Solid Phases from the Liquid System H₂N(CH₂)₂NH₂ CdBr₂ HBr H₂O. An X‐ray and ^79,81Br — NQR Study of (H₃N(CH₂)₂NH₃)CdBr₄, (H₃N(CH₂)₂NH₃) (CdBr₃)₂ · H₂O, and (H₃N(CH₂)₂NH₃)₂CdBr₆

Abstract

Abstract From the aqueous system ethylenediamine, (en),  CdBr 2  HBr  H 2 O of varying component concentrations, at room temperature the compounds (enH 2 )CdBr 4 , (enH 2 )(CdBr 3 ) 2 · H 2 O, and (enH 2 ) 2 CdBr 6 were crystallized. 79,81 Br NQR spectroscopy shows that the three solid phases, crystallized at room temperature, are the stable ones in the temperature range 77 ≤ T /K ≤ 300. (enH 2 )CdBr 4 shows a three line 81 Br NQR spectrum, the frequencies lying in the range 53 ≤ v ( 81 Br)/MHz < 68 and having negative temperature coefficients d v /d t over the whole temperature range covered. A phase transition with hysteresis is observed at 340 K. In the high temperature phase I only one 81 Br NQR line is observed, with negative temperature coefficient, too. (enH 2 )(CdBr 3 )2 · H 2 O shows also a three line 81 Br NQR spectrum, the frequencies lying in the range 38 MHz to 72 MHz at room temperature with positive as well as negative temperature coefficients. (enH 2 ) 2 CdBr 6 reveals a four line 81 Br NQR spectrum, three of the lines lying in the range 40 MHz to 70 MHz; the fourth line was observed at frequencies as low as 10 MHz. (enH 2 )(CdBr 3 ) 2 · H 2 O crystallizes at room temperature monoclinic, space group C 2h 6 ‐C2/ c with Z = 4 (formula units per unit cell) a = 818 pm, b = 2413 pm, c = 801 pm, β = 107.8°. The structure is characterized by chains of units [Cd 4 Br 12 ] 4 . The chains are built up by edge connected octahedra CdBr 6 and the corners of two neighbouring octahedra are part of a bridging tetrahedron CdBr 4 , located alternating cis and trans on the chain. (enH 2 ) 2 CdBr 6 crystallizes also monoclinic, space group C 2h 2 ‐P2 1 / m , a = 669 pm, b = 2050 pm, c = 637 pm, β = 93.4°; Z = 4. The structure corresponds to a formulation ((enH 2 )) 2 (CdBr 4 )2⊖ · 2Br⊖. In both compounds hydrogen bonding is observed. The NQR results are in full agreement with the crystal structures; structure and bond scheme are discussed.