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

AbstractFrom the aqueous system ethylenediamine, (en),  CdBr2  HBr  H2O of varying component concentrations, at room temperature the compounds (enH2)CdBr4, (enH2)(CdBr3)2 · H2O, and (enH2)2CdBr6 were crystallized. 79,81Br NQR spectroscopy shows that the three solid phases, crystallized at room temperature, are the stable ones in the temperature range 77 ≤ T/K ≤ 300. (enH2)CdBr4 shows a three line 81Br NQR spectrum, the frequencies lying in the range 53 ≤ v (81Br)/MHz < 68 and having negative temperature coefficients dv/dt over the whole temperature range covered. A phase transition with hysteresis is observed at 340 K. In the high temperature phase I only one 81Br NQR line is observed, with negative temperature coefficient, too.(enH2)(CdBr3)2 · H2O shows also a three line 81Br NQR spectrum, the frequencies lying in the range 38 MHz to 72 MHz at room temperature with positive as well as negative temperature coefficients.(enH2)2CdBr6 reveals a four line 81Br 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. (enH2)(CdBr3)2 · H2O crystallizes at room temperature monoclinic, space group C2h6‐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 [Cd4Br12]4. The chains are built up by edge connected octahedra CdBr6 and the corners of two neighbouring octahedra are part of a bridging tetrahedron CdBr4, located alternating cis and trans on the chain. (enH2)2CdBr6 crystallizes also monoclinic, space group C2h2‐P21/m, a = 669 pm, b = 2050 pm, c = 637 pm, β = 93.4°; Z = 4. The structure corresponds to a formulation ((enH2))2(CdBr4)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.