The crystal structure of vacuum-dehydrated (activated) zeolite A• of apparent composition Nai2-AI12SÍ12O48 has been determined by single-crystal X-ray diffraction techniques. Least-square refinement in the space group Pm3m (a = 12.263 Á) has led to a conventional R index of 0.069. Sodium ions are distributed among three equipoints in the structure: eight of the twelve ions occupy threefold axis positions 0.20 Á from the plane of three nearest neighbors in the six-oxygen ring; three more lie in the planes of the eight-oxygen rings, displaced from the center by 1.23 Á along a diagonal mirror line; and the twelfth ion is found on a twofold axis opposite a four-oxygen window in the large cavity. The shortest approach distances to framework oxygen atoms are 2.32, 2.4, and 2.5 Á respectively. Small changes in cation positions and in the zeolite framework are observed upon dehydration. The changes due to dehydration in framework angles at the three nonequivalent oxygen atoms, corresponding to rotations of aluminosilicate tetrahedra, are much less than those observed for the Tl-exchanged form of zeolite A. An attempt to sorb C2CI4, although unsuccessful, yielded a structure in which the twelfth sodium ion is about 1.1 Á further from its nearest neighbors in the zeolite framework, presumably because an impurity or decomposition product which associates with that sodium ion was sorbed to a small extent.

The crystal structure of an acetylene sorption complex of zeolite A has been determined by single-crystal X-ray techniques. Fully vacuum-dehydrated zeolite A of approximate composition Na12A112Si1~048 per unit cell was exposed to dry acetylene for 24 hr at a pressure of 650 Torr. The cubic space group Pm3m was used with a = 12.260(5) A. Approximately six CzH2 molecules are absorbed per unit cell and occupy two or perha.ps three kinds of sites in the large cavity. Three C2H2 molecules participate in a symmetric approach to threefold axis sodium ions; each carbon atom in each of these three molecules is 2.8(1) A from a sodium ion. Two other carbon positions have been determined and refined, but the assignment of molecular positions to these is uncertain. One C2H2 appears to be symmetrically associated with the twofold axis sodium ion at distances of 2.6(1) A and simultaneously asymmetrically associated with a sodium ion in the eight-oxygen window with distances of 2.6(1) and 3.0(1) A. Crystallographically refined occupancy parameters indicate that the remaining two CzHz molecules are (nearly) equivalent to this one except that each associates with only one ion, one of the remaining two sodium ions in the eight -oxygen windows. However, a chemically more reasonable distribution of occupancy parameters for these two carbon positions would allow each of these latter two C2H2 molecules to be symmetrically associated (2.6(1) A) with an eight-window sodium ion. In each case, the sorption mechanism involves an ion to induced dipole interaction between Naf and the laterally polarizable A system of acetylene. IntroductionZeolites find extensive commercial application as catalysts for hydrocarbon cracking or for specific high yield rearrangement reartions involving hydrocarbons and other organic mo1ecules.l In order to determine the sorption sites (these may be catalytic sites as well in some cases) selected by a hydrocarbon in a zeolite, this structure was studied. Because unsaturated hydrocarbons are sorbed more tenaciously than saturated ones, and because the carbon atoms i~n acelylene were likely to be equivalent in the complex, C2W:z was selected as the sorbate material. Previous sorption nieasurements2 ,3 indicated that approximately six CzH:! molecules are sorbed per unit cell in zeolitcl 4A; the effect of less absorbent binder materials contained in the commercial pellets was taken into accountThe mechanism for sorption could involve a direct ion to induced dipole interaction4 between the cations and the polarizable T sysitems in these hydrocarbons as spectroscopic5-7 and nuclear magnetic resonance8 evidence indicates, and/or weak hydrogen bonding between the very weakly acidic protons, whose acidity might be enhanced by the former interattion, and the negatively charged zeolite framework oxygen atoms. Specific results9 for acetylene on variounly exchanged zeolite A samples support the former mechanism for zeolite 4A. Ethylene sorbed onto variously exchanged samples1° was found to be freely rotating except in the cas...

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Tris-(2-aminomethylpyridine)Ni(II) perchlorate crystallizes in space group P~t3n with a= 16.95 (1)A. The complexed Ni(II) ion must exhibit threefold symmetry and each pyridine ring must be cis to the other two.Ni(CsHsN-CH2-NH2)3(CIO4)2, tris(2-aminomethylpyridine)nickel(II) perchlorate, was prepared by mixing acetoneethanol solutions of Ni(C104)z and 2-aminomethylpyridinc. Slow evaporation yielded magenta prismatic crystals.From Weissenberg and precession X-ray photographs the crystals were found to be cubic with space group P~3n Fig. 1. View prepared from a scale model of the structure in which an approximate dihedral angle of 15 °, natural to the model, was used for the exocyclic C-C bond.* Author to whom correspondence should be addressed.(hkl, no conditions; hhl, 1= 2n; and cyclicly). These photographs do not suggest disorder. The cell constant, as determined by a least-squares refinement of fifteen intense reflections with 20 values up to 23 °, is a= 16.95 (1) A at 19°C. For this, a Syntex computercontrolled diffractometer with MoKc~ (2=0.71069 /~) radiation was used. Using a molecular weight of 585.06 amu and assuming eight formula-weights per unit cell, a density of 1.595 g.cm -3 can be calculated which agrees well with the observed value, 1.585 g.cm -a, measured by flotation in CCL (0 = 1.585 g.cm-a).Considering the symmetry of the ligand, and assuming that the complexed cation is not disordered, these conclusions follow: the Ni(II) ions are at Wyckoff positions 8(e) of PT~3n on the threefold axes; the complexed Ni(II) ions contain a molecular threefold axis; and each of the three pyridine rings must be cis to each of the other two. It is felt that the coordination configuration is nearer to trigonal antiprismatic (pseudo-octahedral) (see, for instance, Swink & Atoji, 1960) than to trigonal prismatic, which has been found only where the ligands impose particularly severe constraints (see Parks, Wagner & Holm, 1970) on the coordination geometry. Since the five-membered NiNCC'"N rings cannot deviate greatly from planarity due to ring strain, an apt~roximate model (see Fig. 1) of the complexed cation was, e .dily assembled. No further work is planned. Acta Cryst. (1972). B28, 2298Strucfu.'e cristalline du radical nitroxyde: sub6rate de di-t6tram6thy1-2,2,6,6-pip6ridinyl-4-oxyle 1. Par At~-m CAPIOMONT, Laboratoire de Spectromdtrie Physique, Universitd Scientifique et M~dicale de Grenoble, Cedex 53, 38 -Grenoble-Gate, France (Recu le 6 mars 1972) Di-(2,2,6,6-tetramethyl-4-piperidinyl-l-oxyl) suberate C26H46N206 is a nitroxide radical which crystallizes in the monoclinic system, space group P21/c. The dimensions of the unit cell are a= 6.041, b =21.52, c= 13.62 A, ,8= 126.360 °. There are two molecules in a cell, each at a centre of symmetry.Le sub6rate de di-t6tram6thyl-2,2,6,6-piperidinyl-4-oxyle 1 a 6t6 synth6tis6 au Laboratoire de Chimie Organique Physique du CENG (Professor Rassat) (Rey, 1967). II fait, d'autre part, l'objet d'6tudes de r.p.e., de chaleur sp6cifique et de susceptibilit6 magn6tique ~t...