Structural Studies of Antimony Tribromide and Its Molecular Complex with Benzene by Means of the 81Br Nuclear Quadrupole Resonance

The structures of α-antimony tribromide and its molecular complex with benzene 2SbBr3·C6H6 were investigated by studying the Zeeman effect on the nuclear quadrupole resonance (NQR) due to 81Br at room temperature. The bond angles, ∠Br-Sb-Br, of α-antimony tribromide in its solid state were found to be 94°14±4′, 94°31′±8′, and 98°27′±7′. The polar angles specifing the directions of the Sb-Br bonds with respect to the orthorhombic crystal axes are (60°0′±9′, 62°13′±7′), (59°10′±7′, −54°35′±8′), and (57°39′±17′, 180°0′±3′). These values are consistent with those of the X-ray analysis by Gushen et al. within about 1°. The asymmetry parameters at the three bromine atoms were 0.068±0.002, 0.049±0.002, and 0.080±0.004. On the other hand, the bond angles, ∠Br-Sb-Br, of the SbBr3 molecule in the complex 2SbBr3·C6H6 were found to be 92°24′±11′, 97°7′±14′, and 97°40′±6′. The crystal has a monoclinic symmetry, and the three Sb-Br bonds have directions making angles of 29°4′±21′, 76°26′±9′ and 63°33′±11′ relative to the b-axis. The asymmetry parameters at the three bromine atoms in the complex were 0.169±0.001, 0.063±0.002, and 0.094±0.002. The largest value of the asymmetry parameters seems to be the result mostly of the intermolecular bonding. From the coupling constants, the Sb-Br bond characteristics in antimony tribromide and its molecular complex, 2SbBr3·C6H6, are considered to be almost the same.