A low-temperature study of the magnetic properties of the molecular cations O2+, Br2+, and I2+

Magnetic susceptibility measurements to 4.2 K are reported forO2+[AsF6]−, Br2+[Sb3F16]−, and I2+[Sb2F11]−. The data are interpreted utilizing previous results from photoelectron spectroscopy, known crystal structures, magnetic studies on the superoxide ion and the ozonide ion, and in the case of O2+[AsF6]−, previous ESR studies. The magnetic properties of the three materials are quite different. Br2+[Sb3F16]− obeys Curie–Weiss law between 80 and 4 K: Cm = 0.49 ± 0.01 cm3 mol−1 K and 9 = −0.74 ± 0.01 K (with TIP = 120 × 10−6 cm3 mol−1). The magnetic moment decreases slightly from 2.04 μB at room temperature to 1.93 μB at 4 K. I2+[Sb2F11]− exhibits relatively strong antiferromagnetic coupling with a maximum in χM observed at −54 K. The magnetic moment (corrected for a TIP contribution of 68 × 10−6 cm3 mol−1) decreases from 1.92 μB at 124 K to 0.41 μB at 4 K. Experimental susceptibilities for this compound over the range 300–4 K have been compared to values calculated using three different theoretical models for extended chains of antiferromagnetically coupled paramagnetic species. O2+[AsF6]− exhibits Curie–Weiss behaviour over the range 60–2 K (Cm = 0.34 ± 0.01 cm3 mol−1 K, θ = −1.90 ± 0.01 K). The magnetic moment, uncorrected for TIP, varies from 1.63 μB at 80 K to 1.17 μB at 2 K, and the presence of weak antiferromagnetic coupling in this material is suggested. Keywords: magnetic susceptibilities, dihalogen cations, dioxygenyl cation, low temperature behaviour.