest that enantiodiscrimination in these complexes is enthalpic and that locking of methyl rotors in the thermodynamically disfavored complexes is probably not important. Computational methods were also used to determine complex geome...
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