White powdery beta-SrNCN was obtained by the solid-state metathesis between SrI(2) and ZnNCN at 843 K, by the reaction of SrI(2), CsCN, and CsN(3) in tantalum cylinders at the same temperature, and from the direct reaction between elemental Sr and H(2)NCN dissolved in liquid ammonia. The solid-state reactions carried out at a higher 973 K yield white alpha-SrNCN. Both experimental data (X-ray diffraction (XRD), infrared spectroscopy, differential scanning calorimetry (DSC)) as well as GGA density-functional phonon calculations show that the beta-phase is thermochemically more stable, by a minute 2 kJ/mol (electronic-structure theory) and about 6 kJ/mol (DSC), whereas the alpha-phase is slightly more dense. In addition, both XRD and DSC measurements reveal two distinct (endothermic) steps for the beta-to-alpha phase transition, that is, first around 920 +/- 20 K, then at 985 +/- 15 K based on the X-ray data. Thermochemically, the upper heat effect is larger by a factor of 20.

Calorimetric dissolution measurements of the solid compounds mercury carbodiimide HgNCN(I) and mercury cyanamide HgNCN(II) in aqueous HCl that targeted at their thermochemical stabilities show the cyanamide species HgNCN(II) to be the more stable phase in terms of both enthalpy and Gibbs energy with an enthalpy difference of 2-3 kJ mol-1. While the stability ranking of HgNCN(I) and HgNCN(II) thus perfectly matches Pearson's HSAB concept, quantum-chemical stability predictions using common parametrizations of density functional theory appear to be fundamentally flawed. An analysis of the error is attempted on the basis of correlated wave functions for related molecules.

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