Structures of three crystals containing approximately — linear chains of triiodide ions

Abstract

Abstract The structures of the ordered crystals of tetra(n-butyl)ammonium triiodide [I: a = 15.791(8), b = 15.993(8), c = 9.578(5)Å, α = 74.48(8)°, β = 101.52(9), γ = 96.63(9)°, P[unk], Z =4)] and (benzamide)2 · HI3; [II: a = 20.824(10), b = 9.874(5), c = 9.620(5) Å, α = 95.58(9), β = 102.10(10), γ = 94.72(9)°, P[unk], Z=4] have been fully refined, while that of caffeine · H2O · HI3 [III: a = 14.043(5), b = 12.202(5), c = 9.701(5)Å, β = 106.5(1), P21/a, Z = 4] has been determined in outline because of unresolved problems of disorder. Intensity measurements were made on a four-circle diffractometer using graphite-monochromated MoKα. The numbers of reflections used in the final refinement cycle were: 15370, II5295, III 2971.I has cations of D 2 d symmetry (i.e. in the form of flattened crosses) which interleave to form channels of rectangular cross-section with axes along [001]; these channels contain single, almost-linear chains of triiodide ions, with weak I[unk] – I[unk] interactions within the chains. II is a pseudo-Type A basic salt, with pairs of benzamide molecules, (joined by a short, presumably symmetrical, proton bond between carbonyl groups) forming the cations; these cations are arranged so as to leave channels (axes along [001]) of rectangular cross-section which contain double, almost-linear chains of triiodide ions. In III the caffeine molecules are presumably protonated at N(9) and are hydrogen bonded via water molecules to form corrugated sheets, the molecular plane being in the sheets. These sheets are so disposed as to leave channels of rectangular cross-section which contain double chains of polyiodide ions. One of the three iodines is disordered along the chain direction and it was not possible to identify the nature of the chains ( –I[unk]