We were not able to locate the hydrogen atoms on the MeNH + 3 cation and our assignment of the polarity of MeNH + 3 cation, i.e. which atom was carbon and which nitrogen, was thus tentative. We therefore carried out a series of SCND studies on large (≈ 1 mm 3 ) single crystals using instrument D19 at the ILL. These measurements did not allow us to definitively answer these questions because we were unable to obtain an untwinned crystal of sufficient size, but did confirm both the space group symmetry and broad structural features observed via SCXD.