The sequential bond energies for complexes of Mg + with CO, CO 2 , NH 3 , CH 4 , CH 3 OH, and C 6 H 6 are determined by collision-induced dissociation (CID) with xenon or argon in a guided ion beam tandem mass spectrometer. The kinetic energy dependence of the CID and ligand exchange cross sections are analyzed to yield 0 and 298 K bond energies for Mg + -L after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Bond energies (in eV) to Mg + at 0 K are determined for L ) Ar (0.10 ( 0.07), Xe (0.32 ( 0.12), 1-2 CO (0.43 ( 0.06 and 0.40 ( 0.03), 1-3 CO 2 (0.60 ( 0.06, 0.50 ( 0.03, and 0.46 ( 0.06), 1-5 NH 3 (1.60 ( 0.12, 1.27 ( 0.07, 0.99 ( 0.09, 0.45 ( 0.11, and 0.58 ( 0.12), 1-2 CH 4 (0.29 ( 0.07 and 0.15 ( 0.07), 1-3 CH 3 OH (1.51 ( 0.07, 1.25 ( 0.07, and 0.95 ( 0.09), and one C 6 H 6 (1.39 ( 0.10 eV). As expected for largely electrostatic interactions, the sequential bond energies generally decrease monotonically with increasing number of ligands. These values are in good agreement with theoretical values in the literature and ab initio calculations performed here, but the agreement is mixed for comparison with results of photodissociation measurements. Qualitatively, geometries of these complexes are controlled by interactions of the ligands with the single polarized valence electron on Mg + .

In part I, we discussed the chain-propagating and possible competing mechanisms of low-temperature (300−1000 K) dimethyl ether (DME) combustion. Here we consider the chain-branching mechanism that results in explosive combustion, initiated by O2 addition to the ·CH2OCH2OOH intermediate formed in the earlier chain-propagation step. Ideally, chain-branching leads to the formation of two highly reactive ·OH radicals from the ·OOCH2OCH2OOH precursor. Each of these two ·OH radicals can initiate a chain-reaction “branch” with another DME molecule, which, ideally, leads to the formation of four more ·OH, and so on. This exponential increase in ·OH concentration causes an exponential increase in the DME oxidation rate, leading to explosive combustion. Here we show that although the pathway to create the first ·OH from ·OOCH2OCH2OOH in a hydrogen-transfer isomerization step is unambiguous, the formation of the second ·OH from the remaining hydroperoxyformate (HPMF or HOOCH2OC(O)H) fragment is potentially very complicated. HPMF has many possible fates, including HĊO + formic acid (HC(O)OH) + ·OH; H2O + formic acid anhydride (HC(O)OC(O)H); the Criegee intermediate (·CH2OO·) + formic acid; peroxyformic acid (HC(O)OOH) + H2 + CO; dihydroxymethylformate ((HO)2HCOC(O)H); ·OCH2OC(O)H + ·OH; and quite possibly others. The first and last of these products derived from HPMF directly produce ·OH and thus can complete the chain-branching step. Activation energies of 42−44 kcal/mol are needed to overcome barriers to form these two sets of products from HPMF. While these pathways directly form ·OH, they may not be the most favorable. The formation of a Criegee intermediate (·CH2OO·)−formic acid hydrogen-bonded adduct requires ∼15 kcal/mol less enthalpy than paths directly producing ·OH. Formation of the Criegee intermediate has never been considered as an intermediate in DME combustion before, but its formation (along with formic acid) appears to be the most favorable unimolecular path for HPMF decomposition. In atmospheric chemistry, decomposition of vibrationally excited ·CH2OO· can potentially lead to ·OH formation. Thus, we propose ·CH2OO· as a new intermediate that may significantly contribute to dimethyl ether's chain-branching mechanism.

The crystallography of transition Al2O3 has been extensively studied in the past, because of the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al2O3 and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at least two different end-members. By correlating imaging and spectroscopy results with density functional theory (DFT) calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely related to δ-Al2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.

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