The removal of SO2 in the presence of alkene-ozone systems has been studied for ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene-ozone systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (±1.1) × 10(-5) for CH2OO, 26 (±10) × 10(-5) for CH3CHOO derived from cis-but-2-ene, 33 (±10) × 10(-5) for CH3CHOO derived from trans-but-2-ene, and 8.7 (±2.5) × 10(-5) for (CH3)2COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO2) are -2.3 (±3.5) × 10(11) cm(-3) for CH2OO, 13 (±43) × 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, -14 (±31) × 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (±14) × 10(11) cm(-3) for (CH3)2COO. Uncertainties are ±2σ and represent combined systematic and precision components. These values are derived following the approximation that a single SCI is present for each system; a more comprehensive interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented. This yields values of 3.5 (±3.1) × 10(-4) for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (±1.1) × 10(13) for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is also considered, with a derived value for k(CH2OO + (H2O)2)/k(CH2OO + SO2) of 1.4 (±1.8) × 10(-2). The observed SO2 removal rate constants, which technically represent upper limits, are consistent with decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI.

Hexagonal, nonperovskite HoMnO3 oxide, containing a triangular arrangement of Mn3+ cations, has been prepared in polycrystalline form by the thermal decomposition of metal citrates. The crystal structure has been refined from neutron powder diffraction data. Magnetic and specific-heat measurements anticipate a complex phase diagram:  HoMnO3 becomes magnetically ordered at T N ≈ 72 K, and another two magnetic transitions take place at lower temperatures. Neutron powder diffraction measurements demonstrate that, below the ordering temperature, the moments of the Mn3+ cations adopt a triangular spin arrangement, the magnetic moments lying in the basal plane and parallel to the [100] axis. At T = 44.6 K, the moments suddenly reorientate within the basal plane and become aligned perpendicularly to the initial direction. Below T = 25.4 K, an ordered magnetic moment is observed on the Ho atoms at the 4b sites of the crystal structure, whereas those of the 4a site remain in a paramagnetic state. The Ho atoms adopt an antiferromagnetic structure with the moments parallel to the c axis. At 1.7 K, the ordered moment on the Mn3+ cations is 3.05(2) μB, and that on the Ho3+ cations is 2.97(3) MB.