Crystal Structures of Encapsulates within Zeolites. 2. Argon in Zeolite A

Abstract

The positions of Ar atoms encapsulated in the cavities of fully dehydrated Cs3Na8H−A (Cs3−A) have been determined. Cs3−A was exposed to 660 atm of argon gas at 400 °C, followed by cooling at pressure to encapsulate Ar atoms; a second crystal was treated similarly at 1000 atm. The resulting crystal structures, of Cs3−A(5Ar) (a = 12.253(2) Å, R1 = 0.055, R2 = 0.056) and Cs3−A(6Ar) (a = 12.253(2) Å, R1 = 0.064, R2 = 0.065), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3̄m at 21(1) °C and 1 atm. The five and six Ar atoms per unit cell, respectively, are distributed over three crystallographically distinct positions: one Ar atom at Ar(1) lies opposite a six-ring in the sodalite unit, two Ar atoms at Ar(3) occupy a similar position in the large cavity, and two or three Ar atoms in Cs3−A(5Ar) and Cs3−A(6Ar), respectively, are at Ar(2) opposite four-rings in the large cavity. Relatively strong interactions of Ar(1) and Ar(3) atoms with six-ring Na+ ions are observed: Na−Ar(1) = 3.26 and Na−Ar(3) = 3.00 Å. The four Ar atoms in the large cavity of Cs3−A(5Ar) form a rhombus with Ar(2)−Ar(3) = 4.75(8) Å and Ar(2)−Ar(3)−Ar(2) = 88(1)°. The five Ar atoms in the large cavity of Cs3−A(6Ar) have a trigonal bipyramidal arrangement with three Ar(2) atoms at equatorial positions and two Ar(3) atoms at axial positions; Ar(2)−Ar(3) = 4.63(9) Å. Both of the above arrangements are stabilized by alternating dipoles induced on Ar(2) and Ar(3) by four-ring oxygens and six-ring Na+ ions, respectively.