81Br NQR and crystal structure of 4-bromopyridinium pentabromoantimonate(III); 3c-4e bonding and NQR trans influence

The crystal structure of (4-BrC5H4NH)2SbBr5 was determined by a single crystal X-ray diffraction at 193 K. The crystals belong to the triclinic system (P1¯) with a = 9.1861(19) Å, b = 10.622(2) Å, c = 10.703(2) Å, α = 99.665(3)°, β = 97.393(3)°, γ = 108.539(3)°, and Z = 2. There exists an anion dimer Sb2Br104− in the crystal structure which consists of an edge-sharing between two SbBr6 octahedra. The inversion center situated at the center of the shared-edge of the dimer relates two asymmetric units, which consist of a SbBr52− moiety and two nonequivalent cations. Each cation is bound to the apical Br atoms of Sb2Br104− through N-H…Br hydrogen bonds. Seven 81Br NQR resonance lines including those for two Br atoms in the nonequivalent cations have been observed in the temperature range of 77 to ca. 360 K in accordance with the crystal structure. The frequencies of each of the respective pairs of NQR lines for the apical Br and for the equatorial Br atoms showed unusual positive temperature dependence with almost linear changes. For these curves, the other lines of pairs showed symmetrical temperature dependence, maintaining the center of mass almost constant in frequency. These observations may indicate the existence of NQR trans influence which were manifested through the occurrence of substantial electronic redistributions in the 3c-4e (Br-Sb-Br) bonds with temperature changes.