Many polycyclic aromatic hydrocarbons are able to trap singlet oxygen 1O2. Some of the endoperoxides, thus obtained, exhibit the exceptional feature of releasing oxygen, frequently in the excited singlet state, under heating or UV irradiation. In this Account, we provide a short summary of the present knowledge on these endoperoxides:  preparation and thermal and photolytic decomposition, with a special emphasis on the structural requirements to favor cycloreversion. The profitable use of this property in the development of highly reversible photochromic systems and of specific sources or traps of 1O2 in aqueous media is also described.

Reversible Binding of Oxygen to Aromatic Compounds

The presence of thin layers of free metal atoms at around 90 km in the upper atmosphere has been known for about fifty years. Layers of the alkali metals Na, K and Li, as well as Ca and Fe, have been observed. This discovery has posed two important questions. First, what is the source of the metals: interplanetary or terrestrial? Secondly, what is the nature of the chemistry that causes reactive metals such as sodium to exist in their atomic form in the atmosphere? The first part of this review covers the techniques that have been developed to observe the metal layers, including ground-, rocket- and space-based photometers, and in particular metal lidars. The many curious phenomena that have been observed are then described, such as the small scale-heights of the layers, the quite different seasonal variations of the three alkali metals, the large depletions of Ca and Fe relative to Na, and the dramatic appearance of sporadic Na layers. The second part of the review describes the recent advances ...

The chemistry of meteoric metals in the Earth's upper atmosphere

A one-dimensional steady state model of meteoric iron in the Earth's upper atmosphere has been developed. The model covers the altitude range 70 to 110 km at latitude 40°N and allows for seasonal variations in the chemical composition, total density and temperature of the upper atmosphere. It addresses the three processes thought to be most influential in affecting the layer of free Fe atoms at approximately 85 km altitude, that is, the deposition of iron via meteor ablation, the vertical distribution of the ablated material, and the partitioning of the metal among 10 constituent species as a result of neutral and ion gas phase reactions in dynamical steady state. A comparison between calculated and observed average monthly Fe profiles reveals that many features displayed by the meteoric iron layer (such as the peak height and width, the small scale heights at the topside and underside, and the seasonal changes in these parameters) can be successfully reproduced by the model.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JD03075

A model of meteoric iron in the upper atmosphere

We report on a comprehensive set of observations of the upper atmospheric Ca and Ca + layers. The observations were obtained by ground-based lidars at Kuhlungsborn, Germany (54°N, 12°E), between December 1996 and December 1998. During this period, 112 nights of Ca soundings and 58 nights of Ca + soundings were realized. The Ca layer has an average column abundance of 2.1.10 7 cm -2 , centered around 90.3 km with a mean peak density of 22 cm -3 at 89.9 km altitude. The Ca + dominates the total Ca amount above 90 km and has an average column abundance of 4.9.10 7 cm -2 . Because the vaporization of cosmic dust is the most probable source of atmospheric metals, the column densities of the metals within the atmosphere are often compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such as Na and Fe, compared with their relative abundances in CI chondrites. We present a one-dimensional steady state chemistry model of the nighttime Ca and Ca + layers, based on new laboratory studies of CaO reaction kinetics. This model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusual seasonal variation of the Ca layer which exhibits a pronounced summertime enhancement around 87 km.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JA900088

Atmospheric Ca and Ca+ layers: Midlatitude observations and modeling

The homogeneous and heterogeneous reductions of the endoperoxides 9,10-diphenyl-9,10-epidioxyanthracene (DPA-O2) and 9,10-dimethyl-9,10-epidioxyanthracene (DMA-O2) were investigated, and they were found to undergo a dissociative electron-transfer reduction of the O−O bond to yield a distonic radical anion, with no evidence for C−O bond dissociation. A number of thermochemical parameters for each were determined using Saveant's model for dissociative electron transfer (ET), including E°, Δ , and bond dissociation energies. The products of the ET are dependent on the mode of reduction, namely heterogeneous or homogeneous, and on the electrode potential or standard potential of the homogeneous donor, respectively. The dissociative reduction of DMA-O2 under heterogeneous and homogeneous conditions yields the corresponding 9,10-dihydroxyanthracene DMA-(OH)2, quantitatively, in an overall two-electron process. In the case of DPA-O2, ET reduction also yields the corresponding 9,10-dihydroxyanthracene DPA-(OH)2 f...

Elucidation of the electron transfer reduction mechanism of anthracene endoperoxides

The reaction Mg+O 2 +N 2 was studied by a static-flow technique in which the rate coefficient was determined relative to the diffusion coefficient of Mg( 1 S) atoms in N 2 . This yielded k(349K<T<625 K)=(2.15±0.56) ×10 -30 exp[-(19.5±0.7) kJ mol -1 /RT], where the quoted uncertainty is 2σ. This result is in very good accord with the results of a recent flow-tube study which found the rate coefficient to be about 3 times slower when He was the third body. The reaction Ca+O 2 +M (M=N 2 , He) was then studied by the pulsed laser photolysis of a Ca atom precursor, followed by time-resolved laser-induced fluorescence of the resulting Ca( 1 S) atoms.

Unusual kinetic behavior of the reactions magnesium + oxygen + M and calcium + oxygen + M (M = N2, He) over extended temperature ranges

A study is presented of the reaction Ca({sup 1}S) + N{sub 2}O over the temperature range 250-898 K. Above 500 K there is a clear upward curvature in the Arrhenius plot, and the best description of the temperature dependence of the rate constant over the experimental temperature range is given. Such behavior is explained by vibrationally excited N{sub 2}O enhancing the reaction at high temperatures. This rate coefficient expression, and the chemiluminescence cross section obtained in a previous study of the title reaction in a beam-scattering experiment, are used to estimate photon yield of 12.5%. This yield is shown to be about half that predicted by a statistical consideration of the densities of states in the reactant electronic channels.

A kinetic investigation of the calcium/calcium oxide system: non-Arrhenius behavior of the reaction calcium(1S) + nitrous oxide over the temperature range 250-898 K and a study of the reaction calcium oxide + oxygen atoms

This reaction was studied by two techniques. At temperatures below 650 K, where the reaction is extremely slow, a novel static-flow method was used to measure the rate coefficient relative to the diffusion coefficient of Mg( 1 S) atoms in N 2

A kinetic investigation of the reaction magnesium (1S) + nitrous oxide over the temperature range 382-893 K

Unusual kinetic behavior of the reactions magnesium + oxygen + M and calcium + oxygen + M (M = N2, He) over extended temperature ranges. [Erratum to document cited in CA119(2):16021u]

Chia Fu Nien

Papers

Unusual kinetic behavior of the reactions magnesium + oxygen + M and calcium + oxygen + M (M = N2, He) over extended temperature ranges

A kinetic investigation of the calcium/calcium oxide system: non-Arrhenius behavior of the reaction calcium(1S) + nitrous oxide over the temperature range 250-898 K and a study of the reaction calcium oxide + oxygen atoms

A kinetic investigation of the reaction magnesium (1S) + nitrous oxide over the temperature range 382-893 K

Unusual kinetic behavior of the reactions magnesium + oxygen + M and calcium + oxygen + M (M = N2, He) over extended temperature ranges. [Erratum to document cited in CA119(2):16021u]