Polymorphism of potassium ferrocyanide trihydrate as studied by solid-state multinuclear NMR spectroscopy and X-ray...

The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K(4)Fe(CN)(6).3H(2)O, KFCT) has been studied using (1)H, (13)C, and (15)N NMR spectroscopy in combination with X-ray diffraction. At room temperature, KFCT typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below -25 degrees C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal I4(1)/a form is also known to exist at room-temperature. This tetragonal form also converts to the monoclinic Cc form upon cooling, although this phase transition is irreversible and occurs at -60 degrees C. Initial room-temperature (15)N MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of KFCT prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The (13)C MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the (15)N NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the (13)C NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of KFCT. Using the known temperature dependence of these two polymorphs, the (13)C peaks corresponding to each of the C2/c and I4(1)/a forms were assigned. The (13)C NMR spectra and powder X-ray diffraction results demonstrate that upon grinding, a near 50-50 mixture of the two forms is always produced, rather than a new form entirely. The insensitivity of the (15)N NMR spectra to the polymorphism of KFCT is surprising, and likely arises from a fortuitous overlap of the (15)N NMR peaks of the two forms.