Structural study of monomethylammonium and dimethylammonium-exchanged vermiculites

Vermiculite crystals from Santa Olalla, Spain, were first Na exchanged and then intercalated with monomethylammonium (= NH3(CH3)(+), MMA) and dimethyl ammonium (= NH2(CH3)(2)(+), DMA) molecules, respectively, by immersion in 1 M ammonium-chloride solutions at 65 degrees C for 2-3 wk. MMA- and DMA-exc hange with vermiculite resulted in crystals with near perfect three-dimensi onal stacking, suitable for single crystal X-ray diffraction analysis. Unit cell parameters are: a = 5.353(2) Angstrom, b = 9.273(3) Angstrom, c = 11. 950(6) Angstrom, and beta = 98.45(4)degrees for MMA-exchanged vermiculite a nd a = 5.351(2) degrees, b = 9.268(4) Angstrom, c = 12.423(8) Angstrom, and beta = 98.33(5)degrees for DMA-exchanged vermiculite. Refinement results a re R = 0.059 and wR = 0.073 (MMA-exchanged vermiculite) and R = 0.059 and w R = 0.064 (DMA exchanged vermiculite). The results are based on structures which show substitutional disorder, and thus the presented models are deriv ed from average structures. There are two distinct sites for the MMA molecule in MMA-exchanged vermicul ite. One crystallographically unique MMA is oriented such that the N-C axis of the molecule is perpendicular to the basal oxygen plane, with the N ion offset from the center of the interlayer by 1.04 Angstrom. The other MMA i s located such that the N ion is at the center of the interlayer between ad jacent 2:1 layers, presumably with the N-C axis of the molecule oriented pa rallel to the basal oxygen plane. This represents the first known occurrenc e of an organic molecule located exactly between the two adjacent 2:1 layer s. Both sites are located between hexagonal cavities of adjacent layers. DM A molecules in DMA-exchanged vermiculite are located such that the N ion is offset from the central plane in the interlayer by 0.95 Angstrom. A static model is proposed with two orientations of DMA to produce a DMA "zigzag" o rientation of molecules parallel to the (001) plane. The plane defined by t he C-N-C atoms in the molecule is perpendicular to the (001) plane. An alte rnate model is more dynamic, and it involves the rotation of DMA molecules about one C-N axis. Identical starting material was used in previous studies on tetramethylammo nium (TMA)-exchanged vermiculite and tetramethylphosphonium (TMP)-exchanged vermiculite. The effect of onium-ion substitutions on the 2:1 layer shows that the tetrahedral rotation angle, a, is significantly smaller for MMA- a nd DMA-exchanged vermiculite vs. TMA and TMP-exchanged vermiculite. Tetrahe dral and octahedral bond distances of the 2:1 layer of the TMA, TMP, MMA, a nd DMA-exchanged structures may be explained by the location of the organic cation relative to the basal oxygen atom plane and by the differences in t he geometries of the organic molecule. Thus, the 2:1 layer is affected by t he interlayer molecule, and the 2:1 layer is not a rigid substrate, but int eracts significantly with the onium ions.