The methylamine radical cation [CH₃NH₂]+˙ and its stable isomer the methylenammonium radical cation [CH₂NH₃]+˙

The potential energy surface for the [CH 5 N] +˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence‐electron correlation. Two [CH 5 N] +˙ isomers can be distinguished: the well known methylamine radical cation, [CH 3 NH 2 ] +˙ , and the less familiar methylenammonium radical cation, [CH 2 NH 3 ] +˙ . The latter is calculated to lie 8 kJ mol −1 lower in energy. A substantial barrier (176 kJ mol −1 ) is predicted for rearrangement of [CH 2 NH 3 ] +˙ to [CH 3 NH 2 ] +˙ . In addition, a large barrier (202 kJ mol −1 ) is found for loss of a hydrogen radical from [CH 2 NH 3 ] +˙ via direct N—H bond cleavage to give the aminomethyl cation [CH 2 NH 2 ] + . These results are consistent with the existence of the methylenammonium ion [CH 2 NH 3 ] +˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH 3 NH 2 ] +˙ is calculated to be 140 kJ mol −1 .