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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 22, Number 9
BKCSDE 22(9)
September 20, 2001

Title	Kr Atoms and Their Clustering in Zeolite A
Author	Woo Taik Lim, Chang Hwan Chang, Kee Jin Jung, Nam Ho Heo
Keywords	1,6-Methano[10]annulene, Photolysis, Methylene.
Abstract	The positions of Kr atoms encapsulated in the molecular-dimensioned cavities of fully dehydrated zeolite A of unit-cell composition Cs₃Na₈HSi₁₂Al₁₂O₄₈ (Cs₃-A) have been determined. Cs₃-A was exposed to 1025 atm of krypton gas at 400 ℃ for four days, followed by cooling at pressure to encapsulate Kr atoms. The resulting crystal structure of Cs₃-A(6Kr) (a = 12.247(2)Å, R₁ = 0.078, and R₂ = 0.085) has been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m at 21(1)℃ and 1 atm. In the crystal structure of Cs₃-A(6Kr), six Kr atoms per unit cell are distributed over three crystallographically distinct positions: each unit cell contains one Kr atom at Kr(1) on a threefold axis in the sodalite unit, three at Kr(2) opposite four-rings in the large cavity, and two at Kr(3) on threefold axes in the large cavity. Relatively strong interactions of Kr atoms at Kr(1) and Kr(3) with Na⁺ ions of six-rings are observed: Na-Kr(1) = 3.6(1) Å and Na-Kr(3) = 3.08(5)Å. In each sodalite unit, one Kr atom at Kr(1) was displaced 0.74Å from the center of the sodalite unit toward a Na⁺ ion, where it can be polarized by the electrostatic field of the zeolite, avoiding the center of the sodalite unit which by symmetry has no electrostatic field. In each large cavity, five Kr atoms were found, forming a trigonal-bipyramid arrangement with three Kr(2) atoms at equatorial positions and two Kr(3) atoms at axial positions. With various reasonable distances and angles, the existence of Kr₅ cluster was proposed (Kr(2)-Kr(3) = 4.78(6)Å and Kr(2)-Kr(2) = 5.94(7)Å, Kr(2)-Kr(3)-Kr(2) = 76.9(3), Kr(3)-Kr(2)-Kr(3) = 88(1), and Kr(2)-Kr(2)-Kr(2) = 60°). These arrangements of the encapsulated Kr atoms in the large cavity are stabilized by alternating dipoles induced on Kr(2) by four-ring oxygens and Kr(3) by six-ring Na⁺ ions, respectively.
Page	1023 - 1029
Full Text	PDF

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