Location of pyridine guest molecules in an electroneutral {(3)infinity}[SiO4/2] host framework: Single-crystal structures of the As-synthesized and calcined forms of high-silica ferrierite

Single crystals (up to 580 x 480 x 20 mu m) of highly silicious ferrierite (Si-FER, 1), suitable for single-crystal X-ray investigations, are Synthesized under organothermal conditions. The structures of the as-synthesized (1a) and the calcined (1b) Si-FER are determined at room temperature. Both structures are refined in the orthorhombic space group Pnnm (No. 58, standard setting) with a = 743.0(1) pm, b = 1409.2(2) pm, c = 1882.0(2) pm, V = 1970.5(4) x 10(6) pm(3), Z = 1, R = 0.041 (1a) and a = 741.8(1) pm, b = 1407.0(2) pm, c 1871.3(2) pm, V = 1953.1(4) x 10(6) pm(3), Z = 1, R = 0.037 (1b). The structure solution when combined with chemical analysis and H-1 and C-13 MAS NMR gives a unit cell content of [Si36O72]{py((4-x)) ap(x)} (x = 0-1, py = pyridine, ap = 1-amino-n-propane) and [Si36O72] for 1a and 1b, respectively. The structure of 1a shows only weak host-guest interactions between the {(3)(infinity})[SiO4/2] framework and the occluded, orientationally disordered pyridine molecules by means of relatively long organic-to-framework distances, d(C-py...O) greater than or equal to 354(2) pm. Si-29 MAS NMR spectra from the organic-containing Si-FER la and the organic-free form Ib are in good agreement with the crystallographic results in that they conform to the well-known linear relationship between the cosine expression of the T-O-T angles and the chemical shift of the respective tetrahedral sites (T-sites). A new modification of this relationship is presented here and offers an improved linear correlation between the X-ray and NMR data for 1a and 1b, as well as for other high-silica microporous materials. Application of this new correlation to denser SiO2 compounds is discussed.