The structure of an ammonia sorption complex of dehydrated fully Ca2+-exchanged zeolite X, Ca 46
Si 100 Al 92 O 384 β’ ~135NH 3 (a=24.904(4) A Λ), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 : at 21(1)Β°C. The complex was prepared by dehydration of a single crystal of Ca 46 -X at 380Β°C and 2Γ10-6 Torr for 2 days, followed by exposure to 300 Torr of ammonia gas at 21(1)Β°C. The structure was determined in this atmosphere and was refined to the final error indices R 1 =0.074 and R 2 =0.069 with 332 reflections for which I>3s(I ). Ca2+ ions are located at two crystallographic sites. Sixteen Ca2+ ions fill the octahedral sites I at the centers of the hexagonal prisms (Ca-O=2.408(8) A Λ). The remaining 30 Ca2+ ions are at sites II; each extends 0.87 A Λinto the supercage (an increase of 0.57 A Λupon NH 3 sorption) where it coordinates octahedrally to three framework oxygens at 2.382(7) A Λand to three ammonia molecules at 2.75(2) A Λ. Each of the remaining 45(3) NH 3 molecules hydrogen bonds via its lone pair to two coordinating NH 3 molecules and is in position to associate weakly with two or more framework oxygens. To avoid an impossibly short N,N contact, this position has a maximum occupancy of 48 per unit cell; it may be full.