Protonation thermochemistry of aminoacetonitrile

The gas‐phase basicity (GB) of aminoacetonitrile (NH 2 CH 2 CN, 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 · mol −1 ). Molecular orbital calculations up to the G2 level demonstrate that protonation occurs preferentially on the nitrogen atom of the NH 2 group, and provide a theoretical proton affinity (PA(1)) of 824.0 kJ · 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°(1H + ) − S°(1) = 8.6 J · mol −1  · K −1 . Combining this value with experimental GB(1) leads to an ‘experimental’ proton affinity of 819.2 kJ · mol −1 , in close agreement with the G2 expectation. Copyright © 2006 John Wiley & Sons, Ltd.