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Updated: Aug 10, 2026

Determination of the Gas-phase Acidities of Oligopeptides
Published on: June 24, 2013
Protonation thermochemistry of aminoacetonitrile
Guy Bouchoux1, Jean-Claude Guillemin, Nadège Lemahieu
1Laboratoire des Mécanismes Réactionnels, UMR CNRS 7651, Département de Chimie, Ecole Polytechnique, 91128 Palaiseau Cedex, France. bouchoux@dcmr.polytechnique.fr
Abstract:
The gas-phase basicity (GB) of aminoacetonitrile (NH2CH2CN, 1) has been determined from measurement of proton transfer equilibrium constants in an ion cyclotron resonance mass spectrometer (GB(1) = 789.3 +/- 1.0 kJ x mol(-1)). Molecular orbital calculations up to the G2 level demonstrate that protonation occurs preferentially on the nitrogen atom of the NH2 group, and provide a theoretical proton affinity (PA(1)) of 824.0 kJ x mol(-1). Exact calculation of the entropy associated with hindered rotations and consideration of Boltzman distribution of conformers allow a theoretical estimate of the molar protonation entropy S degrees (1H+) - S degrees (1) = 8.6 J x mol(-1) x K(-1). Combining this value with experimental GB(1) leads to an 'experimental' proton affinity of 819.2 kJ x mol(-1), in close agreement with the G2 expectation.
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