Tetraguanidium Hexabromocadmate, [C(NH2)3]4[CdBr6]. Crystal Structure and Bromine Nuclear Quadrupole Resonance

The ^{79, 81}Br NQR spectra of tetraguanidinium hexabromocadmate, [C(NH₂)₃]₄[CdBr₆] have been studied as a function of temperature from 77 K to 390 K and the crystal structure of the compound was determined at room temperature. The title compound crystallizes monoclinic, P2₁/c, with four formula units in the unit cell, a = 839.2(3) pm, b = 1895.8(6) pm, c= 1527.4(5) pm, β= 108.14(1)°. The anion [CdBr₆]^4⊖ is an isolated octahedron, with bond lengths 275≤d(Cd-Br)/pm ≤ 281, and bond angles 88 ≤(Br-Cd-Br)/° ≤95, slightly distorted by hydrogen bonds N -H ··· Br. The ⁸¹Br NQR sextet, in dependence from temperature, shows positive and negative temperature coefficients. At 77 K the ⁸¹Br NQR frequencies have been found between 42.42 MHz and 31.99 MHz; the ⁷⁹Br NQR at the frequencies expected from the nuclear quadrupole moment ratio Q(⁷⁹Br)/Q(⁸¹Br). Relations between the ⁸¹Br NQR spectrum, the crystal structure, and the hydrogen bonds are discussed.