Multinuclear NMR study of zinc dicyanide

The isotropic negative expansion of Zn(CN)₂ has been linked to a temperature induced increase in off-axis tilting of the C-N bond direction and an increase in CN-bond length. However, the bond length could be determined only indirectly based on pair-distribution function analysis and was found to be surprisingly large. Here we study Zn(CN)₂ by nuclear magnetic resonance spectroscopy and first principles calculations. By using samples enriched in ¹³C and ¹⁵N the dipole coupling between carbon and nitrogen is determined, and from this an upper bound on the C-N bond length of 1.19±0.01 Å is derived. This quantity agrees with earlier determinations based on diffraction but is shorter than estimates based on pair distribution function analysis. The relation of this estimate to possible dynamics in the sample is discussed. Finally, ⁶⁷Zn NMR is used together with first principles calculations to assess disorder in the material.